N-i-p-i Infrared photodetectors

Abstract

n-i-p-i doping superlattices, a periodic sequence of n- and p-doped layers, possibly separated by intrinsic layers of the same semiconductor material, exhibit unusual electronic properties which make them suitable for a wide variety of high performance infrared photodetectors. An indirect band gap in real space separates photoelectrons and holes spatially and, therefore, leads to extremely long recombination lifetimes. By the use of selective n- and p-type contacts lifetimes as well as electron and hole concentrations can be externally adjusted, electron and hole transport can be manipulated independently. These features allow for photodetectors with extremely low values of thermal and generation-recombination noise, and giant low-frequency responsitivities. By suitable design a gain-bandwidth product larger than 20 GHz becomes possible, which makes them highly attractive even for high-speed optical communication. The wavelength dependence of the responsivity of n-i-p-i infrared detectors differs from familiar semiconductor detectors by a longwavelength tail at photon energies less than the band gap of the host semiconductor. The decay width of this subband gap response can be tailored by design and tuned by reverse bias over a wide range. These properties are particularly pronounced in narrow-gap semiconductors. In this paper we outline the concept of various versions of n-i-p-i photodetectors and we report on recent experimental results on detectors made from GaAs and PbTe.