A Schottky diode is a semiconductor device featuring a metal-semiconductor junction that enables current flow in one direction with notably low forward voltage drop and rapid switching capabilities. Unlike conventional PN junction diodes, the Schottky diode lacks a depletion layer, allowing faster response times and reduced power loss. It is named after German physicist Walter H. Schottky.
The symbol used to represent a Schottky diode in circuit diagrams is similar to that of a standard diode with a modified cathode to reflect the unique characteristics of a Schottky diode.
The working principle of a Schottky diode revolves around its unique metal-semiconductor junction, which leads to distinctive properties compared to a traditional PN junction diode.
Barrier Formation and Conductivity
The crucial aspect of the Schottky diode’s operation is the formation of a potential energy barrier between the metal and semiconductor materials. When a metal, such as platinum or gold, comes into contact with a semiconductor material like silicon or gallium arsenide, it forms a barrier due to differences in their work functions. This barrier prevents the majority carriers from readily moving across the junction. This barrier height is typically lower than the PN junction diode, resulting in faster switching speeds and lower forward voltage drop characteristics.
Unlike a PN junction diode that relies on a depletion region formed by the diffusion of charge carriers, the Schottky diode’s barrier is not due to the diffusion of carriers. It arises from the difference in the Fermi levels between the metal and semiconductor. The metal-semiconductor junction creates a “rectifying” behavior, allowing current flow more readily in one direction (forward bias) than the other (reverse bias). When a forward voltage is applied across the diode, electrons from the semiconductor material overcome the potential barrier and flow into the metal, resulting in a highly conductive state. However, the majority carriers cannot overcome the barrier in the reverse bias condition, resulting in minimal current flow.
The I-V characteristics of a Schottky diode provide crucial insights into its behavior concerning current flow under varying voltage conditions. The relationship between the current and voltage across the diode primarily exhibits distinct non-linear behavior due to its metal-semiconductor junction construction. It is similar to the exponential curve of a PN junction diode.
Under forward bias conditions, the Schottky diode rapidly increases current with minimal voltage changes due to its low forward voltage drop. This behavior arises from the absence of a depletion region, leading to a quick injection of majority carriers across the metal-semiconductor interface.
Conversely, the reverse bias region of the Schottky diode I-V curve demonstrates distinct characteristics. At reverse bias, the diode showcases minimal leakage current. The absence of minority carriers and the rapid recombination of carriers near the metal-semiconductor interface contribute to this low leakage current. However, Schottky diodes typically exhibit a higher reverse leakage current than conventional PN junction diodes due to the absence of the depletion layer, making them less suitable for applications requiring stringent reverse bias requirements.
The image below compares the characteristics curves of Schottky and PN Diodes. While both diodes exhibit non-linear current-voltage characteristics, the Schottky diode tends to have lower forward voltage drop and faster response but higher reverse leakage voltage than the PN junction diode.
Here are the applications of a Schottky diode:
- Power rectification in power supply circuits
- RF (Radio Frequency) and microwave applications
- High-speed switching circuits in digital electronics
- Voltage clamping in transient voltage suppression devices
- Solar cells and energy harvesting systems
Advantages and Disadvantages
Here are the advantages of a Schottky diode:
- The diode boasts low capacitance owing to its minimal depletion region.
- Its rapid reverse recovery time signifies swift transitioning from ON to OFF states.
- With its negligible depletion region, the diode exhibits high current density.
- Its remarkably low turn-on voltage ranges between 0.2 to 0.3 volts.
The sole drawback of Schottky diodes is their considerable reverse saturation current.
Article was last reviewed on Tuesday, November 21, 2023