Performance of Graded Bandgap HgCdTe Avalanche Photodiode

This paper reports the performance of midwave infrared (MWIR) electron-injection avalanche photodiode (e-APD) fabricated using graded bandgap HgCdTe epilayers. Carrier transport in the e-APD is dominated by drift transport due to the built-in electric field associated with gradient in the bandgap. Carriers encounter fewer collision events before entering the multiplication region as the dead space effect is reduced. On-set of generation and multiplication processes is controlled more effectively. High gain indicates a reduced in-elastic scattering by phonon-emission due to gradient. Quantum efficiency above 80% is achieved in merely 2-3-mu m-thick absorbing layer because of more efficient collection of the photogenerated carriers. Lower generation volume is beneficial in terms of low dark current. The generation is confined in the vicinity of the multiplication region. Generated carriers are readily evacuated from the absorber region under the built-in electric field. An order of magnitude improvement over the state-of-the art performance in MWIR e-APD is achieved by introducing a controlled energy bandgap gradient in the HgCdTe epilayers.