We want to make a Schottky diode on one surface of an n-type...

We want to make a Schottky diode on one surface of an n-type semiconductor, and an ohmic contact on the other side. The electron affinity is $5 \mathrm{eV}$, band gap is $1.5 \mathrm{eV}$, and the Fermi potential is $0.25 \mathrm{eV} ?$ What should be values of work functions of the two metals? (Give your answer as greater than or less than certain values.) Sketch the band diagram of the structure.

Key Concepts

Schottky Barrier

A Schottky barrier is a potential energy barrier for electrons formed at a metal-semiconductor junction. Its height is determined by the difference between the metal work function and the semiconductor’s electron affinity (often modified by additional effects from doping and interface states). This barrier controls the rectifying behavior of the junction, enabling current flow more easily in one direction than the other, which is fundamental for devices like diodes.

Ohmic Contact

An ohmic contact is designed to provide a low-resistance, non-rectifying electrical connection between a metal and a semiconductor. Unlike a Schottky barrier, an ohmic contact allows for efficient bidirectional current flow with minimal voltage drop. It is achieved by choosing a metal with a work function that aligns closely with the semiconductor’s conduction or valence band, ensuring that the energy barrier for carrier injection is negligible.

Work Function

The work function is the minimum energy required to remove an electron from the interior of a material to a point immediately outside its surface (the vacuum level). In metal-semiconductor interfaces, selecting appropriate work functions for the metals is critical because it determines the energy band alignment and influences whether a contact will be Schottky or ohmic.

Electron Affinity

Electron affinity in a semiconductor is the energy difference between its conduction band edge and the vacuum level. It is a crucial parameter in determining the alignment of energy bands when a metal contacts the semiconductor, affecting the formation of potential barriers at the interface.

Band Gap

The band gap is the energy difference between the valence band and conduction band in a semiconductor. It defines the range of forbidden energies where no electron states exist and is a key parameter in determining the electrical and optical properties of the material, influencing how easily electrons can be excited for conduction.

Fermi Level

The Fermi level is the chemical potential for electrons and represents the energy level at which the probability for occupancy is 50% at absolute zero. In semiconductors, the position of the Fermi level relative to the conduction and valence bands is crucial for determining the type of conductivity (n-type or p-type) and for predicting the behavior of carriers at interfaces with metals.

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