Performance enhancement of high-speed SiGe-based heterojunction phototransistor with substrate terminal

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Abstract
Heterojunctionphototransistors (HPTs) are requested to have high electrical bandwidth ( ∼ 1 GHz ) performance for their application of high-speed digital fiber communication. In this letter, a method is disclosed to enhance the speed performance of Si ∕ SiGe -based HPTs, which can overcome the low quantum efficiency drawback ( ∼ 0.1 A ∕ W ) of Si -based high-speed photodetectors at the wavelength of 850 nm due to its large internal gain. By use of the substrate terminal of HPT, the speed performance can be enhanced greatly with much less reduction in optical gain as compared to the traditional technique with base-terminal-bias. Under proper optical power excitation and the common ground of substrate and emitter terminals, we can achieve 1.8 GHz fast-Fourier-transformed electrical bandwidth with 0.7 A ∕ W responsivity simultaneously. The demonstrated device structure can serve as a key component in the short-reach fiber communication system.

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