N-Type Doping

Part of The Transistor

N-type doping adds Group V impurity atoms to a semiconductor crystal, creating an excess of free electrons that dramatically increase conductivity and form the electron-rich regions essential to transistors and diodes.

Why This Matters

A pure silicon crystal conducts electricity almost as poorly as glass. N-type doping transforms it into a useful semiconductor by adding a controlled density of free electrons. Without n-type material, there is no PN junction. Without a PN junction, there are no diodes, no transistors, no rectifiers, no electronics.

The “N” stands for negative — not because the material carries a net negative charge (it is electrically neutral) but because electrons are the dominant charge carriers. In every NPN transistor — the most common transistor type — the emitter and collector are n-type material. Understanding n-type doping means understanding what those regions are made of, why electrons flow from emitter to collector, and what controls that flow.

This knowledge also matters for fabrication: if you are attempting to grow or process semiconductors from raw materials, you need to know which chemicals to add, in what amounts, and how to verify the result.

The Physics of N-Type Doping

Silicon has four valence electrons. Each silicon atom forms four covalent bonds with neighboring silicon atoms, filling its outer shell. This leaves no free electrons — hence the poor conductivity of pure silicon.

Group V elements — phosphorus, arsenic, antimony — have five valence electrons. When one of these atoms replaces a silicon atom in the crystal lattice (substitutional doping), it forms the same four covalent bonds with neighbors. The fifth electron cannot bond to anything. It is very loosely held — only about 0.045 eV of energy separates it from the conduction band (compared to 1.12 eV for silicon’s bandgap). At room temperature (thermal energy ~0.026 eV), this fifth electron is essentially always free.

Result: each donor atom contributes one free electron to the crystal. The atom becomes a positively charged ion (fixed in the lattice), while the free electron roams. The material is electrically neutral, but rich in mobile negative charge carriers.

Donor Elements

Dopant	Atomic Number	Ionization Energy in Si	Natural Source
Phosphorus (P)	15	0.045 eV	Bone ash, apatite rock, phosphate minerals
Arsenic (As)	33	0.054 eV	Arsenopyrite (iron arsenic sulfide)
Antimony (Sb)	51	0.039 eV	Stibnite (antimony sulfide)

Phosphorus is the most commonly used n-type dopant in silicon for several reasons:

• Widely available in phosphate rock (Ca₅(PO₄)₃F)
• Lower ionization energy → fully ionized at room temperature
• Smaller atom than arsenic or antimony → less lattice distortion
• Forms volatile compounds (phosphine, phosphorus oxychloride) suitable for diffusion doping

Arsenic has the advantage of slower diffusion at high temperatures (lower diffusivity), making it useful when precise, shallow n-type junctions are needed.

For germanium (lower-temperature processing), antimony is often preferred because it has lower solubility and diffuses predictably.

Doping Concentration and Its Effects

The amount of dopant added determines the electrical properties of the resulting material:

Doping Level	Phosphorus Atoms/cm³	Resistivity (Ω·cm)	Use
Very light	10¹³–10¹⁴	1–50	High-voltage devices
Light	10¹⁴–10¹⁵	0.1–1	Transistor base, detector diodes
Medium	10¹⁵–10¹⁷	0.001–0.1	Transistor emitter/collector
Heavy (n+)	10¹⁸–10²⁰	<0.001	Ohmic contacts, heavily doped emitters

For a bipolar NPN transistor:

• Emitter: heavily doped n+ (high electron concentration for efficient injection)
• Collector: lightly doped n (to handle voltage without breakdown)
• Base: lightly doped p (thin, for fast transit time)

The doping asymmetry between emitter and base is what gives the transistor its current gain. If both were equally doped, gain would approach 1.

Methods of N-Type Doping

Melt Doping

Add phosphorus to the silicon melt during Czochralski crystal growth:

1. Calculate required phosphorus mass: for 10¹⁶ atoms/cm³ in a 100g silicon crystal, you need approximately 0.005 mg of phosphorus
2. Prepare a master alloy: melt 1 mg phosphorus with 999 mg silicon (0.1% by weight) in a sealed quartz tube
3. Add weighed chips of this master alloy to your silicon melt before pulling the crystal
4. Pull crystal as normal — dopant distributes throughout

Segregation: Phosphorus tends to concentrate in the last-solidified portion of the crystal because its equilibrium segregation coefficient is less than 1. The end of the crystal pulled last will be more heavily doped than the beginning.

Diffusion Doping

Introduce phosphorus into the surface of an already-grown wafer by heating in a phosphorus-bearing atmosphere:

Phosphorus POCl₃ (phosphorus oxychloride) diffusion:

1. Clean the silicon wafer thoroughly
2. Place in a quartz tube furnace at 900–1,000°C
3. Pass nitrogen gas through liquid POCl₃ — it carries phosphorus vapor into the tube
4. Phosphorus atoms land on the wafer surface and diffuse inward
5. Diffusion depth depends on temperature and time

Depth vs. conditions:

Temperature	30 min	2 hr	8 hr
900°C	~0.05 µm	~0.1 µm	~0.2 µm
1000°C	~0.2 µm	~0.5 µm	~1 µm
1100°C	~0.8 µm	~2 µm	~4 µm

Starting material matters: Diffuse into p-type silicon to create an n-type surface layer, forming a PN junction at the boundary where phosphorus concentration equals boron concentration.

Alloying (for Germanium)

Press a small antimony or phosphorus-containing pellet against germanium and heat:

1. Weigh a small pellet of antimony metal (0.1–0.5 mm diameter works well)
2. Place on cleaned n-type germanium surface (or p-type if creating an n-on-p junction)
3. Heat to 650–700°C in nitrogen atmosphere for 2–5 minutes
4. Cool slowly over 20–30 minutes
5. The alloyed region is heavily n-type

This is how early n-type regions in point-contact transistors were formed.

Verifying N-Type Doping

Hot-Point Probe Test

1. Heat a metal probe to ~200°C
2. Touch hot probe and a cold probe to the semiconductor surface, 5–10 mm apart
3. Connect a voltmeter: positive terminal to hot probe, negative to cold
4. N-type result: voltmeter reads negative (hot probe is the negative terminal)
5. This happens because electrons — the majority carrier — diffuse from the hot end (more thermal energy) to the cold end, leaving a positive charge at the hot end and negative at the cold end

Resistance Measurement

Measure four-point probe resistivity and compare to expected values from the table above to verify doping level.

Diode Test

Form a junction with a p-type contact and measure forward/reverse resistance ratio. N-type material should form a clear rectifying junction.

Summary

N-Type Doping — At a Glance

• N-type doping adds Group V atoms (phosphorus, arsenic, antimony) to semiconductor crystal
• Each donor atom contributes one free electron — material has excess mobile electrons
• Phosphorus is most common for silicon; antimony often used for germanium
• Resistivity drops from thousands of Ω·cm (intrinsic) to milliohm-cm (heavily doped)
• In NPN transistors: emitter and collector are n-type; emitter is more heavily doped
• Melt doping adds phosphorus during crystal growth; diffusion doping adds it to a wafer surface post-growth
• Verify with hot-point probe: n-type shows negative voltage at the hot probe