A bipolar junction transistor (BJT) is a type of transistor. It is a three-terminal device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes. Assorted discrete transistors A transistor is a semiconductor device, commonly used as an amplifier. ... In semiconductor production, doping refers to the process of intentionally introducing impurities into an intrinsic semiconductor in order to change its electrical properties. ... A semiconductor is a solid whose electrical conductivity can be controlled over a wide range, either permanently or dynamically. ... e- redirects here. ... For the following two reasons the electron hole was introduced into calculations: If an electron is excited into higher state it leaves a hole in its old state. ...

Although a small part of the transistor current is due to the flow of majority carriers, most of the transistor current is due to the flow of minority carriers and so BJTs are classified as 'minority-carrier' devices. Charge carrier denotes in physics a free (mobile, unbound) particle carrying an electric charge. ... Charge carrier denotes in physics a free (mobile, unbound) particle carrying an electric charge. ...

	PNP
	NPN

The schematic
symbols for PNP-
and NPN-type
BJTs.

Image File history File links BJT_symbol_PNP.svg File links The following pages link to this file: Bipolar junction transistor ... Image File history File links BJT_symbol_NPN.svg File links The following pages link to this file: Bipolar junction transistor NPN ...

Introduction

NPN BJT with forward-biased E–B junction and reverse-biased B–C junction

An NPN transistor can be considered as two diodes with a shared anode region. In typical operation, the emitter–base junction is forward biased and the base–collector junction is reverse biased. In an NPN transistor, for example, when a positive voltage is applied to the base–emitter junction, the equilibrium between thermally generated carriers and the repelling electric field of the depletion region becomes unbalanced, allowing thermally excited electrons to inject into the base region. These electrons wander (or "diffuse") through the base from the region of high concentration near the emitter towards the region of low concentration near the collector. The electrons in the base are called minority carriers because the base is doped p-type which would make holes the majority carrier in the base. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... Types of diodes closeup, showing germanium crystal In electronics, a diode is a component that restricts the direction of movement of charge carriers. ... Diagram of a zinc anode in a galvanic cell. ... A p-n junction is formed by combining N-type and P-type semiconductors together in very close contact. ... A p-n junction is formed by combining N-type and P-type semiconductors together in very close contact. ... A p-n junction is formed by combining N-type and P-type semiconductors together in very close contact. ... Charge carrier denotes in physics a free (mobile, unbound) particle carrying an electric charge. ... In semiconductor physics, the depletion region, also called depletion layer or depletion zone, is an insulating region within a conductive, doped semiconductor material where the charge carriers have been swept away through recombination. ... This article or section does not cite any references or sources. ... Charge carrier denotes in physics a free (mobile, unbound) particle carrying an electric charge. ... In semiconductor production, doping refers to the process of intentionally introducing impurities into an intrinsic semiconductor in order to change its electrical properties. ... Charge carrier denotes in physics a free (mobile, unbound) particle carrying an electric charge. ...

The base region of the transistor must be made thin, so that carriers can diffuse across it in much less time than the semiconductor's minority carrier lifetime, to minimize the percentage of carriers that recombine before reaching the collector–base junction. The thickness of the base should be less than the diffusion length of the electrons. The collector–base junction is reverse-biased, so little electron injection occurs from the collector to the base, but electrons that diffuse through the base towards the collector are swept into the collector by the electric field in the depletion region of the collector–base junction. In the solid state physics of semiconductors, carrier generation and recombination are processes by which mobile electrons and electron holes are created and eliminated. ...

Importance of BJTs and Non-technical Overview

Consider a variable light switch such as those with round, rotating control knobs found in many home's dining rooms. An operating BJT is similar to that light and control knob. There are three signals in this case: (i) electricity input by the home to the light, (ii) the output of the lightbulbs in form of light, but ultimately traceable to electricity flowing through the bulbs interior, and (iii) the adjustment of a the control knob by a person. In analogy to the variable dining-room light, the emitter (E) is the input electricity, the collector (C) is the output light, and the base (B) is the controlling person or the position of the rotating knob. For example, consider the case where the BJT emitter current, I_e, is about 1 A, which translates to a current at the collector, I_c, being anywhere from 0 to 1 A. The voltage placed on the base, V_b, determines the value of I_c. The usefulness in an electrical circuit can be immediately imagined. For example, if a signal representing music is input to the base of a BJT, the output at the collector can be used as an amplified signal boosting the sound to a speaker. This is the same concept as described below by the value beta. Understand that these are very simplistic explanations which will be clarified in more technically correct ways below. However, this should provide the non-technical reader with a conceptual understanding of what a BJT and its three electrodes (E, B, and C) do.

