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An n-p-n InGaP/GaAs heterojunction bipolar transistor (HBT) using a graded base doping profile has been fabricated by low pressure metalorganic chemical vapor deposition. A current gain of 77 and a base sheet resistance of 251 Ω/sq are achieved in the graded-base HBT. Compared to the graded-base structure, the nongraded-base structure has a lower current gain (68) and a higher base sheet resistance (294 Ω/sq). Furthermore, the studied graded-base HBT device also shows better microwave characteristics. The measured unity current-gain cutoff frequency (fT) can be improved from 18 to 22 GHz. The functional dependences of current gain, base sheet resistance, and microwave characteristics on the base doping profile are attributed to the graded-doping enhanced built-in field across the base and higher base doping at the emitter edge.
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Chen et al. (2003) studied this question.
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