1. Definition
A PN 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 conductivity and serves as the core foundation for photoelectric conversion in image sensors.

Figure 1: Schematic diagram of the physical structure and circuit symbol of a PN junction

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

2. Materials
The formation of a PN junction relies on two semiconductor materials with different doping types. By doping with specific impurity elements, the conductivity of an intrinsic semiconductor (pure silicon crystal) can increase by millions of times, forming two types of extrinsic semiconductors:

2.1 P-type Semiconductor (Positive-type, Hole-type)

• Doping Process: Doping with trace amounts of trivalent elements (e.g., boron).
• Microscopic Mechanism: When trivalent boron atoms replace silicon atoms in the crystal lattice, they lack one valence electron, initially forming bound holes. At room temperature, acceptor ionization occurs: boron atoms capture valence electrons from surrounding silicon atoms, becoming immobile negative acceptor ions while generating freely mobile holes.

• Carrier Characteristics: Majority carriers are holes; minority carriers are electrons.

2.2 N-type Semiconductor (Negative-type, Electron-type)

• Doping Process: Doping with trace amounts of pentavalent elements (e.g., phosphorus, arsenic).
• Microscopic Mechanism: When pentavalent phosphorus atoms replace silicon atoms in the crystal lattice, they have one extra valence electron, initially forming bound electrons. At room temperature, donor ionization occurs: phosphorus atoms release this extra valence electron, becoming immobile positive donor ions while generating freely mobile free electrons.

• Carrier Characteristics: Majority carriers are electrons; minority carriers are holes.

Core Concept Notes:

• Carrier: Microscopic particles in a semiconductor that can move freely and carry electric charge (including free electrons, free holes, etc.). These freely moving charges do not alter the overall electrical neutrality of the material.
• Majority Carriers: In extrinsic semiconductors, carriers generated by the ionization of impurity atoms that hold an overwhelming numerical advantage, determining the primary conductive properties of the material.
• Minority Carriers: In extrinsic semiconductors, carriers present in very small numbers, primarily generated by thermal excitation from the ambient temperature.

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 come 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 PN junction.

4. Operating Principle
An external voltage applied to a PN junction disrupts the original dynamic balance, causing changes in its space charge region and thereby exhibiting unidirectional conductivity:

• Forward Bias (Positive terminal connected to P, negative to N)

The external electric field opposes the built-in electric field, weakening the built-in electric field. The space charge region narrows, majority carriers resume diffusion and form a large current, and the PN junction exhibits a low-resistance conducting state.

• Reverse Bias (Positive terminal connected to N, negative to P)

The external electric field aligns with the built-in electric field, strengthening the built-in electric field. The space charge region widens, the diffusion of majority carriers is completely blocked, and only an extremely weak drift current of minority carriers exists, causing the PN junction to exhibit a high-resistance cut-off state.

5. Application: Image Sensors
The core component of image sensors (such as CMOS) is the silicon photodiode, which is essentially a PN junction operating in a reverse bias (cut-off) state, utilizing the photovoltaic effect to convert optical signals into electrical signals. Its photosensitive mechanism is divided into the following two scenarios:

• Absorbed in the space charge region: Photons excite electron-hole pairs, which directly drift toward the N/P neutral regions under the action of the built-in electric field; upon reaching the edge of the neutral regions, they diffuse as majority carriers along the concentration gradient to the electrodes on both sides, forming a current.
• Absorbed in the neutral region: Photons excite electron-hole pairs, whose minority carriers first diffuse toward the interface driven by the concentration gradient; after entering the space charge region, they drift toward the opposite side under the action of the built-in electric field; upon reaching the edge of the opposite neutral region, they similarly diffuse as majority carriers to the electrodes, forming a current.

Core Reason for Choosing Reverse Bias: It is to significantly widen the space charge region and strengthen the built-in electric field, thereby allowing more photons to participate in the highly efficient “direct drift” process. This prevents charge recombination losses while significantly improving the sensor's response speed and suppressing background noise (dark current).