Oscillations in collector current of a double heterostructure bipolar transistor: a quantum effect

摘要

In spite of many advantages, GaInP/GaAs/GaInP double heterostructure bipolar transistors (DHBT) suffer from the disadvantage of energy barrier to flow of carriers at the collector-base junction due to non-zero conduction band edge discontinuity which results in higher V_(D,sat). To circumvent this critical problem, Liu et al. IEEE Transactions on Electron Devices, 40, 1384-1389 (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 current in the forward active mode with the output voltage (V_(BC)), the origin of which they proposed to be the presence and absence of resonant energy 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 performed at the base-set-back layer and set-back layer-collector junctions to determine the distribution of the output voltage, V_(BC), at these junctions 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 linearized. The transmission coefficient as a function of energy was obtained using Airy and exponential function solutions to Schroedinger 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 results are in excellent agreement with the experimental results with the V_(BC)s at which the peaks of the collector current occurs matching closely for the first two peaks.