SUMMARYnMOS-type and pMOS-type silicon avalanche photodiodes (APDs) were fabricated by standard 0.18 µm CMOS process, and the current-voltage characteristic and the frequency response of the APDs with and without guard ring structure were measured. The role of the guard ring is cancellation of photo-generated carriers in a deep layer and a substrate. The bandwidth of the APD is enhanced with the guard ring structure at a sacrifice of the responsivity. Based on comparison of nMOS-type and pMOS-type APDs, the nMOS-type APD is more suitable for high-speed operation. The bandwidth is enhanced with decreasing the spacing of interdigital electrodes due to decreased carrier transit time and with decreasing the detection area and the PAD size for RF probing due to decreased device capacitance. The maximum bandwidth was achieved with the avalanche gain of about 10. Finally, we fabricated a nMOS-type APD with the electrode spacing of 0.84 µm, the detection area of 10 × 10 µm 2 , the PAD size for RF probing of 30 × 30 µm 2 , and with the guard ring structure. The maximum bandwidth of 8.4 GHz was achieved along with the gain-bandwidth product of 280 GHz.

Avalanche photodiodes fabricated by CMOS process (CMOS-APDs) have features of high avalanche gain below 10 V, wide bandwidth over 5 GHz, and easy integration with electronic circuits. In CMOS-APDs, guard ring structure is introduced for high-speed operation by canceling photo-generated carriers in the substrate at the sacrifice of the responsivity. We describe here wavelength dependence of the responsivity and the bandwidth of the CMOS-APDs with opened and shorted guard ring structure.

SUMMARYQuadrant silicon avalanche photodiodes (APDs) were fabricated by standard 0.18 µm CMOS process, and were characterized at 405 nm wavelength for Blu-ray applications. The size of each APD element is 50 × 50 µm 2 . The dark current was 10 pA at low bias voltage, and low crosstalk of about −80 dB between adjacent APD elements was achieved. Although the responsivity is less than 0.1 A/W at low bias voltage, the responsivity is enhanced to more than 1 A/W at less than 10 V bias voltage due to avalanche amplification. The wide bandwidth of 1.5 GHz was achieved with the responsivity of more than 1 A/W, which is limited by the capacitance of the APD. We believe that the fabricated quadrant APD is a promising photodiode for multi-layer Blu-ray system.

Abstract-Silicon avalanche photodiode (APD) was fabricated by standard 0.18 μm CMOS process. The current-voltage characteristic and frequency response was measured for the APD with and without guard ring. With the existent of guard ring around the perimeter of the diode junction, it shows a better performance for the maximum bandwidth but in contrast it shows lower in responsivity. For the first time, an optimizing of electrode spacing, detection area and the PAD size for RF probing shows an enhancement of the bandwidth. The detection area and the PAD size for RF probing are reduced to 10 x 10 μm² and 30 x 30 μm², respectively, to decrease the device capacitance, the spacing of interdigital electrode is narrowed to 0.84 μm to decrease carrier transit time, and by cancelling the carriers photo-generated in the deep layer and the substrate because the carriers are slow diffusion carriers. As a result, the maximum bandwidth of 8.0 GHz was achieved along with gain-bandwidth product of 280 GHz.