Electronic devices

Imporatnt modes of transistor operation

Active region :

Base emitter junction                                                       collector base junction

Junction BE is forward biased. Hence holes from P move towards N (I_pe) and electrons move from N to P side (I_ne) .

Ie= Ipe + Ine

Ipe is made up of holes going from P to N and part of these combines with electrons in base and constittues current Ib= Ipe-Ipc1. Rest of holes cross to collector and constitute current Ipc1 (collector current).

Junction CB is reverse biased as in figure. So electrons move from P to N (Ipc0) and holes move from N to P (Inc0).

|Ic0| = Ipc0 + Inc0 ( mod because directions are opposite as per figure.)

Ico = Icbo (figure 1)

Ic = -αIe + Icbo ( apply KCL in figure).

Saturation region :

Veb = constant so that Ie = constant.

Ic = Ic’ – Ie

If Vcb is negative then CB junction is reverse biased so only small reverse saturation current flows. When Vcb increased from –ve to + ve CB becomes forward biased and large Ic starts flowing. Hence in saturation maximum collector current flows.