current gain decrease in both high and low current?

[pugongying]

The current gain β=Ic/Ib is almost constant for moderate bias current,but if bias current is too high or too low,the β decreases,can anyone explain it ?expecially in terms of semiconductor device,thanks.

 

[A.Anand Srinivasan]

when the transistor is in cutoff there is base current due to leakage and collector current is also due to leakage and is very low and hence β is low.... considering the other extreme i.e. saturation both the base collector and base emitter diode are in forward bias and hence the base current increases drastically leading to reduction in β.....