Voltage, current, and charge control

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 the current–voltage relation of the base–emitter junction, which is just the usual exponential current–voltage curve of a p-n junction (diode). A p-n junction is formed by combining N-type and P-type semiconductors together in very close contact. ...

The physical explanation for collector current is the amount of minority-carrier charge in the base region. Detailed models of transistor action, such as the Gummel–Poon model, account for this charge explicitly to explain transistor behavior more exactly. The charge-control view easily handles photo-transistors, where minority carriers in the base region are created by the absorption of photons, and handles the dynamics of turn-off, or recovery time, which depends on charge in the base region recombining. However, since base charge is not a signal that is visible at the terminals, the current- and voltage-control views are usually used in circuit design and analysis. The word light is defined here as electromagnetic radiation of any wavelength; thus, X-rays, gamma rays, ultraviolet light, infrared radiation, microwaves, radio waves, and visible light are all forms of light. ...

In linear circuit design, the current-control view is often preferred, since it is approximately linear. That is, the collector current is approximately 'beta' times the base current. The voltage-control model requires an exponential function to be taken into account.

Transistor 'alpha' and 'beta'

The proportion of electrons able to cross 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 cause many more electrons to be injected from the emitter into the base than holes to be injected from the base into the emitter. The common emitter current gain is represented by β_F or h_fe. It is approximately the ratio of the DC collector current to the DC base current in forward-active mode and common-emitter configuration and is typically greater than 100. Another important parameter is the common-base current gain, α_F. The common base current gain is approximately the gain of current from emitter to collector in common-base configuration. This ratio usually has a value close to unity; between 0.98 and 0.998. Alpha and beta are more precisely related by the following identities (NPN transistor):

Structure

Simplified cross section of an npn bipolar junction transistor

Die of a KSY34 high-frequency NPN transistor, base and emitter connected via bonded wires

A BJT consists of three differently doped semiconductor regions, the emitter region, the base region and the collector region. These regions are, respectively, p type, n type and p type in a PNP, and n type, p type and n type in a NPN transistor. Each semiconductor region is connected to a terminal, appropriately labeled: emitter (E), base (B) and collector (C). Image File history File links Npn_BJT_cross_section. ... Image File history File links Npn_BJT_cross_section. ... Image File history File links Download high resolution version (1024x768, 62 KB) Summary Description: w:Transistor die, wedge w:wire bonding Author, date of creation: selfmade by Shaddack, 5 December 2005 Source: self-made Copyright: Public Domain (PD) Comments: KSY34 transistor die microphotograph Licensing File links The following pages link... Image File history File links Download high resolution version (1024x768, 62 KB) Summary Description: w:Transistor die, wedge w:wire bonding Author, date of creation: selfmade by Shaddack, 5 December 2005 Source: self-made Copyright: Public Domain (PD) Comments: KSY34 transistor die microphotograph Licensing File links The following pages link... In semiconductor production, doping refers to the process of intentionally introducing impurities into an extremely pure (also referred to as intrinsic) semiconductor in order to change its electrical properties. ... A bipolar junction transistor (BJT) is a type of transistor. ... NPN is one of the two types of bipolar transistors, the second being PNP. The letters N and P refer to the majority charge carriers inside the different regions of the transistor. ...

The base is physically located between the emitter and the collector and 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 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 the emitter–base junction.

The bipolar junction transistor, unlike other transistors, is 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 to forward-mode operation, interchanging the collector and the emitter makes the values of α and β in reverse operation much smaller than those found 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 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.

Small changes in the voltage applied across the base–emitter terminals causes the current that flows 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. It has been suggested that this article or section be merged with Voltage source. ...

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). General Name, Symbol, Number germanium, Ge, 32 Chemical series metalloids Group, Period, Block 14, 4, p Appearance grayish white Atomic mass 72. ... It has been suggested that Silicons ranking be merged into this article or section. ... This article is about the chemical compound. ...

NPN

The symbol of an NPN Bipolar Junction Transistor.

