|| PNP || NPN

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 the main conduction channel employs both electrons and holes to carry the main electric current.

History and present applications

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The bipolar junction transistor was invented in 1948 at the Bell Telephone Laboratories by John Bardeen and Walter Brattain under the direction of William Shockley, and 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 integrated circuits. Nevertheless, 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 transistors.

Basics of transistor operation

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An NPN transistor can be considered as two diodes connected anode to anode. In typical operation, the emitter-base junction is forward biased and the base-collector junction is reverse biased. As an NPN transistor 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 (this should not be interpreted that the level of injected electrons is small.) A critical feature of BJT design, the base is made very thin so that electrons spend little time in the base: most of the electrons diffuse over to the collector before they can recombine with holes in the base. The collector-base junction is reverse biased thus no electron injection occurs from the collector to the base, however electrons which 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.

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 base current is the sum of the holes injected into the emitter and the electrons that recombine in the base–both small proportions of the emitter to collector current. Hence, a small change of the base current can translate to a large change in electron flow between emitter and collector. The ratio of these currents I_c/I_b, called the current gain, and represented by β or h_fe, is typically between 100 and 1000 for a forward biased diode, it is normally smaller than 1 for a reverse biased diode. Another important parameter is the 'alpha' of the device which is defined as the common base forward short circuit current gain This has values usually between 0.98 and 0.998. Alpha and beta are related by the following identities:

$\backslash alpha\_F\; =\; \backslash frac\{I\_\backslash mathrm\{C\}\}\{I\_\backslash mathrm\{E\}\}$

$\backslash beta\_F\; =\; \backslash frac\{I\_\backslash mathrm\{C\}\}\{I\_\backslash mathrm\{B\}\}$

$\backslash beta\_F\; =\; \backslash frac\{\backslash alpha\_F\}\{1\; -\; \backslash alpha\_F\}$

Structure

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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 transistor, 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).

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 the interchange of the collector and the emitter makes the transistor leave the forward active mode, starting to operate in the reverse mode. Because the transistor internal structure is usually optimized to forward mode operation, interchanging the collector and the emitter makes the values of α and β of reverse operation much smaller than those found in the forward operation, usually, the α of the reverse mode is lower than 0.5.

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.

Early transistors were made from germanium but most modern BJTs are made from silicon.

Transistors in circuits

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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:

$I\_E\; =\; I\_B\; +\; I\_C$

(Note: in this 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 this collector current to this base current is called the DC current gain. This gain is usually quite large and is often 100 or more.

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 several different regions of operation:

• In the linear region, collector-emitter current is approximately proportional to the base current but many times larger, making this the ideal mode of operation for current amplification.
• The BJT enters saturation when the base current is increased to a point where the external circuitry prevents the collector current from growing any larger. At this point, the C-B junction also becomes forward biased. A residual voltage drop of approximately 100 mV to 300 mV (depending on the amount of base and collector current) then remains between collector and emitter.
• In the cut-off region the base-emitter voltage is too small for any significant current to flow. In typical BJTs manufactured from silicon, this is the case below 0.6 V or so.

BJTs that are operated only in 'cut off' and 'saturation' regions can by viewed as electronic switches.

'Inverted' mode operation

Less commonly, bipolar transistors are operated with emitter and collector reversed, thus a base-collector current can control the emitter-collector current. The current gain in this mode is often much smaller (i.e., 2 instead of 100), and it is not a value that is controlled by manufacturers so it can vary dramatically among transistors.

Temperature sensitivity

Because of its temperature sensitivity, the BJT can be used to measure temperature. Its nonlinear characteristics can also be used to compute logarithms.

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.

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.

Theory and modeling

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Ebers-Moll model

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

$I\_\backslash mathrm\{E\}\; =\; I\_\backslash mathrm\{ES\}\; \backslash left(e^\{\backslash frac\{V\_\backslash mathrm\{BE\}\}\{V\_\backslash mathrm\{T\}\}\}\; -\; 1\backslash right)$

$I\_\backslash mathrm\{C\}\; =\; \backslash alpha\_F\; I\_\backslash mathrm\{ES\}\; \backslash left(e^\{\backslash frac\{V\_\backslash mathrm\{BE\}\}\{V\_\backslash mathrm\{T\}\}\}\; -\; 1\backslash right)$

The base internal current is mainly by diffusion and

$J\_p(Base)\; =\; \backslash frac\{q\; D\_p\; p\_\{bo\}\}\{W\}\; \backslash left*$

Where

• $I\_\backslash mathrm\{E\}$ is the emitter current
• $I\_\backslash mathrm\{C\}$ is the collector current
• $\backslash alpha\_F$ is the common base forward short circuit current gain (0.98 to 0.998)
• $I\_\backslash mathrm\{ES\}$ is the reverse saturation current of the base-emitter diode (on the order of 10⁻¹⁵ to 10⁻¹² amperes)
• $V\_\backslash mathrm\{T\}$ is the thermal voltage (approximately 26 mV at room temperature ≈ 300 K).
• $V\_\backslash mathrm\{BE\}$ is the base-emitter voltage
• W is the base width

The collector current is slightly less than the emitter current, since the value of $\backslash alpha\_F$ 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 $h\_\backslash mathrm\{FE\}$. 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:

$\backslash alpha\_F\; =\; \backslash frac\{I\_\backslash mathrm\{C\}\}\{I\_\backslash mathrm\{E\}\}$

$\backslash beta\_F\; =\; \backslash frac\{I\_\backslash mathrm\{C\}\}\{I\_\backslash mathrm\{B\}\}$

$\backslash beta\_F\; =\; \backslash frac\{\backslash alpha\_F\}\{1\; -\; \backslash alpha\_F\}$

Emitter Efficiency: $\backslash eta\; =\; \backslash frac\{J\_p(Base)\}\{J\_E\}$

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.

