Top-Illuminated Annular Ge-on-Si APD in 45-nm Lateral SOI Process With High Gain–Bandwidth Product at 4 V Reverse Bias

A top-illuminated Ge-on-Si avalanche photodiode is designed and fabricated in GlobalFoundries’ 45-nm SOI platform (Si Photonic). The device utilizes an annular-shaped germanium layer as the absorption region, enclosed by cathode contacts to ensure avalanche multiplication of photocarriers and flow of photocurrent from all directions to the central anode. The proposed device structure modification results in an increased gain within a small silicon area. The device utilizes a germanium width of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.8~\mu $ </tex-math></inline-formula>m to reduce the carrier transit time and achieve high optoelectronic bandwidth. Testing is performed using 940- and 1550-nm wavelength continuous wave lasers. At a reverse bias of 4 V, the device shows a peak gain of 3.5 and 8.6 at 940- and 1550-nm wavelength, respectively. At the same bias, a -3-dB optoelectronic bandwidth of around 13.4 GHz is observed for 1550-nm wavelength. This results in a gain-bandwidth product (GBP) of ~115 GHz at 1550-nm wavelength for a low reverse bias of 4 V.