P-Type Doping

Part of Semiconductors

Adding acceptor atoms to create hole-rich semiconductor material for device fabrication.

Why This Matters

P-type semiconductor — where holes are the majority carriers — is half of every p-n junction. Diodes, bipolar transistors, solar cells, and LEDs all require controlled regions of both p-type and n-type material. Understanding p-type doping means understanding how to make and characterize one of the two fundamental semiconductor polarities.

Holes as charge carriers can seem abstract — a “hole” is an absence of an electron, moving in the opposite direction. But holes are real physical entities with measurable properties: mobility, diffusion coefficient, lifetime. They carry current in precisely the same way electrons do, just with opposite sign. Devices exploit both carriers: in a PNP transistor, holes are the majority carriers in emitter and collector; in an NPN transistor, holes are the minority carriers being injected into the base and swept across.

For practical fabrication, the choice of p-type dopant and technique affects every downstream process. Boron is the standard p-type dopant for silicon. Indium is the standard for germanium. Each has specific chemical behavior, diffusivity, solid solubility limit, and practical handling considerations that matter when setting up a fabrication process.

Acceptor Atoms and the Hole Mechanism

Group III elements (3 valence electrons) replace Group IV lattice atoms (4 valence electrons). Three of the four neighboring bonds are filled; the fourth has only one electron from the Group III atom instead of two. This bond “wants” to be complete — it accepts an electron from a neighboring bond to complete itself. This acceptance process generates a hole in the adjacent bond.

The accepted electron is only loosely held — the ionization energy is 0.01-0.05 eV for common acceptors. At room temperature, virtually all acceptor atoms are ionized: each one captures a valence electron, generating one hole. The number of free holes ≈ acceptor concentration N_A.

The ionized acceptor atom becomes a fixed negative ion embedded in the crystal. It cannot move (it is part of the lattice), but the hole it created can move freely through the crystal under field or concentration gradient.

Common p-type dopants for silicon:

• Boron (B): Overwhelmingly standard. Ionization energy 0.045 eV, ideal diffusivity for junction formation, no precipitation problems at normal doping levels, widely available. Introduced by BBr3 diffusion, ion implantation, or diborane (B2H6) gas (highly toxic).
• Aluminum (Al): Faster diffusion than boron. Historically used for diffused junction transistors in early silicon work. Aluminum metallization that diffuses in can inadvertently create p-type regions — this is a contamination concern.
• Gallium (Ga): Useful when boron is unavailable. Lower vapor pressure than boron, different diffusion temperature profile.
• Indium (In): Used in older silicon technology; now rare. Has deep acceptor level in silicon — not fully ionized at room temperature.

Common p-type dopants for germanium:

• Indium (In): Standard p-type dopant for germanium and the alloy material for PNP transistor fabrication. Ionization energy 0.012 eV, essentially fully ionized at room temperature. Melts at 156°C — suitable for alloy junction technique.
• Gallium (Ga): Also works for germanium.
• Boron (B): Less commonly used in germanium due to relatively deeper acceptor level.

Practically: boron for silicon (BBr3 diffusion is accessible), indium for germanium alloy junctions.

Resistivity and Doping Concentration

For p-type silicon at room temperature (µ_p ≈ 480 cm²/V·s for lightly doped material):

• N_A = 10^14 cm^-3 → ρ ≈ 130 Ω·cm
• N_A = 10^15 cm^-3 → ρ ≈ 13 Ω·cm
• N_A = 10^16 cm^-3 → ρ ≈ 1.3 Ω·cm
• N_A = 10^17 cm^-3 → ρ ≈ 0.13 Ω·cm

P-type silicon has higher resistivity than n-type silicon at the same doping concentration because holes have lower mobility (480 vs 1350 cm²/V·s). This means p-type transistor bases, collectors, and resistors made in p-type silicon have higher sheet resistance than equivalent n-type structures — a constraint that affects circuit design.

For germanium, hole mobility is 1900 cm²/V·s — substantially higher than silicon. This narrower mobility gap between n and p germanium makes PNP and NPN germanium transistors more symmetrically performing than their silicon counterparts.

Measuring p-type carrier type: Thermoelectric probe. The hot probe accumulates holes that diffuse from hot to cold region, making the hot probe positive. This is opposite to n-type, where the hot probe is negative. The polarity reversal is the diagnostic test.

