The effects of nonlinear C/sub BC/ on the linearity of HBT

It is commonly known that C/sub BC/ is a very strong nonlinear source in the HBT. To investigate the contribution of the nonlinear C/sub BC/ to the linearity, HBTs with a punch-through collector and normal collector were fabricated. At 2 GHz, the punch-through HBT and normal one exhibit the power gain of 16.6 dB and 16.9 dB, output power of 28.9 dBm and 28.6 dBm, and PAE of 51% and 59%, respectively at 1 dB gain compression point. Two-tone tests were carried out for both power devices. The normal and punch-through HBTs showed IMD3 (third-order intermodulation distortion) levels of -25 dBc and -20 dBc, respectively at 1 dB gain compression point; for both cases, IMD3 dropped to -30 dBc at 1.6 dB output power back off. Because C/sub BC/ of HBT with punch-through is linear at small input signal, the IP3 power of the HBT is higher than that of the normal HBT by 15 dB. We found that RF grounding at the collector terminal is an efficient method for third-order IM power reduction.