# PN Junction Diode 3D

By Ubermensch

Topic: Physics & Chemistry

[Open the interactive Jig](https://ask.jig.com/j/lHljhse4eONA)

## Guide

### 1. A tiny one\-way door

This is a diode, shown 100 times bigger than life\. Electric current can pass through it one way but not the other\. The silver band marks the end where current comes out\. Drag anywhere to turn it around\.

A real one is about 5 mm long, smaller than a grain of rice\.

### 2. Look inside

Under the black coating sits a tiny chip of silicon between the two wire heads\. One half of the chip is p\-type silicon and the other half is n\-type\. Where the two halves meet is the p\-n junction\.

The chip is only about 1 mm across\.

### 3. Two kinds of silicon

We zoom in a million times\. The p side \(red\) is full of holes: empty spots where an electron is missing, which act like positive charges\. The n side \(blue\) is full of free electrons\. Each side also has a few of the other kind, called minority carriers\.

The many carriers are the majority carriers\. Both pieces are still neutral overall\.

### 4. Join them: diffusion

Push the halves together\. Carriers jiggle with heat and spread from where they are crowded to where they are rare, like perfume across a room\. Electrons wander from n into p and holes from p into n\. This flow is the diffusion current\.

Electrons move one way and holes the other, yet both make current point from p to n\.

### 5. The depletion region

When a wandering electron meets a hole, they cancel out and both vanish\. Near the junction almost every free carrier is used up\. What stays behind are fixed charged atoms: minus on the p side, plus on the n side\.

The width of this empty zone is the depletion width\. In a real chip it is less than a thousandth of a millimetre\.

### 6. A hill called the barrier

The fixed plus and minus charges create an electric field across the gap\. It acts like a hill that pushes back the majority carriers\. Most of them bounce off, and only the most energetic climb over\.

In silicon this potential barrier is about 0\.7 volts\.

### 7. Drift current

The same field is a slide for minority carriers\. Any stray electron on the p side or hole on the n side that reaches the edge is swept straight across\. This flow is the drift current, and it points from n to p\.

Heat keeps making a few new minority carriers, so the drift never stops\.

### 8. Two currents cancel

Now both flows run at once\. A few energetic carriers climb the hill \(diffusion\), and an equal number slide down it \(drift\)\. The two currents are equal and opposite, so the net current is zero\. The junction settles and the diode is formed\.

Zero net current does not mean nothing moves\. Carriers keep crossing both ways\.

### 9. Why it is a one\-way door

Add a battery to tip the balance\. Forward bias lowers the hill and thins the depletion region, so diffusion floods across\. Reverse bias raises the hill and widens the gap, so only the tiny drift current remains\.

Forward current starts to flow strongly at about 0\.7 volts in silicon\.