Performance potential of high-frequency heterojunction transistors

Assuming a state-of-the-art microwave planar geometry, the maximum frequency of oscillation has been calculated for GaAs-Ge heterojunction transistors utilizing either doped or high-resistivity space-charge-limited emitters. This is compared with a Ge homojunction transistor of the same geometry. A detailed equivalent circuit is used which accounts for the parasitics of the chip. It is shown that if chip parasitics are neglected, GaA-Ge devices should outperform Ge devices by about 4 to 1 in power gain. In the geometry assumed, however both heterojunction and homojunction transistors are limited by wafer parasitics, particularly base contact resistance. The calculated figures of merit of the two types of devices are therefore quite similar.