With continuing scaling down in microelectronic devices, the current density and the power consumption in the devices must increase. Hence, device reliability under high current density is an issue for ultralarge-scale integration technology. This study investigates the heating behavior of the heavily doped Si channels under high current stress. Thermal and electrical characterization of the channels in bulk Si and in silicon-on-insulator were conducted. An abnormal asymmetrical heating along the channels in bulk Si has been observed. We propose a junction leakage mechanism to explain the phenomenon observed. Other asymmetrical thermal effects, such as electron-hole recombination and Peltier effect, have also been discussed.