Bipolar Junction Transistor (BJT)


A bipolar junction transistor (BJT or bipolar transistor) is a type of transistor that relies on the contact of two types of semiconductor for its operation. BJTs can be used as amplifiers, switches, or in oscillators. BJTs can be found either as individual discrete components, or in large numbers as parts of integrated circuits.
Bipolar transistors are so named because their operation involves both electrons and holes. These two kinds of charge carriers are characteristic of the two kinds of doped semiconductor material.
The invention of the BJT in 1948 at the Bell Telephone Laboratories ushered in the era of solid state circuits. The basic principle involved is the use of the voltage between two terminals to control the current flowing in the third terminal. In this way, a three terminal device can be used to realize a controlled source.
The BJT is still the preferred device in very demanding analog circuit applications, both integrated and discrete. This is especially true in very-high-frequency applications, such as radio frequency (RF) circuits for wireless systems. Bipolar transistors can be combined with MOSFETs to create innovative circuits that take advantage of the high-input-impedance and low-power operation of MOSFETs and the very-high-frequency operation and high-current-driving capability of bipolar transistors.

BJT consists of three semiconductor regions: the emitter region (n type), the base region (p type), and the collector region (n type). Such a transistor is called an npn transistor. The terminals labelled Emitter (E), base (B), and collector (C).

The transistor consists of two pn junctions, the emitter–base junction (EBJ) and the collector–base junction (CBJ).

Figure: A simplified structure of the pnp transistor


The active mode, which is also called forward active mode, is the one used if the transistor is to operate as an amplifier. Switching applications (e.g., logic circuits) utilize both the Cut-off and the saturation modes. The reverse active (or inverse active) mode has very limited application.

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).

Transistor parameters: alpha (α) and beta (β)
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 causes 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 βF or hFE; it is approximately the ratio of the DC collector current to the DC base current in forward-active region. It is typically greater than 100 for small-signal transistors but can be smaller in transistors designed for high power applications.
Another important parameter is the common-base current gain, αF. 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 0.98 and 0.998. It is less than unity due to recombination of charge carriers as they cross the base region. Alpha and beta are more precisely related by the following identities (NPN transistor):
αF =IC /IE
βF = IC /IB
βF= αF/(1- αF)
αF= βF/( βF+1)

Figure: The relationship between , VCE and Ib

The relationship between  , VCE and Ib, Ic
The relationship between , VCE and Ib, Ic