>> The directions of actual currents in both npn and pnp transistors are shown in the figure : ( Conventional currents direction always taken outward to node )
Transistor current Components
IpE, InE, and Ipc : The emitter current IE consists of hole current IpE (holes crossing from emitter into base) and electron current InE (electrons crossing from base into emitter).
>> The ratio of holes to electron currents (IpE / InE), crossing the emitter base junction is proportional to the ratio of the conductivity of the P material to that of the N material and the conductivity is proportional to the doping level.
>> The holes crossing the emitter base junction and reaching collector base junction constitutes collector current Ipc. Not all the holes crossing the emitter base junction reach collector base junction, because some of them combine with the electrons in the n- type base. Since base width is very small, most of holes cross the collector base junction and very few recombine, constituting the recombination current (IpE – IpC).
>> During recombination, base region looses its electrons, if electrons are continuously lost than base region become positive in nature and it may start opposing movement of holes passing from Emitter to Collector.
>> Therefore compensating electrons should be supplied to the base region from external biasing supply. These compensating electrons flowing into the base, generate a current equal to recombination current. In BJT base current is almost equal to recombination current.
>> If electrons supplied to the base are fixed → Base region remains neutral and fixed no. of holes travel from Emitter to Collector.
>> If electrons supplied to the base are increased → Base becomes negative in nature and hence attract greater no. of holes from Emitter, due to which net flow from E to C increases.
>> If electron supplied to the base are reduced → Base becomes positive in nature and attract less no. of holes from Emitter, due to which net flow from E to C decreases.
>> Thus electrons supplied to the base or base current controls the flow of holes from E to C, therefore BJT is called current controlled device.
Emitter Injunction Efficiency (γ) : It is the ratio of current through the emitter junction due to carrier of emitter and the total current through the emitter junction.
γ = IpE / IE
>> If NE >> NB ⇒ γ → 1
Base Transport Factor : It is the ratio of hole current through collector junction and hole current through emitter junction.
β* = IpC / IpE
>> If WB << LB and NE >> NB ⇒ β* → 1
>> It is the measurement of efficient transportation of carrier through base region.
Reverse Saturation Current (ICO) : Due to the reverse voltage VBC, the reverse saturation current also flows through collector junction.
Large Signal Current Gain (α) : It is the ratio of the current due to injected carriers IpC to the total emitter current IE.
α = IpC / IE
α = ( IpC / IpE ) ( IpE / IE )
>> α = β* γ
Expression of Collector Current in Active Region
IC = IpC + ICO and α = IpC / IE
>> IC = α IE + ICO
- Reverse Collector Saturation Current (ICBO) : It is defined as the current when emitter current is zero.
>> Since the emitter is open, no carriers are injected from emitter into the base and emitter current is zero. Under this condition the collector base junction acts as a reverse biased diode, and the collector current is equal to the reverse saturation current ICBO.
ICBO = ICO (When Emitter current = 0)
ICEO : It is the collector current when base is open circuit (i.e. IB = 0)
ICEO = (1+β) ICO