A chemical junction is defined by the dopant profile where the relevant concentrations balance. Depletion boundaries delimit a region with reduced mobile-carrier populations under the electrical state of interest. They are not two names for the same geometrical line.
Begin with the profile definition
For a simple diffused region, the textbook defines the metallurgical junction where the introduced chemical concentration meets the background concentration. This is a profile-based definition. Electrical carrier distributions can differ from that chemical boundary, and real compensated or complex profiles require more careful interpretation.
A lithographic opening does not directly measure that profile. Implant scattering and later redistribution can place dopants beyond the originally exposed outline.
Process checkpoint
Understand N FD (Between T1 and T2) Ion Implantation in context
Identify the FD dopant-defined region and the reverse-biased depletion contribution to its capacitance; explain why the boundaries are different.
Process context for “Chemical Junction and Depletion Boundary Are Different Locations”: 40nm BSI CMOS Image Sensor · NFD · Step 96
Depletion belongs to an electrical state
At a p–n junction, charge redistribution and electrostatic conditions determine the depleted region on the two sides. The FD capacitance study relates junction capacitance to depletion width and junction area. Its analysis distinguishes those contributions from gate overlap and metal capacitance.
Consequently, a changed electrical observation need not mean that the chemical junction moved during that observation. A changed chemical profile can also affect the electrical boundary and capacitance, so the two descriptions interact without becoming identical.
Conditions and counterexamples
The same fabricated region can have different depletion conditions under different electrical states. Two regions with similar mask outlines can have different chemical profiles. Neither a layout boundary nor a single capacitance result uniquely supplies the missing profile.
The article uses a simple junction picture to clarify definitions. It does not treat an operating depletion boundary as a sharp material interface or prescribe bias and implant settings.
Identify both boundaries at FD
Read the FD station and name the introduced dopant region. Then identify the depleted region that contributes to capacitance when the junction is reverse biased. Explain which statement concerns fabrication and which concerns an electrical state.
A complete answer reserves the chemical-junction location for profile evidence and the depletion extent for electrostatic evidence. This is a spatial-boundary task, distinct from comparing total dose and active carriers.
References
Silicon VLSI Technology - Full
James D. Plummer, Michael D. Deal, Peter B. Griffin
Silicon VLSI Technology · ISBN 978-0130850379
[Paper] Analysis and Reduction Technologies of Floating Diffusion Capacitance in CMOS Image Sensor for Photon-Countable Sensitivity
Fumiaki Kusuhara, Shunichi Wakashima, Satoshi Nasuno, R. Kuroda, S. Sugawa