12-Ann Trippe - HBTs

12-Ann Trippe - HBTs - Heterojunction Bipolar Transistors...

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Heterojunction Bipolar Transistors Ann Trippe 6 April 2008
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HBT Basics Advantages: ± Base can be thinner (50-1000 Å) and more highly doped (10 18 10 20 /cm 3) . ± High Early voltage. ± Current density is rarely limited by high injection effects in the base. ± Current gain decreases with increasing T. High current gains possible at low temperatures when diffusion is not effective (with appropriate base grading).
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Modeling HBTs Gummel-Poon Model has 55+ parameters and is mainly used in SPICE simulations. Small signal model is more developed than large-signal model. BJT models can often be adapted to model HBTs.
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Designing an HBT Emitter-Up Structure: Easier to make and higher emitter injection efficiency. Collector-Up Structure: Reduced base-collector capacitance. More susceptible to self-heating since the collector holds the largest part of the device voltage.
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Designing an HBT Narrower fingers are better for higher frequencies and low temperatures, but have lower current gain and B-C capacitance is increased. Microwave performance of a single cell represents the best gain, efficiency, and power density of the HBT.
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12-Ann Trippe - HBTs - Heterojunction Bipolar Transistors...

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