I-V characteristics of the unpassivated InAs p i n diodes with various i-layer thickness have been systematically studied. At the same reverse bi;ls of -0.5 V, the reverse current density is -0.83 and -0.18 A/cm(2) for the p n diodes and p i n diodes with a 0.72-mum-thick i-layer, respectively. It is obvious that the surface shunt and bulk leakage current are dramatically decreased owing to increasing of the i-layer thickness between p and n layers of InAs p n junction. Meanwhile, an odd current temperature behavior; the reverse-bias current decreases with increasing temperature, has been observed in the InAs p i n photodetector with a 0.72-mum-thick i-layer. Below 90 K, the photocurrent generated by the background thermal radiation should dominate the I-V characteristics of the p-i-n diode and the photocurrent increases a little as temperature drops due to the improvement of minority carrier diffusion length.