High frequency characterization of small geometry bipolar transistors

Small-signal high-frequency measurements of small bipolar transistors, carried out `on wafer´ up to a frequency of 18 GHz, are presented. Current gain, transconductance, and maximum power gain characteristics as a function of frequency and DC bias conditions have been obtained with accurate calibration and correction techniques. The figures of merit, f_tf_y, and f_max, associated with these characteristics have been discussed, and a simplified analysis of the relevant time constants has been given. Simulation results with a modified Gummel/Poon model show that good high-frequency modeling can be achieved by taking into account the current dependence of the base resistance and the base transit time, and the modeling of excess phase shift. Finally the importance of modeling the distributed base-collector capacitance, which is accomplished in the Gummel/Poon model with the parameter x _cjc, has been emphasized. It is very important even for frequencies well below the cutoff frequency