Unconditionally thermally stable cascode GaAs HBT's for microwave applications

This letter describes the performance of a thermally stabilized cascode-heterojuction bipolar transistor (TSC-HBT) that exhibits unconditional thermal stability without the use of ballast resistors. A thermal isolation inserted between the current source (CE stage) and the power stage (CB stage) eliminates the positive electrothermal feedback that causes thermal runaway in bipolar transistors, The TSC-HBT cell designs with f(max) values in excess of 100 GHz demonstrated about 300% improvement in de power dissipation capability compared to conventional cascode HBT's in a direct comparison.