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What is A Schottky Diode? Basics of Schottky Diode



When it comes to low-power, high-current, and ultra-high-speed semiconductor devices, many electronics hobbyists or engineers must first think of Schottky diodes (SBD). But do you really know how to use Schottky diodes? Compared with other diodes, what is special about Schottky diodes? This article will answer these questions for you and introduce Schottky diodes in details.

This short video gives a brief introduction to Schottky Diode


Catalog

I. Schottky Diode Brief Introduction

II. How does Schottky Diode Work?

III. The Structure of Schottky Diode

IV. How to Test Schottky Diode?

V. Pros and Cons of Schottky Diode

VI. Where to Use Schottky Diode?

VII. How to Use Schottky Diode Correctly?

FAQ


I. Brief Introduction to Schottky Diode 


Schottky diodes are named after their inventor, Dr. Schottky. The full name is: Schottky RecTIfier Diode (abbreviated as SR), also called: Schottky barrier diode, or SBD.

 

Schottky diode is a low-power, ultra-high-speed semiconductor device. The most notable feature is its extremely short reverse recovery time (can be as small as a few nanoseconds), and the forward voltage drop is only about 0.4V. It is mostly used as high-frequency, low-voltage, high-current rectifier diodes, freewheeling diodes, protection diodes, and also useful as rectifier diodes and small-signal detector diodes in circuits such as microwave communications. It is more common in communication power supplies, inverters, etc.

 

A typical application of Schottky diodes is in the switching circuit of a bipolar transistor BJT. By connecting a Shockley diode to the BJT to clamp, the transistor is actually close to the off state when the transistor is on, thereby improving the transistor’s performance. Switching speed. This method is a technique used in the TTL internal circuits of typical digital ICs such as 74LS, 74ALS, and 74AS.

 

The biggest feature of Schottky diodes is that the forward voltage drop VF is relatively small. In the case of the same current, its forward voltage drop is much smaller. In addition, its recovery time is short. It also has some shortcomings: the withstand voltage is relatively low, and the leakage current is slightly larger. It must be fully considered when selecting.



II. How does Schottky Diode Work?


Schottky diodes are metal-semiconductor devices made of precious metals (gold, silver, aluminum, platinum, etc.) A as the anode and N-type semiconductor B as the cathode. The barrier formed on the contact surface of the two has rectification characteristics.

 

Because there are a large number of electrons in N-type semiconductors, and there are only a small amount of free electrons in noble metals, electrons diffuse from the high concentration of B to the low concentration of A. Obviously, there are no holes in metal A, and there is no diffusion movement of holes from A to B.

 

As electrons continue to diffuse from B to A, the electron concentration on the surface of B gradually decreases, and the electrical neutrality of the surface is destroyed, so a potential barrier is formed, and the direction of the electric field is B→A. But under the action of this electric field, the electrons in A will also produce a drifting movement from A→B, thereby weakening the electric field formed by the diffusion movement.

 

When a space charge region with a certain width is established, the drifting movement of electrons caused by the electric field and the diffusion movement of electrons caused by different concentrations reach a relative balance, forming a Schottky barrier.

 

The internal circuit structure of a typical Schottky rectifier is based on an N-type semiconductor, and an N-epitaxial layer with arsenic as a dopant is formed on it. The anode uses materials such as molybdenum or aluminum to make a barrier layer. Use silicon dioxide (SiO2) to eliminate the electric field in the edge area and improve the withstand voltage of the tube.

 

The N-type substrate has a small on-state resistance, and its doping concentration is 100% higher than that of the H-layer. An N+ cathode layer is formed under the substrate, and its function is to reduce the contact resistance of the cathode. By adjusting the structural parameters, a Schottky barrier is formed between the N-type substrate and the anode metal.

 

When a forward bias is applied to both ends of the Schottky barrier (the anode metal is connected to the positive pole of the power supply, and the N-type substrate is connected to the negative pole of the power supply), the Schottky barrier layer becomes narrower and its internal resistance becomes smaller; on the contrary, if When reverse bias is applied to both ends of the Schottky barrier, the Schottky barrier layer becomes wider and its internal resistance becomes larger.

 

In summary, the structure principle of Schottky rectifier is very different from PN junction rectifier. The PN junction rectifier is usually called the junction rectifier, and the metal-semi-conductor rectifier is called the Schottky rectifier.

