OSCILLATIONS IN COLLECTOR CURRENT OF A DOUBLE-HETEROSTRUCTURE BIPOLAR-TRANSISTOR - A QUANTUM EFFECT

In spite of many advantages, GaInP/GaAs/GaInP double heterostructure b ipolar transistors (DHBT) suffer from the disadvantage of energy barri er to flow of carriers at the collector-base junction due to non-zero conduction band edge discontinuity which results in higher V-D,V-sat. To circumvent this critical problem, Liu et al. IEEE Transactions on E lectron Devices, 40 1381-1389 (1993) have employed a set-back layer of undoped GaAs between the base and the collector. As a consequence of the set-back layer, they observed oscillations in the collector curren t in the forward active mode with the output voltage (V-BC), the origi n of which they proposed to be the presence and absence of resonant en ergy levels at the energy equal to the conduction band edge, E(c), of the base. In this work, we have investigated the origin and conditions of these oscillations theoretically. Energy band balance was performe d at the base-set-back layer and set-back layer-collector junctions to determine the distribution of the output voltage, V-BC, at these junc tions using degenerate statistics. This calculation also provided the electric field and potential drop on the set-back layer. The parabolic E(c) profiles of the base and collector depletion layers were lineari zed. The transmission coefficient as a function of energy was obtained using Airy and exponential function solutions to Schrodinger equation . The transmission coefficient was energy averaged for various V(BC)s and thus, a transmission parameter for the collector-base junction was obtained and used in a DC I-V characteristics model. Theoretical resu lts are in excellent agreement with the experimental results with the V(BC)s at which the peaks of the collector current occurs matching clo sely for the first two peaks. (C) 1995 Academic Press Limited