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143-492: BJTs use two p–n junctions between two semiconductor types, n-type and p-type, which are regions in a single crystal of material. The junctions can be made in several different ways, such as changing the doping of the semiconductor material as it is grown, by depositing metal pellets to form alloy junctions, or by such methods as diffusion of n-type and p-type doping substances into the crystal. The superior predictability and performance of junction transistors quickly displaced

286-440: A current requires the flow of electrons, and semiconductors have their valence bands filled, preventing the entire flow of new electrons. Several developed techniques allow semiconducting materials to behave like conducting materials, such as doping or gating . These modifications have two outcomes: n-type and p-type . These refer to the excess or shortage of electrons, respectively. A balanced number of electrons would cause

429-439: A cut-off frequency of one cycle per second, too low for any practical applications, but an effective application of the available theory. At Bell Labs , William Shockley and A. Holden started investigating solid-state amplifiers in 1938. The first p–n junction in silicon was observed by Russell Ohl about 1941 when a specimen was found to be light-sensitive, with a sharp boundary between p-type impurity at one end and n-type at

572-442: A hole . It is an important parameter in optoelectronics where radiative recombination is required to produce a photon; if the non-radiative life time is shorter than the radiative, a carrier is more likely to recombine non-radiatively. This results in low internal quantum efficiency . In Shockley-Read-Hall recombination ( SRH ), also called trap-assisted recombination , the electron in transition between bands passes through

715-407: A mass-production basis, which limited them to a number of specialised applications. Carrier generation and recombination In solid-state physics of semiconductors , carrier generation and carrier recombination are processes by which mobile charge carriers ( electrons and electron holes ) are created and eliminated. Carrier generation and recombination processes are fundamental to

858-462: A common region that minority carriers can move through. A PNP BJT will function like two diodes that share an N-type cathode region, and the NPN like two diodes sharing a P-type anode region. Connecting two diodes with wires will not make a BJT, since minority carriers will not be able to get from one p–n junction to the other through the wire. Both types of BJT function by letting a small current input to

1001-513: A common semi-insulator is gallium arsenide . Some materials, such as titanium dioxide , can even be used as insulating materials for some applications, while being treated as wide-gap semiconductors for other applications. The partial filling of the states at the bottom of the conduction band can be understood as adding electrons to that band. The electrons do not stay indefinitely (due to the natural thermal recombination ) but they can move around for some time. The actual concentration of electrons

1144-423: A completely full valence band is inert, not conducting any current. If an electron is taken out of the valence band, then the trajectory that the electron would normally have taken is now missing its charge. For the purposes of electric current, this combination of the full valence band, minus the electron, can be converted into a picture of a completely empty band containing a positively charged particle that moves in

1287-482: A conduction band electron loses energy and re-occupies the energy state of an electron hole in the valence band. These processes must conserve quantized energy crystal momentum , and the vibrating lattice which plays a large role in conserving momentum as in collisions, photons can transfer very little momentum in relation to their energy. Recombination and generation are always happening in semiconductors, both optically and thermally. As predicted by thermodynamics ,

1430-474: A current to flow throughout the material. Homojunctions occur when two differently doped semiconducting materials are joined. For example, a configuration could consist of p-doped and n-doped germanium . This results in an exchange of electrons and holes between the differently doped semiconducting materials. The n-doped germanium would have an excess of electrons, and the p-doped germanium would have an excess of holes. The transfer occurs until an equilibrium

1573-406: A few hundred millivolts) biases. For example, in the typical grounded-emitter configuration of an NPN BJT used as a pulldown switch in digital logic, the "off" state never involves a reverse-biased junction because the base voltage never goes below ground; nevertheless the forward bias is close enough to zero that essentially no current flows, so this end of the forward active region can be regarded as

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1716-410: A guide to the construction of more capable and reliable devices. Alexander Graham Bell used the light-sensitive property of selenium to transmit sound over a beam of light in 1880. A working solar cell, of low efficiency, was constructed by Charles Fritts in 1883, using a metal plate coated with selenium and a thin layer of gold; the device became commercially useful in photographic light meters in

1859-399: A junction between two regions of different charge carrier concentration. The regions of a BJT are called emitter , base , and collector . A discrete transistor has three leads for connection to these regions. Typically, the emitter region is heavily doped compared to the other two layers, and the collector is doped more lightly (typically ten times lighter) than the base. By design, most of

2002-408: A logical process. Bipolar transistors, and particularly power transistors, have long base-storage times when they are driven into saturation; the base storage limits turn-off time in switching applications. A Baker clamp can prevent the transistor from heavily saturating, which reduces the amount of charge stored in the base and thus improves switching time. The proportion of carriers able to cross

2145-445: A low-pressure chamber to create plasma . A common etch gas is chlorofluorocarbon , or more commonly known Freon . A high radio-frequency voltage between the cathode and anode is what creates the plasma in the chamber. The silicon wafer is located on the cathode, which causes it to be hit by the positively charged ions that are released from the plasma. The result is silicon that is etched anisotropically . The last process

2288-421: A material at thermal equilibrium will have generation and recombination rates that are balanced so that the net charge carrier density remains constant. The resulting probability of occupation of energy states in each energy band is given by Fermi–Dirac statistics . The product of the electron and hole densities ( n {\displaystyle n} and p {\displaystyle p} )

2431-450: A material that is between the conductor and insulator in ability to conduct electrical current. In many cases their conducting properties may be altered in useful ways by introducing impurities (" doping ") into the crystal structure . When two differently doped regions exist in the same crystal, a semiconductor junction is created. The behavior of charge carriers , which include electrons , ions , and electron holes , at these junctions

2574-506: A more positive potential than the n-doped side, and the base–collector junction is reverse biased . When forward bias is applied to the base–emitter junction, the equilibrium between the thermally generated carriers and the repelling electric field of the emitter depletion region is disturbed. This allows thermally excited carriers (electrons in NPNs, holes in PNPs) to inject from the emitter into

2717-410: A new energy state (localized state) created within the band gap by a dopant or a defect in the crystal lattice ; such energy states are called traps . Non-radiative recombination occurs primarily at such sites. The energy is exchanged in the form of lattice vibration, a phonon exchanging thermal energy with the material. Since traps can absorb differences in momentum between the carriers, SRH

2860-626: A non-equilibrium situation. This introduces electrons and holes to the system, which interact via a process called ambipolar diffusion . Whenever thermal equilibrium is disturbed in a semiconducting material, the number of holes and electrons changes. Such disruptions can occur as a result of a temperature difference or photons , which can enter the system and create electrons and holes. The processes that create or annihilate electrons and holes are called generation and recombination, respectively. In certain semiconductors, excited electrons can relax by emitting light instead of producing heat. Controlling

