As an alternative to HgCdTe, InAsxSb1-x is an important alloy providing bandgaps suitable for infrared detection in both 3-5 and 8-12 mum atmospheric windows. There has been interest in high-quality growth of this material on GaAs and Si substrates. This paper reports the fabrication and characteristics of mid-infrared InAs0.8Sb0.2 photodetectors grown on GaAs substrates by molecular beam epitaxy. Peak detectivities of 1.4 x 10(10) and 7.5 x 10(8) cm Hz(1/2) W-1 were measured at 77 K and 240 K. respectively. showing that the alloy is promising as an alternative to thermal detectors for near room temperature operation. A detailed analysis of the dark current showed that the reverse-bias differential resistance was limited by trap-assisted tunneling in a wide temperature range up to 150 K under low and moderately large reverse bias voltages. Noise measurements have shown that trap assisted tunneling is the source of I If noise which is increased with the addition of band to band tunneling under large reverse bias. Our observations on InAs0.8Sb0.2 photodiodes show resemblance to those reported for HgCdTe detectors. The results are encouraging for the development of mid-infrared large area focal plane arrays on GaAs substrates.