DC characterisation of HBTs using the observed kink effect on the base current

Large parasitic series resistances of the heterostructure bipolar transistors are shown to cause a sharp rise in the base current of the Gummel plot at high current levels. This effect is analysed and attributed to a forward-biased base-collector junction due to the large voltage drop across the collector series resistance of the device. The occurrence or this effect results in a severe reduction in current gain of the device at high current levels. A DC Ebers-Moll model is presented with ail the parameters extracted from DC characterisation of the HBT, and an excellent fit between the model and experimental data is obtained. The method can be used to determine the base, collector and emitter series resistances of the bipolar transistors for DC applications. Finally. the effect of temperature on the onset of this effect is demonstrated. (C) 1998 Elsevier Science Ltd. All rights reserved.