 

Aluminum-silicon Schottky diodes manufactured by the silicon plane process have also come out, which not only saves precious metals, but also Significantly reduce costs and improve the consistency of parameters.


III. The Structure of Schottky Diode


The structure and materials of the new high-voltage SBD are different from the traditional SBD. Traditional SBD is formed by contacting metal and semiconductor. The metal material can be aluminum, gold, molybdenum, nickel, titanium, etc., and the semiconductor is usually silicon (Si) or gallium arsenide (GaAs).

 

Since electrons have higher mobility than holes, in order to obtain good frequency characteristics, N-type semiconductor materials are selected as the substrate. In order to reduce the junction capacitance of the SBD and increase the reverse breakdown voltage without making the series resistance too large, a high-resistance N-thin layer is usually epitaxially on the N+ substrate.

 

CP is the parallel capacitance of the shell and tube, LS is the lead inductance, RS is the series resistance including the semiconductor body resistance and lead resistance, and Cj and Rj are the junction capacitance and junction resistance (both are functions of bias current and bias voltage), respectively.

 

As we all know, there are a large number of conductive electrons inside a metal conductor. When the metal is in contact with the semiconductor (the distance between the two is only an order of magnitude of the atom), the Fermi level of the metal is lower than the Fermi level of the semiconductor. At the sub-energy level corresponding to the conduction band of the semiconductor inside the metal, the electron density is less than that of the conduction band of the semiconductor.

 

Therefore, after the two contact, electrons will diffuse from the semiconductor to the metal, so that the metal is negatively charged and the semiconductor is positively charged. Since metal is an ideal conductor, negative charges are only distributed in a thin layer with the size of an atom on the surface.

 

For N-type semiconductors, the donor impurity atoms that have lost electrons become positive ions, which are distributed in a larger thickness. As a result of the diffusion and movement of electrons from the semiconductor to the metal, a space charge zone, self-built electric field and potential barrier are formed, and the depletion layer is only on the side of the N-type semiconductor (all the barrier zone falls on the semiconductor side).

 

The direction of the self-built electric field in the barrier zone points from the N-type region to the metal. With the increase of the thermionic self-built field, the drift current opposite to the diffusion current direction increases, and finally a dynamic equilibrium is reached, forming a contact potential between the metal and the semiconductor Barrier, this is the Schottky barrier.

 

When the applied voltage is zero, the diffusion current of electrons is equal to the reverse drift current, achieving dynamic equilibrium. When a forward bias is applied (that is, a positive voltage is applied to a metal and a negative voltage is applied to a semiconductor), the self-built field is weakened and the barrier on the semiconductor side is lowered, thus forming a positive current from the metal to the semiconductor.

 

When a reverse bias is applied, the self-built field increases, and the barrier height increases, forming a smaller reverse current from the semiconductor to the metal. Therefore, the SBD, like the PN junction diode, is a non-linear device with unidirectional conductivity.


IV. How to Test Schottky Diode?


Here we show you three testing method for three different diodes.


1. Detect low-power crystal diodes

 

A. Discrimination of positive and negative electrodes

 

(1) Observe the symbol mark on the housing. Usually the diode is marked with the symbol of the diode on the housing of the diode, one end with a triangular arrow is the positive electrode, and the other end is the negative electrode.

 

(2) Observe the color dots on the shell. The case of point contact diodes is usually marked with polar color points (white or red). Generally, the end marked with a colored dot is the positive electrode. Other diodes are marked with a color ring, and the end with the color ring is the negative electrode.

 

(3) Based on a measurement with a smaller resistance value, the end connected to the black test lead is the positive electrode, and the end connected to the red test lead is the negative electrode.

 

B. Detect the highest working frequency fM. The operating frequency of crystal diodes can be found in the relevant characteristic table. In practice, they are often distinguished by observing the contact wires inside the diode.

 

 For example, point contact diodes are high-frequency tubes, and surface contact diodes are mostly low-frequency tubes. In addition, you can also use the multimeter R×1k block to test, generally the forward resistance is less than 1k high frequency tube.

 

C. Detect the highest reverse breakdown voltage VRM. For alternating current, because of constant changes, the highest reverse working voltage is also the peak alternating current voltage that the diode bears.

