Effects of a p–n junction on heterojunction far infrared detectors

Abstract

HEterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) far infrared detectors based on the GaAs/AlGaAs material system have shown promise for operation at wavelengths up to a few hundred microns. HEIWIP detectors with GaAs emitters have been shown to operate out to 92μm. Recent modifications to use AlGaAs emitters have extended the zero response threshold out to 128μm. Extension to longer wavelengths will require reducing the dark current in the devices. An approach using the addition of a p–n junction in the detector, which was shown to work in QWIP and homojunction detectors is considered here. Differences between the predicted and observed threshold behavior could be explained by the presence of space charge within the device. The band bending from this space charge produces the observed variation in the threshold. The space charge can also be used to explain anomalous conduction observed at low biases. When the device is forward biased, the current is expected, to be small until the bias voltage is similar to the bandgap of 1.4eV, above which the current should increase rapidly. Dark current was observed for biases significantly less than the bandgap. The threshold bias decreased with temperature, and was as low as 0.25V for a temperature of 300K. This is much lower than could be explained by thermal effects alone.