Negative Photoconductivity in Ultranarrow-Gap Semiconductors

A decrease in conductivity under illumination, known as negative photoconductivity, has been observed in various semiconductors and is commonly attributed to trapping of excess carriers by deep centers. Here, we demonstrate that negative photoconductivity can instead arise from a rapid increase in carrier scattering in ultranarrow-gap semiconductors with degenerate carrier statistics. This behavior is explained by the combined effects of enhanced optical phonon emission scattering and an increase in effective mass due to band filling. Experimentally, photoconductivity was measured over wide ranges of excitation and temperature in unintentionally doped n-type short-period InAsSb_0.6/InAsSb_0.3 strained-layer superlattices (SLS), relevant for long-wavelength infrared optoelectronic devices. The resistive device impedance, weakly dependent on excess carrier concentration, simplifies broadband impedance matching to low-voltage CMOS driver electronics. At 77 K, 10.6 µm laser excitation led to an initial rise in conductivity, with a decrease observed above 10 W/cm².