A novel base feed design for high power, high frequency heterojunction bipolar transistors

HBTs for power amplifiers are typically designed to provide high power per unit cell to minimize chip area and losses in the on-chip combiner circuitry. A high power unit cell is obtained by combining the power of several emitter fingers ("subcells"). A problem arises when the magnitude and phase of signals driving each sub-cell differ significantly from one to the other. The imbalance results in reduced total power and efficiency and the problem gets worse with increasing frequency. Our experience is that at X-band the performance of the unit cell is compromised when more than four to six subcells (40 /spl mu/m/sup 2/ each) are combined in one device using the conventional "fishbone" method. This paper describes a new method of feeding signal to the sub-cells for overcoming this problem. An example is provided where the powers of the sixteen 40 /spl mu/m/sup 2/ subcells are combined such that the output scales as expected and there is minimal loss in the efficiency. The unit cell produced 29.6 dBm power with a 10 dB gain and PAE of 58% at 10.5 GHz. A two-stage X-Band power MMIC using this cell yielded 5 W with a gain of 13 dB and 43% PAE.