edance Extraction olar Transistors

Abstruct- This paper describes a method for extracting the thermal impedance of bipolar transistors. The measurement is a two-step process: first the fractional temperature coefficients are calibrated at dc and then a transient step response is measured to extract the thermal spreading impedance. Measurement configurations and an example measurement cycle are shown. The measurement results can be fitted to multiple-pole models for use in compact circuit modeling in SPICE. HERMAL effects must be accounted for if bipolar transistors (BJT’s) are to be accurately modeled [l], [a]. This applies to all BJT’s, to heterojunction BJT’s where band-gap engineering leads to large and compEex temperature dependences of the current gain, and to SO1 and dielectrically isolated transistors where the low thermal conductivity of isolating layers enhances self-heating [3], 147. Two components must be characterized for accurate modeling: the thermal spreading impedance (temperature response to a change in dissipated power) and the temperature dependence of the current-voltage relationships of the transistor. At first glance measuring the temperature dependence of the currents appears easy; the ambient temperature is varied by a known amount and the resulting changes in the terminal currents are measured. In practice, the situation is complicated by the rise of the actual device temperature above the ambient by an unknown amount because of self-heating. This unknown temperature change can be significant compared to ambient variation especially in high current regions. This paper shows a way to overcome this problem, and how the results can be used to extract the thermal impedance from a transient response. If properly accounted for, the errors created by selfheating can be reduced in precision circuit design as well as in the extraction of the device model itself [5], (61. The base current is treated as the thermometer in the techniques demonstrated here. The technique is based on the assumption that modulation of the base recombination current by the collector-base voltage is negligible compared to the effect of self-heating on the total base current. This Manuscript received April 3, 1995; revised August 28, 1995. The review of this paper was arranged by Associate Editor D. P. Venet. This work was supported by the Seminconductor Research Corporation, Contract 91-SP-087. D. T. Zweideinger, R. M. Fox, and J. S. Brodsky are with the Department of Electrical and Computer Engineering, University of Florida, Gainesville, FL 3261 1-6200 USA. T. Jung is with Harris Semiconductor, Melbourne, FL 32902 USA. S.-G. Lee is with the School of Computer Science and Electronic Engineer