NPN is one of the two types of bipolar transistors, in which the letters "N" and "P" refer to the majority charge carriers inside the different regions of the transistor. Most bipolar transistors used today are NPN, because electron mobility is higher than hole mobility in semiconductors, allowing greater currents and faster operation. Image File history File links BJT_symbol_NPN.svg File links The following pages link to this file: Bipolar junction transistor NPN ... Image File history File links BJT_symbol_NPN.svg File links The following pages link to this file: Bipolar junction transistor NPN ... Charge carrier denotes in physics a free (mobile, unbound) particle carrying an electric charge. ... This does not adequately cite its references or sources. ...

NPN transistors consist of a layer of P-doped semiconductor (the "base") between two N-doped layers. A small current entering the base in common-emitter mode is amplified in the collector output. Doping is generally the practice of adding impurities to something. ...

The arrow in the NPN transistor symbol is on the emitter leg and points in the direction of the conventional current flow when the device is in forward active mode. In electricity, current is the rate of flow of charges, usually through a metal wire or some other electrical conductor. ...

PNP

The other type of BJT is the PNP with the letters "P" and "N" referring to the majority charge carriers inside the different regions of the transistor. Few transistors used today are PNP, since the NPN type gives better performance in most circumstances. Charge carrier denotes in physics a free (mobile, unbound) particle carrying an electric charge. ...

The symbol of a PNP BJT.

PNP transistors consist of a layer of N-doped semiconductor between two layers of P-doped material. PNP transistors are commonly operated with the collector at ground and the emitter connected to a positive voltage through an electric load. A small current flowing from the base allows a much greater current to flow from the emitter to the collector. Image File history File links BJT_symbol_PNP.svg File links The following pages link to this file: Bipolar junction transistor ... Image File history File links BJT_symbol_PNP.svg File links The following pages link to this file: Bipolar junction transistor ... Doping is generally the practice of adding impurities to something. ... It has been suggested that Ground conductor be merged into this article or section. ... International safety symbol Caution, risk of electric shock (ISO 3864), colloquially known as high voltage symbol. ... If an electric circuit has a well-defined output terminal, the circuit connected to this terminal (or its input impedance) is the load. ...

Heterojunction bipolar transistor

The heterojunction bipolar transistor (HBT) is an improvement of the BJT that can handle signals of very high frequencies up to several hundred GHz. It is common nowadays in ultrafast circuits, mostly RF systems. The Heterojunction Bipolar Transistor (HBT) is an improvement of the bipolar junction transistor (BJT) that can handle signals of very high frequencies up to several hundred GHz. ... HBT is a TLA that could mean: Handelsschule Berliner Tor [1] Healthcare Benefit Trust [2] Heritage Bank & Trust [3] Hostage Barricade Team Houston Belt and Terminal Railway – AAR reporting mark HBT HydroBall Technics [4] This is a disambiguation page — a navigational aid which lists other pages that might... A gigahertz is a billion hertz or a thousand megahertz, a measure of frequency. ...

Heterojunction transistors have different semiconductors for the elements of the transistor. Usually the emitter is composed of a larger bandgap material than the base. This helps reduce minority carrier injection from the base when the emitter-base junction is under forward bias and increases emitter injection efficiency. The improved injection of carriers into the base allows the base to have a higher doping level, resulting in lower resistance to access the base electrode. With a more traditional BJT, also referred to as homojunction BJT, the efficiency of carrier injection from the emitter to the base is primarily determined by the doping ratio between the emitter and base. Because the base must be lightly doped to allow the high injection efficiency, its resistance is relatively high. Higher doping in the base can improve figures of merit like the Early voltage. The Early voltage as seen in the output-characteristic plot of a BJT The Early effect is the variation in the width of the base in a BJT due to a variation in the applied collector voltage. ...

Two commonly used HBTs are silicon–germanium and aluminum gallium arsenide. Silicon–germanium is widely used because it is compatible with standard silicon digital processes, allowing integration of very high speed circuitry with complex lower speed digital circuitry.

Transistors in circuits

Structure and use of npn transistor

The diagram opposite is a schematic representation of an npn transistor connected to two voltage sources. To make the transistor conduct appreciable current (on the order of 1 mA) from C to E, V_BE must be above a threshold voltage sometimes referred to as the cut-in voltage. The cut-in voltage is usually about 600 mV for silicon BJTs. This applied voltage causes the lower p-n junction to 'turn-on' allowing a flow of electrons from the emitter into the base. Because of the electric field existing between base and collector (caused by V_CE), the majority of these electrons 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 the other terminal currents. That is: npn transistor structure and circuit. ... npn transistor structure and circuit. ... Depletion Region of an NMOS The threshold voltage of a MOSFET is usually defined as the gate voltage where a depletion region forms in the substrate (body) of the transistor. ...

