Area and Bandwidth Enhancement of an n⁺/p-Well Dot Avalanche Photodiode in 0.35 μm CMOS Technology

This paper presents a CMOS-integrated dot avalanche photodiode (dot-APD) that features a small central n+/p-well hemispherical cathode/p-well structure circularly surrounded by an anode ring. The dot-APD enables wide hemispherical depletion, charge collection from a large volume, and a small multiplication region. These features result in a large light-sensitive area, high responsivity and bandwidth, and exceptionally low junction capacitance. The active area can be further expanded using a multi-dot structure, which is an array of several cathode/p-well dots with a shared anode. Experimental results show that a 5 × 5 multi-dot APD with an active area of 70 μm × 70 μm achieves a bandwidth of 1.8 GHz, a responsivity of 9.7 A/W, and a capacitance of 27 fF. The structure of the multi-dot APD allows for the design of APDs in various sizes that offer high bandwidth and responsivity as an optical detector for various applications while still maintaining a small capacitance

A Near-Infrared Enhanced Field-Line Crowding Based CMOS-Integrated Avalanche Photodiode

This paper presents a CMOS-integrated linear-mode avalanche photodiode based on electric field-line crowding (EFLC-APD) to form an effective multiplication zone and a wide absorption zone. The EFLC-APD possesses a hemispherical avalanching electric field at the n-well/p- epi junction formed due to the curvature of the half-sphere cathode. A lower electric field extends radially across the entire volume of the EFLC-APD towards the substrate and towards the surface anode. Because of such a distribution of the electric field, electrons photogenerated within the whole volume drift towards the cathode. Therefore, the EFLC-APD provides a large sensitive-area to total-area ratio while offering high responsivity and bandwidth for red and near-infrared light due to its thick absorption zone and drift-based carrier transport. It is shown that the electric field distribution can be modified by the design parameters such as cathode radius and diode size in addition to doping profiles. The EFLC-APD achieves a responsivity-bandwidth (R-BW) product of 49.5 $\frac{\mathrm{A}}{\mathrm{W}}\cdot$GHz, corresponding to the responsivity and bandwidth of 33 A/W and 1.5 GHz, respectively, at the wavelength of 850 nm. In addition, a maximum responsivity of 3.05 $\times\, 10^{\mathrm{3}}$ A/W at 2 nW optical power is achieved for the red and near-infrared spectral range. Noise characterization resulted in an excess noise factor F = 6 measured at an avalanche gain of 56.7. Due to the high sensitive-area to total-area ratio, high responsivity, large bandwidth, and CMOS compatibility, this APD is a promising optical detector for many applications