P-Type Semiconductor — Definition, Doping, and Properties

Pure Semiconductor: Silicon (Si) has 4 valence electrons and forms a perfect covalent lattice at 0 K with no free charge carriers.

Doping with Trivalent Impurity: When a trivalent atom (3 valence electrons) such as Boron (B), Aluminium (Al), or Indium (In) is added to silicon: • The dopant atom replaces a silicon atom in the lattice • It can only form 3 covalent bonds with neighbouring Si atoms • One bond is incomplete — a missing electron creates a HOLE • The trivalent atom accepts an electron from a neighbouring bond → called ACCEPTOR impurity

Hole creation: B (3 valence e⁻) + Si lattice → 3 covalent bonds formed + 1 bond deficient → A positive hole is created at that position → Neighbouring electrons can jump to fill this hole, moving the hole in the process

Common P-type dopants: • Boron (B) — most common in silicon technology • Aluminium (Al) • Indium (In) • Gallium (Ga)

Doping level: Typically 1 dopant atom per 10⁶ to 10⁸ silicon atoms.

In a P-type semiconductor at room temperature:

Majority Carriers: HOLES (positive charge carriers) • Created by trivalent dopant atoms • Concentration: p ≈ N_A (acceptor concentration) • Move in the direction of electric field (opposite to electron movement)

Minority Carriers: FREE ELECTRONS • Generated by thermal excitation (electron-hole pair generation) • Concentration much lower than holes • n_i² = n × p (mass action law)

Mass Action Law: p × n = n_i² Where: • p = hole concentration • n = electron concentration • n_i = intrinsic carrier concentration of pure semiconductor

For silicon at 300 K: n_i ≈ 1.5 × 10¹⁰ cm⁻³

Net charge of P-type semiconductor: Although holes are majority carriers, the overall semiconductor is ELECTRICALLY NEUTRAL — the positive holes are balanced by the negatively ionised acceptor atoms fixed in the lattice.

Fermi level in P-type: The Fermi level shifts DOWNWARD toward the valence band (closer to valence band than conduction band).

Both types: • Are electrically neutral overall • Have higher conductivity than intrinsic (pure) semiconductor • Conductivity increases with doping concentration and temperature • Obey mass action law: np = n_i²

When P-type and N-type semiconductors are joined, a P-N junction is formed:

Depletion Region: • Holes from P-side diffuse into N-side • Electrons from N-side diffuse into P-side • Leaves behind ionised acceptors (P-side) and donors (N-side) • Creates a built-in electric field opposing further diffusion • Width: ~1 μm at equilibrium

Forward Bias (P connected to + of battery): • Applied field opposes built-in field • Depletion region narrows • Majority carriers (holes from P, electrons from N) can cross the junction • Large current flows (low resistance)

Reverse Bias (P connected to − of battery): • Applied field adds to built-in field • Depletion region widens • Only minority carriers cross → very small reverse saturation current • Diode acts as open circuit (high resistance)

Diode equation: I = I₀(e^(V/V_T) − 1) Where I₀ = reverse saturation current, V_T = thermal voltage ≈ 26 mV at 300 K

1. P-N Junction Diode: • Rectification (AC → DC) in power supplies • Signal detection and demodulation • Clipping and clamping circuits

2. Bipolar Junction Transistor (BJT — PNP type): • PNP transistor: emitter (P) → base (N) → collector (P) • Used in amplifiers, switches • P-type emitter injects holes into base

3. Solar Cells (Photovoltaic): • P-type silicon is the base layer in most solar cells • Photons create electron-hole pairs across P-N junction • Electron and holes separated by junction → electric current

4. Light Emitting Diode (LED): • P-type and N-type semiconductor junction • Electrons and holes recombine → emit photons (light) • Colour depends on bandgap energy

5. CMOS Integrated Circuits: • P-channel MOSFET uses P-type source and drain regions • Complementary to N-channel MOSFET • Foundation of modern digital electronics (CPUs, RAM)

6. Zener Diode: • Heavily doped P and N regions • Used as voltage regulator in reverse bias