Cost Effective MeV Isolation Mask for Heterojunction Bipolar Transistors
J. Gillespie, C. Bozada, R. Dettmer, R. Fitch, K. Nakano, J.
Sewell, D. Via
Sensors Directorate, Air Force Research Laboratory Wright-Pattereson, AFB, OH 45433
R. Bhattacharya, H. Evans
Universal Energy Systems Dayton, OH 45432
Heterojunction Bipolar Transistors (HBTs) have become popular microwave power devices for commercial applications as well as military. Commercial vendors include cell phone manufacturers, future automotive electronics and satellite communication systems. They are attracted to HBTs for their superior power handling capabilities, high efficiency, single bias requirement as well as high linearity. The military's power requirements and need for high efficient devices for phased array radar, wide band electronic warfare (EW), communication systems and advanced power supplies generates the need for thicker collector structures. The load line for an HBT shows that for high power capability you need high voltage at low collector current and high current at low collector voltage. In order to increase the collector breakdown voltage for HBTs, the collector thickness should be increased. This increase in collector thickness will in turn raise the power capability for HBTs. Also, to reduce the extrinsic collector resistance thick sub-collectors are desirable for high frequency operation.
These requirements can make an HBT structure as thick as 3mm. By increasing the structure thickness, we introduce problems associated with fabrication of the device. Thick epitaxial layers make it difficult to isolate the device. Thicknesses of this magnitude make standard KeV implant technology marginally acceptable. There is typically a trade off in performance in order to maintain an acceptable structure thickness. If performance cannot be traded away, other options need to be considered.