The Basics of Schottky Diodes: A Comprehensive Guide

Schottky diodes are important in modern electronics for their speed and efficiency. Unlike traditional diodes, they use a metal semiconductor junction and this results in lower voltage drop and faster switching times.

The design makes Schottky diodes ideal for several high frequency applications that we will discuss in this article. We hope the article helps you understand the working principle of this kind of diode together with its applications.

Understanding Schottky Diodes

A Schottky diode is a semiconductor device characterized by its metal semiconductor junction, rather than the typical p-n junction found in conventional diodes. 

Named after the physicist Walter H. Schottky, these diodes are designed to offer specific advantages such as lower forward voltage drop and faster switching speeds.

Construction of Schottky Diodes

The main feature of a Schottky diode is its metal semiconductor junction. This involves bonding a metal like platinum or molybdenum to an n-type semiconductor such as silicon. Unlike a p-n junction, this structure doesn’t create a depletion region, resulting in different electrical behavior.

Typical Materials Used

To understand the materials used in this diode, we look at both the metal electrode and the semi conductor.

• Metal Electrode: Metals like tungsten, platinum and molybdenum are commonly used for the metal electrode. These metals are chosen for their low resistance and good electrical properties.

• Semiconductor: The n-type semiconductor is typically silicon. Silicon is chosen because it provides efficient conduction properties.

Electrical Characteristics

One of the standout electrical features of Schottky diodes is their low forward voltage drop. When the diode is forward biased, the voltage required to initiate conduction is notably lower compared to convectional p-n junction diodes.

Basically, Schottky diodes have forward voltage drop between 0.2 to 0.4 volts. This is advantageous in reducing power loss in electronic circuits.

To illustrate, let’s consider a Schottky diode with a forward voltage drop of 0.3 volts conducting a current of 1 ampere. The power dissipation in the diode primarily as heat, can be calculated as:

P = vf x I

Where:

• P is the power dissipation

• Vf is the forward voltage drop (0.3 V)

• I is the current (1 A)

Thus:

P = 0.3 V x 1 A

= 0.3 W

This calculation demonstrates that the diode dissipates 0.3 watts of power, highlighting its efficiency compared to other types of diodes.

In addition to their low forward voltage, Schottky diodes are known for their rapid switching speed. The feature is due to the absence of minority carrier storage which is typical in pn-junction diodes. When reverse biased, these diodes can switch off almost instantaneously.

However, Schottky diodes also exhibit a higher reverse leakage current than the pn-junction diodes. This leakage current occurs when the diode is reverse biased and although it is generally small, it can be significant in certain applications.

For instance, if the reverse leakage current is 10 microamperes (µA) and the reverse voltage is 50 volts, the power loss due to leakage can be computed as:

Pleakage = IR x VR

Where:

• Pleakage is the power loss due to the leakage

• IR is the reverse leakage current (10 µA)

• VR is the reverse voltage (50 V)

Thus:

Pleakage = 10 µA x 50 V

= 500 µW

= 0.5 mW

How a Schottky Diode Works

A Schottky diode functions through a metal semiconductor junction offering the above electrical characteristics we have seen above.  It operates in the following steps:

Forward Bias Operation

When a Schottky diode is forward biased, the metal side (cathode) is connected to the negative potential and the n-type semiconductor side (anode) is connected to the positive potential. 

This arrangement lowers the potential barrier at the metal semiconductor interface allowing electrons from the n-type semiconductors to move into the metal.

The movement of electrons in this setup constitutes the forward current.

Calculation of forward current:

The current through a Schottky diode in the forward direction can be approximated using the thermionic emission equation:

Where:

• IF is the forward current

• A⋅ is the effective Richardson constant

• A is the area of the diode

• T is the temperature in Kelvin

• ΦB is the Schottky barrier height

• k is the Boltzmann’s constant

• q is the electron charge

• v is the applied forward voltage

This equation shows that the forward current increases exponentially with the applied voltage, similar to a pn-junction diode but with a lower threshold voltage due to the smaller Schottky barrier height.

Reverse Bias Operation

In reverse bias, the metal is connected to a positive potential and the n-type semiconductor is connected to a negative potential. This increases the potential barrier, preventing electrons in the n-type semiconductor in moving into the metal.

As a result, only a small leakage current flows which is due to minority carriers and thermionic emission.

Sample calculation:

The reverse leakage current IR can be approximated as:

The equation above indicates that the reverse leakage current depends on the temperature and the Schottky barrier height. 

Unlike in forward bias, the current does not depend significantly on the reverse voltage due to the lack of minority carrier injection, which is a primary feature in pn-junction diodes.