3003-519: A pair is completed. Such carrier traps are sometimes purposely added to reduce the time needed to reach the steady-state. The conductivity of semiconductors may easily be modified by introducing impurities into their crystal lattice . The process of adding controlled impurities to a semiconductor is known as doping . The amount of impurity, or dopant, added to an intrinsic (pure) semiconductor varies its level of conductivity. Doped semiconductors are referred to as extrinsic . By adding impurity to

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3146-407: A photon with the same properties as the incident photon , in terms of phase , frequency , polarization , and direction of travel. Stimulated emission together with the principle of population inversion are at the heart of operation of lasers and masers . It has been shown by Einstein at the beginning of the twentieth century that if the excited and the ground level are non degenerate then

3289-425: A proportionality constant B r to eliminate the ∝ {\displaystyle \propto } sign: R r = B r n p {\displaystyle R_{r}=B_{r}np} If the semiconductor is in thermal equilibrium, the rate at which electrons and holes recombine must be balanced by the rate at which they are generated by the spontaneous transition of an electron from

3432-501: A silicon atom in the crystal, a vacant state (an electron "hole") is created, which can move around the lattice and function as a charge carrier. Group V elements have five valence electrons, which allows them to act as a donor; substitution of these atoms for silicon creates an extra free electron. Therefore, a silicon crystal doped with boron creates a p-type semiconductor whereas one doped with phosphorus results in an n-type material. During manufacture , dopants can be diffused into

3575-783: A theory of solid-state physics , which developed greatly in the first half of the 20th century. In 1878 Edwin Herbert Hall demonstrated the deflection of flowing charge carriers by an applied magnetic field, the Hall effect . The discovery of the electron by J.J. Thomson in 1897 prompted theories of electron-based conduction in solids. Karl Baedeker , by observing a Hall effect with the reverse sign to that in metals, theorized that copper iodide had positive charge carriers. Johan Koenigsberger  [ de ] classified solid materials like metals, insulators, and "variable conductors" in 1914 although his student Josef Weiss already introduced

3718-423: A thin p-doped region, and a PNP transistor comprises two semiconductor junctions that share a thin n-doped region. N-type means doped with impurities (such as phosphorus or arsenic ) that provide mobile electrons, while p-type means doped with impurities (such as boron ) that provide holes that readily accept electrons. Charge flow in a BJT is due to diffusion of charge carriers (electrons and holes) across

3861-476: A vacuum, though with a different effective mass . Because the electrons behave like an ideal gas, one may also think about conduction in very simplistic terms such as the Drude model , and introduce concepts such as electron mobility . For partial filling at the top of the valence band, it is helpful to introduce the concept of an electron hole . Although the electrons in the valence band are always moving around,

4004-567: A variety of proportions. These compounds share with better-known semiconductors the properties of intermediate conductivity and a rapid variation of conductivity with temperature, as well as occasional negative resistance . Such disordered materials lack the rigid crystalline structure of conventional semiconductors such as silicon. They are generally used in thin film structures, which do not require material of higher electronic quality, being relatively insensitive to impurities and radiation damage. Almost all of today's electronic technology involves

4147-415: Is a combination of processes that are used to prepare semiconducting materials for ICs. One process is called thermal oxidation , which forms silicon dioxide on the surface of the silicon . This is used as a gate insulator and field oxide . Other processes are called photomasks and photolithography . This process is what creates the patterns on the circuit in the integrated circuit. Ultraviolet light

4290-431: Is a constant ( n o p o = n i 2 ) {\displaystyle (n_{o}p_{o}=n_{i}^{2})} at equilibrium, maintained by recombination and generation occurring at equal rates. When there is a surplus of carriers (i.e., n p > n i 2 {\displaystyle np>n_{i}^{2}} ), the rate of recombination becomes greater than

4433-547: Is a critical element for fabricating most electronic circuits . Semiconductor devices can display a range of different useful properties, such as passing current more easily in one direction than the other, showing variable resistance, and having sensitivity to light or heat. Because the electrical properties of a semiconductor material can be modified by doping and by the application of electrical fields or light, devices made from semiconductors can be used for amplification, switching, and energy conversion . The term semiconductor

Bipolar junction transistor - Misplaced Pages Continue

4576-472: Is a function of the temperature, as the probability of getting enough thermal energy to produce a pair increases with temperature, being approximately exp(− E G / kT ) , where k is the Boltzmann constant , T is the absolute temperature and E G is bandgap. The probability of meeting is increased by carrier traps – impurities or dislocations which can trap an electron or hole and hold it until

4719-434: Is a multistep process wherein a carrier falls into defect-related wave states in the middle of the bandgap. A trap is a defect capable of holding a carrier. The trap emission process recombines electrons with holes and emits photons to conserve energy. Due to the multistep nature of trap emission, a phonon is also often emitted. Trap emission can proceed by use of bulk defects or surface defects. Non-radiative recombination

4862-408: Is a process in phosphors and semiconductors , whereby charge carriers recombine releasing phonons instead of photons. Non-radiative recombination in optoelectronics and phosphors is an unwanted process, lowering the light generation efficiency and increasing heat losses. Non-radiative life time is the average time before an electron in the conduction band of a semiconductor recombines with

5005-428: Is also used to describe materials used in high capacity, medium- to high-voltage cables as part of their insulation, and these materials are often plastic XLPE ( Cross-linked polyethylene ) with carbon black. The conductivity of silicon is increased by adding a small amount (of the order of 1 in 10 ) of pentavalent ( antimony , phosphorus , or arsenic ) or trivalent ( boron , gallium , indium ) atoms. This process

5148-720: Is an improvement of the BJT that can handle signals of very high frequencies up to several hundred GHz . It is common in modern ultrafast circuits, mostly RF systems. Two commonly used HBTs are silicon–germanium and aluminum gallium arsenide, though a wide variety of semiconductors may be used for the HBT structure. HBT structures are usually grown by epitaxy techniques like MOCVD and MBE . Bipolar transistors have four distinct regions of operation, defined by BJT junction biases: Although these regions are well defined for sufficiently large applied voltage, they overlap somewhat for small (less than

5291-459: Is approximately β F {\displaystyle \beta _{\text{F}}} times the base current. Some basic circuits can be designed by assuming that the base–emitter voltage is approximately constant and that collector current is β times the base current. However, to accurately and reliably design production BJT circuits, the voltage-control model (e.g. the Ebers–Moll model )

