Abstract: An electric thermal model of PIN diodes was established with the T-CAD software, the steady-state and transient characteristics of PIN diodes have been numerically investigated. The electric thermal properties of PIN diodes were simulated under pulse and stable signals with forward and reverse voltage biases respectively, and the effects of the thickness of the intrinsic layer on the temperature inside the diode were studied with their steady-state and transient response curves. The simulation results show that, when the thickness of intrinsic layer increases, the peak temperature inside the diode increases slowly and the position of the high-temperature region moves from the junction to the middle of the diode.