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

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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 VD,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 (VBC), 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, Ec, 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, VBC, at these junctions using degenerate statistics. This calculation also provided the electric field and potential drop on the set-back layer. The parabolic Ec 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 Schrödinger equation. The transmission coefficient was energy averaged for various VBCS 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 VBCS at which the peaks of the collector current occurs matching closely for the first two peaks.


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