5434-406: Is called conventional current . However, current in metal conductors is generally due to the flow of electrons. Because electrons carry a negative charge, they move in the direction opposite to conventional current. On the other hand, inside a bipolar transistor, currents can be composed of both positively charged holes and negatively charged electrons. In this article, current arrows are shown in

5577-404: Is called diffusion . This is the process that gives the semiconducting material its desired semiconducting properties. It is also known as doping . The process introduces an impure atom to the system, which creates the p–n junction . To get the impure atoms embedded in the silicon wafer, the wafer is first put in a 1,100 degree Celsius chamber. The atoms are injected in and eventually diffuse with

5720-411: Is called active mode, the base–emitter voltage V BE {\displaystyle V_{\text{BE}}} and collector–base voltage V CB {\displaystyle V_{\text{CB}}} are positive, forward biasing the emitter–base junction and reverse-biasing the collector–base junction. In this mode, electrons are injected from the forward biased n-type emitter region into

5863-780: Is inert, blocking the passage of other electrons via that state. The energies of these quantum states are critical since a state is partially filled only if its energy is near the Fermi level (see Fermi–Dirac statistics ). High conductivity in material comes from it having many partially filled states and much state delocalization. Metals are good electrical conductors and have many partially filled states with energies near their Fermi level. Insulators , by contrast, have few partially filled states, their Fermi levels sit within band gaps with few energy states to occupy. Importantly, an insulator can be made to conduct by increasing its temperature: heating provides energy to promote some electrons across

Bipolar junction transistor - Misplaced Pages Continue

6006-447: Is key to the operation, the transistors are usually modeled as voltage-controlled current sources whose transconductance is proportional to their collector current. In general, transistor-level circuit analysis is performed using SPICE or a comparable analog-circuit simulator, so mathematical model complexity is usually not of much concern to the designer, but a simplified view of the characteristics allows designs to be created following

6149-418: Is known as doping, and the resulting semiconductors are known as doped or extrinsic semiconductors . Apart from doping, the conductivity of a semiconductor can be improved by increasing its temperature. This is contrary to the behavior of a metal, in which conductivity decreases with an increase in temperature. The modern understanding of the properties of a semiconductor relies on quantum physics to explain

6292-409: Is made from lightly doped, high-resistivity material. The collector surrounds the emitter region, making it almost impossible for the electrons injected into the base region to escape without being collected, thus making the resulting value of α very close to unity, and so, giving the transistor a large β. A cross-section view of a BJT indicates that the collector–base junction has a much larger area than

6435-845: Is neither a very good insulator nor a very good conductor. However, one important feature of semiconductors (and some insulators, known as semi-insulators ) is that their conductivity can be increased and controlled by doping with impurities and gating with electric fields. Doping and gating move either the conduction or valence band much closer to the Fermi level and greatly increase the number of partially filled states. Some wider-bandgap semiconductor materials are sometimes referred to as semi-insulators . When undoped, these have electrical conductivity nearer to that of electrical insulators, however they can be doped (making them as useful as semiconductors). Semi-insulators find niche applications in micro-electronics, such as substrates for HEMT . An example of

6578-404: Is reached by a process called recombination , which causes the migrating electrons from the n-type to come in contact with the migrating holes from the p-type. The result of this process is a narrow strip of immobile ions , which causes an electric field across the junction. A difference in electric potential on a semiconducting material would cause it to leave thermal equilibrium and create

6721-447: Is related to V BE {\displaystyle V_{\text{BE}}} exponentially. At room temperature , an increase in V BE {\displaystyle V_{\text{BE}}} by approximately 60 mV increases the emitter current by a factor of 10. Because the base current is approximately proportional to the collector and emitter currents, they vary in the same way. The bipolar point-contact transistor

6864-419: Is replaced by the density of trapped electrons/holes N t ( 1 − f t ) {\displaystyle N_{t}(1-f_{t})} . R n t = B n n N t ( 1 − f t ) {\displaystyle R_{nt}=B_{n}nN_{t}(1-f_{t})} Where N t {\displaystyle N_{t}}

7007-469: Is required. The voltage-control model requires an exponential function to be taken into account, but when it is linearized such that the transistor can be modeled as a transconductance, as in the Ebers–Moll model, design for circuits such as differential amplifiers again becomes a mostly linear problem, so the voltage-control view is often preferred. For translinear circuits , in which the exponential I–V curve

7150-577: Is reverse biased in normal operation. The reason the emitter is heavily doped is to increase the emitter injection efficiency: the ratio of carriers injected by the emitter to those injected by the base. For high current gain, most of the carriers injected into the emitter–base junction must come from the emitter. The low-performance "lateral" bipolar transistors sometimes used in CMOS processes are sometimes designed symmetrically, that is, with no difference between forward and backward operation. Small changes in

7293-481: Is significant only in direct bandgap materials. This process is also known as bimolecular recombination . This type of recombination depends on the density of electrons and holes in the excited state, denoted by n ( t ) {\displaystyle n(t)} and p ( t ) {\displaystyle p(t)} respectively. Let us represent the radiative recombination as R r {\displaystyle R_{r}} and

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7436-413: Is the basis of diodes , transistors , and most modern electronics . Some examples of semiconductors are silicon , germanium , gallium arsenide , and elements near the so-called " metalloid staircase " on the periodic table . After silicon, gallium arsenide is the second-most common semiconductor and is used in laser diodes , solar cells , microwave-frequency integrated circuits , and others. Silicon

7579-728: Is the density of trap states and f t {\displaystyle f_{t}} is the probability of that occupied state. Considering a material containing both types of traps, we can define two trapping coefficients B n , B p {\displaystyle B_{n},B_{p}} and two de-trapping coefficients G n , G p {\displaystyle G_{n},G_{p}} . In equilibrium, both trapping and de-trapping should be balanced ( R n t = G n {\displaystyle R_{nt}=G_{n}} and R p t = G p {\displaystyle R_{pt}=G_{p}} ). Then,

7722-456: Is the dominant recombination process in silicon and other indirect bandgap materials. However, trap-assisted recombination can also dominate in direct bandgap materials under conditions of very low carrier densities (very low level injection) or in materials with high density of traps such as perovskites . The process is named after William Shockley , William Thornton Read and Robert N. Hall , who published it in 1952. Even though all

7865-470: Is the name for the process of electrons jumping down from the conduction band to the valence band in a radiative manner. During band-to-band recombination, a form of spontaneous emission , the energy absorbed by a material is released in the form of photons. Generally these photons contain the same or less energy than those initially absorbed. This effect is how LEDs create light. Because the photon carries relatively little momentum , radiative recombination

