Effects of polarization fields on avalanche breakdown of AlGaN quantum-well photodiode

Polarization-induced electric fields in AlGaN quantum wells have important effects on avalanche breakdown of AlGaN quantum-well photodiodes. When the polarization-induced fields within the AlGaN well layers have the same direction as applied electric field, they can help enhance impact ionization rate and decrease threshold voltage of avalanche breakdown of AlGaN avalanche photodiodes. However, according to previous research on avalanche breakdown of AlGaN photodiodes, no distinct breakdown threshold was observed from current-voltage curve. Instead, a soft avalanche breakdown was observed across applied voltage ranging from zero to a few volts while electroluminescence spectra show a threshold of about 10 V for avalanche breakdown. In this work, by considering impact ionization of defect levels and carrier screening effect, impact ionization coefficients are calculated as functions of applied voltage and the soft breakdown is well explained. It is also found that strong carrier screening effect will decrease impact ionization rate in a certain range of voltage thus affecting device performance.