Hall measurement: current and magnetic field produce a transverse Hall voltage opposite in sign to n-type material. With a known test circuit (four contacts, known current, measured Hall voltage, and known magnetic field), Hall coefficient = (V_H × t)/(I × B), which gives carrier concentration p = 1/(q × R_H) and mobility µ_p = σ × R_H.

Boron Diffusion into Silicon: Practical Details

BBr3 (boron tribromide) source diffusion is the most accessible p-type doping method for silicon:

BBr3 properties: Liquid at room temperature, boiling point 91°C. Highly reactive with water — store sealed, handle in dry conditions. Reacts with skin moisture causing chemical burns — wear gloves, eye protection, and work in ventilated space.

Tube furnace setup:

1. Clean silicon wafers: HF dip (10% HF, 30 seconds) removes native oxide, rinse in distilled water, dry in nitrogen stream.
2. Load wafers in quartz boat in horizontal tube furnace.
3. Prepare BBr3 bubbler: nitrogen carrier gas bubbles through liquid BBr3 at controlled temperature (typically 0-20°C, controlled by ice bath). Temperature sets the BBr3 vapor pressure and thus the boron delivery rate.
4. Set furnace to 900-1000°C. Flow nitrogen only initially for 10 minutes to stabilize temperature.
5. Switch to BBr3/nitrogen mix. A borosilicate glass layer deposits on the silicon surface, acting as a boron source for diffusion.
6. Deposition/diffusion time: 20-60 minutes for typical base diffusion depth (1-5 µm).
7. Switch back to pure nitrogen; remove wafers while hot, or cool in nitrogen.

Post-diffusion steps: Remove the borosilicate glass by a brief HF etch. Measure sheet resistance with four-point probe. Sheet resistance (Ω/square) = resistivity / junction depth. Target for transistor base: 100-1000 Ω/square. Lower resistance means heavier doping and/or deeper junction.

Defect glass problem: If BBr3 flow rate is too high, a thick glassy layer forms that cannot be fully removed by HF. This leaves boron-rich contaminants on the surface. Keep BBr3 partial pressure low (bubbler temperature near 0°C) and diffusion time moderate.

P-Type Doping in Germanium via Alloying

For PNP germanium transistors, indium-alloy junction formation simultaneously creates the p-type emitter and collector regions:

1. Start with n-type germanium wafer (antimony-doped, ~1 Ω·cm).
2. Prepare indium metal pieces: cut from pure indium sheet (99.99% purity minimum). Weigh to ~1 mg precision. Uniformity of dot size ensures matching emitter/collector.
3. Alloy at 530-560°C for 2-5 minutes in reducing atmosphere.
4. The recrystallized indium-germanium alloy region is p-type. The concentration of indium in the recrystallized zone is determined by the solubility of indium in germanium at the alloying temperature — approximately 2×10^19 cm^-3. This is heavily p-type, suitable for emitter and collector.
5. Base region is the original n-type wafer between the two alloy dots.

For diodes only, alloy indium on one face only. For diodes requiring specific junction characteristics, control the indium dot area (junction area determines current capacity and capacitance).

Matching P and N Type for Specific Devices

Different device regions require specific p-type or n-type doping levels:

Diode p-side: Moderate doping (10^16-10^17 cm^-3) for good current capacity without tunneling breakdown. Match to n-side doping for symmetric junction characteristics.

BJT base (p-type, in NPN): Light doping (10^15-10^16 cm^-3) for long minority carrier lifetime (electrons must cross the base without recombining). Too-heavy doping reduces electron mobility and lifetime; too-light doping increases base resistance.

PNP transistor emitter (p-type): Heavy doping (> 10^17 cm^-3) for high injection efficiency. The emitter must inject many holes into the base for each electron that flows back through the base contact.

Solar cell p-base: Light to moderate doping (10^15-10^16 cm^-3) for long diffusion length — photogenerated minority carriers must diffuse to the junction before recombining.

Choosing doping levels is a design decision, not an arbitrary one. Each device type has optimum ranges derived from the carrier transport physics. Fabricating at those ranges requires measuring and controlling your doping — hence the emphasis on resistivity measurement and carrier type characterization throughout the fabrication process.