 

It should be pointed out that the highest reverse working voltage is not the breakdown voltage of the diode. Under normal circumstances, the breakdown voltage of the diode is much higher than the maximum reverse working voltage (about twice as high).

 

2. Detection of high frequency varistor diodes

 

A. identification diode positive and negative

 

The difference in appearance between high-frequency varistor diodes and ordinary diodes is that their color code is different. The color code of ordinary diodes is generally black, while the color code of high-frequency varistor diodes is light. Its polarity law is similar to that of ordinary diodes, that is, the end with the green ring is the cathode, and the end with the green ring is the anode.

 

B. Measure the forward and reverse resistance to judge whether it is good or bad

 

The specific method is the same as the method of measuring the forward and reverse resistance of ordinary diodes. When using a 500-type multimeter to measure the R×1k gear, the forward resistance of a normal high-frequency varistor diode is 5k~55k, and the reverse resistance is infinity.

 

3. Transient voltage suppression diode (TVS) detection

 

Use a multimeter to measure the quality of the tube. For a unipolar TVS, according to the method of measuring ordinary diodes, the forward and reverse resistance can be measured. Generally, the forward resistance is about 4kΩ, and the reverse resistance is infinite.

 

For the two-way polar TVS, the resistance between the two pins should be infinite when the red and black test leads are arbitrarily exchanged. Otherwise, the tube has poor performance or has been damaged.


V. Pros and Cons of Schottky Diode


Pros: 

Schottky diodes have the advantages of high switching frequency and reduced forward voltage, but their reverse breakdown voltage is relatively low, mostly not higher than 60V, and the highest is only about 100V, which limits its application range.

 

Like in the switching power supply (SMPS) and power factor correction (PFC) circuit, the freewheeling diode of the power switch device, the high frequency rectifier diode of 100V or more used in the transformer secondary, the 600V~1.2kV high speed diode in the RCD snubber circuit, and For PFC boosting 600V diodes, only fast recovery epitaxial diodes (FRED) and ultra-fast recovery diodes (UFRD) are used.

 

The reverse recovery time Trr of UFRD is also above 20ns, which cannot meet the needs of 1MHz~3MHz SMPS in fields such as space stations. Even for SMPS with hard switching of 100kHz, due to the large conduction loss and switching loss of UFRD, the case temperature is very high, and a larger heat sink is required, which increases the size and weight of SMPS, which does not meet the requirements of miniaturization and lightness. Development trend.

 

Therefore, the development of high-voltage SBDs above 100V has always been a research topic and a hot spot of concern. In recent years, SBD has made breakthrough progress. High-voltage SBDs of 150V and 200V have been put on the market, and SBDs with more than 1kV made of new materials have also been successfully developed, thus injecting new vitality and vitality into their applications.

 

Cons: 

The biggest disadvantage of Schottky diodes is their low reverse bias voltage and large reverse leakage current. For example, Schottky diodes using silicon and metal as materials have the highest reverse bias voltage rating. To 50V, and the reverse leakage current value is a positive temperature characteristic, it is easy to increase rapidly as the temperature rises, and it is necessary to pay attention to the hidden concern of thermal runaway in practical design.

 

In order to avoid the above-mentioned problems, the reverse bias voltage of the Schottky diode in actual use will be much smaller than its rated value. However, the technology of Schottky diodes has also progressed, and its reverse bias voltage rating can reach up to 200V.


VI. Where to Use Schottky Diode?


The structure and characteristics of SBD make it suitable for high-frequency rectification in low-voltage and high-current output occasions. It is used for detection and mixing at very high frequencies (such as X-band, C-band, S-band and Ku-band). Used as a clamp in high-speed logic circuits. SBD is often used in ICs. SBD*TTL integrated circuits have long become the mainstream of TTL circuits and are widely used in high-speed computers.

 

In addition to the characteristic parameters of ordinary PN junction diodes, SBD electrical parameters used for detection and mixing also include intermediate frequency impedance (referring to the impedance presented by the SBD to the specified intermediate frequency when the rated local oscillator power is applied, generally between 200Ω and 600Ω) , Voltage standing wave ratio (generally ≤ 2) and noise figure, etc.


