Name: schottky and ohmic metal contacts
Uploaded: 2024-07-08T13:20:09.000Z
Duration: 1 min 24 s
Channel: JoVE

Contact between a Metal and a semiconductor forms a junction with either Schottky or Ohmic behavior.
If the metal&#39;s work function exceeds that of the semiconductor, a Schottky junction is formed. Before contact, the energy band diagrams are aligned at the vacuum level, with the Fermi levels at different positions.
Upon contact, the Fermi levels align as electrons transfer from the semiconductor to the metal, reaching equilibrium.
Electron loss in the semiconductor decreases its potential, while their addition to the metal increases the potential, forming a Schottky barrier at the junction. The barrier height equals the difference between the metal&#39;s work function and the semiconductor&#39;s electron affinity.
The junction potential difference resists electron transfer from the semiconductor to the metal and is determined by the difference in their work functions.
If the metal&#39;s work function is smaller than that of the semiconductor, an Ohmic junction is formed. Here, the Fermi levels align as electrons transfer from the metal to the semiconductor.
This alignment raises the semiconductor electron energies relative to the metal, allowing electron flow from the semiconductor to the metal. So, the Ohmic junction conducts current in both directions.

metal-semiconductor junctions

Metal-Semiconductor Junctions

The contact of metal and semiconductor can lead to the formation of a junction with either Schottky or Ohmic behavior.
Schottky Barriers
Schottky barriers arise when a metal with a work function (&#934;m) contacts a semiconductor with a different work function (&#934;s). Initially, electrons transfer until the Fermi levels of the metal and semiconductor align at equilibrium. For instance, if &#934;m &#62; &#934;s, the semiconductor Fermi level is higher than the metal&#39;s before contact. The semiconductor&#39;s electrostatic potential must be raised to align the Fermi levels, resulting in a depletion region where positive charges from uncompensated donor ions balance the negative charge on the metal. The depletion width in the semiconductor can be calculated similarly to that in p-n junctions.
The equilibrium contact potential (Vo) prevents further electron diffusion from the semiconductor&#39;s conduction band into the metal. This potential is the difference in work function potentials (&#934;m - &#934;s). The potential barrier height (&#934;B) for electron injection from the metal into the semiconductor conduction band is given by &#934;m - &#967;, where &#967; is the electron affinity.
Ohmic Contacts
In many applications, such as integrated circuits, it is crucial to have ohmic metal-semiconductor contacts with a linear I-V characteristic in both biasing directions. Ohmic contacts are formed when the charge induced in the semiconductor to align the Fermi levels is provided by majority carriers. For example, in an n-type semiconductor where &#934;m &#60; &#934;s, electrons transfer from the metal to the semiconductor to align the Fermi levels, raising the semiconductor&#39;s electron energies. This results in a small barrier to electron flow, easily overcome by a small voltage. Similarly, for p-type semiconductors where &#934;m &#62; &#934;s, hole flow across the junction is facilitated, ensuring minimal resistance and no signal rectification.

Contact between a Metal and a semiconductor forms a junction with either Schottky or Ohmic behavior.
If the metal&#39;s work function exceeds that of the ...

Education

JoVE Core

Electrical Engineering

Unit 1

Basics of Semiconductors

KEY TERMS AND CONCEPTS

schottky and ohmic metal contacts

Metal contacts with semiconductors can form either Schottky or Ohmic junctions. The result depends on the work function of the metal, the work function of the semiconductor, and how electrons move when the two materials touch. At equilibrium, the Fermi levels align, so charge shifts until both sides match in energy.

Schottky barriers form when a metal with work function Φm contacts a semiconductor with a different work function Φs. If Φm is greater than Φs, the semiconductor Fermi level starts higher than the metal's before contact. The semiconductor potential then rises, and a depletion region forms where positive donor ions balance the negative charge on the metal.

The depletion width in the semiconductor can be calculated in a way similar to a p-n junction. The equilibrium contact potential Vo stops more electrons from diffusing from the semiconductor conduction band into the metal. This potential equals the work function difference, Φm - Φs. The barrier height ΦB for electron injection from the metal into the semiconductor conduction band is Φm - χ, where χ is the electron affinity.

Ohmic contacts are needed in many devices, including integrated circuits, because they give a linear I-V characteristic in both bias directions. They form when the charge needed to align the Fermi levels comes from majority carriers. In an n-type semiconductor with Φm less than Φs, electrons move from the metal to the semiconductor, and only a small barrier remains. In a p-type semiconductor with Φm greater than Φs, hole flow is favored, so the contact has low resistance and does not rectify the signal.

