Imporatnt modes of transistor operation
|Junction bias condition|
Active region :
Junction BE is forward biased. Hence holes from P move towards N (Ipe) and electrons move from N to P side (Ine) .
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.