1 TD / CIS Team / Sian-Min Chung

IEEE ED 2000,No,11;P2110

The PPD basically consists of a p+/n/p- structure where both p layers are on substrate potential (GND).

As the voltage applied to the n-layer is increased, the Depletion regions of both pn-junctions grow towardeach other. At a certain voltage, the pinned voltage , the depletion regions meet and no more majority carriers can be extracted from the device.The device is fully depleted . The potential then Remains fixed inside the device and cannot be increased any further. The voltage is pinned.

What’s the purpose of Pinned Photo Diode ?PHOTO Diode pinning potential is designed for Complete Photodiode-to-floating diffusion electron transfer at Operate Voltage .

2 TD / CIS Team / Sian-Min Chung

For light sensing operation, the PPD is initially fully depleted. During the integration phase, photo generated majority carriers are stored in the depletion region, decreasing the potential of the PPD below . For readout ,the floating diffusion FD is first reset to VDD. This reset potential may now first be read out for true CDS. Next, the transfer gate TG is opened and the completephotocharge is transferred to FD, which ensures lag-free operation. The complete transfer takes place if the voltage on FD remains above the pinning voltage while the PPD operates below this voltage.

The greatest benefit of this structure lies in the complete charge transfer from a large area, large capacitance diode into the small floating diode capacitance. The second advantage is the separation of the charge collection region away from the silicon surface into the bulk through the top p layer, equivalent to a buried channel CCD structure . This surface state pinning results in a great reduction of dark current and white point defects that stem from a large density of recombination centers at the surface.

3 TD / CIS Team / Sian-Min Chung

4 TD / CIS Team / Sian-Min Chung

Issue for PPD IEDM-99 36.5 (p.884)

Incomplete charge transfer at the low voltage caused By a potential pocket or a potential barrier for signalCharge on a charge transfer Gate !

Lest Figure :Potential barrier height and the potential pocket depthDepend on the offset length of P+ layer to N- layerEdge !

The Potential Barrier and Pocket make residue Charge In Photo Diode !

5 TD / CIS Team / Sian-Min Chung

Issue for PPD IEDM-99 36.5.2 (p.884)

The Potential barrier is suppressed By longer P+ to N- distance

The Potential pocket is suppressed By a counter doping with extra surface P-layer