ANALYSIS OF THE OPERATION OF GAALAS/GAAS HBTS

Drift-diffusion modeling in two dimensions has been used to characterize and analyze storage, transport, and recombination effects in GaAlAs/GaAs heterostructure bipolar transistors. Both intrinsic and parasitic effects have been studied, and their relationship to the design of the device is discussed. For conventional dopings and high current densities, the heterojunction grading potential causes a barrier in the base-emitter junction, which results in a large increase in the dynamic resistance. In heterojunction collectors, a similar barrier leads to a large increase in base charge storage and to spreading of the collector current. It is shown that increased doping levels can successfully suppress these barrier effects. The capacitance and transport phenomena at the base-emitter junction are also analyzed under conditions of large forward bias, where the junction space-charge region is shorter than the alloy grading length. Recombination is analyzed in the limit of high surface recombination velocities using Shockley-Read-Hall theory in the presence of Fermi-level pinning due to surface states. The pinning results in a potential energy saddle point at the edge of the base-emitter junction, which largely determines the surface recombination behavior of the transistor when the recombination velocity is high. The characteristics of this behavior are found to agree with the experimentally observed recombination behavior of conventional devices.