Doping Techniques

Part of Semiconductors

Doping is the controlled addition of specific impurities to pure semiconductor crystals to create n-type or p-type material. This is what transforms an insulating crystal into the building block of diodes and transistors.

Why Doping Is the Key Step

A pure semiconductor crystal is nearly an insulator — it conducts electricity poorly because its electrons are tightly bound in covalent bonds. Doping adds atoms with either one extra electron (n-type) or one fewer electron (p-type), dramatically increasing conductivity in a controlled way. The boundary between n-type and p-type regions — the PN junction — is where all semiconductor magic happens: rectification, amplification, and switching.

Without precise doping, you have a piece of shiny rock. With doping, you have the fundamental building block of all electronics.

N-Type Doping: Adding Electron Donors

N-type (negative-type) semiconductor has extra free electrons available to carry current.

Donor Elements

Element	Group	Extra Electrons	Source
Phosphorus	V	1 per atom	Bone ash, phosphate rock
Arsenic	V	1 per atom	Arsenopyrite mineral
Antimony	V	1 per atom	Stibnite mineral

These elements have five valence electrons. When they replace a silicon atom (which has four), the fifth electron is loosely bound and free to move, creating a negative charge carrier.

Concentration

Typical doping concentrations are astronomically small:

Doping Level	Atoms per cm3	Silicon Atoms per cm3	Ratio
Light doping	10^14 - 10^15	5 x 10^22	1 in 10 million
Medium doping	10^16 - 10^17	5 x 10^22	1 in 100,000
Heavy doping	10^18 - 10^19	5 x 10^22	1 in 1,000

Less Is More

Semiconductor doping uses vanishingly small amounts of impurity. Adding too much dopant destroys the semiconductor behavior and creates a mediocre metal. The challenge is adding just enough — parts per million to parts per billion.

P-Type Doping: Adding Electron Acceptors

P-type (positive-type) semiconductor has “holes” — missing electrons that behave as positive charge carriers.

Acceptor Elements

Element	Group	Holes Created	Source
Boron	III	1 per atom	Borax mineral
Gallium	III	1 per atom	Zinc and aluminum ores
Indium	III	1 per atom	Zinc ore byproduct
Aluminum	III	1 per atom	Bauxite, common clay

These elements have three valence electrons. When they replace a silicon atom, there is one missing bond — a “hole.” Neighboring electrons can jump into this hole, effectively making the hole move through the crystal like a positive charge carrier.

Doping Methods

Method 1: Melt Doping (During Crystal Growth)

The simplest method — add the dopant directly to the semiconductor melt before pulling the crystal:

1. Calculate the required dopant mass based on desired concentration
2. Add a precisely weighed amount of dopant element to the melt
3. Stir thoroughly to distribute uniformly
4. Pull the crystal as normal

Pre-Alloying

Rather than adding pure phosphorus or boron (which are difficult to weigh in microgram quantities), pre-alloy the dopant with the semiconductor at a known ratio. For example, melt 1 gram of phosphorus with 999 grams of silicon to create a 0.1% master alloy. Then add small, measurable amounts of this master alloy to your pure melt.

Advantages

• Uniform doping throughout the crystal
• Simple — no additional equipment needed
• Good for bulk material (diodes, power devices)

Limitations

• Cannot create junctions (need both n and p regions in the same device)
• Concentration is fixed at crystal-growing time
• Difficult to make very lightly doped material

Method 2: Diffusion Doping

Diffusion doping introduces impurity atoms into the surface of an already-grown crystal by heating it in a dopant atmosphere:

1. Place the semiconductor wafer in a quartz tube furnace
2. Heat to 900-1,200 degrees C (depending on dopant and semiconductor)
3. Introduce dopant vapor:
   • For phosphorus: heat red phosphorus to produce vapor
   • For boron: use boron tribromide (BBr3) vapor or powdered boron oxide (B2O3) on the surface
4. Dopant atoms diffuse into the crystal surface
5. Diffusion depth depends on temperature and time

Diffusion Depth vs. Time

Temperature	30 Minutes	2 Hours	8 Hours
900 C	~0.1 microns	~0.3 microns	~0.5 microns
1000 C	~0.5 microns	~1 micron	~2 microns
1100 C	~2 microns	~5 microns	~10 microns
1200 C	~5 microns	~15 microns	~30 microns

Creating PN Junctions by Diffusion

This is how you make working devices. Start with an n-type wafer (phosphorus-doped during crystal growth). Diffuse boron into one face. The boron concentration exceeds the phosphorus concentration near the surface, creating a p-type layer on top of n-type bulk. The boundary is a PN junction — the heart of a diode.

