Schottky Diodes: Principle, Functions, and Applications

30 March

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Schottky diodes are metal-semiconductor devices made of precious metals (gold, silver, aluminum, platinum, etc.) A as the positive electrode and N-type semiconductor B as the negative electrode. Schottky diodes are unidirectionally conductive and can convert alternating currents into pulsed direct currents in a single direction.

Schottky diodes are named after their inventor, Dr. Schottky, and SBD is short for Schottky Barrier Diode. SBDs are not made using the principle of forming a PN junction between a P-type semiconductor and an N-type semiconductor but are made using the metal-semiconductor principle formed by the contact between a metal and a semiconductor. Therefore, SBD is also called a metal-semiconductor diode or surface barrier diode, which is a kind of hot carrier diode.

Schottky diodes are metal-semiconductor devices made of precious metals (gold, silver, aluminum, platinum, etc.) A as the positive electrode and N-type semiconductor B as the negative electrode. Schottky diodes are unidirectionally conductive and can convert alternating currents into pulsed direct currents in a single direction. By using the switching characteristics of Schottky diodes, various logic circuits can be composed. A Schottky diode can limit the amplitude of the signal to the required range while preventing reverse breakdown. Schottky diodes use their unidirectional conductivity to extract low-frequency or audio signals from high-frequency or intermediate-frequency radio signals. It can be used in high-frequency circuits for automatic tuning, frequency modulation, and equalization. For example, in televisions, the Schottky diode is used in the receiver's tuning loop as a variable capacitor. Schottky diodes can be used as AND gates or OR gates, and can also be applied to dual power supplies.

Catalog

I Working principle of Schottky diodes

Schottky diodes are metal-semiconductor devices made of precious metals (gold, silver, aluminum, platinum, etc.) A as the positive electrode, N-type semiconductor B as the negative electrode. And the potential barrier formed on the contact surface of the two has the rectifying characteristics. Because there are a large number of electrons in the N-type semiconductor, and there are only a small number of free electrons in the precious metal, the electrons diffuse from the high concentration B to the low concentration A. Obviously, there are no holes in metal A, so there is no diffusion movement of holes from A to B. As the electrons continue to diffuse from B to A, the electron concentration on the surface of B gradually decreases. And the surface neutrality is destroyed, so a potential barrier is formed, and its electric field direction is B → A. However, under the action of this electric field, the electrons in A will also drift from A → B, thereby weakening the electric field formed by the diffusion motion. After the space charge region of a certain width is established, the electron drift motion caused by the electric field and the electron diffusion motion caused by different concentrations reaches a relative equilibrium, 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 using arsenic as a dopant is formed thereon. The anode uses a material such as molybdenum or aluminum to make a barrier layer. Silicon dioxide (SiO2) is used 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 role 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, as shown in the figure. When a forward bias is applied across the Schottky barrier (the anode metal is connected to the positive electrode of the power supply, and the N-type substrate is connected to the negative electrode of the power supply), the Schottky barrier layer becomes narrower and its internal resistance becomes smaller; otherwise, if When a reverse bias is applied to both ends of the Schottky barrier, the Schottky barrier layer becomes wider and its internal resistance becomes larger.

Figure 1. The internal circuit structure of the Schottky diode

In summary, the structural principle of Schottky rectifiers is very different from that of PN junction rectifiers. Generally, PN junction rectifiers are called junction rectifiers, and metal-semiconductor rectifiers are called Schottky rectifiers. Aluminium-silicon Schottky diodes manufactured using a silicon planar process have also been introduced, which not only saves precious metals, greatly reduces costs, but also improves the consistency of parameters.

II Functions of Schottky diodes

1. Rectification

With the unidirectional conductivity of Schottky diodes, alternating currents with alternating directions can be converted into pulsed direct currents in a single direction.

In the circuit, current can only flow from the positive pole of the Schottky diode and flows out from the negative pole. The carriers in the P region are holes, and the carriers in the N region are electrons. A certain potential barrier is formed between the P region and the N region. When the applied voltage makes the P-region positive to the N-region, the potential barrier decreases, and storage carriers are generated near both sides of the potential barrier. The circuit can pass a large current and have a low voltage drop (typically 0.7V), which is called a forwarding state. When the opposite voltage is added and the barrier is increased, a small reverse leakage current flows, and we call it a reverse blocking state.

