10.9: Biasing of Metal-Semiconductor Junctions
Biasing metal-semiconductor junctions involves applying a voltage across the junction. Specifically, the metal is connected to a voltage source, while the semiconductor is grounded. This technique is essential for controlling the direction and magnitude of current flow in electronic devices, including diodes, transistors, and photovoltaic cells.
In Schottky junctions, where the semiconductor is n-type, applying a positive voltage to the metal relative to the semiconductor reduces its Fermi level. It lowers the energy barrier that the electrons in the semiconductor need to overcome to move into the metal. This enables a significant flow of electrons from the semiconductor to the metal resulting in a rapidly increasing current when the junction is forward-biased. When a negative voltage is applied, the situation reverses. The Fermi level of the metal rises, enhancing the barrier against electron flow from the semiconductor to the metal. A few electrons can overcome the barrier despite this, generating a minor reverse bias current.
Ohmic junctions behave differently. Due to the absence of a significant barrier, even a slight positive voltage can induce a large forward bias current, allowing easy electron flow from the semiconductor to the metal. In reverse bias, there exists a minimal barrier for electron flow from the metal to the semiconductor, but this barrier effectively disappears if the reverse bias voltage exceeds a few tenths of a volt.
The interaction dynamics change with p-type semiconductors. The behavior described for n-type semiconductors in both Schottky and Ohmic junctions is reversed. This ability to manipulate current flow through biasing is essential for the operation of many electronic components, providing a foundation for the functionality of a wide range of devices.
