Transistor Theory: Bipolar Junction Transistor BJT

The theory of operation of the bipolar junction transistor, BJT, involves many elements. We have aimed to simplify it, but give a correct summary.


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There are several different elements that govern the theory and understanding of the way the bipolar junction transistor, BJT, works.

By understanding the way the bipolar junction transistor works, it is possible to utilise it better in circuit design, understanding its mode of operation, limitations and advantages.

Although some of the mathematics can become complicated, it is not necessary to cover all of this to gain a good idea of how the bipolar transistor works and operates.

Transistor theory of operation

A bipolar transistor can be operated in one of four different regimes dependent upon the bias levels on the two diodes in the transistor.

Of the four regimes, the active or normal mode where the emitter base junction is forward biased and the collector base junction is reverse biased is the most important. It is in this operating mode that the transistor is able to provide current gain.

Operational modes for a bipolar transistor
Operational mode Emitter base junction Collector base junction
Active / normal Forward Reverse
Cutoff Reverse Reverse
Saturation Forward Forward
Inverse Reverse Forward

Transistor theory & energy band diagram

The energy band diagram reveals an important aspect of the transistor theory of operation.

Transistor theory:  energy band diagram for normal operational mode
Transistor energy band diagram for normal operational mode

The diagram reveals some of the major current components. The emitter base junction the forward current consists of electron and hole diffusion current InE and Ip as well as the recombination currents in the depletion region IrD and in the base IrB.

Transistor theory:  regions in active mode operation
Transistor regions-in active mode operation

It is possible to calculate the current components if we assume that the doping levels are uniform.

InE = q   A Dn ni 2 NA xB exp ( q Vbe k T )
Ip = q   A Dp ni 2 NDE xE exp ( q Vbe k T )
IrD = Ir exp ( q VBE 2 k T )

Where:
    NDE = the donor concentration in the emitter
    xB = neutral base
    xE = neutral emitter
    InE = standard pn junction diffusion current
    Ip = standard junction diffusion current

Important transistor theory parameters

Some of the important transistor theory equations are given below:

Emitter injection efficiency:

γ = InE IE

Base transport factor:

α = InC InE

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