I. Definition
A p–n junction is a microstructure formed at the interface where a p-type semiconductor and an n-type semiconductor are in close contact. It exhibits unidirectional (direction-dependent) electrical behavior and is a fundamental basis for photoelectric conversion in image sensors.

Figure 1: Schematic showing the physical structure and circuit symbol of a p–n junction

(Image source: https://en.wikipedia.org/wiki/P%E2%80%93n_junction#/media/File:PN_diode_with_electrical_symbol.svg)

II. Materials
The formation of a p–n junction depends on two semiconductor materials with different doping types. By doping an intrinsic semiconductor (such as a pure silicon crystal) with selected impurity elements, its conductivity can increase by millions of times, producing two types of extrinsic semiconductors: n-type and p-type.

(1) P-type semiconductor (hole-type)

• Doping Process: Doping with trace amounts of trivalent elements (e.g., boron).
• Microscopic Mechanism: When trivalent boron atoms substitute for silicon atoms in the crystal lattice, they create an initial bound hole due to the missing valence electron. At room temperature, the acceptor ionizes: the boron atom captures a valence electron from a neighboring silicon atom, forming an immobile negatively charged acceptor ion and producing mobile holes.
• Carrier Characteristics: Majority carriers are holes; minority carriers are electrons.

(2) N-type semiconductor (electron-type)

• Doping Process: Doping with trace amounts of pentavalent elements (e.g., phosphorus, arsenic)..
• Microscopic Mechanism: When pentavalent donor atoms substitute for silicon atoms in the crystal lattice, they provide one extra valence electron, initially forming a bound electron. At room temperature, the donor ionizes: the donor atom releases the extra valence electron, becoming an immobile positively charged donor ion while generating freely mobile electrons.
• Carrier Characteristics: Majority carriers are electrons; minority carriers are holes.

Notes:

• Carrier: Mobile charge carriers in a semiconductor (e.g., free electrons, free holes). Their presence preserves the material’s overall electrical neutrality.
• Majority carriers: Carriers produced by impurity ionization in extrinsic semiconductors that greatly outnumber the others and dominate conduction.
• Minority carriers: Carriers present in much smaller numbers in extrinsic semiconductors, largely determined by thermal excitation and carrier balance.

3. Formation
The formation of a PN junction begins with the diffusion of majority carriers driven by the concentration gradient. When p-type and n-type semiconductors are brought into contact, free holes in the P-region and free electrons in the N-region diffuse into the opposite region and recombine. Subsequently,positively charged ionized donors are left at the boundary of the N-region, and negatively charged ionized acceptors are left at the boundary of the P-region, thereby forming a built-in electric field (space charge region)directed from the N-region to the P-region at the interface. The drift motion generated by this electric field prevents further diffusion of majority carriers. Ultimately, under thermal equilibrium, the diffusion current of majority carriers and the drift current of minority carriers reach a dynamic balance, forming a stable p–n junction.

4. Operating Principle
An external voltage applied to a p–n junction disrupts the original dynamic equilibrium, causing changes in its space-charge region and resulting in unidirectional conductivity:

Forward bias (positive terminal connected to p, negative terminal to n):
The external electric field opposes the built-in electric field, thereby reducing it. The space-charge region narrows; majority carriers resume diffusion, resulting in a large current. The p–n junction therefore exhibits a low-resistance conducting state.

Reverse bias (positive terminal connected to n, negative terminal to p):
The external electric field aligns with the built-in electric field, strengthening it. The space-charge region widens; majority-carrier diffusion is effectively blocked, and only a very small minority-carrier drift current flows. As a result, the p–n junction exhibits a high-resistance cutoff state.

5. Application: Image Sensors
The core component of image sensors (such as CMOS) is the silicon photodiode, which is essentially a p–n junction operated under reverse bias (cut-off). It converts optical signals into electrical signals through photogeneration and carrier separation.

Its photosensitive mechanism can be described in two cases:

• Generated in the space-charge region: Photons generate electron–hole pairs in the space-charge region. Under the built-in electric field, electrons and holes drift toward the neutral n-side and p-side, respectively. After reaching the edges of the neutral regions, they diffuse to the electrodes as majority carriers driven by the concentration gradient, producing a current.

• Generated in the neutral region: Photons generate electron–hole pairs in the neutral region. The minority carriers first diffuse toward the junction/interface driven by the concentration gradient. After entering the space-charge region, they drift across the junction under the built-in electric field toward the opposite side. Upon reaching the edge of the opposite neutral region, they diffuse to the electrodes as majority carriers, forming a current.

Core reason for using reverse bias: Reverse bias widens the space-charge region and strengthens the built-in electric field, enabling more photogenerated carriers to be separated by the efficient “direct drift” process. This reduces recombination losses, improves response speed, and suppresses dark current.