# PN Junction Formation

By Ubermensch

Topic: Physics & Chemistry

[Open the interactive Jig](https://ask.jig.com/j/fnQ840axORgz)

## Guide

### 1. A crystal of silicon

This glowing block is a tiny piece of silicon crystal, magnified millions of times\. Each silver atom holds on to four neighbours by sharing its outer electrons\. With every electron busy in a bond, almost nothing can move, so pure silicon carries very little electric current\.

Drag anywhere to turn the crystal and look around\.

### 2. Add phosphorus: n\-type

On the right, a few silicon atoms are swapped for phosphorus atoms \(violet\)\. Phosphorus brings one more electron than its bonds need, so each one sets a free electron \(blue dot\) wandering\. This side is called n\-type, for the negative electrons\.

It is still neutral: each phosphorus atom keeps one extra proton that balances its loose electron\.

### 3. Add boron: p\-type

On the left, boron atoms \(green\) are added instead\. Boron has one electron too few, leaving a gap in a bond called a hole \(amber ring\)\. Nearby electrons hop into the gap, so the hole wanders like a tiny positive bubble\. This side is called p\-type\.

Real junctions are made in one crystal, by adding the two impurities to neighbouring areas, not by gluing two pieces together\.

### 4. Spreading across the border

Look at the border in the middle\. The right side is crowded with free electrons and the left side with holes, so each spreads toward the side that has few\. This spreading is called diffusion, like a smell drifting across a room\.

### 5. Electrons fill holes

When a wandering electron drops into a hole, the gap is filled and both disappear\. This is called recombination\. It keeps happening near the border, clearing out a strip with no free electrons or holes\.

Each small flash is one electron settling into one hole\.

### 6. Charges left behind

Phosphorus atoms near the border have lost their electrons, so each is now a fixed positive ion \(\+\)\. Boron atoms have gained one, so each is a fixed negative ion \(−\)\. They are locked in the crystal and cannot move\. This empty strip is the depletion region\.

It is drawn wide here\. In a real diode it is often less than a thousandth of a millimetre across\.

### 7. A built\-in push

The plus and minus ions create an electric field across the border \(gold arrows\), pointing from the n side to the p side\. It pushes electrons back to the right and holes back to the left\. When this push balances the spreading, the strip stops growing: the PN junction has formed\.

In silicon this built\-in barrier is about 0\.7 volts, and it settles in a tiny fraction of a second\.

### 8. A one\-way gate

The barrier turns the junction into a diode, a one\-way gate for electricity\. Push forward, with plus on the p side, and the strip shrinks so current flows\. Push in reverse and it widens, blocking the flow\. Try both\.

Silicon diodes start to conduct at about 0\.6 to 0\.7 volts\. LEDs and solar cells are built around this same junction\.