Abrupt interface AlGaAs/GaAs heterojunction bipolar transistors: Carrier heating and junction characteristics

Abstract

We have fabricated and characterized a series of abrupt interface Npn AlGaAs/GaAs single heterojunction bipolar transistors in which the aluminum arsenide mole fraction in the emitter has been varied in the range 0% to 55%. Tunneling through the spike in the emitter conduction band is shown to be important. It affects the initial energy of carriers that are injected into the base, their effective mass distribution, and also the collector turn-on voltage. Hot electron injection heats the minority carriers in the base. In an Al0.35Ga0.65As emitter device held at room temperature we have optically measured a carrier temperature Tc=640 K. From diffusive transport theory it follows that the carrier heating reduces the base transit time by 70%. The elevated carrier temperature in this device is shown to be confirmed by the gain (hFE=3500). The gains of all the other devices are limited by imperfect injection efficiencies. Although electroluminescence (EL) spectra reveal that hole injection into the bulk of the emitter takes place in low aluminum composition structures, space-charge recombination is shown to be a more important mechanism. Appendices present Richardson’s constant for AlGaAs as a function of composition and the derivation from EL spectra of the ratio of electron-to-hole current across a heterojunction.