Impact Ionization and Light-emission In AlGaAs/GaAs HEMTs

Impact ionization and light emission phenomena have been studied in AlGaAs/GaAs HEMT's biased at high drain voltages by measuring the gate excess current due to holes generated by impact ionization and by analyzing the energy distribution of the light emitted from devices in the 1.1-3.1 eV energy range. The emitted spectra in this energy range can be divided into three energy regions: i) around 1.4 eV light emission is dominated by band-to-band recombination between cold electrons and holes in GaAs; ii) in the energy range from 1.5 to 2.6 eV energy distribution of the emitted photons is approximately Maxwellian; iii) beyond 2.6 eV the spectra are markedly distorted due to light absorption in the n+ GaAs cap layer. The integrated intensity of photons with energies larger than 1.7 eV is proportional to the product of the drain and gate currents. This suggests recombination of channel electrons with holes generated by impact ionization as the dominant emission mechanism of visible light.