On the energy band structure of the GaInP/GaAs heterojunction bipolar transistor

Abstract

The GaInP/GaAs heterojunction bipolar transistor energy band gap close to the junction is determined through the behavior of its collector and base currents and current gain as a function of the temperature. It results that the emitter-base junction is a gradual one with the emitter bandgap, at the place where the carrier injection takes place, being wider than that of the GaAs by just ~13 meV. It is also shown that for T > 200 K the diffusion contribution to the base current is due to injected holes into the emitter, imposing a strong limitation on the device current gain. Besides, such bandgap difference between the emitter and the base makes the device current gain to decrease as the temperature increases. In view of these results technological modifications are discussed to improve the transistor performance. . Their superior performances stem from an emitter forbidden bandgap larger than that of the base, introducing additional degrees of freedom in the design of the device. In the case of an abrupt heterojunction, such bandgap difference is partitioned between the conduction and the valence bands producing energy band discontinuities, introducing different barriers heights for the injection of carriers into the other material. The HBT's take advantage of this unequal minority carrier injection rate through the heterojunction, to achieve a higher emitter efficiency than in a homojunction transistor bearing the same majority carriers concentration at each side of the junction. The GaInP alloy matched to the GaAs lattice has a forbidden energy gap wider than that of GaAs by ~430 meV. Most of this bandgap difference appears in the valence band with ∆E v = 240-300 meV and the respective discontinuity in the conduction band, ∆E c being much smaller. Such heterojunction bands condition reduces, in principle, in an important way the injection of holes from the GaAs into the GaInP, leaving almost unchanged the ability to inject electrons from the GaInP into the GaAs [2-5]. These unequal injection rates are well adapted for getting an Npn: n-GaInP/p-GaAs, heterojunction bipolar transistor in which the n-GaInP as emitter should severely confine the base holes. Although most of these promises are now confirmed, there is still a poor understanding of the HBT currents origin and mechanisms, complicating the transistor optimization. In this work we report on the emitter-base heterojunction energy bands diagram at the place where the minority carriers are injected at each side of the heterojunction. We have probed the energy band gap at those places of the heterostructure by measuring the collector and base currents as well as the current gain as a function of the emitter-base junction temperature.