Dependence of base crowding effect on base doping and thickness for Npn AlGaAs/GaAs HBTs

The dependence of the base current crowding effect on base doping level and thickness in Npn AlGaAs/GaAs heterojunction bipolar transistors is examined. In contrast to the intuitive view that the base current crowding effect should decrease when the base resistivity is reduced by either/or both increasing base doping level or thickness, it is demonstrated that the base current crowding effect actually becomes more severe. The physical cause for this phenomenon is described. The effective emitter width as a function of device geometry for various base doping levels and thicknesses is also calculated.