Simulation · Physics · Class 12
How the depletion layer forms
From the lesson Formation of a p-n junction in Semiconductor Electronics: Materials, Devices and Simple Circuits. Change the values and watch what happens.
How the depletion layer formsPhysics · Class 12
The idea behind it
NCERT §14.5.1
- A p-n junction underlies the diode, the transistor and many other devices. It is made by adding a precise small amount of pentavalent impurity to part of a thin p-type Si wafer, turning that part n-type.
- Two processes shape the junction: diffusion and drift.
- Diffusion: holes are concentrated on the p-side and electrons on the n-side, so holes spread p → n and electrons spread n → p. This gives the diffusion current.
- Each electron leaving the n-side leaves an immobile ionised donor (positive) behind; each hole leaving the p-side leaves an immobile ionised acceptor (negative). A positive space-charge layer builds on the n-side and a negative one on the p-side.
- Together these layers form the depletion region, emptied of free carriers. It is about one-tenth of a micrometre thick.
- The space charges set up an electric field from the n-side (positive) to the p-side (negative). It pushes electrons from p to n and holes from n to p; this field-driven motion is drift, and the drift current runs opposite to the diffusion current.
- At first diffusion dominates. As the space charge grows, so do the field and the drift current, until drift equals diffusion. In equilibrium there is no net current across the junction.
- The n-side has lost electrons and the p-side gained them, so the n-side is positive relative to the p-side. This barrier potential opposes further flow of electrons from n to p.
- Example 14.3: two separate slabs pressed together cannot make a junction. Even the flattest surface is far rougher than the interatomic spacing (about 2 to 3 Å), so there is no continuous atomic contact and carriers meet a break.