8008-451: Is the principle of operation in laser diodes . Besides light excitation, carriers in semiconductors can also be generated by an external electric field, for example in light-emitting diodes and transistors . When light with sufficient energy hits a semiconductor, it can excite electrons across the band gap. This generates additional charge carriers, temporarily lowering the electrical resistance of materials. This higher conductivity in

8151-587: Is the rate at which excess holes Δ p {\displaystyle \Delta p} disappear Solve this differential equation to get a standard exponential decay where p max is the maximum excess hole concentration when t = 0. (It can be proved that p max = G L B n 0 {\displaystyle p_{\max }={\frac {G_{L}}{Bn_{0}}}} , but here we will not discuss that). When t = 1 B n 0 {\displaystyle t={\frac {1}{Bn_{0}}}} , all of

8294-887: Is then obtained from the rate of both type of events according to: 1 τ = 1 τ r + 1 τ n r {\displaystyle {\frac {1}{\tau }}={\frac {1}{\tau _{r}}}+{\frac {1}{\tau _{nr}}}} From which we can also define the internal quantum efficiency or quantum yield, η {\displaystyle \eta } as: η = 1 / τ r 1 / τ r + 1 / τ n r = radiative recombination total recombination ≤ 1. {\displaystyle \eta ={\frac {1/\tau _{r}}{1/\tau _{r}+1/\tau _{nr}}}={\frac {\text{radiative recombination}}{\text{total recombination}}}\leq 1.} Band-to-band recombination

8437-504: Is typically very dilute, and so (unlike in metals) it is possible to think of the electrons in the conduction band of a semiconductor as a sort of classical ideal gas , where the electrons fly around freely without being subject to the Pauli exclusion principle . In most semiconductors, the conduction bands have a parabolic dispersion relation , and so these electrons respond to forces (electric field, magnetic field, etc.) much as they would in

8580-402: Is used along with a photoresist layer to create a chemical change that generates the patterns for the circuit. The etching is the next process that is required. The part of the silicon that was not covered by the photoresist layer from the previous step can now be etched. The main process typically used today is called plasma etching . Plasma etching usually involves an etch gas pumped in

8723-408: Is usually 100 or more, but robust circuit designs do not depend on the exact value (for example see op-amp ). The value of this gain for DC signals is referred to as h FE {\displaystyle h_{\text{FE}}} , and the value of this gain for small signals is referred to as h fe {\displaystyle h_{\text{fe}}} . That is, when a small change in

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8866-532: The Annalen der Physik und Chemie in 1835; Rosenschöld's findings were ignored. Simon Sze stated that Braun's research was the earliest systematic study of semiconductor devices. Also in 1874, Arthur Schuster found that a copper oxide layer on wires had rectification properties that ceased when the wires are cleaned. William Grylls Adams and Richard Evans Day observed the photovoltaic effect in selenium in 1876. A unified explanation of these phenomena required

9009-429: The Pauli exclusion principle ). These states are associated with the electronic band structure of the material. Electrical conductivity arises due to the presence of electrons in states that are delocalized (extending through the material), however in order to transport electrons a state must be partially filled , containing an electron only part of the time. If the state is always occupied with an electron, then it

9152-454: The Siege of Leningrad after successful completion. In 1926, Julius Edgar Lilienfeld patented a device resembling a field-effect transistor , but it was not practical. R. Hilsch  [ de ] and R. W. Pohl  [ de ] in 1938 demonstrated a solid-state amplifier using a structure resembling the control grid of a vacuum tube; although the device displayed power gain, it had

9295-445: The band gap , be accompanied by the emission of thermal energy (in the form of phonons ) or radiation (in the form of photons ). In some states, the generation and recombination of electron–hole pairs are in equipoise. The number of electron-hole pairs in the steady state at a given temperature is determined by quantum statistical mechanics . The precise quantum mechanical mechanisms of generation and recombination are governed by

9438-470: The conservation of energy and conservation of momentum . As the probability that electrons and holes meet together is proportional to the product of their numbers, the product is in the steady-state nearly constant at a given temperature, providing that there is no significant electric field (which might "flush" carriers of both types, or move them from neighbor regions containing more of them to meet together) or externally driven pair generation. The product

9581-416: The emitter region, the base region and the collector region. These regions are, respectively, p type, n type and p type in a PNP transistor, and n type, p type and n type in an NPN transistor. Each semiconductor region is connected to a terminal, appropriately labeled: emitter (E), base (B) and collector (C). The base is physically located between the emitter and the collector and

9724-404: The valence band they become hole traps. The distinction between shallow and deep traps is commonly made depending on how close electron traps are to the conduction band and how close hole traps are to the valence band. If the difference between trap and band is smaller than the thermal energy k B T it is often said that it is a shallow trap . Alternatively, if the difference is larger than

9867-419: The vibrating crystal lattice itself , it can flow freely among the nearly empty conduction band energy states. Furthermore, it will also leave behind a hole that can flow like a physically charged particle. Carrier generation describes processes by which electrons gain energy and move from the valence band to the conduction band, producing two mobile carriers; while recombination describes processes by which

10010-461: The 1930s. Point-contact microwave detector rectifiers made of lead sulfide were used by Jagadish Chandra Bose in 1904; the cat's-whisker detector using natural galena or other materials became a common device in the development of radio . However, it was somewhat unpredictable in operation and required manual adjustment for best performance. In 1906, H.J. Round observed light emission when electric current passed through silicon carbide crystals,

10153-401: The BJT collector current is due to the flow of charge carriers injected from a heavily doped emitter into the base where they are minority carriers (electrons in NPNs, holes in PNPs) that diffuse toward the collector, so BJTs are classified as minority-carrier devices . In typical operation, the base–emitter junction is forward biased , which means that the p-doped side of the junction is at

10296-450: The Ebers–Moll model: The base internal current is mainly by diffusion (see Fick's law ) and where The α {\displaystyle \alpha } and forward β {\displaystyle \beta } parameters are as described previously. A reverse β {\displaystyle \beta } is sometimes included in the model. The unapproximated Ebers–Moll equations used to describe

10439-579: The absorption rate W 12 {\displaystyle W_{12}} and the stimulated emission rate W 21 {\displaystyle W_{21}} are the same. Else if level 1 and level 2 are g 1 {\displaystyle g_{1}} -fold and g 2 {\displaystyle g_{2}} -fold degenerate respectively, the new relation is: g 1 W 12 = g 2 W 21 . {\displaystyle g_{1}W_{12}=g_{2}W_{21}.} Trap emission