In the diagram, the arrows representing current point in the direction of the electric or conventional current—the flow of electrons is in the opposite direction of the arrows since electrons carry negative electric charge. The ratio of the collector current to the base current is called the DC current gain. This gain is usually quite large and is often 100 or more. In electricity, current is the rate of flow of charges, usually through a metal wire or some other electrical conductor. ... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ...

It should also be noted that the emitter current is related to V_BE exponentially. At room temperature, increasing V_BE by about 60 mV increases the emitter current by a factor of 10. The base current is approximately proportional to the emitter current, so it varies the same way.

Regions of operation

Bipolar transistors have five distinct regions of operation, defined mostly by applied bias:

• Forward-active (or simply, active): The emitter-base junction is forward biased and the base-collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest common-emitter current gain, β_f in forward-active mode. If this is the case, the collector-emitter current is approximately proportional to the base current, but many times larger, for small base current variations.
• Reverse-active (or inverse-active or inverted): By reversing the biasing conditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. Since most BJTs are designed to maximise current gain in forward-active mode, the β_f in inverted mode is several (2 - 3 for the ordinary germanium transistor) times smaller. This transistor mode is seldom used, usually being considered only for failsafe conditions and some types of bipolar logic. The reverse bias breakdown voltage to the base may be an order of magnitude lower in this region.
• Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates high current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch.
• Cutoff: In cutoff, biasing conditions opposite of saturation (both junctions reverse biased) are present. There is very little current flow, which corresponds to a logical "off", or an open switch.
• Avalanche breakdown region

While these regions are well defined for sufficiently large applied voltage, they overlap somewhat for small (less than 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 the cutoff region. In mathematics, two quantities are called proportional if they vary in such a way that one of the quantities is a constant multiple of the other, or equivalently if they have a constant ratio. ... Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials. ...

History

The bipolar (point-contact) transistor 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, invented by Shockley in 1951, enjoyed three decades as the device of choice in the design of discrete and integrated circuits. Nowadays, the use of the BJT has declined in favour of CMOS technology in the design of digital integrated circuits. Bell Telephone Laboratories or Bell Labs was originally the research and development arm of the United States Bell System, and was the premier corporate facility of its type, developing a range of revolutionary technologies from telephone switches to specialized coverings for telephone cables, to the transistor. ... John Bardeen (May 23, 1908 – January 30, 1991) was an American physicist and electrical engineer. ... Walter Houser Brattain (February 10, 1902 – October 13, 1987) was a physicist who, along with John Bardeen, invented the transistor. ... William Bradford Shockley (February 13, 1910 – August 12, 1989) was a British-born American physicist and inventor. ... An integrated circuit (IC) is a thin chip consisting of at least two interconnected semiconductor devices, mainly transistors, as well as passive components like resistors. ... Static CMOS Inverter Complementary metal–oxide–semiconductor (CMOS) (see-moss, IPA: ), is a major class of integrated circuits. ...

Germanium transistors

The germanium transistor was more common in the 1950s and 1960s, and while it exhibits a lower "cut off" voltage, making it more suitable for some applications, it also has a greater tendency to exhibit thermal runaway. General Name, Symbol, Number germanium, Ge, 32 Chemical series metalloids Group, Period, Block 14, 4, p Appearance grayish white Atomic mass 72. ... This does not cite any references or sources. ... The 1960s decade refers to the years from January 1, 1960 to December 31, 1969, inclusive. ...

Theory and modeling

Large-signal models

Ebers–Moll model

The DC emitter and collector currents in normal operation are well modeled by the Ebers–Moll model:

Ebers-Moll Model for NPN Transistor

Ebers-Moll Model for PNP Transistor

The base internal current is mainly by diffusion and Image File history File links Ebers-Moll_Model_NPN.PNG Summary Drawn using Klunky and added the minor details using MS-WORD Licensing Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1. ... Image File history File links Ebers-Moll_Model_NPN.PNG Summary Drawn using Klunky and added the minor details using MS-WORD Licensing Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1. ... Image File history File links Ebers-Moll_Model_PNP.PNG Summary Drawn using Klunky and added the minor details using MS-WORD Licensing Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1. ... Image File history File links Ebers-Moll_Model_PNP.PNG Summary Drawn using Klunky and added the minor details using MS-WORD Licensing Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1. ...