Schottky and Ohmic Metal Contacts

energy bands in solids

Valence and Conduction Bands in Silicon

Valence and conduction bands explain how electrons behave in solids. In isolated atoms, electrons occupy discrete energy levels that the Bohr model describes well. The hydrogen atom, for example, has specific electron energies labeled by the quantum number n, and higher n values give levels that are less negative and closer together. When atoms move close together in a solid, those discrete levels start to split. This happens because electron orbitals from neighboring atoms overlap. The Pauli...

semiconductors

Energy Bands in Metals and Insulators

Materials conduct electricity in different ways because their energy bands are arranged differently. Metals, semiconductors, and insulators can be compared with energy band diagrams. These diagrams show how easily electrons can move when an electric field is applied. Metals such as copper, zinc, and lead have low resistivity. Their conduction band is not fully occupied, or it overlaps with the valence band, so a band gap is essentially absent. Electrons in the highest valence energy levels can...

types of semiconductors

Intrinsic and Doped Semiconductors

Intrinsic semiconductors are very pure materials with no impurities. At absolute zero, they act as perfect insulators because the valence electrons are bound and the conduction band is empty. That means electrical conduction cannot occur under those conditions. The Fermi level helps describe the chance that an energy level is occupied by an electron at thermal equilibrium. In an intrinsic semiconductor, the Fermi level sits near the middle of the band gap at absolute zero. As temperature...

carrier generation and recombination

Semiconductor Charge Carriers: Creation and Loss

Carrier generation and recombination control how free charge carriers change inside a semiconductor. Carrier generation creates electron-hole pairs, or EHPs, when electrons move from the valence band into the conduction band and leave holes behind. Carrier recombination does the opposite and lowers the number of free carriers. In direct-bandgap semiconductors such as gallium arsenide, generation happens efficiently because energy absorption can move a valence electron straight to the...

carrier transport

Drift and Diffusion in Semiconductors

Drift and diffusion are the two main ways electric current forms in semiconductors. These charge-carrier motions help determine how semiconductor devices work and how well they perform. The carriers involved are electrons and holes. Drift current begins when an external electric field acts on charge carriers. Electrons and holes move between collisions with lattice atoms, and this motion creates drift velocity. For electrons, drift velocity depends on electron mobility and the electric field...

p-n junction

P-N Junction Depletion Region

A p-n junction forms when p-type and n-type semiconductor materials are joined together. At the boundary, holes from the p-side and electrons from the n-side diffuse into the opposite side because of the concentration gradient. This movement leaves a region with no free charge carriers near the junction, called the depletion region. The charge density in the depletion region can be described on the n-side and p-side using the donor and acceptor doping concentrations, ND and NA. The fixed...

biasing of p-n junction

P-N Junction Diode Under Bias

P-N junction diodes change behavior under equilibrium, forward bias, and reverse bias. In equilibrium, no external voltage is applied across the junction. Carriers diffuse at the boundary and leave behind charged dopants. Acceptors remain on the p-side, and donors remain on the n-side. These fixed charges form a depletion region and an electric field that blocks further diffusion. The band diagram at equilibrium shows the Fermi levels on both sides aligned. That alignment means the junction is...

Metal contacts with semiconductors can form either Schottky or Ohmic junctions. The result depends on the work function of the metal, the work function of the semiconductor, and how electrons move when the two materials touch. At equilibrium, the Fermi levels align, so charge shifts until both sides match in energy. Schottky barriers form when a metal with work function Φm contacts a semiconductor with a different work function Φs. If Φm is greater than Φs, the semiconductor Fermi level starts...

biasing of metal-semiconductor junctions

How Bias Changes Metal-Semiconductor Current

Biasing metal-semiconductor junctions means applying a voltage across the junction. The metal connects to the voltage source, and the semiconductor is grounded. This setup helps control the direction and size of current in devices such as diodes, transistors, and photovoltaic cells. In a Schottky junction with an n-type semiconductor, a positive voltage on the metal lowers its Fermi level. A Fermi level is the energy level that helps describe how electrons are distributed. Lowering this level...

fermi level

Fermi Level in Semiconductors

The Fermi level is a key energy level in semiconductors. It sits between the lower-energy valence band and the higher-energy conduction band. The Fermi-Dirac function, which follows an S-shaped curve, shows the chance that an energy state is filled by an electron at a given temperature. At absolute zero, electrons occupy every energy state up to the Fermi level. States above it stay empty. As temperature rises, electrons gain energy and can move into vacant states above the Fermi level. In an...

fermi level dynamics

Vacuum Level and Schottky Barriers

Vacuum level and Schottky barriers help explain how electrons move between materials. The vacuum level is the energy needed for an electron to escape a material surface. In a semiconductor, it usually sits above the conduction band and serves as a reference for comparing electron energy. Electron affinity is the energy gap between the conduction band minimum and the vacuum level. It shows how easily a semiconductor can accept extra electrons. The work function is the least energy needed to...

key terms and concepts

Learn how metal-semiconductor contacts form Schottky barriers or Ohmic contacts, and why work function differences control current flow and resistance.

Schottky and Ohmic Metal Contacts | JoVE K12

JoVE Core Electrical Engineering offers easy-to-understand animated videos that cover fundamental concepts of electric circuits along with circuit analysis techniques. It incorporates first-order and second-order circuits using operational amplifiers and circuit transformation concepts. The inclusion of practical design examples and real-world applications through scientist-in-action videos adds significant educational value, making complex concepts accessible.

electrical engineering

Schottky and Ohmic Metal Contacts

Description

Transcript

Tags