10582-416: The band gap, inducing partially filled states in both the band of states beneath the band gap ( valence band ) and the band of states above the band gap ( conduction band ). An (intrinsic) semiconductor has a band gap that is smaller than that of an insulator and at room temperature, significant numbers of electrons can be excited to cross the band gap. A pure semiconductor, however, is not very useful, as it

10725-400: The base and reach the collector is a measure of the BJT efficiency. The heavy doping of the emitter region and light doping of the base region causes many more electrons to be injected from the emitter into the base than holes to be injected from the base into the emitter. A thin and lightly doped base region means that most of the minority carriers that are injected into the base will diffuse to

10868-465: The base control an amplified output from the collector. The result is that the BJT makes a good switch that is controlled by its base input. The BJT also makes a good amplifier, since it can multiply a weak input signal to about 100 times its original strength. Networks of BJTs are used to make powerful amplifiers with many different applications. In the discussion below, focus is on the NPN BJT. In what

11011-427: The base reduce the BJT gain. Another useful characteristic is the common-base current gain , α F . The common-base current gain is approximately the gain of current from emitter to collector in the forward-active region. This ratio usually has a value close to unity; between 0.980 and 0.998. It is less than unity due to recombination of charge carriers as they cross the base region. Alpha and beta are related by

11154-400: The base region. These carriers create a diffusion current through the base from the region of high concentration near the emitter toward the region of low concentration near the collector. To minimize the fraction of carriers that recombine before reaching the collector–base junction, the transistor's base region must be thin enough that carriers can diffuse across it in much less time than

11297-434: The base–emitter junction and recombination in the base). In many designs beta is assumed high enough so that base current has a negligible effect on the circuit. In some circuits (generally switching circuits), sufficient base current is supplied so that even the lowest beta value a particular device may have will still allow the required collector current to flow. BJTs consists of three differently doped semiconductor regions:

11440-533: The carrier generation rate as G. Total generation is the sum of thermal generation G 0 and generation due to light shining on the semiconductor G L : G = G 0 + G L {\displaystyle G=G_{0}+G_{L}} Here we will consider the case in which there is no illumination on the semiconductor. Therefore G L = 0 {\displaystyle G_{L}=0} and G = G 0 {\displaystyle G=G_{0}} , and we can express

11583-564: The change in carrier density as a function of time as d n d t = G − R r = G 0 − R r {\displaystyle {dn \over dt}=G-R_{r}=G_{0}-R_{r}} Because the rate of recombination is affected by both the concentration of free electrons and the concentration of holes that are available to them, we know that R r should be proportional to np: R r ∝ n p {\displaystyle R_{r}\propto np} and we add

11726-507: The collector and not recombine. The common-emitter current gain is represented by β F or the h -parameter h FE ; it is approximately the ratio of the collector's direct current to the base's direct current in forward-active region. (The F subscript is used to indicate the forward-active mode of operation.) It is typically greater than 50 for small-signal transistors, but can be smaller in transistors designed for high-power applications. Both injection efficiency and recombination in

11869-458: The collector–base depletion region, are swept into the collector by the electric field in the depletion region. The thin shared base and asymmetric collector–emitter doping are what differentiates a bipolar transistor from two separate diodes connected in series. The collector–emitter current can be viewed as being controlled by the base–emitter current (current control), or by the base–emitter voltage (voltage control). These views are related by

12012-466: The collector–base voltage, for example, causes a greater reverse bias across the collector–base junction, increasing the collector–base depletion region width, and decreasing the width of the base. This variation in base width often is called the Early effect after its discoverer James M. Early . Narrowing of the base width has two consequences: Both factors increase the collector or "output" current of

12155-406: The concentration and regions of p- and n-type dopants. A single semiconductor device crystal can have many p- and n-type regions; the p–n junctions between these regions are responsible for the useful electronic behavior. Using a hot-point probe , one can determine quickly whether a semiconductor sample is p- or n-type. A few of the properties of semiconductor materials were observed throughout

12298-489: The concept of band gaps had been developed. Walter H. Schottky and Nevill Francis Mott developed models of the potential barrier and of the characteristics of a metal–semiconductor junction . By 1938, Boris Davydov had developed a theory of the copper-oxide rectifier, identifying the effect of the p–n junction and the importance of minority carriers and surface states. Agreement between theoretical predictions (based on developing quantum mechanics) and experimental results

12441-465: The conventional direction, but labels for the movement of holes and electrons show their actual direction inside the transistor. The arrow on the symbol for bipolar transistors indicates the p–n junction between base and emitter and points in the direction in which conventional current travels. BJTs exist as PNP and NPN types, based on the doping types of the three main terminal regions. An NPN transistor comprises two semiconductor junctions that share

12584-506: The currents occurs, and sufficient time has passed for the new condition to reach a steady state h fe {\displaystyle h_{\text{fe}}} is the ratio of the change in collector current to the change in base current. The symbol β {\displaystyle \beta } is used for both h FE {\displaystyle h_{\text{FE}}} and h fe {\displaystyle h_{\text{fe}}} . The emitter current

12727-448: The current–voltage relation of the base–emitter junction, which is the usual exponential current–voltage curve of a p–n junction (diode). The explanation for collector current is the concentration gradient of minority carriers in the base region. Due to low-level injection (in which there are much fewer excess carriers than normal majority carriers) the ambipolar transport rates (in which the excess majority and minority carriers flow at

12870-476: The cutoff region. The diagram shows a schematic representation of an NPN transistor connected to two voltage sources. (The same description applies to a PNP transistor with reversed directions of current flow and applied voltage.) This applied voltage causes the lower p–n junction to become forward biased, allowing a flow of electrons from the emitter into the base. In active mode, the electric field existing between base and collector (caused by V CE ) will cause

13013-671: The design of digital integrated circuits. The incidental low performance BJTs inherent in CMOS ICs, however, are often utilized as bandgap voltage reference , silicon bandgap temperature sensor and to handle electrostatic discharge . The germanium transistor was more common in the 1950s and 1960s but has a greater tendency to exhibit thermal runaway . Since germanium p-n junctions have a lower forward bias than silicon, germanium transistors turn on at lower voltage. Various methods of manufacturing bipolar transistors were developed. BJTs can be thought of as two diodes (p–n junctions) sharing

13156-408: The dynamics of turn-off, or recovery time, which depends on charge in the base region recombining. However, because base charge is not a signal that is visible at the terminals, the current- and voltage-control views are generally used in circuit design and analysis. In analog circuit design, the current-control view is sometimes used because it is approximately linear. That is, the collector current