Where

• I_E is the emitter current
• I_C is the collector current
• α_T is the common base forward short circuit current gain (0.98 to 0.998)
• I_ES is the reverse saturation current of the base–emitter diode (on the order of 10⁻¹⁵ to 10⁻¹² amperes)
• V_T is the thermal voltage kT / q (approximately 26 mV at room temperature ≈ 300 K).
• V_BE is the base–emitter voltage
• W is the base width

The collector current is slightly less than the emitter current, since the value of α_T is very close to 1.0. In the BJT a small amount of base–emitter current causes a larger amount of collector–emitter current. The ratio of the allowed collector–emitter current to the base–emitter current is called current gain, β or h_FE. A β value of 100 is typical for small bipolar transistors. In a typical configuration, a very small signal current flows through the base–emitter junction to control the emitter–collector current. β is related to α through the following relations: Ludwig Boltzmann The Boltzmann constant (k or kB) is the physical constant relating temperature to energy. ...

Emitter Efficiency :

Another set of equations used to describe the three currents in the any operating region are given below. These equations are based on the transport model for a Bipolar Junction Transistor.

Where

• i_C is the collector current
• i_B is the base current
• i_E is the emitter current
• β_F is the forward common emitter current gain (20 to 500)
• β_R is the reverse common emitter current gain (0 to 20)
• I_S is the reverse saturation current (on the order of 10⁻¹⁵ to 10⁻¹² amperes)
• V_T is the thermal voltage (approximately 26 mV at room temperature ≈ 300 K).
• V_BE is the base–emitter voltage
• V_BC is the base–collector voltage

Ludwig Boltzmann The Boltzmann constant (k or kB) is the physical constant relating temperature to energy. ...

Base-width modulation

Graded hetero-junction bipolar transistor. Top=closed, bottom=open, grey=band gap, blue=electrons, white=holes, y-axis=energy. Left=emitter, middle=base, right=collector

As the applied collector–base voltage (V_BC) varies, the collector–base depletion region varies in size. This is often called the "Early effect" after its discoverer James M. Early. Image File history File links Graded_hetero_bipolar_transistor. ... Image File history File links Graded_hetero_bipolar_transistor. ... Early effect voltage as seen in the output-characteristic plot of a BJT . The Early effect is the variation in the width of the base in a BJT due to a variation in the applied collector voltage. ... James M. Early (1922–2004) was an American engineer, best known for his work on transistors and charge-coupled device imagers. ...

This effectively means a variation in the width of the base region of the BJT. An increase in the collector–base voltage, for example, causes a greater reverse bias across the collector–base junction, increasing the collector–base depletion region width, decreasing the width of the base. This has two consequences :

• There is a lesser chance for recombination within the "smaller" base region.
• The charge gradient is increased across the base, and consequently, the current of minority carriers injected across the emitter junction increases.

Both factors increase the collector or "output" current of the transistor due to an increase in the collector–base voltage.

In the forward active region the Early effect modifies the collector current (i_C) and the forward common emitter current gain (β_F) to the following equations.

Where

• V_CB is the collector–base voltage
• V_A is the Early voltage (15 V to 150 V)
• β_F0 is forward common-emitter current gain when V_CB = 0 V

Punchthrough

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.

Gummel–Poon charge-control model

The Gummel–Poon model^[1] is a detailed charge-controlled model of BJT dynamics, which has been adopted and elaborated by others to explain transistor dynamics in greater detail than the terminal-based models typically do [1]. The Gummel-Poon Model is a model of the bipolar junction transistor. ...

Small-signal models

h-parameter model

Generalized h-parameter model of an NPN BJT.
replace x with e, b or c for CE, CB and CC topologies respectively.

Another model commonly used to analyse BJT circuits is the h-parameter model. This model is a 2-port network particularly suited to BJTs as it lends itself easily to the analysis of circuit behaviour, and may be used to develop further accurate models. As shown, the term "x" in the model represents the BJT lead depending on the topology used. For common-emitter mode the various symbols take on the specific values as – Image File history File links H-parameters. ... Image File history File links H-parameters. ...

