Compositionally graded contact layers in quantum-well infrared photodetectors

We have observed a threshold voltage, below which no photocurrent flows, in a class of midwave quantum-well infrared photodetectors using (In,Ga)As/(In,Al)As lattice matched to InP substrates. We propose that this threshold voltage is the result of internal fields established at the interface between doped contact layers and the doped quantum wells in the detector. We show that compositionally graded contact layers can be used to eliminate the threshold voltage and to enhance the low-bias performance of the detectors. (C) 2004 American Institute of Physics.