BASIC KNOWLEDGE – SCHOTTKY DIODES Schottky diode: Definition, applications, and more

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Schottky diode is a rectifying diode made from a junction of metal and semiconductor. Also known as the hot carrier or Schottky barrier diodes, these diodes are capable to operate at high frequencies with faster switching capabilities and lower forward voltage compared to silicon diodes. The article explores the Schottky diode, and its symbol, types, working, and applications.

1. What is a Schottky diode

Schottky diodes operate on the basis of a barrier potential named after the German physicist Walter H. Schottky. While studying under Max Planck in 1914, Walter H. Schottky discovered a type of noise observed due to electrons at the anode of thermionic tubes. In 1938, Schottky formulated a theory to explain the rectifying behavior of the metal-semiconductor junction. The random noise and the constructed device were later named in honor of the German physicist.

What does a Schottky diode do?

A Schottky diode primarily performs rectification. The solid-state device with an M-S junction conducts current only in one direction and blocks in the other. The metal acts as an anode and the semiconductor behaves like a cathode in the Schottky diode.

2. Schottky diode symbol

The Schottky diode symbol shows a triangular-shaped anode, similar to the PN junction diode. The cathode of the Schottky diode is represented in a slightly different manner.

3. Schottky diode working principles

A Schottky diode is of two types- N-type and P-type. The n-type Schottky diode consists of an N-type semiconductor bonded to metal. Alternatively, the P-type Schottky diode contains the P-type semiconducting material bonded to the metal to form the junction.

Metals such as Molybdenum (Mo = 42), Tungsten (W = 74), Chromium (Cr = 23), Platinum (Pt = 78), Palladium (Pd = 46), Aluminium ( Al= 13), etc, form a junction with the semiconducting material. Typically for metal + n-type semiconductor Schottky diodes, the Schottky height ranges between 0.7 V - 0.9 eV.

Fermi level: The Fermi level is defined as the highest level of energy bands that electrons can occupy at absolute zero. When the state of thermal equilibrium ceases to persist, the fermi level is said to become the “quasi fermi level” or imref.

Work function: Workfunction is the minimum thermodynamic energy required to extract an electron from the solid to a point in a vacuum (outside the solid). All types of metals, insulators, and semiconductors have different work functions. Depending upon the work function of the metal and semiconductor used, the Schottky barrier exhibits rectifying or non-rectifying properties. The non-rectifying property of the Schottky barrier provides “near ohmic contacts” used as external contact electrodes in transistors and various semiconductor devices.

Note: Let us consider metal + n-type semiconductor for understanding

Case 1:

When the work function of the metal is larger than the n-type semiconductor, the M-S junction exhibits rectifying character. The obtained device is called an n-type Schottky diode or Schottky barrier diode.

Case 2:

If the work function of the metal is smaller than the n-type semiconductor, the M-S junction behaves like a “near ohmic contact”- conducting current in both directions.

Electronic affinity: The electron affinity is the amount of energy released when an electron from outside the semiconductor (in the vacuum) adds to the neutral atom within the semiconductor. The electron enters the semiconductor to reach the lowest energy state of the conduction band. In simple words, electron affinity is the amount of energy released when a semiconductor atom accepts an electron. Electron affinity for semiconductors is denoted by

and is expressed in kilojoules per mole.

Metal is often referred to as a “pool of electrons” as all the electrons are packed in the energy band below the Fermi level. The higher bands of the metal are vacant. In a semiconductor, free electrons above the edge of the conduction band are in the higher vacant energy states. Under isolated conditions, metals and semiconductors have different Fermi levels.

In an n-type semiconductor, the fermi level lies below the higher-level conduction band. On the other hand, the fermi level of a p-type semiconductor is above the lower-energy valence band.

TYPES OF DIODES - OVERVIEW

The different diode types explained

Zero bias Schottky diode

The fermi levels of metal and semiconductor must align at equilibrium. At the M-S junction, the excess electrons from semiconductors migrate to metals as there are vacant states in the upper bands of the metal. The potential difference between metal and semiconductors gives rise to an electric field. The energy bands start to move down due to the built-in electric field. After the diffusion of these charge carriers, the Fermi levels of metal and semiconductor align at the thermal equilibrium.

