A SiGe/Si Phototransistor With High FOM of Gain*VA Using 0.35-μm BiCMOS Technology

Abstract

A novel SiGe/Si phototransistor with high figure of merit (FOM) of gain*early voltage for optical communication was designed and fabricated in a standard 0.35- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> BiCMOS process. A compound base consisting of a P-Si layer and a P+-SiGe layer was designed to achieve a good optical response through relieving the negative influence induced by doped ion’s mutual diffusion in device fabrication. The fabricated surface-illuminated SiGe/Si heterojunction phototransistor (HPT) was measured under 850-nm incident light with a variety of optical powers at 1-V collector bias. When the incident optical power was <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.29~\mu \text{W}$ </tex-math></inline-formula> , the collector current reached <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4.45~\mu \text{A}$ </tex-math></inline-formula> . The maximum gain exceeded 6.925 with the dark current of about 100 pA. The FOM of gain*early voltage of the proposed SiGe/Si HPT can achieve 100.97. Its optical DC and RF responses under high light power were analyzed with a physically based simulation model, which considered the process effects of device fabrication detailed. The optical gain may promote to 13.3 but with the deteriorated output performance and a smaller early voltage. The maximum optical characteristic frequency may reach 17.6 GHz.