# Power electronics - BJT

Tutorial on power electronics 2020-11-20 02:02:17# Power electronics - BJT

A bipolar junction transistor (BJT) is a transistor whose operation depends on the contact made by two semiconductors. It can act as a switch, amplifier or oscillator. It is known as a bipolar transistor because its operation requires two types of charge carriers (holes and electrons). Holes are the dominant charge carriers in P-type semiconductors while electrons are the main charge carriers in N-type semiconductors.

## Symbols of a BJT

## Structure of a BJT

A BJT has two PN junctions connected back to back and sharing a common B region (base). This guarantees contacts in all regions which are the base, the collector and the emitter. The structure of a PNP bipolar transistor is shown below.

The BJT shown above consists of two connect diodesback to back, which leads to the depletion of so-called quasi-neutral regions. The quasi-neutral widths of the emitter, base and collector are indicated above as W _{ E } ', W _{ B } 'and W _{ C } '. They are obtained as follows -

The conventional signs of the currents for the transmitter, base and collector are denoted by *I _{E} *,

*I*and

_{ B }*I*respectively. Therefore, the collector and base current are positive when positive current meets the collector or base contact. In addition, the current of the emitter is positive when the current leaves the contact of the emitter. Thus,

_{C}When apositive voltage is applied to the base contact relative to the collector and the emitter, the base-collector voltage as well as the base-emitter voltage become positive.

For simplicity, V _{ CE } is assumed to be zero.

The electron scattering occurs from the emitter to the base while the hole scattering comes from the base to the emitter. Once the electrons reach the base-collector depleted region, they are swept through the region by an electric field. These electrons form the collector current.

When a BJT is polarized in direct active mode, the total current of the emitter is obtained by adding the electron scattering current (* I _{ E, n } *), hole diffusion current (

*I*) and the current of the base transmitter.

_{ E, p } The total collection currentur is given by the electron scattering current (* I _{ E, n } *), minus the base recombination current (

*I*).

_{ r, B } The sum of the base current * I _{ B } * is obtained by adding the hole diffusion current (

*I*), the recombination current base (

_{ E, p }*I*) and the base-emitter recombination current of the depletion layer (

_{ r, B }*I*).

_{ r, d }## Transport factor

This is given by the ratio of collector current and emitter current.

$$ alpha = frac {I_ {C}} {I_ {E}} $$By applying Kirchhoff's current law, we see that the base current is given by the difference between the current of the emitter and the current of the collector.

## Current gain

This is given by the ratio of the collector current to the base current.

$$ beta = frac {I_ {C}} {I_ {B}} = frac {alpha} {1 - alpha} $$The above explains how a BJT can produce current amplification. The transport factor (α) approaches one if the collector current is almost equivalent to the emitter current. The current gain (β) then becomes greater than one.

For further analysis, the transport factor (α) is rewritten as a product of the emitter efficiency (γ _{ E }) the base transport factor (α _{ T }) and the depletion layer recombination factor (δ _{ r }). It is rewritten as follows -

The following is a summary of the effectiveness of the 'emitter, the base transport factor and the r factorecombination of the depletion layer discussed.