E_C = The edge of the conduction band

E_V = The edge of the valence band

E_F = Fermi level

The discontinuity in the energy band alignment of the M-S junction gives rise to an energy barrier known as the “Schottky barrier”. The Schottky barrier opposes further movement of the electrons for current flow. There is no net current at zero bias (or equilibrium) because the Schottky barrier balances the movement of electrons from semiconductor to metal.

The height of the Schottky barrier is called Schottky height or barrier height, which is denoted by

The Schottky height is expressed in electron volts (eV).

The above equation is known as the “Schottky-Mott” relationship in semiconductor and solid-state physics.

In metal + n-type semiconductor, Schottky height is the difference between the fermi level of the metal and the lowest edge of the conduction band. While in the case of metal + p-type semiconductor, the Schottky height is the difference between the metal work function and valence band maximum edge.

In simple words, the Schottky barrier is the depletion region (or discontinuity) formed at the M-S junction. Due to high permissible energy states and carrier density, the depletion region is negligible inside the metal. Hence, the depletion extends more towards the semiconductor in the M-S junction.

Forward bias Schottky diode

In forward bias, metal is connected to the positive, and the n-type semiconductor is connected to the negative of the battery. The forward voltage supports the current flow in the Schottky diode due to the electric field.

Upon application of forward voltage, the quasi-fermi level of the semiconductor starts to rise above the metallic Fermi level. Both the valence and conduction bands start to lift about the same distance without changing the band gap of the semiconductor.

The electrons gain sufficient energy to cross over the Schottky barrier from semiconductor to metal. These electrons are now called hot carriers, naming Schottky diodes as hot carrier diodes.

The process where an external electric field injects hot carrier electrons over the Schottky barrier from semiconductor to metal is called the “Schottky effect”. It is a form of increasing thermionic emission due to the electric field. The current in the forward-biased Schottky diode mainly flows through the thermionic emission. However, a small leakage current flows through carrier tunneling.

Reverse bias Schottky diode

In reverse bias, metal is connected to the negative, and the n-type semiconductor is connected to the positive terminal of the battery. The quasi-fermi level of the semiconductor goes down. The conduction and valence band bend about the same distance.

The probability of electrons crossing the barrier becomes almost zero as reverse bias opposes the flow of current. However, a reverse current flows from metal to semiconductor. When the quantum states of charge carriers from the metal and semiconductor match and align, the charge carriers tunnel through the barrier.

The probability of tunneling increases with the increasing reverse biasing potential. The tunneling current of the Schottky diode flows in order of microamperes. The reverse current can be increased to a desirable level upon the reduction in Schottky barrier potential width.

4. Schottky diode applications

What is a Schottky diode used for?

A Schottky diode is primarily used for rectification in high-power applications. The Commercial Schottky barrier diodes (SBDs) provide a low forward voltage drop, fast reverse recovery time, high switching speed, and enough thermal stability.

A recent advancement in constructing Schottky diodes is using SiC (Silicon Carbide).

Semiconductors

What are diodes and rectifiers?

Applications

• Schottky diodes are used in power rectifiers, and clamp diodes.

• The high-frequency switching allows Schottky diodes to be used in RF mixers, detectors, and other microwave devices.

• Anti-reverse flow Schottky diodes operate like an electrical valve for current flow. Cat’s whisker is said to be the Schottky diode predecessor.

• Schottky diodes provide low voltage drop in solar cell systems.

• SiC Schottky diodes are efficient to hinder discharging of batteries in an energy storage renewable system.

5. Schottky diode advantages

• The forward voltage drop of Schottky diodes is between 1.5 V - 4.5 V, much lower than 0.7 V of silicon diodes.

• Schottky diodes have faster reverse recovery time as the storage time is negligible.

• Schottky diodes offer fast switching speed.

• Schottky diodes witness low parasitic switching losses.

6. Schottky diode disadvantages

• Schottky diodes have low reverse breakdown voltage and higher leakage current.

• Schottky diodes have low peak inverse voltage.

• Schottky diodes with low forward voltage drops may hinder the ability to operate at high temperatures.

• Schottky diodes have lower surge current capabilities.

7. Schottky diode vs. Zener diode

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