Useful full well is not determined solely by the physical photodiode volume. The storage well must hold the signal, the charge must transfer, and FD and the readout chain must accommodate the result. Increasing one capacity can reveal another bottleneck.
Two nodes, two capacity questions
The photodiode storage well holds collected carriers before readout. Its potential boundary and capacitance shape how much charge can be retained. FD receives the transferred charge and converts it into a voltage change. With finite permitted voltage swing, its receiving capacity is also finite.
The review identifies the smaller of storage-well and FD capacity as a possible full-well limit when the readout chain is not itself limiting. This is a conditional relationship, not a value assigned to the linked Flow.
Process checkpoint
Understand P-Pinning Ion Implantation in context
Identify the buried storage well and its potential boundary; explain why capacity alone does not establish complete transfer.
Process context for “Full Well Can Be Limited by the Photodiode or Floating Diffusion”: 40nm BSI CMOS Image Sensor · PD · Step 99
More storage can make transfer harder
Changing storage-region doping can increase capacity while also changing the empty-well potential. The transfer-gate and FD potentials must still support transfer. A larger nominal storage capacity does not therefore guarantee a larger completely readable signal.
Likewise, increasing FD capacity for a given voltage range reduces voltage change per carrier. The resulting input-referred readout uncertainty can change. Capacity and sensitivity must be discussed together, rather than treating every capacitance increase as an improvement.
Nominal capacity is not unlimited retention
A potential well is not a rigid container with an absolute brim. Carrier escape, diffusion and holding time affect the practical charge that can be retained. Illumination and operating conditions can also matter. A geometric drawing cannot certify that nominal and useful capacities coincide.
This distinction also prevents a misleading counterexample: a larger drawn photodiode does not prove that the receiving node or readout voltage range grew with it.
Locate the bottleneck in the linked pair
At pinning, identify the storage region and its boundaries. At the FD station, identify the receiving capacitance and charge-to-voltage role. Explain what additional evidence would be needed to tell which limit controls useful full well.
The existing PD–TG–FD article establishes the functional handoff. This article asks a different question: which part of that handoff constrains maximum useful charge, and why changing one node does not settle the whole limit.
References
A Review of the Pinned Photodiode for CCD and CMOS Image Sensors
E. Fossum, Donald B. Hondongwa · IEEE Journal of the Electron Devices Society