A diode is the simplest possible semiconductor device, the most interesting thing of which is the p- and n-type junction and the surrounding transition zone, where the two semiconductor crystals with different impurities connect and interact. If a p-type and an n-type semiconductor crystal are attached to each other, a rearrangement of the charges will take place in a very narrow strip along the contact surface. In an n-type silicon crystal the fifth valence electrons acquired by the donor phosphorus atoms not involved in the set up of the bond will move freely among the nuclear cores in the Si-crystal, while in a p-type crystal there is a hole (deficiency of electrons) in the bond beside the acceptor boron atoms. Following the assembly of the p- and n-type semiconductor crystals stray electrons would migrate (by diffusion) into a very narrow band of the p-layer following the attraction force of the acceptor holes found in the p-type crystal, where electrons recombine with the holes. This initially unobstructed migration results in very great changes. As a result of the electron diffusion from n-type silicon to p-type silicon the charge equilibrium is turned over in this very thin junction n-type layer. Electron deficiency develops in the junction n-type layer of the n-type semiconductor, and surplus electrons appear in the p-type silicon. The layer thus formed, poor in charged particles will be called the depletion layer (non-conductive layer). In other words, electron surplus and electron deficiency was formed on the p-type half and the n-type side of the depletion layer, respectively. As a consequence, contact potential difference (voltage) – called built-in potential –  will be formed on the two sides of the depletion layer. Direction of the electric field strength-vector (Einternal) points from the direction of the n-type crystal towards the p-type crystal. Since the movement of the electrons is always reversed with the direction of the electric field strength, Einternal  would prevent further movement of the electrons into the p-type crystal, and hence, any further recombination in the deeper layers. Thus, the potential in the n-type silicon of the depletion layer – holding additional positive charges – will be higher than that of the p-type silicon which in turn has electron surplus. This is the potential barrier preventing additional thermal diffusion of electrons from n-type silicon toward p-type silicon.

The diode

A diode consists of a pair of assembled contaminated p- and n-type semiconductor crystals in which the depletion layer was formed [1][2]. In fact, the diode is produced from a clear silicon crystal both half of which is contaminated separately.

Forward biased diode

Operation of a diode will be set forth more in details on the page The diode as an element of the electric circuit.

Key statements

1. Following the assembly of the p- and n-type semiconductor crystals stray electrons would migrate (by diffusion) into a very narrow band of the p-layer following the attraction force of the acceptor holes found in the p-type crystal, where electrons recombine with the holes.

2. A shortage of electrons will be formed in the depletion layer of the n-type semiconductor, while surplus electrons will appear in the p-type layer.

3. The layer thus formed, poor in free charge carriers is called depletion layer.

4. Due to the upset charge equilibrium an Einternal  electric field (built-in potential) fill be created in the depletion layer.

5. The Einternal  will prevent further (thermal) diffusion of the electrons.