Electro-thermal characteristics of the Si MOSFET in transient state are reported using a non-isothermal device simulator where both the transistor's self-heating and the thermal influence of its neighboring devices are duly taken into account. The thermal influence is estimated using a three-dimensional thermal simulator. Based on this set-up, we predict time-dependent electro-thermal characteristics of the Si MOSFET at gate switching and its drain breakdown conditions. We show that the time delay between the electrical response and the lattice temperature rise, is significant and thus can not be neglected. In addition, we found that avalanche and thermal breakdown characteristics largely depend on the slope of the drain input voltage.