$i\_\backslash mathrm\{C\}\; =\; I\_\backslash mathrm\{S\}\backslash left(e^\{\backslash frac\{V\_\backslash mathrm\{BE\}\}\{V\_\backslash mathrm\{T\}\}\}\; -\; e^\{\backslash frac\{V\_\backslash mathrm\{BC\}\}\{V\_\backslash mathrm\{T\}\}\}\backslash right)\; -\; \backslash frac\{I\_\backslash mathrm\{S\}\}\{\backslash beta\_\backslash mathrm\{R\}\}\backslash left(e^\{\backslash frac\{V\_\backslash mathrm\{BC\}\}\{V\_\backslash mathrm\{T\}\}\}\; -\; 1\backslash right)$

$i\_\backslash mathrm\{B\}\; =\; \backslash frac\{I\_\backslash mathrm\{S\}\}\{\backslash beta\_\backslash mathrm\{F\}\}\backslash left(e^\{\backslash frac\{V\_\backslash mathrm\{BC\}\}\{V\_\backslash mathrm\{T\}\}\}\; -\; 1\backslash right)\; +\; \backslash frac\{I\_\backslash mathrm\{S\}\}\{\backslash beta\_\backslash mathrm\{R\}\}\backslash left(e^\{\backslash frac\{V\_\backslash mathrm\{BE\}\}\{V\_\backslash mathrm\{T\}\}\}\; -\; 1\backslash right)$

$i\_\backslash mathrm\{E\}\; =\; I\_\backslash mathrm\{S\}\backslash left(e^\{\backslash frac\{V\_\backslash mathrm\{BE\}\}\{V\_\backslash mathrm\{T\}\}\}\; -\; e^\{\backslash frac\{V\_\backslash mathrm\{BC\}\}\{V\_\backslash mathrm\{T\}\}\}\backslash right)\; +\; \backslash frac\{I\_\backslash mathrm\{S\}\}\{\backslash beta\_\backslash mathrm\{F\}\}\backslash left(e^\{\backslash frac\{V\_\backslash mathrm\{BE\}\}\{V\_\backslash mathrm\{T\}\}\}\; -\; 1\backslash right)$

Where

• $i\_\backslash mathrm\{C\}$ is the collector current
• $i\_\backslash mathrm\{B\}$ is the base current
• $i\_\backslash mathrm\{E\}$ is the emitter current
• $\backslash beta\_\backslash mathrm\{F\}$ is the forward common emitter current gain (20 to 50)
• $\backslash beta\_\backslash mathrm\{R\}$ is the reverse common emitter current gain (0 to 20)
• $I\_\backslash mathrm\{S\}$ is the reverse saturation current (on the order of 10⁻¹⁵ to 10⁻¹² amperes)
• $V\_\backslash mathrm\{T\}$ is the thermal voltage (approximately 26 mV at room temperature ≈ 300 K).
• $V\_\backslash mathrm\{BE\}$ is the base-emitter voltage
• $V\_\backslash mathrm\{BC\}$ is the base-collector voltage

Base-width modulation

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.

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 carries 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 ($i\_\backslash mathrm\{C\}$) and the forward common emitter current gain ($\backslash beta\_\backslash mathrm\{F\}$) to the following equations.

$i\_\backslash mathrm\{C\}\; =\; I\_\backslash mathrm\{S\}\; e^\{\backslash frac\{v\_\backslash mathrm\{BE\}\}\{V\_\backslash mathrm\{T\}\}\}\; \backslash left(1\; +\; \backslash frac\{V\_\backslash mathrm\{CB\}\}\{V\_\backslash mathrm\{A\}\}\backslash right)$

$\backslash beta\_\backslash mathrm\{F\}\; =\; \backslash beta\_\backslash mathrm\{F0\}\backslash left(1\; +\; \backslash frac\{V\_\backslash mathrm\{CB\}\}\{V\_\backslash mathrm\{A\}\}\backslash right)$

Where

• $V\_\backslash mathrm\{CB\}$ is the collector-base voltage
• $V\_\backslash mathrm\{A\}$ is the Early voltage (15 V to 150 V)
• $\backslash beta\_\backslash mathrm\{F0\}$ 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.

h-parameter model

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 –

• 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.

Vulnerabilities of transistors

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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. This causes gradual loss of gain of the transistor.

See also

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• Transistor Models

External links

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• 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 wrong way around)
• Characteristic curves
• A water analogy (See also hydraulic analogy)
• 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

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