Design optimization of microwave power heterojunction bipolar transistor cells

Results of a design optimization study of power heterojunction bipolar transistor (HBT) cells are presented. AlGaAs-GaAs HBTs have been fabricated using a simple heterostructure design grown by molecular beam epitaxy and a novel self-aligned fabrication process which offers relatively low parasitics. The influence of power transistor cell design on device performance is emphasized. The design optimization study involved simultaneous fabrication of transistor cells with a relatively wide range of geometries. Transistors with a wide range of emitter finger sizes and number, but with the same number of collector contacts and the same basic cell design approach, have been fabricated and characterized by DC and microwave testing. The test results indicate that HBTs with more than three fingers do not exhibit a higher maximum current and that adding emitter fingers in this design merely increases the parasitic base-collector capacitance and drastically reduces f/sub T/ and f/sub MAG/. In contrast, using a similar number of longer emitter fingers yields devices with the highest maximum current density while maintaining acceptable high-frequency performance. >