Method 3: Alloying (The Simplest Junction Method)

Alloying is the most practical junction-formation method for a rebuilding scenario:

1. Place a small pellet of dopant metal (indium for p-type, antimony for n-type) on the wafer surface
2. Heat until the pellet melts and dissolves into the semiconductor surface
3. Cool slowly — the semiconductor recrystallizes with the dopant included
4. The recrystallized region has the opposite doping type, forming a PN junction

This is how the first practical transistors were made: indium pellets alloyed into germanium wafers.

Alloying Procedure for Germanium

1. Clean a germanium wafer (n-type, lightly doped)
2. Place a small indium bead (1-2mm diameter) on the center of each face
3. Heat to 500-550 degrees C in a hydrogen or nitrogen atmosphere for 2-5 minutes
4. The indium melts (157 degrees C) and dissolves into the germanium
5. Cool slowly over 15-30 minutes
6. The indium-germanium alloy zone is now p-type
7. You have created PNP junctions: two p-type regions (from indium) sandwiching the n-type bulk

Point-Contact Method (Simplest of All)

The very first transistor used pointed wires pressed against a semiconductor surface. Press two sharpened phosphor bronze wires onto a germanium crystal, 0.05-0.1mm apart. Pass a brief pulse of high current through each contact to form tiny alloy junctions. This crude method actually works for making transistors, though reliability is poor.

Measuring Doping Results

Hot-Point Probe Test

The simplest test to determine if semiconductor is n-type or p-type:

1. Heat a soldering iron tip
2. Touch the hot tip and a cold probe to the semiconductor surface, both connected to a sensitive voltmeter
3. N-type: Voltmeter deflects negative at the hot probe
4. P-type: Voltmeter deflects positive at the hot probe

The heat creates more charge carriers at the hot end, which diffuse toward the cold end, creating a measurable voltage.

Resistivity Measurement

Use a four-point probe or simple two-point resistance measurement:

Resistivity (ohm-cm)	Doping Level	Type
>100	Very light or intrinsic	Near-pure
1-100	Light doping	Suitable for detectors
0.01-1	Medium doping	Suitable for transistors
<0.01	Heavy doping	Contact regions

Junction Testing

To verify a PN junction has formed:

1. Connect an ohmmeter across the junction
2. Measure resistance in both directions
3. Forward resistance (p to positive, n to negative) should be low (10-1000 ohms)
4. Reverse resistance should be high (>100,000 ohms)
5. The ratio of reverse to forward resistance indicates junction quality: >100:1 is usable, >1000:1 is good

Common Mistakes

1. Too much dopant: Overdoping converts the semiconductor into a poor conductor with no semiconductor properties. Start with very small amounts and increase gradually.
2. Contamination during diffusion: Any unwanted impurity that enters during high-temperature processing changes the result unpredictably. Keep the furnace tube clean and use high-purity atmospheres.
3. Oxidized dopant surface: Indium and gallium oxidize in air. Clean the pellet surface immediately before alloying. Use a hydrogen atmosphere during heating to prevent oxidation.
4. Uneven heating during alloying: Hot spots cause the dopant to alloy unevenly, creating irregular junctions with poor characteristics. Heat the entire wafer uniformly.
5. Cooling too fast: Rapid cooling after alloying creates crystal defects (dislocations) at the junction that increase leakage current. Cool slowly over 15-30 minutes.

Summary

Doping Techniques -- At a Glance

• N-type doping uses Group V elements (phosphorus, arsenic, antimony) to add free electrons
• P-type doping uses Group III elements (boron, gallium, indium, aluminum) to create holes
• Melt doping adds impurities during crystal growth — simplest but cannot create junctions
• Diffusion doping heats the wafer in dopant vapor at 900-1,200 degrees C — creates controlled PN junctions
• Alloying melts a dopant pellet into the crystal surface — the most practical junction method for rebuilding; used in the first transistors
• Use the hot-point probe test to determine n-type vs p-type, and forward/reverse resistance ratio to verify junction quality