13299-453: The electrical properties of materials. The properties of the time-temperature coefficient of resistance, rectification, and light-sensitivity were observed starting in the early 19th century. Thomas Johann Seebeck was the first to notice that semiconductors exhibit special feature such that experiment concerning an Seebeck effect emerged with much stronger result when applying semiconductors, in 1821. In 1833, Michael Faraday reported that

13442-415: The electron and hole densities when the quasi Fermi level matches the trap energy. In steady-state condition, the net recombination rate of electrons should match the net recombination rate for holes, in other words: R n t − G n = R p t − G p {\displaystyle R_{nt}-G_{n}=R_{pt}-G_{p}} . This eliminates

13585-530: The electrons in the conduction band). When ionizing radiation strikes a semiconductor, it may excite an electron out of its energy level and consequently leave a hole. This process is known as electron-hole pair generation . Electron-hole pairs are constantly generated from thermal energy as well, in the absence of any external energy source. Electron-hole pairs are also apt to recombine. Conservation of energy demands that these recombination events, in which an electron loses an amount of energy larger than

13728-420: The electrons. At absolute zero temperature, all of the electrons have energy below the Fermi level; but at non-zero temperatures the energy levels are filled following a Fermi-Dirac distribution . In undoped semiconductors the Fermi level lies in the middle of a forbidden band or band gap between two allowed bands called the valence band and the conduction band . The valence band, immediately below

13871-416: The emitter–base junction. The bipolar junction transistor, unlike other transistors, is usually not a symmetrical device. This means that interchanging the collector and the emitter makes the transistor leave the forward active mode and start to operate in reverse mode. Because the transistor's internal structure is usually optimized for forward-mode operation, interchanging the collector and the emitter makes

14014-800: The equilibrium carrier densities. Using the mass action law n p = n i 2 {\displaystyle np=n_{i}^{2}} ，with n i {\displaystyle n_{i}} being the intrinsic carrier density, we can rewrite it as R 0 = G 0 = B r n 0 p 0 = B r n i 2 {\displaystyle R_{0}=G_{0}=B_{r}n_{0}p_{0}=B_{r}n_{i}^{2}} The non-equilibrium carrier densities are given by n = n 0 + Δ n , {\displaystyle n=n_{0}+\Delta n,} p = p 0 + Δ p {\displaystyle p=p_{0}+\Delta p} Then

14157-441: The excess energy is converted into heat by phonon emission after the mean lifetime τ n r {\displaystyle \tau _{nr}} , whereas in the former at least part of the energy is released by light emission or luminescence after a radiative lifetime τ r {\displaystyle \tau _{r}} . The carrier lifetime τ {\displaystyle \tau }

14300-479: The excess holes will have disappeared. Therefore, we can define the lifetime of the excess holes in the material τ p = 1 B n 0 {\displaystyle \tau _{p}={\frac {1}{Bn_{0}}}} So the lifetime of the minority carrier is dependent upon the majority carrier concentration. Stimulated emission is a process where an incident photon interacts with an excited electron causing it to recombine and emit

14443-514: The fast response of crystal detectors. Considerable research and development of silicon materials occurred during the war to develop detectors of consistent quality. Detector and power rectifiers could not amplify a signal. Many efforts were made to develop a solid-state amplifier and were successful in developing a device called the point contact transistor which could amplify 20 dB or more. In 1922, Oleg Losev developed two-terminal, negative resistance amplifiers for radio, but he died in

14586-414: The field-effect transistor (FET). Bipolar transistors are still used for amplification of signals, switching, and in mixed-signal integrated circuits using BiCMOS . Specialized types are used for high voltage switches, for radio-frequency (RF) amplifiers, or for switching high currents. By convention, the direction of current on diagrams is shown as the direction that a positive charge would move. This

14729-410: The following identities: Beta is a convenient figure of merit to describe the performance of a bipolar transistor, but is not a fundamental physical property of the device. Bipolar transistors can be considered voltage-controlled devices (fundamentally the collector current is controlled by the base–emitter voltage; the base current could be considered a defect and is controlled by the characteristics of

14872-426: The forbidden band, is normally very nearly completely occupied. The conduction band, above the Fermi level, is normally nearly completely empty. Because the valence band is so nearly full, its electrons are not mobile, and cannot flow as electric current. However, if an electron in the valence band acquires enough energy to reach the conduction band as a result of interaction with other electrons , holes , photons, or

15015-887: The four rates as a function of f t {\displaystyle f_{t}} become: R n t = B n n N t ( 1 − f t ) G n = B n n t N t f t R p t = B p p N t f t G p = B p p t N t ( 1 − f t ) {\displaystyle {\begin{array}{l l}R_{nt}=B_{n}nN_{t}(1-f_{t})&G_{n}=B_{n}n_{t}N_{t}f_{t}\\R_{pt}=B_{p}pN_{t}f_{t}&G_{p}=B_{p}p_{t}N_{t}(1-f_{t})\end{array}}} Where n t {\displaystyle n_{t}} and p t {\displaystyle p_{t}} are

15158-400: The majority of these electrons to cross the upper p–n junction into the collector to form the collector current I C . The remainder of the electrons recombine with holes, the majority carriers in the base, making a current through the base connection to form the base current, I B . As shown in the diagram, the emitter current, I E , is the total transistor current, which is the sum of

15301-543: The material's majority carrier . The opposite carrier is called the minority carrier , which exists due to thermal excitation at a much lower concentration compared to the majority carrier. For example, the pure semiconductor silicon has four valence electrons that bond each silicon atom to its neighbors. In silicon, the most common dopants are group III and group V elements. Group III elements all contain three valence electrons, causing them to function as acceptors when used to dope silicon. When an acceptor atom replaces

15444-435: The mid-19th and first decades of the 20th century. The first practical application of semiconductors in electronics was the 1904 development of the cat's-whisker detector , a primitive semiconductor diode used in early radio receivers. Developments in quantum physics led in turn to the invention of the transistor in 1947 and the integrated circuit in 1958. Semiconductors in their natural state are poor conductors because

15587-505: The movement of charge carriers in a crystal lattice . Doping greatly increases the number of charge carriers within the crystal. When a semiconductor is doped by Group V elements, they will behave like donors creating free electrons , known as " n-type " doping. When a semiconductor is doped by Group III elements, they will behave like acceptors creating free holes, known as " p-type " doping. The semiconductor materials used in electronic devices are doped under precise conditions to control

