Recently, a new "XBn" device architecture, based on heterostructures, has been proposed as an alternative to a homojunction photodiode. The main difference is that no depletion layer exists in any narrow bandgap region of the device. Instead, the depletion layer is confined to a wide bandgap barrier material. The Generation-Recombination (G-R) contribution to the dark current is then almost totally suppressed and the dark current becomes diffusion limited. This lowering of the dark current allows the device operating temperature to be raised relative to that of a standard photodiode made from the same photon absorbing material, with essentially no loss of performance. At SCD we have been developing XBn devices grown on GaSb substrates with an InAsSb photon absorbing layer and an AlSbAs barrier layer. The results of optical and electrical measurements are presented on devices with a bandgap wavelength of about 4.1μm. Strong suppression of the G-R current is demonstrated over a range of almost two orders of magnitude in the doping of the photon absorbing active layer (AL), while at the same time very high internal quantum efficiencies are achieved. A model of the spectral response is developed which can reproduce the observed behaviour very well at 88K and 150K over the whole AL doping range. In properly optimized devices, the BLIP temperature is shown to be in the region of 160K at f/3.