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Schottky diodes

Schottky diode is named after its inventor Dr. Schottky (Schottky), also known as Schottky barrier diode ((abbreviated as SBD). SBD is not made using the principle of forming a PN junction by contacting a P-type semiconductor with an N-type semiconductor, but is made using the principle of a metal-semiconductor junction formed by contacting a metal with a semiconductor. Therefore, SBD is also called metal-semiconductor (contact) diode or surface barrier diode, it is a kind of hot carrier diode.

Principle

A Schottky diode is a metal-semiconductor device made of precious metal (gold, silver, aluminum, platinum, etc.), A as the positive electrode, and N-type semiconductor B as the negative electrode, using the barrier formed on the contact surface of the two to have rectifying characteristics. Because there are a large number of electrons in the N-type semiconductor, there are only a few free electrons in the precious metal, so 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 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 surface electrical neutrality is destroyed, so a potential barrier is formed, and the direction of the electric field is B→A. However, under the action of this electric field, the electrons in A will also drift from A→B, thus weakening the electric field formed by the diffusion motion. When a space charge region with a certain width is established, the electron drift motion caused by the electric field and the electron diffusion motion caused by different concentrations reach a relative balance, and a Schottky barrier is formed.

The internal circuit structure of a typical Schottky rectifier tube is based on an N-type semiconductor, on which an N-epitaxial layer using arsenic as a dopant is formed. The anode uses molybdenum or aluminum as the barrier layer. Use silicon dioxide (SiO2) to eliminate the electric field in the edge area and improve the pressure resistance of the tube. The N-type substrate has a very small on-state resistance, and its doping concentration is 100 times higher than that of the H-layer. 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 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 across the Schottky barrier, the Schottky barrier layer becomes wider and its internal resistance becomes larger.

In summary, the structural principle of Schottky rectifiers is very different from PN junction rectifiers. Usually PN junction rectifiers are called junction rectifiers, and metal-semiconductor rectifiers are called Schottky rectifiers. Aluminum-silicon Schottky diodes manufactured using silicon planar technology already developed, which not only saves precious metals, greatly reduces costs, but also improves the consistency of parameters.

Advantage

1) High switching frequency. Since SBD is a majority carrier conductive device, there are no minority carrier lifetime and reverse recovery problems. The reverse recovery time of SBD is only the charging and discharging time of Schottky barrier capacitor, which is completely different from the reverse recovery time of PN junction diode. Because the reverse recovery charge of SBD is very small, the switching speed is very fast, and the switching loss is also particularly small, which is especially suitable for high frequency applications.

2) Low forward voltage drop. Because the Schottky barrier height is lower than the PN junction barrier height, so its forward conduction threshold voltage and forward voltage drop are lower than the PN junction diode (about 0.2V lower).

Disadvantages

The biggest disadvantage of Schottky diodes is their low reverse bias voltage and large reverse leakage current. For example the Schottky diodes using silicon and metal material, their reverse bias voltage rating is only up to 50V, while the reverse leakage current value is a positive temperature characteristic, which tends to increase rapidly as the temperature rises. In practical design, it is necessary to pay attention to the hidden worry of thermal runaway. 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 been improved, and its reverse bias voltage rating can be up to 200V.

Application

SBD is a low-power, ultra-high-speed semiconductor device, the most notable feature is the extremely short reverse recovery time (which can be as small as a few nanoseconds), and the forward voltage drop is only about 0.4V. Widely used in switching power supplies, inverters, drivers and other circuits, used as high-frequency, low-voltage, high-current rectifier diodes, freewheeling diodes, protection diodes, or in microwave communications, etc. used as rectifier diode and small signal detection diode in the circuit.

The structure and characteristics of SBD make it suitable for high-frequency rectification in low-voltage, 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 also often used in ICs, and SBD TTL integrated circuits have long been the mainstream of TTL circuits and are widely used in high-speed computers.

In addition to the characteristic parameters of ordinary PN junction diodes, the SBD electrical parameters used for detection and mixing also include the 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 coefficient.