Image lag can involve signal charge left after its intended transfer and appearing in a later observation. Blooming involves excess charge escaping or spreading when storage is overfilled. Both concern charge history, but they describe different failures and need different evidence.
Ask when the charge became unwanted
For transfer-related lag, the problem is incomplete delivery: some of the previous signal remains when it should have left. The potential transition through the transfer gate, the receiving FD state and the available transfer time matter. A correctly drawn gate does not prove that every carrier crossed.
For blooming, the problem begins when generated charge exceeds practical storage capacity. Excess carriers follow the surrounding potential landscape. In a CMOS PPD example, they can preferentially move toward FD and the reset path, although neighboring pixels can still be affected. This is an architecture-dependent description.
Process checkpoint
Understand P-Pinning Ion Implantation in context
Trace storage, transfer and overflow as different states and explain why a pinning layer alone does not guarantee either endpoint.
Process context for “Image Lag vs Blooming: Residual Charge and Excess Charge”: 40nm BSI CMOS Image Sensor · PD · Step 99
A remedy for one is not proof of the other
A transfer path that efficiently empties the well need not provide enough capacity for a stronger exposure. A favorable overflow path need not eliminate trapped or incompletely transferred previous signal. The two questions cannot be combined into “charge management passed.”
Surface pinning contributes to a useful boundary, but the rest of the operating potentials and local defects remain relevant. Manufacturing order is not the same as the charge movement sequence during exposure and readout.
Conditions and counterexamples
A persistent image can have causes beyond incomplete PPD transfer, including trapping elsewhere. A bright spatial artifact need not uniquely identify blooming. Illumination history, later observations and structure-sensitive evidence must distinguish competing explanations.
Likewise, the existence of a reset drain does not guarantee that all excess charge reaches it before affecting a neighbor. A diagram of an intended path is a hypothesis about operation, not a measurement of containment.
Explain two histories at the same station
At the linked station, identify the storage well and transfer boundary. Tell one history in which intended signal remains, and another in which additional signal exceeds storage. For each, state when the charge becomes unwanted and what further observation would support the explanation.
This task distinguishes residual and excess charge without prescribing exposure, pulse or implant settings.
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