Thermal switchback in high ftepitaxial transistors

Maximum power dissipation of transistors is limited by a thermal switchback of the VCEresulting often in a complete melting of a small area of the device even under conditions of maximum thermal conduction between collector junction and case. This thermal switchback occurs when an isolated area of the transistor, because of normal random variations of electrical behavior, or of non-uniform electrical conductivity, or of a physical defect, draws higher currents than other regions; this increase in current density will heat the small area; the VBE, having a negative temperature coefficient, will decrease. As a result IBwill increase followed by a much larger increase in IC. This rapid heating avalanche will cause the thermal runaway and eventually a burning out of the isolated area.