Modeling and characterization of abrupt heterojunction bipolar transistors.

Abstract

A one-dimensional self-consistent boundary-condition numerical model was developed which can account for tunneling-emission and drift-diffusion transport processes across heterojunctions in a single coupled numerical formulation based on the derived thermionic-field emission boundary conditions. The model was applied to investigate the I-V characteristics of an nN isotype, a pN anisotype and a triangular graded GaAs/AlGaAs heterojunction diode. The simulation revealed that the contribution of tunneling to the total current conduction is significant in these abrupt heterojunctions. In the case of an MBE-grown AlGaAs triangular heterojunction diode, good agreement was obtained between theory and experiment when the tunneling process was taken into account. A new GaAs-based npn AlAs Hole Barrier Bipolar Transistor (HBBT) structure was proposed. Based on the detailed experimental and theoretical investigations of barrier transport, an AlAs HBBT structure consisting of an Al$\sb{0.21}$Ga$\sb{0.79}$As emitter and a graded Al$\sb{x}$Ga$\sb{1-x}$As base (x = 0.21 $\rightarrow$ 0) was designed. The performance of the proposed HBBT was simulated using the boundary condition model and compared with the conventional AlGaAs/GaAs Heterojunction Bipolar Transistor (HBT). The injection performance of Npn AlGaAs/GaAs abrupt emitter HBT's was investigated. The analysis illustrated that the presence of abrupt discontinuities in the conduction and valence bands at the emitter-base junction brings several different features to the injection efficiency and current drive capability compared to graded emitter HBT's. A small displacement of the p-n junction into the narrow bandgap semiconductor was found to be very attractive for the performance optimization of abrupt emitter HBT's. The characteristics of Npn InP/InGaAs and InAlAs/InGaAs single HBT's with abrupt emitter-base junctions were studied. The abrupt heterojunction transport and electrical junction displacement from the emitter-base heterojunction resulted in a profound effect on the forward injection characteristics of the devices. Experimental data from a fabricated InAlAs/InGaAs abrupt emitter HBT was compared to the theoretical results of the model. The analysis revealed that physical mechanisms related to the small bandgap of InGaAs, such as Auger recombination, ballistic electron transport and high impact ionization, are responsible for several important features experimentally observed from the fabricated InGaAs SHBT.