Semiconductors are the foundation of modern electronics—from smartphones and laptops to LEDs and solar panels. By adding tiny amounts of impurity atoms (a process called doping) to a pure semiconductor like silicon or germanium, we can create two useful types: p-type and n-type semiconductors. Although they share the same crystal lattice, they differ in the dopant used, the dominant charge carriers, the energy levels created inside the bandgap, and how current flows through them. This guide explains those differences clearly and concisely.
What is a P-Type Semiconductor?
A p-type semiconductor is formed by adding trivalent impurities (Group III elements) such as boron (B), aluminium (Al), or gallium (Ga) to a pure semiconductor. These atoms have three valence electrons—one fewer than silicon’s four—so they leave an incomplete bond known as a hole. Holes behave like positively charged mobile carriers.
- Dopant type: Trivalent (Group III) — acts as an acceptor.
- Majority carriers: Holes; minority carriers: Electrons.
- Energy level: Acceptor level appears close to the valence band.
- Fermi level: Shifts toward the valence band (between acceptor level and valence band).
- Motion: Majority carriers (holes) move from higher to lower potential.
In circuit terms, when an electric field is applied, neighboring electrons jump into holes, making holes appear to move in the opposite direction of electrons. The higher the acceptor concentration, the higher the hole density and hence the conductivity of the p-type region.
What is an N-Type Semiconductor?
An n-type semiconductor is produced by adding pentavalent impurities (Group V elements) such as phosphorus (P), arsenic (As), antimony (Sb), or bismuth (Bi). These atoms have five valence electrons—one more than silicon’s four—so the extra electron is loosely bound and becomes a free electron with minimal energy.
- Dopant type: Pentavalent (Group V) — acts as a donor.
- Majority carriers: Electrons; minority carriers: Holes.
- Energy level: Donor level appears close to the conduction band.
- Fermi level: Shifts toward the conduction band (between donor level and conduction band).
- Motion: Majority carriers (electrons) move from lower to higher potential.
Electrons typically have higher mobility than holes in the same material, so for equal doping levels, n-type regions often exhibit slightly higher conductivity than p-type regions.
Energy Bands & Fermi Level
In an intrinsic (pure) semiconductor, the Fermi level sits roughly midway between the conduction and valence bands. Doping introduces allowed energy states inside the bandgap:
- P-type: Acceptor levels near the valence band reduce the energy required to create mobile holes.
- N-type: Donor levels near the conduction band reduce the energy required to create free electrons.
This shift of the Fermi level explains why one type favors hole conduction while the other favors electron conduction. It also sets up the essential conditions for forming a p-n junction, the heart of diodes, BJTs, MOSFETs, LEDs and solar cells.
P-Type vs N-Type: Comparison Table
| Basis | P-Type Semiconductor | N-Type Semiconductor |
|---|---|---|
| Group of Doping Element | Group III (B, Al, Ga, In) | Group V (P, As, Sb, Bi) |
| Nature of Dopant | Acceptor (creates holes) | Donor (provides electrons) |
| Type of Impurity | Trivalent impurity | Pentavalent impurity |
| Majority / Minority Carriers | Majority: Holes | Minority: Electrons | Majority: Electrons | Minority: Holes |
| Carrier Density | Hole density nh ≫ ne | Electron density ne ≫ nh |
| Energy Level | Acceptor level near the valence band | Donor level near the conduction band |
| Fermi Level Position | Closer to the valence band | Closer to the conduction band |
| Direction of Majority Carrier Motion | Holes move high → low potential | Electrons move low → high potential |
| Conductivity Trend | Controlled by hole concentration | Generally higher (electron mobility > hole mobility) |
| Typical Use | P-regions in diodes, base/PMOS wells, LEDs | N-regions in diodes, emitter/NMOS wells, rectifiers |
Applications & Practical Notes
- P-N Junctions: Joining p-type and n-type regions forms a depletion region and a built-in electric field. This is the key to rectification (diodes), amplification (BJTs), and switching (MOSFETs).
- Doping Control: Adjusting dopant concentration tunes resistivity, threshold voltages, leakage, and device speed.
- Temperature Effects: At higher temperatures, intrinsic carriers increase; very heavy doping can approach degenerate (metal-like) behavior.
- Mobility Insight: Electron mobility is typically higher than hole mobility in the same material (e.g., Si), which is why n-type paths often conduct better for the same doping level.
Frequently Asked Questions
1) Why are holes considered positive charge carriers?
A hole is an empty state in the valence band where an electron is missing. When nearby electrons move to fill it, the hole appears to move in the opposite direction, behaving like a particle with positive charge.
2) Can I convert p-type to n-type (or vice versa)?
Yes, by changing the dopant species and concentration. In integrated circuits, regions are selectively doped using implantation and diffusion steps to create p- and n-wells as required.
3) Which has higher conductivity, p-type or n-type?
For the same base material and comparable dopant density, n-type usually shows slightly higher conductivity because electrons have higher mobility than holes.
4) Where are p-type and n-type used in real devices?
Everywhere—diodes (p-n junction), BJTs (p-n-p or n-p-n stacks), MOSFETs (p-channel and n-channel), LEDs and solar cells (p-n junctions engineered for light emission or collection).