Schottky diodes are mainly used in various low-frequency half-wave rectifier circuits and full-wave rectifiers. The rectifier bridge encloses the rectifier tube in a shell, which is divided into a full bridge and half-bridge. A full bridge is a package of four Schottky diodes connected to a bridge rectifier circuit. The half-bridge is to seal the halves of four Schottky diode bridge rectifiers together. Two bridges can be used to form a bridge rectifier circuit, and one half-bridge can also be used to form a full-wave rectifier circuit with a center tap on the transformer. During each working cycle of the bridge, only two Schottky diodes work at the same time. Through the one-way conduction function of the Schottky diodes, the alternating current is converted into a unidirectional DC pulsating voltage.  

Figure 2. rectifier

2.  Switch

Schottky diodes’ resistance under the action of forwarding voltage is very low and they are in an on the state, which is equivalent to an on the switch; under the action of reverse voltage, their resistance is very large, and they are in an off state, just like an off switch. By using the switching characteristics of Schottky diodes, various logic circuits can be composed. Because the Schottky diode has the characteristic of unidirectional conduction, the PN junction is turned on under positive bias. The resistance in the on-state is very small, about tens to hundreds of ohms. Under reverse bias, it is in the off state, so its resistance is very large. Generally, silicon Schottky diodes are above 10 megaohms, and germanium tubes also have tens of thousands of ohms to hundreds of thousands of ohms. With this feature, the Schottky diode will play a role in controlling the current on or off in the circuit and become an ideal electronic switch.

Figure 3. Switch circuit 1

The most basic switching circuit is shown in the figure. In this circuit, the two ends of the diode are connected to Vcc and GND through a resistor, respectively. The diode is in a reverse-biased state and will not conduct. The AC voltage applied through point C1 cannot pass through the diode, and AC components cannot be detected after C2.

Figure 4. Switch circuit 2

In this figure, the connection of the diode is opposite to the above figure. The diode in the forward conduction state can make the AC signal applied at point C1 pass through the diode and appear at the output of C2. This is the state when the diode is on, and we can also call it the "on" state of the switch.

This is the simplest circuit that adjusts the on-state of the Schottky diode through the state of the DC bias. In order to achieve control of the AC signal. In a practical process, we make sure that the level of one side is unchanged and adjust the level of the other side to control the conduction of the diode. In RF circuits, this design often adds measures to prevent RF components from mixing into the logic/power supply lines on the line that provides the bias to reduce interference, but in general, this design is still very common.

3.  Amplitude limiting

The so-called limiting Schottky diode is to limit the amplitude of the signal to the required range. Because the circuits that usually require limiting are mostly high-frequency pulse circuits, high-frequency carrier circuits, high-frequency signal amplifier circuits, and high-frequency modulation circuits, the limiting Schottky diodes are required to have steep UI characteristics so that they have good switching performance.

Amplitude limiting Schottky diode characteristics: 1. Mostly used for medium, high frequency, and audio circuits; 2. Fast turn-on speed and short recovery time; 3. Stable diode voltage drop under positive bias; 4. It can be implemented in series and parallel and limits amplitude in all directions and values; 5. Temperature compensation can be achieved while limiting.

After the Schottky diode is turned on in the forward direction, its forward voltage drop remains basically unchanged (0.7V for silicon tubes and 0.3V for germanium tubes). Using this feature, as a limiting element in the circuit, the signal amplitude can be limited to a certain range.

4.  Freewheeling

Schottky diodes are connected in parallel at both ends of the line. When the current flowing in the coil disappears, the induced electromotive force generated by the coil is consumed by the work formed by the diode and the coil. This protects the safety of other components in the circuit. In the circuit, the two ends of the relay or the inductor are connected in an anti-parallel. When the inductor is powered off, the electromotive force at both ends does not disappear immediately. At this time, the residual electromotive force is released through a Schottky diode. Freewheeling diodes are used in inductor coils, relays, and thyristor circuits to prevent reverse breakdown.

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Figure 5. Freewheeling diodes circuit

Freewheeling diodes must be connected to both ends of the relay coil and the solenoid valve interface in the circuit. The connection method is as shown in the figure above. The negative pole of the Schottky diode is connected to the positive pole of the coil, and the positive pole of the Schottky diode is connected to the negative pole of the coil. However, you have to be clear that the freewheeling diode does not use the reverse voltage withstand characteristic of the Schottky diode, but uses the unidirectional forward conduction characteristic of the Schottky diode.

5.  Detection

Detection (also called demodulation) Schottky diodes use their unidirectional conductivity to extract low-frequency or audio signals from high-frequency or intermediate-frequency radio signals. They are widely used in semiconductor radios, radio cassette recorders, televisions, and communications. In the small-signal circuit of the device, its working frequency is high, and the amplitude of the processed signal is weak.

