Diode or p-n Junction

A p-n junction is one of the most important concepts in semiconductor electronics because it forms the basis of devices such as diodes, transistors, LEDs, and rectifiers. When a p-type semiconductor and an n-type semiconductor are formed in a single crystal, a special boundary region is produced that controls the movement of charge carriers. This boundary region gives the diode its unidirectional conduction property, which makes it useful in electronic circuits.

The boundary formed when p-type and n-type semiconductor regions are joined in a single crystal is called a p-n junction.

The region around the junction that is free from mobile charge carriers is called the depletion region.

The potential difference developed across the depletion layer due to immobile ions is called the barrier potential.

The two-terminal semiconductor device that allows current mainly in one direction is called a p-n junction diode.

The process in which charge carriers move from a higher concentration to a lower concentration is called diffusion.

The motion of charge carriers under the influence of an electric field is called drift.

The minimum forward voltage after which the current rises sharply is called the knee voltage.

The small current flowing in reverse bias due to minority carriers is called the reverse saturation current.

The reverse voltage at which the current suddenly increases rapidly is called the breakdown voltage.

The ratio of voltage to current at any operating point of a diode is called static resistance.

The ratio of a small change in voltage to the corresponding small change in current is called dynamic resistance.

R = \[\frac {V}{I}\]

\[r_d=\frac{\Delta V}{\Delta I}\]

When p-type and n-type semiconductor regions are brought together in a single crystal, holes from the p-side and electrons from the n-side begin to diffuse across the junction due to the concentration difference. Electrons entering the p-side recombine with holes, and holes entering the n-side recombine with electrons. As a result, the region near the junction loses mobile charge carriers, and only fixed ions remain on both sides.

This region without free charge carriers is the depletion region. The fixed ions establish an electric field from the n-side to the p-side, which produces a barrier potential that opposes further diffusion. Finally, equilibrium is reached when the diffusion and drift currents are equal, so the net current is zero.

A practical p-n junction diode is made from a single semiconductor crystal by controlled doping, not by simply pressing together separate p-type and n-type pieces. In fabrication, one side of a semiconductor crystal is doped to produce p-type material and the other side is doped to produce n-type material, creating a continuous junction at the atomic level. This arrangement ensures proper formation of the depletion region and stable diode behaviour.

A diode conducts mainly in one direction because the applied external voltage changes the barrier potential differently in the two biasing conditions. In forward bias, the external voltage reduces the barrier potential and narrows the depletion region, so majority carriers cross the junction more easily. In reverse bias, the external voltage increases the barrier potential and widens the depletion region, so majority carriers are blocked and only a very small current flows.

This is why a diode behaves like a one-way valve for electric current.

A p-n junction diode is said to be in forward bias when the p-side is connected to the positive terminal of the battery and the n-side is connected to the negative terminal.

Effects of forward bias

• Barrier potential decreases.
• The depletion region becomes thinner.
• Majority carriers move easily across the junction.
• Current rises sharply after the knee voltage is reached.
• A diode offers low resistance under these conditions.

The forward current is generally of the order of milliamperes.

A p-n junction diode is said to be in reverse bias when the p-side is connected to the negative terminal of the battery and the n-side is connected to the positive terminal.

Effects of reverse bias

• Barrier potential increases.
• The depletion region becomes wider.
• Majority carriers cannot cross the junction.
• Only minority carriers contribute to the current.
• The diode offers very high resistance under these conditions.

The reverse current is very small, typically on the order of microamperes, until breakdown occurs.

A zero biased junction diode is a p-n junction diode with no external voltage applied across it. Under this condition, electrons diffuse from the n-side to the p-side and holes diffuse from the p-side to the n-side due to concentration difference. This diffusion process creates the depletion region and develops the barrier potential.

At equilibrium, drift current caused by the electric field balances the diffusion current, so the net current through the diode is zero.

In a zero-bias p-n junction diode, the junction is in its natural equilibrium state. The depletion layer already exists, the barrier potential is present, and no net conduction current flows through the diode. This state is important because forward and reverse bias are understood by comparing them with the zero-bias condition.

Key observation

The zero-biased diode does not mean that charges are absent; it means the opposing processes of diffusion and drift balance each other.

The graph of voltage across the diode versus current through the diode is known as the V-I characteristic of the p-n junction diode.

Forward characteristic

• For small forward voltages, the current is almost negligible.
• After the knee voltage, the current increases rapidly.
• The diode then behaves as a good conductor in the forward direction.

Reverse characteristic

• For reverse bias, only a small reverse saturation current flows initially.
• At very high reverse voltage, breakdown occurs and current rises sharply.

Can a p-type and an n-type semiconductor be physically joined to form a p-n junction?

Answer: No.

1. A p-n junction cannot be formed by simply placing a p-type slab against an n-type slab.
2. Even if both surfaces appear smooth, they have microscopic roughness that is much larger than the spacing between atoms (about 2–3 Å).
3. Because of this roughness, the atoms of the two slabs do not make continuous contact, preventing the formation of a proper crystal lattice.
4. As a result, the interface acts as a discontinuity (barrier) to charge carriers, so a true p-n junction is not formed.

Conclusion:
A p-n junction is created by doping different regions of the same semiconductor crystal, not by physically joining separate p-type and n-type semiconductor pieces.

• Rectifiers: Diodes are used in power supplies and chargers to convert AC into DC.
• Electronic switching: Diodes control the direction of current in circuits.
• Signal detection: Diodes are used in communication and detection circuits.
• Semiconductor devices: The p-n junction principle is used in LEDs, photodiodes, and many other electronic components.

• A p-n junction is formed by joining p-type and n-type semiconductor regions in a single crystal.
• Diffusion of carriers creates a depletion region and barrier potential.
• A p-n junction diode conducts mainly in one direction.
• In forward bias, the barrier potential decreases, and the current becomes large.
• In reverse bias, the barrier potential increases and only a small minority-carrier current flows.
• In zero bias, the diffusion and drift currents balance, so the net current is zero.
• The knee voltage is about 0.3 V for germanium and 0.7 V for silicon.
• Static resistance is given by R = V/I, and dynamic resistance is given by r_d = ΔV/ΔI.