A method for heat flow resistance measurements in avalanche diodes

Avalanche transit time oscillators are operating at power densities approaching 10⁶W/cm², unprecedented in semiconductor device history. At such power densities, heat flow resistance problems at the interface between the flip-chip mounted silicon chips and the metal substrate, as well as between the package and the heat sink, are extremely critical. This paper describes a new, nondestructive and accurate method of measuring the heat flow resistance between junction and heat sink by utilizing the temperature dependent breakdown voltage Vb(T) as a conveniently built-in temperature sensor. Variations in junction temperature ΔT with power ΔP= VbΔI are, therefore, related to variations in breakdown voltage ΔVbwith current ΔI resulting in a contribution to the electrical small signal resistance of the diode. This thermal resistance contribution Rthcan be separated readily from spreading and space charge resistance Rapand Rscbecause of the frequency dependence of Rth(ω). Furthermore, the frequency dependence of Rth(ω) allows the separation of heat flow resistance contributions originating in the immediate vicinity of the junction (Si-metal interface) from contributions originating at a poor thermal contact between package and heat sink. In keeping with calculations on simplified geometrical configurations, for which analytical solutions of the frequency dependent heat flow in a distributed circuit could be obtained, experimental results are presented which indicate that both heat flow resistance contributions can be extracted and separated with sufficient accuracy from as few as three electrical resistance measurements, e.g., at dc, 100 Hz, and 1 MHz. The simplicity of such measurements and their evaluation make this technique ideal for in-line testing of production devices.