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

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-
$\mu \text{m}$
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
$2.29~\mu \text{W}$
, the collector current reached
$4.45~\mu \text{A}$
. 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.