Ultra-Low Capacitance Spot PIN Photodiodes

Spot PIN photodiodes were integrated without any process modifications in a high-voltage 0.18 μm CMOS technology. These photodiodes are a combination of vertical and lateral PIN photodiodes using the P+ bulk wafer and a P-type ring at the surface as anodes. Devices with N+ cathode and N+/N-well cathode are compared. A small N+/N-well cathode spot reduces the capacitance to 1.47 fF and the N+ cathode spot leads to a capacitance of 1.07 fF. The light sensitive area of these photodiodes is 707 μm². Simulated electric field distributions show the full depletion of the spot PIN photodiodes. Responsivities from 0.12 A/W to 0.16 A/W and from 0.50 A/W to 0.52 A/W for 405 nm and 675 nm, respectively, are achieved. The measured bandwidths for 675 nm light are from 520 MHz to 690 MHz at reverse biases from 15 V to 30 V.