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This paper is primarily an overview of our work on the technology, material and electronic properties, and performance limitations of compound semiconductor heterostructure bipolar transistors. Graded-gap epitaxial n-type ohmic contacts and p-type shallow diffusion ohmic contacts are important in the fabrication of high-performance (Al,Ga)As/GaAs devices. In the device structure implemented, the presence of a wide-gap p-type (Al,Ga)As extrinsic base region at the surface suppresses surface recombination, thereby enhancing the current gain at small device dimensions. We discuss experimental and theoretical results concerning the limiting physical effects due to heterostructure design and intrinsic and extrinsic bulk phenomena of compound semiconductors, emphasizing the understanding developed and the discoveries made during the course of our efforts. As device speeds have increased with coordinated scaling, dispersive effects have become increasingly important. We show how these may be included by modifying the conventional quasi-static modeling of the bipolar transistor, in order to obtain a realistic simulation of fast switching transients. Finally, we discuss scaling of heterostructure bipolar transistors, and implications of the use of lower-bandgap materials and operation at cryogenic temperatures.
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