VII. How to Use Schottky Diode Correctly?


Schottky diodes are widely used in circuits such as switching power supplies, frequency converters, and drivers. In different applications, different factors need to be considered, and different devices have different performances. Therefore, when selecting Schottky diodes, the following key parameters need to be considered comprehensively.

 

1. The conduction voltage drop VFVF is the voltage drop across the diode when the diode is forward-conducting. When the current through the diode is larger, the VF is larger; when the diode temperature is higher, the VF is smaller.

 

2. The reverse saturation leakage current IRIR refers to the current that flows through the diode when the reverse voltage is added to the two ends of the diode. The reverse leakage current of the Schottky diode is relatively large. The choice of Schottky diode is to choose a diode with a smaller IR as much as possible.

 

3. The rated current IF refers to the average current value calculated according to the allowable temperature rise during long-term operation of the diode.

 

4. The maximum surge current IFSM allows excessive forward current to flow. It is not a normal current, but an instantaneous current, which is quite large.

 

5. Even if the maximum reverse peak voltage VRM does not have reverse current, as long as the reverse voltage is continuously increased, the diode will be damaged sooner or later.

 

This reverse voltage that can be applied is not an instantaneous voltage, but a forward and reverse voltage repeatedly applied. Because the AC voltage is added to the rectifier, its maximum value is a specified important factor.

 

The maximum reverse peak voltage VRM refers to the maximum reverse voltage that can be applied to avoid breakdown. Currently Schottky's highest VRM value is 150V.


FAQ


1. What is Schottky diode used for?

Schottky diodes are used for their low turn-on voltage, fast recovery time and low-loss energy at higher frequencies. These characteristics make Schottky diodes capable of rectifying a current by facilitating a quick transition from conducting to blocking state.


2. What is the difference between Schottky diode and normal diode?

In the normal rectifier grade PN junction diode, the junction is formed between P type semiconductor to N type semiconductor. Whereas in Schottky diode the junction is in between N type semiconductor to Metal plate. The schottky barrier diode has electrons as majority carriers on both sides of the junction.


3. How does Schottky diode work?

In a Schottky diode, a semiconductor–metal junction is formed between a semiconductor and a metal, thus 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 both a low forward voltage drop and very fast switching.


4. What are the two important features of a Schottky diode?

We have seen here that the Schottky Diode also known as a Schottky Barrier Diode is a solid-state semiconductor diode in which a metal electrode and an n-type semiconductor form the diodes ms-junction giving it two major advantages over traditional pn-junction diodes, a faster switching speed, and a low forward bias.


5. What is Schottky diode made of?

Schottky diodes made from palladium silicide (PdSi)[clarification needed] are excellent due to their lower forward voltage (which has to be lower than the forward voltage of the base-collector junction).


6. Why Schottky is called hot carrier diode?

When a Schottky diode is in unbiased condition, the electrons lying on the semiconductor side have a very low energy level when compared to the electrons present in the metal.Thus, the electrons cannot flow through the junction barrier which is called the Schottky barrier. If the diode is forward biased, electrons present in the N-side get sufficient energy to cross the junction barrier and enters the metal.These electrons enter into the metal with tremendous energy. Consequently, these electrons are known as hot carriers. Thus the diode is called a hot-carrier diode.


7. What is Schottky barrier rectifier?

The Schottky diode or Schottky Barrier Rectifier is named after the German physicist “Walter H. Schottky”, is a semiconductor diode designed with a metal by the semiconductor junction. It has a low-forward voltage drop and a very rapid switching act. ... Actually, it is one of the oldest semiconductor devices in reality.


8. What is meant by Schottky effect?

Schottky effect, increase in the discharge of electrons from the surface of a heated material by application of an electric field that reduces the value of the energy required for electron emission. ... The effect is named after its discoverer, the German physicist Walter Schottky.


9. Why Schottky barrier is formed?

When a metal is put in direct contact with a semiconductor, a so called Schottky barrier can be formed, leading to a rectifying behavior of the electrical contact.


10. What is the barrier potential of Schottky diode?

The forward voltage drop ranges from 0.3 volts to 0.5 volts. The barrier of forward voltage drop is made of silicon. The forward voltage drop is proportional to the doping concentration of N type semiconductor. Due to high concentration of current carriers, the V-I characteristic of Schottky diode is steeper.