• x = 'e' since it is a CE topology
• Terminal 1 = Base
• Terminal 2 = Collector
• Terminal 3 = Emitter
• i_in = Base current (i_b)
• i_o = Collector current (i_c)
• V_in = Base-to-emitter voltage (V_BE)
• V_o = Collector-to-emitter voltage (V_CE)

and the h-parameters are given by –

• h_ix = h_ie - The input impedance of the transistor (corresponding to the emitter resistance r_e).
• h_rx = h_re - Represents the dependence of the transistor's I_B–V_BE curve on the value of V_CE. It is usually very small and is often neglected (assumed to be zero).
• h_fx = h_fe - The current-gain of the transistor. This parameter is often specified as h_FE or the DC current-gain (β_DC) in datasheets.
• h_ox = h_oe - The output impedance of transistor. This term is usually specified as an admittance and has to be inverted to convert it to an impedance.

As shown, the h-parameters have lower-case subscripts and hence signify AC conditions or analyses. For DC conditions they are specified in upper-case. For the CE topology, an approximate h-parameter model is commonly used which further simplifies the circuit analysis. For this the h_oe and h_re parameters are ignored (rather, they are set to infinity and zero, respectively). It should also be noted that the h-parameter model is suited to low-frequency, small-signal analysis. For high-frequency analyses this model is not used since it ignores the inter-electrode capacitances which come into effect at high frequencies.

Applications of transistors

The BJT remains a device that excels in some applications, such as discrete circuit design, due to the very wide selection of BJT types available and because of knowledge about the bipolar transistor characteristics. The BJT is also the choice for demanding analog circuits, both integrated and discrete. This is especially true in very-high-frequency applications, such as radio-frequency circuits for wireless systems. The bipolar transistors can be combined with MOSFET's in an integrated circuit by using a BiCMOS process to create innovative circuits that take advantage of the best characteristics of both types of transistor. Very high frequency (VHF) is the radio frequency range from 30 MHz to 300 MHz. ... It has been suggested that this article or section be merged with Radio waves. ... The metal-oxide-semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is by far the most common field-effect transistor in both digital and analog circuits. ... In integrated circuit technologies, BiCMOS, also called BiMOS, refers to the integration of bipolar junction transistors and CMOS technology into a single device. ...

Temperature sensors

Because of the known temperature and current dependence of the forward-biased base–emitter junction voltage, the BJT can be used to measure temperature by subtracting two voltages at two different bias currents in a known ratio [2].

Logarithmic converters

Since base–emitter voltage varies as the log of the base–emitter and collector–emitter currents, a BJT can also be used to compute logarithms and anti-logarithms. A diode can also perform these nonlinear functions, but the transistor provides more circuit flexibility. Above is the graph plots of Logarithms to various bases: is to base e, is to base 10, and is to base 1. ...

Vulnerabilities of transistors

Exposure of the transistor to ionizing radiation causes radiation damage. Radiation causes a buildup of 'defects' in the base region that act as recombination centers. The resulting reduction in minority carrier lifetime causes gradual loss of gain of the transistor. Power BJT's are subject to a failure mode called secondary breakdown. In this failure mode, certain parts of the die (the actual piece of silicon inside the device) get hotter than the others. As a result, the hottest part of the die conducts the most current causing it to get hotter still until the device short-circuits internally. Radiation hazard symbol. ... Microelectronics designed for environments with high levels of ionizing radiation have special design challenges. ... In the solid state physics of semiconductors, carrier generation and recombination are processes by which mobile electrons and electron holes are created and eliminated. ... Secondary breakdown, in reference to semiconductors, are the destructive means that a primary breakdown may introduce and they may include the following: Wirebond/solder or weld failures (wire bonds are used to attach the semiconductors metalization pads like the gate and the top electrode to the device carrier pads. ...

See also

Electronics Portal

• Transistor models
• SPICE
• Technology CAD (TCAD)

Image File history File links Nuvola_apps_ksim. ... Transistors are complicated devices. ... “Spiciness” redirects here. ... Technology CAD (or Technology Computer Aided Design, or TCAD) is a branch of electronic design automation that models semiconductor fabrication. ...

References

1. ^ H. K. Gummel and R. C. Poon, "An integral charge control model of bipolar transistors," Bell Syst. Tech. J., vol. 49, pp. 827--852, May-June 1970

External links

• Lessons In Electric Circuits - Biploar Junction Transistors (Note: this site shows current as a flow of electrons, rather than the convention of showing it as a flow of holes, so the arrows may appear the other way around)
• Characteristic curves
• The transistor at play-hookey.com
• How Do Transistors Work? by William Beaty
• www.brookdale.cc.nj.us/fac/engtech/aandersen/engi242/bjt_models.pdf