Capacitance and Switching Speed

A significant advantage of Schottky diodes is their minimal capacitance and fast switching speeds. The absence of depletion layer and minority carrier storage means that the capacitance is primarily due to the metal- semiconductor interface.

This capacitance allows the diode to switch on and off quickly, making it ideal for high frequency applications.

The junction capacitance Cj can be given by:

Where:

• ϵ is the permittivity of the semiconductor

• A is the area of the junction

• D is the thickness of the depletion region

Here is an image to illustrate the above operation of the Schottky diode.

(Image source)

The x-axis of the above image represents the voltage applied across the diode, while the y-axis represents the current flowing through it.

The curve is divided is divided into three regions:

1. Reverse Bias Region:

In this, the diode is under a negative voltage with anode connected to negative terminal and cathode to the positive terminal. As a result, only a small leakage current flows though the diode.

2. Breakdown Region:

At a specific reverse voltage known as the breakdown voltage (VB), the diode experiences a sudden increase in current. This region is typically avoided in normal operations.

3. Forward Bias Region:

Here, a positive voltage is applied across the diode with the anode connected to the positive terminal and the cathode to the negative terminal.

As the voltage increases, the current through the diode rapidly rises. The point at which the current starts to increase significantly is known as the knee voltage (VK) which is around 0.7 volts for a Schottky diode.

The forward current (IF) increases exponentially with the applied voltage as described by our thermionic emission equation.

Advantages of Schottky Diodes

Schottky diodes offer several advantages that make them valuable in various applications.

Low Forward Voltage Drop

One of the primary benefits is their low forward voltage drop. Typically around 0.3 to 0.4 volts. This feature results in lower power loss and higher efficiency especially in low voltage circuits. 

This efficiency makes it ideal for power sensitive applications such as solar panels and battery powered devices.

Fast Switching Speed

Another importance is their switching speed. Due to their metal semiconductor junction, Schottky diodes can switch on and off much faster than conventional p-n junction diodes.

Low Capacitance

Schottky diodes also have low capacitance which further enhances their performance in high speed circuits. This low capacitance reduces the delay in diode’s response time ensuring that signals can be processed quickly and accurately.

However, Schottky diodes also have some disadvantages.

Disadvantages of Schottky Diodes

Below are some of the most common limitations of these diodes:

Higher Reverse Leakage Current

Compared to p-n junction diodes, Schottky diodes tend to allow more current to leak in the reverse direction which can be problematic in certain applications where reverse leakage is not required.

Lower Reverse Breakdown Voltage

Schottky diode have a lower maximum reverse voltage rating usually around 50 volts. This limitation means they are not suitable for high voltage applications where higher breakdown voltages are required.

Higher Cost

The manufacturing process of Schottky diodes involving specific materials and methods to create tends to be more expensive than that of standard p-n junction diodes. 

The higher cost can be a factor in applications where budget constraints are critical.

Applications of Schottky Diodes

Feature of Schottky diodes makes them particularly valuable in several fields:

Power Rectification

The low forward voltage drop reduces power loss and heat generation making them ideal for power supplies in devices like computers and consumer electronics. They are often found in AC to DC converters where efficiency is important.

Radio Frequency Systems

The fast switching speed of Schottky diodes makes them well suited for RF applications including mixes and detectors in communication systems. Their low capacitance and fast time response help in processing high frequency signals with minimal distortion.

Clamping and Protection Circuits

Clamping circuits also use these diodes to protect sensitive electronic components from voltage spikes. Their low forward voltage drop and quick response time enable them to clamp voltage effectively.

Logic Gates and Digital Circuits

Schottky diodes are integral parts in digital circuits especially is Schottky TTL logic gates. Their fast switching speeds and low forward voltage drop reduce power consumption and enhance performance in high speed digital systems.

They are commonly used in data communication and memory devices.

FAQs

Commonly asked questions on Schottky diodes are:

What is the basic operation of a Schottky Diode?

Schottky diode operates by allowing current to flow easily in one direction while blocking it in the opposite direction.

What is the concept of Schottky diode?

The concept of a Schottky diode revolves around its metal semiconductor junction which provides lower forward voltage drop and faster switching.

What are the current components of Schottky diode?

The current components include forward current due to majority carriers and the reverse leakage current caused by minority carriers and thermionic emission.

Conclusion

Schottky diodes with their unique metal semiconductor junction offer advantages like low forward voltage drop and fast switching speeds which are ideal for several applications like rectification and high frequency systems.

Understanding their construction and characteristics helps in leveraging their benefits effectively in modern electronics.