The PN Junction

Part of The Transistor

The PN junction is the boundary between p-type and n-type semiconductor material — the fundamental structure from which all diodes, transistors, and solar cells are built.

Why This Matters

The PN junction is the atom of semiconductor electronics. Every useful semiconductor device — diode, transistor, thyristor, LED, solar cell — contains at least one PN junction. The junction’s ability to conduct current in one direction but not the other (rectification) underpins power supplies and radio detectors. The junction’s ability to be controlled by voltage underpins transistor amplification and switching.

Before you can understand any semiconductor device, you must understand what happens at the boundary between p-type and n-type material. This is not obvious from first principles — the junction does things that seem counterintuitive. Why does it block current in one direction? Why does it have a voltage associated with it even when nothing is connected? How does a tiny base voltage control a large collector current? All of these answers begin here.

Formation of the Junction

When p-type and n-type semiconductor are brought into intimate contact (not merely pressed together, but actually joined at the atomic level), three simultaneous processes occur:

Step 1: Diffusion

The p-side has a high concentration of holes and a low concentration of electrons. The n-side has a high concentration of electrons and a low concentration of holes. Any particle in a high-concentration region will naturally diffuse toward lower-concentration regions.

Therefore:

• Electrons diffuse from n-side → p-side
• Holes diffuse from p-side → n-side

This diffusion is driven by concentration gradient, not by any electric field.

Step 2: Recombination and Depletion

As electrons and holes cross the boundary and meet, they recombine and annihilate each other. An electron fills a hole — both disappear.

The result is that the region near the junction becomes depleted of free carriers on both sides. Left behind are:

• On the n-side: positive donor ions (fixed in the lattice — cannot move)
• On the p-side: negative acceptor ions (fixed in the lattice — cannot move)

This depleted region is called the depletion region or space charge region, typically 0.1–1 µm wide.

Step 3: Built-In Electric Field

The fixed positive and negative ions create an electric field pointing from n to p (from positive charges toward negative charges). This field opposes further diffusion — electrons wanting to diffuse from n to p must fight against this field.

Equilibrium is reached when the drift current (electrons pushed back toward n by the field) exactly equals the diffusion current (electrons trying to cross from n to p due to concentration gradient). At this point, no net current flows even though there is motion in both directions.

The Built-In Voltage

The built-in electric field corresponds to a built-in voltage (V_bi) across the depletion region:

Semiconductor	Typical V_bi at 25°C
Silicon	0.6–0.8 V
Germanium	0.2–0.4 V
Gallium arsenide	1.1–1.4 V

This built-in voltage exists even when the junction is disconnected from anything. You cannot directly measure it with a voltmeter, however — the voltmeter’s connecting wires form their own metal-semiconductor junctions whose voltages exactly cancel the built-in voltage. The built-in voltage manifests only when you apply an external voltage that either adds to or subtracts from it.

Depletion Region Properties

The depletion region is not symmetric. It extends further into the more lightly doped side:

If the p-side is more lightly doped than the n-side, most of the depletion region is on the p-side. If the n-side is more lightly doped, most depletion is on the n-side.

Depletion width vs. doping:

W = √(2ε(V_bi + V_reverse) / q × (1/N_A + 1/N_D))

Where N_A and N_D are acceptor and donor concentrations. Key insight: higher doping → narrower depletion region. A heavily doped junction has very narrow depletion and therefore breaks down at lower reverse voltage. Lightly doped junctions handle higher reverse voltages.

Junction capacitance: The depletion region acts like a parallel-plate capacitor. Capacitance decreases as the reverse voltage increases (because the depletion width increases). This voltage-variable capacitance is exploited in varactor diodes used in tunable RF circuits.

Behavior Under Applied Voltage

Zero Bias

No external voltage → depletion region at equilibrium width, no net current.

Forward Bias

Apply positive voltage to p-side, negative to n-side. This external voltage opposes the built-in field, narrowing the depletion region. When the applied voltage approaches V_bi (~0.6 V for silicon):

• Depletion region nearly collapses
• Electrons can now flow from n-side into p-side and continue through the circuit
• Holes flow from p-side into n-side

Current increases exponentially with voltage: each 60 mV increase multiplies current by ~10×.

Reverse Bias

Apply positive voltage to n-side, negative to p-side. This adds to the built-in field, widening the depletion region. Almost no current flows except a small reverse saturation current (I₀) from thermally generated minority carriers:

Device Type	I₀ (silicon)	I₀ (germanium)
Small signal diode	1–100 nA	1–50 µA
Power rectifier	100 nA–10 µA	10–500 µA

Germanium has 100–1,000× more leakage — a significant disadvantage for precision and power circuits.

Junctions in Transistors

A bipolar transistor has two PN junctions:

NPN transistor:

• Junction 1: Base (p) — Emitter (n) → forward biased in active mode
• Junction 2: Collector (n) — Base (p) → reverse biased in active mode

The forward-biased emitter junction injects electrons into the thin base. The reverse-biased collector junction creates a strong electric field that sweeps these electrons into the collector before they can recombine. The transistor “steers” electrons from emitter to collector using the base voltage as a control signal.

If both junctions were reverse biased: no current → transistor cut off. If both junctions were forward biased: transistor in saturation → maximum current. One forward, one reverse: active mode → linear amplification.

Making a PN Junction

Method 1: Alloy Junction (Germanium)

1. Start with n-type germanium (resistivity 1–10 Ω·cm, phosphorus or antimony doped)
2. Place a small indium pellet (0.5 mm diameter) on the cleaned surface
3. Heat to 520°C in nitrogen for 3 minutes; cool over 20 minutes
4. The indium dissolves into the germanium surface and recrystallizes as p-type
5. Junction forms at the boundary of the resolidified region

Method 2: Diffused Junction (Silicon)

1. Start with n-type silicon wafer
2. Paint or coat one face with boron oxide (B₂O₃) mixed with a binder
3. Heat to 1,000°C in a tube furnace for 2 hours
4. Boron diffuses into the silicon surface, converting it to p-type
5. Junction forms at the depth where boron concentration equals background n-type doping

Verifying Junction Quality

Measurement	Good Junction	Poor Junction
Forward resistance (ohmmeter)	10–500 Ω	<10 Ω (shorted) or >10 kΩ (open)
Reverse resistance	>100 kΩ	<10 kΩ (leaky)
F/R resistance ratio	>1,000:1	<100:1
Forward voltage (multimeter diode test)	0.15–0.35 V (Ge), 0.5–0.7 V (Si)	Out of range

Summary

The PN Junction — At a Glance

• PN junction forms when p-type and n-type semiconductor make atomic contact
• Diffusion of carriers creates a depletion region with a built-in electric field (~0.6 V for silicon)
• Forward bias (+ to p-side) collapses depletion region → exponential current flow
• Reverse bias (+ to n-side) widens depletion region → near-zero leakage current
• Two PN junctions in a transistor: emitter-base (forward biased) and collector-base (reverse biased)
• Alloy-junction method: melt indium onto n-type germanium at 520°C
• Verify junction with ohmmeter: forward/reverse ratio >1,000:1 indicates good quality