Design of gallium arsenide- and indium phosphide-based heterojunction bipolar transistors for high-speed performance.

Abstract

GaAs- and InP-based Heterojunction Bipolar Transistors (HBT's) have been studied for high speed/frequency performance. A new approach to the transient analysis of HBT's was developed using a self-consistent Monte Carlo (MC) technique. The approach, together with steady-state analysis, can provide design guidelines for high speed operation. The MC analysis of AlGaAs/GaAs and InP/InGaAs HBT's with the inverted-field and the undoped collectors demonstrated that the special collector designs offer higher average velocity and shorter switching time. The switching times are 2.8, 1.7 and 2.2 ps for the conventional, inverted field and undoped collector designs, respectively, for AlGaAs/GaAs HBT's. The corresponding switching times for InP/InGaAs HBT's are 1.79, 0.98 and 1.29 ps, respectively. The total base and collector transit times in the latter devices are 2.17, 1.02 and 1.11 ps, respectively. The investigation of the effect of the emitter junction grading on the electron transport and the switching performance of InAlAs/InGaAs HBT's showed that short transit time (1.18 ps) can be achieved when the graded Al composition is $\sim$0.6 at the emitter-base interface; the corresponding transit time is 3.0 ps for abrupt and 2.4 ps for fully graded junctions. Base dopant diffusion into the emitter was found to influence the optimum grading condition significantly. Conventional and self-aligned HBT technologies were developed. During the development, several processing techniques such as wet chemical etching, ohmic contact quality, lift-off process and ion implantation were studied. A special trench etching technique was also studied suitable for HBT's with small emitter dimensions and low parasitic capacitances. Characterization techniques were developed to provide better insight to the device properties and DC and microwave characteristics are reported for HBT's fabricated using the developed technology. The time constants obtained from equivalent circuit modeling of self-aligned HBT's reveal the same bias dependence trends as in the MC simulation. GaInP/GaAs shows distinct advantages over AlGaAs/GaAs for HBT's because of its smaller conduction band and larger valence band discontinuity. GaInP/GaAs HBT's have been studied theoretically and experimentally and their performance is reported.