When it comes to low power consumption, high current, and ultra-high-speed semiconductor devices, many electronics enthusiasts and electronic engineers first think of Schottky diodes. But do you really know how to use Schottky diodes? What is so special about Schottky diodes compared to other diodes? In this article, I will solve these problems for you and introduce Schottky diodes in detail.
What is a Schottky diode?
Schottky diode definition
A Schottky diode, also known as a hot carrier diode, is a semiconductor diode with low forward voltage drop and very fast switching action. When current flows through a Schottky diode, there is a small voltage drop across the Schottky diode terminals. The voltage drop of ordinary diodes is between 0.6V-1.7V, while the voltage drop of Schottky diodes is usually between 0.15V-0.45V.
This lower voltage drop provides better system efficiency and higher switching speed. In a Schottky diode, a semiconductor-metal junction forms between the semiconductor and the metal, creating a Schottky barrier. The N-type semiconductor acts as the cathode, and the metal side acts as the anode of the diode. This Schottky barrier results in low forward voltage drop and very fast switching.
Schottky diode circuit symbol
Schottky Diode Internal Structure
Schottky diodes are formed by connecting a doped semiconductor region (usually N-type) with a metal (eg gold, silver, platinum). Not a PN junction, but a metal-semiconductor, as shown in the figure below.
Schottky diode structure diagram
Equivalent circuit of a Schottky diode
Equivalent circuit of a Schottky diode
How Schottky Diodes Work
When a metal combines with an N-type semiconductor, an MS junction is formed. This junction is called the Schottky barrier. The Schottky barrier behaves differently depending on whether the diode is unbiased, forward biased, or reverse biased.
Unbiased Schottky diode
When a metal is combined with an N-type semiconductor, conduction band electrons (free electrons) in the N-type semiconductor move from the N-type semiconductor to the metal, establishing an equilibrium state.
We know that when a neutral atom loses an electron, it becomes a positive ion, and when a neutral atom gains an extra electron, it becomes a negative ion.
Conduction band electrons or free electrons across the junction will provide additional electrons to the atoms in the metal. As a result, the atoms at the metal junction gain extra electrons, while the atoms at the N-side junction lose electrons.
Unbiased Schottky diode
Atoms that lose electrons at the N-side junction will become positive ions, while atoms that gain extra electrons at the metal junction will become negative ions. Therefore, positive ions are generated at the N-side junction and negative ions are generated at the metal junction. These positive and negative ions are nothing but depletion regions.
Since metals have a large number of free electrons, the width of these electrons moving into the metal is negligible compared to the width inside the N-type semiconductor. Therefore, the built-in potential or built-in voltage mainly exists inside the N-type semiconductor. The built-in voltage is the barrier that the conduction band electrons of the N-type semiconductor see when trying to enter the metal.
To overcome this obstacle, free electrons need more energy than the built-in voltage. In an unbiased Schottky diode, only a small amount of electrons will flow from the N-type semiconductor to the metal. A built-in voltage prevents further flow of electrons from the semiconductor conduction band into the metal. The transfer of free electrons from the N-type semiconductor into the metal causes band bending near the contact.
Forward Biased Schottky Diodes
If the positive pole of the battery is connected to metal and the negative pole is connected to N-type semiconductor, then the Schottky diode is called forward biased.
When a forward bias is applied to a Schottky diode, a large number of free electrons are generated in N-type semiconductors and metals. However, free electrons in N-type semiconductors and metals cannot cross the junction unless a voltage greater than 0.2 volts is applied.
Forward Biased Schottky Diodes
If the applied voltage is greater than 0.2 volts, the free electrons gain enough energy and overcome the built-in voltage in the depletion region. As a result, current starts flowing through the Schottky diode. If the applied voltage is continuously increased, the depletion region becomes very thin and eventually disappears.
Reverse Biased Schottky Diode
If the negative pole of the battery is connected to metal and the positive pole is connected to N-type semiconductor, it is said that the Schottky diode is reverse biased.
When a reverse bias voltage is applied to the Schottky diode, the depletion width increases. As a result the current stops flowing. A small amount of leakage current occurs due to thermally excited electrons in the metal.
Reverse Biased Schottky Diode
If the reverse bias continues to increase, the current gradually increases due to the weaker barrier.
If the reverse bias voltage is increased significantly, the current will suddenly rise. This sudden rise in current can cause damage to the depletion region, which can permanently damage the device.
VI Characteristics of Schottky Barrier Diodes
It can be seen from the VI characteristics that the VI characteristics of Schottky barrier diodes are similar to ordinary PN junction diodes, but there are still the following differences.
VI Characteristics of Schottky Barrier Diodes
Schottky barrier diodes have a lower forward voltage drop than ordinary PN junction diodes. Schottky barrier diodes made of silicon exhibit a forward voltage drop of 0.3 volts to 0.5 volts.
The forward voltage drop increases with increasing n-type semiconductor doping concentration. Due to the high concentration of carriers, the VI characteristics of Schottky barrier diodes are steeper than those of ordinary PN junction diodes.
Advantages and Disadvantages of Schottky Diodes
Advantage 1: Low forward voltage drop
The turn-on voltage of a Schottky diode is between 0.2V-0.3V for a silicon diode, while the turn-on voltage of a standard silicon diode is between 0.6 and 0.7 volts. This gives it the very same turn-on voltage as a germanium diode.
Advantage 2: Fast recovery time
The fast recovery time due to the small amount of stored charge means it can be used in high-speed switching applications.
Advantage 3: Low Junction Capacitance
Given the very small active area, typically due to the use of line-point contacts on silicon, capacitance levels are very small.
Advantage 4: High current density
Schottky diodes have a negligible depletion region. So applying a small voltage is enough to generate a large current.
Advantage 5: less noise
Schottky diodes generate less unwanted noise than typical PN junction diodes.
Advantage 6: Better performance
Schottky diodes will dissipate less power and can easily meet the requirements of low voltage applications.
Disadvantage 1: Higher reverse current
Since the Schottky diode is a metal-semiconductor structure, it is easier to leak current when the voltage is reversed.
Disadvantage 2: Lower maximum reverse voltage
Reverse voltage is the voltage at which a diode will break down and start conducting a significant amount of current when the voltage is connected in reverse (cathode to anode). This means that Schottky diodes cannot withstand very large reverse voltages without breaking down and conducting a large amount of current. Even before reaching this maximum reverse value, it still leaks a small amount of current.
What is special about Schottky diodes
Like we said earlier, Schottky diodes look and behave very much like normal diodes, but a unique feature of Schottky diodes is their extremely low voltage drop and high switching speed.
Schottky diode function and application
Schottky diodes have many useful applications ranging from rectification, signal conditioning, switching, voltage clamping, solar cells to TTL and CMOS logic gates, mainly because of their low power consumption and fast switching speed. TTL Schottky logic gates are identified by the letters LS appearing somewhere in their logic gate code, for example 74LS00.
RF Mixers and Detector Diodes
Schottky diodes are unique in RF applications due to their high switching speed and high frequency capability. For this reason, Schottky barrier diodes are used in many high performance diode ring mixers. In addition to this, their low turn-on voltage and high frequency capability and low capacitance make them ideal for RF detectors.
Power Rectifier
Schottky barrier diodes are also used in high power applications as rectifiers. Their high current density and low forward voltage drop means that less power is wasted than with ordinary PN junction diodes. This increased efficiency means less heat needs to be dissipated and smaller heatsinks can be incorporated into the design.
