Bipolar junction transistor

	PNP
	NPN

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

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

The bipolar junction transistor was invented in 1948 at the Bell Telephone Laboratories and enjoyed nearly 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 a 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.

Structure

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

A BJT consists of three semiconductor differently doped 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. Varying the voltage across the base-emitter terminals very slightly causes the current that flows between the emitter and the collector to change. This effect can be used to amplify the input current. BJTs can be thought of as voltage-controlled current sources but are usually characterized as current amplifiers due to the low impedance at the base. Early transistors were made from germanium but most modern BJTs are made from silicon.

Basics of transistor operation

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 electric field of the depletion region becomes unbalanced, causing electrons to be injected into the base region. These electrons wander (or "diffuse") through the base from a region of high concentration near the emitter towards a 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 under equilibrium (this shoud 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 recombine with holes in the base. The collector-base junction is reverse biased thus no electron injection occurs from the collector to the base, but electrons which diffuse across the base towards the collector are swept into the collector by the electric field gradient within the depletion region around the collector-base junction. The proportion of electrons able to cross the base and reach the collector is a measure of the BJT efficency. 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 100 or more.

Transistors in circuits

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 equal to or slightly greater than the cut-in voltage. The cut-in voltage is usually between 600 mV and 700 mV for silicon based 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 exit 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 base current is related to $V_{BE}$ exponentially. For a typical transistor, increasing $V_{BE}$ by just 60 mV increases the base current by a factor of 10!

Transistors have 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 current) then remains between collector and emitter.

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 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.

A transistor is said to operate in the "cut off" region when the base-emitter voltage is too small for any significant current to flow. In typical BJTs manufactured from silicon, this is the case below 0.7 V or so. BJTs that operate only in 'cut off' and 'saturation' regions can by viewed as electronic switches.

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

The emitter and collector currents in normal operation is given by the Ebers-Moll model:

I_{\mathrm {E} }=I_{\mathrm {ES} }(e^{\frac {V_{\mathrm {BE} }}{V_{\mathrm {T} }}}-1)

I_{\mathrm {C} }=\alpha _{F}I_{\mathrm {ES} }(e^{\frac {V_{\mathrm {BE} }}{V_{\mathrm {T} }}}-1)

The base internal current is mainly by diffusion and

J_{p}(Base)={\frac {qD_{p}p_{bo}}{W}}\left[exp\left({\frac {V_{EB}}{V_{T}}}\right)\right]

Where

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

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

\alpha _{F}={\frac {I_{\mathrm {C} }}{I_{\mathrm {E} }}}

\beta _{F}={\frac {I_{\mathrm {C} }}{I_{\mathrm {B} }}}

\beta _{F}={\frac {\alpha _{F}}{1-\alpha _{F}}}

Emitter Efficiency: $\eta ={\frac {J_{p}(Base)}{J_{E}}}$

Base Width Modulation

As the applied base-collector voltage ( $V_{BC}$ ) varies, the base-collector depletion region varies in size. This variation causes the gain of the device to change, since the gain is related to the width of the effective base region. This is often called the "Early Effect".

Punch Through

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.

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. This causes gradual loss of gain of the transistor.

External links

A good introduction to BJTs (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)
http://www.play-hookey.com/semiconductors/transistor.html
How Do Transistors Work? by William Beaty 1995