Direct Extraction of HBT Equivalent Circuit Model Using Dominant Pole Approximation

A new direct parameter extraction technique has been developed for HBT smallsignal hybrid-¿ equivalent circuit. The fictitious input (Rs) and output (RI) resistances are added to HBT. Based on 20 dB/decade frequency roll-off characteristics, the 3 dB current gain cut-off frequencies are measured for the overall circuit as a function of Rs and RL, to exactly determine the total base-collector capacitance(CT=C¿i+C¿x) the effective base resistance (rbx + rbi ×(C¿i /CT)), and the base-emitter capacitance (C¿). Most important is the ability to clearly separate the effective base resistance from the frequency-dependent base resistance, Re(Z11-Z12). Assuming that (C¿I /CT) ~ emitter area/B-C junction area, an additional measurement of extrinsic base resistance (rbx), under the open collector bias condition, uniquely gives rbi, C¿i, and C¿x values. The modeled S-parameters closely fit the measured ones within 3 % error.