Both uniform temperature distribution and crowded temperature distribution for a large-geometry power transistor are studied. Different concepts are used for calculating these two types of temperature distribution. In the first instance, in order to calculate the uniform temperature distribution, it is assumed that the power is uniformly dissipated over the entire emitter surface. In the second instance, in calculating the crowded temperature distribution, the power dissipation is assumed to be concentrated at the edges of the emitter. The time-dependent diffusion equation is solved to calculate the temperature rises in these two cases.