Hot-electron transport in a graded-bandgap base heterojunction bipolar transistor

The direct observation of electron heating in an electric field using hot-electron spectroscopy is reported. The device structure used for the study was a graded-bandgap base heterojunction bipolar transistor, fabricated in the GaAs-AlGaAs semiconductor alloy system and grown by molecular-beam epitaxy (MBE). The transistor had a quasielectric field in the base of 20 kV/cm, obtained by compositional grading of the Al. The emitter-base junction was also compositionally graded so that an equilibrium electron distribution could be injected into the 900-Å-wide base. The results are consistent with previous calculations showing that the energy loss rate for hot electrons varies with average energy but is in the range of 0.05 ps for energies in excess of 60 meV. While the effective electron temperature measured in the graded-base transistor is a factor of four times the temperature obtained in earlier experiments, this is still well below the maximum temperature expected for the case where electron-hole scattering is neglected. For this case, Monte Carlo simulations predict an electron temperature of about 2000 K for a energy gain of 0.18 eV. The temperature rise is limited by the onset of intervalley scattering at higher energies