Detector Schottky diodes are used in electronic circuits to detect low-frequency signals (such as audio signals) modulated on high-frequency electromagnetic waves. In general, the high-frequency detection circuit uses a germanium point-contact type detection diode. Its junction capacitance is small, the reverse current is small, and the operating frequency is high.

Figure 6. demodulation

6.  Variable capacitance

Varactor Diodes, also called "variable reactance diodes," are made by using the characteristic that the junction capacitance changes with the applied voltage when the pN junction is reverse biased. When the reverse bias voltage increases, the junction capacitance decreases, and conversely, the capacitance of the varactor Schottky diode is generally small. The maximum value is from tens of picofarads to hundreds of picofarads. The ratio of maximum capacitance and minimum capacitance is approximately 5: 1. It is mainly used in high-frequency circuits for automatic tuning, frequency modulation, and equalization, for example, as a variable capacitor in the tuning loop of a television receiver.

When a forward bias voltage is applied, a large amount of current is generated, the depletion region of the PN (positive and negative electrode) junction is narrowed, the capacitance is increased, and a diffusion capacitance effect is generated; when a reverse bias voltage is applied, a transition capacitance effect is generated. However, due to the leakage current generated when the forward bias is applied, a reverse bias is applied in the application.

III Application of Schottky diodes in digital circuit

1. Schottky diode applied to dual power

At present, in the electronic design with the main controller, the real-time clock (RTC) is basically used. The RTC needs an extra button battery to supply power to prevent the time information from being lost after the system is powered off. At the same time, after the system is started, in order to extend the battery life, the main system is often powered. Therefore, RTCs often require dual power supplies, and diodes can provide power isolation due to their unidirectional conductance. Taking the small-signal Schottky diode BAT54C as an example, the maximum forward voltage drop is only 0.24v (at a forward current of 0.1ma), and the RTC current consumption is also ua level. After adding the Schottky diode isolated power supply, it can also fully meet the requirements.

Figure 7. Schottky diode applied to dual power

2. Schottky diodes used as AND gate

As shown in the figure below, n Schottky diodes form an n-input AND gate. As long as there is a signal output logic 0 in A1 ~ An, Output is logic 0. Only all signals in A1 ~ An output logic 1 and Output can be logic 1. That is, the AND of the signals A1 ~ An is realized. Because in the digital circuit, the signal input stage of the chip is basically high-impedance, the overall current of the AND circuit composed of Schottky diodes is ua level. The voltage drop of the Schottky diode is extremely small, and the ping can still meet the design requirements.

Figure 8. Schottky diodes used as AND gate

3.  Schottky diodes used as OR gate

As shown in the figure below, n Schottky diodes form an n-input OR gate. As long as there is a signal output logic 1 in A1 ~ An, Output will be logic 1. Only all signals in A1 ~ An output logic 0, Output can be logic 0. That is, the phase OR of the signals A1 ~ An is realized.

Figure 9. Schottky diodes used as OR gate

4. Schottky diode application examples

In digital circuit design, it is often necessary to implement simple phase AND, phase OR, or phase inversion of some signals. If you directly use logic chips such as 74 series, not only the layout area is greatly increased, but also the wiring is not flexible. And the use of small-signal Schottky diodes to form the AND and Or gate circuit will be more flexible and easy to use. The figure below is a simple two-way reset circuit. JTAG needs to reset the master controller to generate a reset signal, and the external reset key needs to be reset when the master controller is pressed. If JTAG reset and key reset are wired directly to the reset pin of the host controller, it may cause damage to the JTAG emulator. For example, when the reset key is pressed, the JTAG output reset pin will be directly low. And through the Schottky diode BAT54A constitutes the phase and circuit, each signal output will not affect each other. As shown in the figure below, as long as JTAG outputs 0 logic or key reset output 0 logic, the master controller can be reset.

Figure 10. simple two-way reset circuit

IV Conclusion

The structure and characteristics of Schottky diodes make them suitable for high-frequency rectification in low-voltage, high-current output, etc., and for detection and mixing at high frequencies. Schottky diodes are used as clamps in high-speed logic circuits. Schottky diodes are also commonly used in ICs and are widely used in high-speed computers. In addition to the characteristics of ordinary PN junction diodes, the electrical parameters of Schottky diodes for detection and mixing also include intermediate frequency impedance, which refers to the impedance presented by the Schottky diode to the specified intermediate frequency when the rated local oscillator power is applied.

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