15730-951: The new recombination rate R net {\displaystyle R_{\text{net}}} becomes, R net = R r − G 0 = B r n p − G 0 = B r ( n 0 + Δ n ) ( p 0 + Δ p ) − G 0 {\displaystyle R_{\text{net}}=R_{r}-G_{0}=B_{r}np-G_{0}=B_{r}(n_{0}+\Delta n)(p_{0}+\Delta p)-G_{0}} Because n 0 ≫ Δ n {\displaystyle n_{0}\gg \Delta n} and p 0 ≫ Δ p {\displaystyle p_{0}\gg \Delta p} , we can say that Δ n Δ p ≈ 0 {\displaystyle \Delta n\Delta p\approx 0} In an n-type semiconductor, thus Net recombination

15873-429: The operation of many optoelectronic semiconductor devices , such as photodiodes , light-emitting diodes and laser diodes . They are also critical to a full analysis of p-n junction devices such as bipolar junction transistors and p-n junction diodes . The electron–hole pair is the fundamental unit of generation and recombination in inorganic semiconductors , corresponding to an electron transitioning between

16016-488: The original point-contact transistor . Diffused transistors, along with other components, are elements of integrated circuits for analog and digital functions. Hundreds of bipolar junction transistors can be made in one circuit at a very low cost. Bipolar transistor integrated circuits were the main active devices of a generation of mainframe and minicomputers , but most computer systems now use Complementary metal–oxide–semiconductor ( CMOS ) integrated circuits relying on

16159-499: The other particles involved in the process (e.g. photons , electron , or the system of vibrating lattice atoms ). When light interacts with a material, it can either be absorbed (generating a pair of free carriers or an exciton ) or it can stimulate a recombination event. The generated photon has similar properties to the one responsible for the event. Absorption is the active process in photodiodes , solar cells and other semiconductor photodetectors , while stimulated emission

16302-469: The other terminal currents, (i.e. I E  =  I B  +  I C ). In the diagram, the arrows representing current point in the direction of conventional current – the flow of electrons is in the opposite direction of the arrows because electrons carry negative electric charge . In active mode, the ratio of the collector current to the base current is called the DC current gain . This gain

16445-449: The other. A slice cut from the specimen at the p–n boundary developed a voltage when exposed to light. The first working transistor was a point-contact transistor invented by John Bardeen , Walter Houser Brattain , and William Shockley at Bell Labs in 1947. Shockley had earlier theorized a field-effect amplifier made from germanium and silicon, but he failed to build such a working device, before eventually using germanium to invent

16588-469: The p-type base where they diffuse as minority carriers to the reverse-biased n-type collector and are swept away by the electric field in the reverse-biased collector–base junction. For an illustration of forward and reverse bias, see semiconductor diodes . In 1954, Jewell James Ebers and John L. Moll introduced their mathematical model of transistor currents: The DC emitter and collector currents in active mode are well modeled by an approximation to

16731-508: The point-contact transistor. In France, during the war, Herbert Mataré had observed amplification between adjacent point contacts on a germanium base. After the war, Mataré's group announced their " Transistron " amplifier only shortly after Bell Labs announced the " transistor ". In 1954, physical chemist Morris Tanenbaum fabricated the first silicon junction transistor at Bell Labs . However, early junction transistors were relatively bulky devices that were difficult to manufacture on

16874-452: The presence of light is known as photoconductivity . This conversion of light into electricity is widely used in photodiodes . Carrier recombination can happen through multiple relaxation channels. The main ones are band-to-band recombination, Shockley–Read–Hall (SRH) trap-assisted recombination, Auger recombination and surface recombination. These decay channels can be separated into radiative and non-radiative. The latter occurs when

17017-524: The principle behind the light-emitting diode . Oleg Losev observed similar light emission in 1922, but at the time the effect had no practical use. Power rectifiers, using copper oxide and selenium, were developed in the 1920s and became commercially important as an alternative to vacuum tube rectifiers. The first semiconductor devices used galena , including German physicist Ferdinand Braun's crystal detector in 1874 and Indian physicist Jagadish Chandra Bose's radio crystal detector in 1901. In

17160-574: The pure semiconductors, the electrical conductivity may be varied by factors of thousands or millions. A 1 cm specimen of a metal or semiconductor has the order of 10 atoms. In a metal, every atom donates at least one free electron for conduction, thus 1 cm of metal contains on the order of 10 free electrons, whereas a 1 cm sample of pure germanium at 20   °C contains about 4.2 × 10 atoms, but only 2.5 × 10 free electrons and 2.5 × 10 holes. The addition of 0.001% of arsenic (an impurity) donates an extra 10 free electrons in

17303-471: The rate of generation, driving the system back towards equilibrium. Likewise, when there is a deficit of carriers (i.e., n p < n i 2 {\displaystyle np<n_{i}^{2}} ), the generation rate becomes greater than the recombination rate, again driving the system back towards equilibrium. As the electron moves from one energy band to another, the energy and momentum that it has lost or gained must go to or come from

17446-409: The recombination events can be described in terms of electron movements, it is common to visualize the different processes in terms of excited electron and the electron holes they leave behind. In this context, if trap levels are close to the conduction band , they can temporarily immobilize excited electrons or in other words, they are electron traps . On the other hand, if their energy lies close to

17589-629: The resistance of specimens of silver sulfide decreases when they are heated. This is contrary to the behavior of metallic substances such as copper. In 1839, Alexandre Edmond Becquerel reported observation of a voltage between a solid and a liquid electrolyte, when struck by light, the photovoltaic effect . In 1873, Willoughby Smith observed that selenium resistors exhibit decreasing resistance when light falls on them. In 1874, Karl Ferdinand Braun observed conduction and rectification in metallic sulfides , although this effect had been discovered earlier by Peter Munck af Rosenschöld ( sv ) writing for

17732-469: The same rate) is in effect determined by the excess minority carriers. Detailed transistor models of transistor action, such as the Gummel–Poon model , account for the distribution of this charge explicitly to explain transistor behaviour more exactly. The charge-control view easily handles phototransistors , where minority carriers in the base region are created by the absorption of photons , and handles

17875-534: The same volume and the electrical conductivity is increased by a factor of 10,000. The materials chosen as suitable dopants depend on the atomic properties of both the dopant and the material to be doped. In general, dopants that produce the desired controlled changes are classified as either electron acceptors or donors . Semiconductors doped with donor impurities are called n-type , while those doped with acceptor impurities are known as p-type . The n and p type designations indicate which charge carrier acts as

18018-472: The same way as the electron. Combined with the negative effective mass of the electrons at the top of the valence band, we arrive at a picture of a positively charged particle that responds to electric and magnetic fields just as a normal positively charged particle would do in a vacuum, again with some positive effective mass. This particle is called a hole, and the collection of holes in the valence band can again be understood in simple classical terms (as with

