SSC JE Electrical Engineering 2017 Set 3 Paper

Thermal run way in a transistor biased in the active range is due to change in reverse collector saturation currents due to rise in temperature because reverse collector saturation current (${\mathit{I}}_0$)depends on temperature. If ${\mathit{I}}_{\mathit{C}}$ increase then ${{\mathit{I}}^2}_{\mathrm{C}}\mathrm{R}$ loss increase and the temperature increase. So ${\mathit{I}}_0$ increase, ${\mathit{I}}_{\mathit{C}}=\mathrm{\beta }{\mathit{I}}_{\mathit{B}}+{\mathit{I}}_0$