Base transit time of a bipolar junction transistor considering majority-carrier current

In this paper an analytical expression for base transit time tau_B for an npn bipolar transistor considering majority-carrier current density is obtained. For finding tau_B, expressions for minority-carrier electron current density J_n, majority-carrier hole current density J_p and electron concentration, n(x) are analytically derived. In the model energy-bandgap-narrowing effects due to heavy doping, velocity saturation as well as doping and field dependent mobility are considered. It is found that, in the low-injection condition, tau_B depends on J_p and its value is found greater than the value if Jp is neglected. In finding tau_B, previous works neglected J_p. The closed-form analytical expressions offer a clear physical insight into device operations at various bias conditions.