Transistor Current Components

>>  The directions of actual currents in both NPN and PNP transistors are shown in the figure : ( Conventional currents direction always is taken outward to node ) 

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Transistor current Components

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\displaystyle I_{pE}, I_{nE}, I_{pC}    : The emitter current I_{E}  consists of hole current I_{pE}  (holes crossing from the emitter into the base) and electron current I_{nE}  (electrons crossing from base into emitter).

>>  The ratio of holes to electron currents \displaystyle \frac {I_{pE}}{I_{nE}}  , 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 \displaystyle I_{pC}  . Not all the holes crossing the emitter-base junction reach the collector-base junction, because some of them combine with the electrons in the n-type base. Since the base width is very small, most of the holes cross the collector-base junction and very few recombine, constituting the recombination current \displaystyle (I_{pE} - I_{pC})  .

>>  During recombination, base region loses 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 an 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 (\gamma  ): It is the ratio of the current through the emitter junction due to the carrier of the emitter and the total current through the emitter junction.

\displaystyle \gamma = \frac {I_{pE}}{I_E} 

If  \displaystyle N_E >> N_B \Rightarrow \gamma \rightarrow 1 

Base Transport Factor: It is the ratio of hole current through collector junction and hole current through emitter junction.

\displaystyle \beta^{*} = \frac {I_{pC}}{I_{pE}} 

 If  \displaystyle W_B << L_B  and \displaystyle N_E >> N_B \Rightarrow \beta^{*} \rightarrow 1 

>>  It is the measurement of efficient transportation of carrier through base region.

Reverse Saturation Current (ICO): Due to the reverse voltage V_{BC}  , the reverse saturation current also flows through collector junction.

Large Signal Current Gain (\alpha  ) : It is the ratio of the current due to injected carriers I_{pC}  to the total emitter current I_{E}  .

\displaystyle \alpha = \frac {I_{pC}}{I_E} 

\displaystyle \alpha = \biggl ( \frac {I_{pC}}{I_{pE}} \biggr ) \biggl ( \frac {I_{pE}}{I_E} \biggr ) 

\displaystyle \Rightarrow \alpha = \beta^{*} \gamma 

Expression of Collector Current in Active Region

\displaystyle I_C = I_{pC} + I_{CO} 

\displaystyle \alpha = \frac {I_{pC}}{I_E} 

\displaystyle I_C = \alpha I_E + I_{CO} 

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\displaystyle I_E = I_C + I_B 

\displaystyle I_C = \alpha (I_C + I_B) + I_{CO} 

\displaystyle I_C = \frac {\alpha I_B+I_{CO}}{1 -\alpha} 

\displaystyle \beta = \frac {\alpha}{1-\alpha} 

\displaystyle I_C = \beta I_B + (1+\beta) I_{CO} 

  • Reverse Collector Saturation Current (I_{CBO}  )  : It is defined as the current when emitter current is zero.

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Since the emitter is open, no carriers are injected from the 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 I_{CBO}  .

\displaystyle I_{CBO} = I_{CO} 

I_{CEO}  : It is the collector current when the base is open circuit (\displaystyle I_B = 0  )

\displaystyle I_{CEO} = (1+\beta) I_{CO} 

 

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