18161-591: The scale at which the materials are used. A high degree of crystalline perfection is also required, since faults in the crystal structure (such as dislocations , twins , and stacking faults ) interfere with the semiconducting properties of the material. Crystalline faults are a major cause of defective semiconductor devices. The larger the crystal, the more difficult it is to achieve the necessary perfection. Current mass production processes use crystal ingots between 100 and 300 mm (3.9 and 11.8 in) in diameter, grown as cylinders and sliced into wafers . There

18304-425: The semiconductor body by contact with gaseous compounds of the desired element, or ion implantation can be used to accurately position the doped regions. Some materials, when rapidly cooled to a glassy amorphous state, have semiconducting properties. These include B, Si , Ge, Se, and Te, and there are multiple theories to explain them. The history of the understanding of semiconductors begins with experiments on

18447-1007: The semiconductor composition and electrical current allows for the manipulation of the emitted light's properties. These semiconductors are used in the construction of light-emitting diodes and fluorescent quantum dots . Semiconductors with high thermal conductivity can be used for heat dissipation and improving thermal management of electronics. They play a crucial role in electric vehicles , high-brightness LEDs and power modules , among other applications. Semiconductors have large thermoelectric power factors making them useful in thermoelectric generators , as well as high thermoelectric figures of merit making them useful in thermoelectric coolers . A large number of elements and compounds have semiconducting properties, including: The most common semiconducting materials are crystalline solids, but amorphous and liquid semiconductors are also known. These include hydrogenated amorphous silicon and mixtures of arsenic , selenium , and tellurium in

18590-421: The semiconductor's minority-carrier lifetime. Having a lightly doped base ensures recombination rates are low. In particular, the thickness of the base must be much less than the diffusion length of the carriers. The collector–base junction is reverse-biased, and so negligible carrier injection occurs from the collector to the base, but carriers that are injected into the base from the emitter, and diffuse to reach

18733-458: The silicon. After the process is completed and the silicon has reached room temperature, the doping process is done and the semiconducting wafer is almost prepared. Semiconductors are defined by their unique electric conductive behavior, somewhere between that of a conductor and an insulator. The differences between these materials can be understood in terms of the quantum states for electrons, each of which may contain zero or one electron (by

18876-407: The term Halbleiter (a semiconductor in modern meaning) in his Ph.D. thesis in 1910. Felix Bloch published a theory of the movement of electrons through atomic lattices in 1928. In 1930, B. Gudden  [ de ] stated that conductivity in semiconductors was due to minor concentrations of impurities. By 1931, the band theory of conduction had been established by Alan Herries Wilson and

19019-517: The thermal energy, it is called a deep trap . This difference is useful because shallow traps can be emptied more easily and thus are often not as detrimental to the performance of optoelectronic devices. In the SRH model, four things can happen involving trap levels: When carrier recombination occurs through traps, we can replace the valence density of states by that of the intragap state. The term p ( n ) {\displaystyle p(n)}

19162-416: The three currents in any operating region are given below. These equations are based on the transport model for a bipolar junction transistor. where As the collector–base voltage ( V CB = V CE − V BE {\displaystyle V_{\text{CB}}=V_{\text{CE}}-V_{\text{BE}}} ) varies, the collector–base depletion region varies in size. An increase in

19305-401: The transistor in response to an increase in the collector–base voltage. When the base–collector voltage reaches a certain (device-specific) value, the base–collector depletion region boundary meets the base–emitter depletion region boundary. When in this state the transistor effectively has no base. The device thus loses all gain when in this state. Semiconductor A semiconductor is

19448-406: The use of semiconductors, with the most important aspect being the integrated circuit (IC), which are found in desktops , laptops , scanners, cell-phones , and other electronic devices. Semiconductors for ICs are mass-produced. To create an ideal semiconducting material, chemical purity is paramount. Any small imperfection can have a drastic effect on how the semiconducting material behaves due to

19591-412: The valence band and the conduction band where generation of an electron is a transition from the valence band to the conduction band and recombination leads to a reverse transition. Like other solids, semiconductor materials have an electronic band structure determined by the crystal properties of the material. Energy distribution among electrons is described by the Fermi level and the temperature of

19734-567: The valence band to the conduction band. The recombination rate R 0 {\displaystyle R_{0}} must be exactly balanced by the thermal generation rate G 0 {\displaystyle G_{0}} . Therefore： R 0 = G 0 = B r n 0 p 0 {\displaystyle R_{0}=G_{0}=B_{r}n_{0}p_{0}} where n 0 {\displaystyle n_{0}} and p 0 {\displaystyle p_{0}} are

19877-428: The values of α and β in reverse operation much smaller than those in forward operation; often the α of the reverse mode is lower than 0.5. The lack of symmetry is primarily due to the doping ratios of the emitter and the collector. The emitter is heavily doped, while the collector is lightly doped, allowing a large reverse bias voltage to be applied before the collector–base junction breaks down. The collector–base junction

20020-670: The voltage applied across the base–emitter terminals cause the current between the emitter and the collector to change significantly. This effect can be used to amplify the input voltage or current. BJTs can be thought of as voltage-controlled current sources , but are more simply characterized as current-controlled current sources, or current amplifiers, due to the low impedance at the base. Early transistors were made from germanium but most modern BJTs are made from silicon . A significant minority are also now made from gallium arsenide , especially for very high speed applications (see HBT, below). The heterojunction bipolar transistor (HBT)

20163-467: The years preceding World War II, infrared detection and communications devices prompted research into lead-sulfide and lead-selenide materials. These devices were used for detecting ships and aircraft, for infrared rangefinders, and for voice communication systems. The point-contact crystal detector became vital for microwave radio systems since available vacuum tube devices could not serve as detectors above about 4000 MHz; advanced radar systems relied on

20306-553: Was invented in December 1947 at the Bell Telephone Laboratories by John Bardeen and Walter Brattain under the direction of William Shockley . The junction version known as the bipolar junction transistor (BJT), invented by Shockley in 1948, was for three decades the device of choice in the design of discrete and integrated circuits . Nowadays, the use of the BJT has declined in favor of CMOS technology in

20449-637: Was sometimes poor. This was later explained by John Bardeen as due to the extreme "structure sensitive" behavior of semiconductors, whose properties change dramatically based on tiny amounts of impurities. Commercially pure materials of the 1920s containing varying proportions of trace contaminants produced differing experimental results. This spurred the development of improved material refining techniques, culminating in modern semiconductor refineries producing materials with parts-per-trillion purity. Devices using semiconductors were at first constructed based on empirical knowledge before semiconductor theory provided

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