Semiconductor Diode Operation

Fundamentals of Semiconductor Behavior

Intrinsic semiconductors, such as silicon (Si) and germanium (Ge), possess a crystalline structure with covalent bonds. At room temperature, thermal energy causes a small number of electrons to break free, creating electron-hole pairs. Doping these materials with impurities alters their electrical properties, leading to extrinsic semiconductors.

P-N Junction Formation

A p-n junction is formed by joining a p-type semiconductor (doped with acceptor impurities, creating an excess of holes) and an n-type semiconductor (doped with donor impurities, creating an excess of electrons). At the junction, electrons from the n-type material diffuse into the p-type material, and holes from the p-type material diffuse into the n-type material. This diffusion creates a depletion region depleted of free charge carriers.

Depletion Region and Built-in Potential

The diffusion of charge carriers across the p-n junction creates a space charge region, known as the depletion region. The uncompensated ions within this region establish an electric field, which opposes further diffusion. This electric field leads to a potential difference across the junction, referred to as the built-in potential (Vbi). The magnitude of Vbi depends on the doping concentrations and temperature.

Forward Bias Condition

Applying a positive voltage to the p-type material and a negative voltage to the n-type material (forward bias) reduces the width of the depletion region. When the applied voltage exceeds the built-in potential, the depletion region collapses, allowing a large current to flow. The diode exhibits a low resistance in this state.

Reverse Bias Condition

Applying a negative voltage to the p-type material and a positive voltage to the n-type material (reverse bias) widens the depletion region. This increases the resistance to current flow, allowing only a small leakage current to pass. The diode effectively blocks current in this state until the reverse breakdown voltage is reached.

Diode Characteristics

The current-voltage (I-V) characteristic of a diode exhibits a non-linear behavior. In the forward bias region, the current increases exponentially with voltage. In the reverse bias region, the current remains small until the breakdown voltage, where it increases rapidly.

Breakdown Mechanisms

Reverse breakdown can occur through two primary mechanisms: Zener breakdown and avalanche breakdown. Zener breakdown occurs in heavily doped diodes due to quantum mechanical tunneling. Avalanche breakdown occurs in lightly doped diodes due to impact ionization.

Types of Diodes

• Rectifier Diodes: Used for converting AC to DC.
• Zener Diodes: Used for voltage regulation.
• Light-Emitting Diodes (LEDs): Emit light when forward biased.
• Photodiodes: Conduct current when exposed to light.
• Schottky Diodes: Characterized by a fast switching speed and low forward voltage drop.
• Varactor Diodes (Varicaps): Used as voltage-controlled capacitors.