In ToF and many other medical field application, the SPAD is quenched by either passive or active quench circuit otherwise the current after SPAD is on by a photon is self-sustained. The quenching time and recharge time for passive quench circuit (PQC) is typically around ns and us respectively, while for active quench circuit the recharge time can be as low as ns range, as far as I know. In PQC The quench time Tq after SPAD on is governed by (RD//RL)*Ctotal~=RD*(CD+C) and the recharge time Tr is (RL)*Ctotal~=RL*(CD+C).

I come across this simple idea, why cannot we design a circuit with a capacitor connected in parallel with SPAD, with voltage source Vs(designed to be have Vbreakdown+Vexcess bias) and switch as shown in below.

Then in this simple setup, at time instance t0 the switch is on, and thus the Vd becomes Vs immediately, then switch is off. The SPAD is still in off-state, then Vd will slightly reduces as the current leakage though the SPAD diode resistive path. However, as the SPAD is still off the resistance is significant large and the voltage drop across the diode is much slow ensuring the Vd is still above Vbreakdown after a while (say even after few us). Then when the photon comes, the resistance in SPAD becomes RD (100ohm-1kohm range), the avalanche current triggers and the junction capacitance is typically 0.1-1 pF. The Tq is calculated to be RD(CD+C), if I set C as small as CD and assume RD=500 ohm, CD=0.5pF, then the Tq is 500ps. As for recharge phase, we can turn the switch on, and there is no large quench resistor RL, and SPAD is off, and ideally speaking the resistor is infinitely small, the voltage is charged to Vs instantsly. so the total Tq+Tr is as small as in ps scale. Why people not use this simple design for fast quenching and recharge?