Abstract A thermal breakdown analysis of planar GaAs metal-semiconductor diodes is presented and three models relating failure power ( P f ) to corresponding failure times ( t f ) are compared. The standard Wunsch and Bell model, a modified form of the Tasca model, and a three-dimensional model (developed by the authors) are fitted to experimental data. The results indicate that, within given failure time domains, GaAs structures of this kind follow the same general patterns, P f αt f − q , as those previously reported for silicon semiconductor devices. There is one exception. In the time domain roughly between 1 and 20 μs, the failure power is given by P f α 1/ log e ( t f ). Analytic expressions are used to extract three “defect” dimensions from forward bias experimental data. These “defect” dimensions are discussed with reference to the device dimensions and the ambient temperature. Two distinct visible and electrical failure modes have been observed and these are linked to changes in the channel conductance subsequent to an applied pulse.