Experimental method to thermally deembed pads from R/sub TH/ measurements

Both compound semiconductor and silicon-based bipolar junction transistors or heterojunction bipolar transistors (HBTs) require the efficient removal of heat in order to achieve a maximum level of performance and reliability. In order to satisfy both of these criteria, the electrothermal behavior of each device must be captured in a compact model. The model parameter that determines the junction temperature is R/sub TH/, the thermal resistance. Experimental methods to determine R/sub TH/ often require a relatively small device with a large R/sub TH/ to be attached to a set of relatively large metal pads with a low R/sub TH/. The pads act as a thermal shunt to the substrate and artificially lower the measured R/sub TH/. In order to obtain a suitable R/sub TH/ value for a device located in an IC, the pads must be deembedded from the measured data, much like pad deembedding for an S-parameter measurement. Test structures with various width metal traces between the emitter pad and device's emitter have been fabricated in a 200-GHz InP double HBT process. A method of using the measured R/sub TH/ of these structures and a simple resistive network model to deembed the pads is presented. It is shown that deembedded values can be as much as 30% higher than the measured R/sub TH/.