What is the working principle and application of Schottky diodes? — SUNMATE

What is the working principle and application of Schottky diodes?

Schottky diode is a metal semiconductor device made from precious metals (gold, silver, aluminum, platinum, etc.) A as the positive electrode and N-type semiconductor B as the negative electrode, utilizing the rectification characteristics of the potential barrier formed on the contact surface between the two. Due to the presence of a large number of electrons in N-type semiconductors and only a very small number of free electrons in precious metals, electrons diffuse from high concentration B to low concentration A. Obviously, there are no holes in metal A, so there is no diffusion motion 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 electroneutrality is disrupted, forming a potential barrier with an electric field direction of B → A. But under the action of this electric field, the electrons in A will also experience drift motion from A to B, thereby weakening the electric field formed by diffusion motion. When a certain width of space charge region is established, the electron drift motion caused by the electric field and the electron diffusion motion caused by different concentrations reach a relative equilibrium, forming a Schottky barrier.

The internal circuit structure of a typical Schottky rectifier tube is based on an N-type semiconductor substrate, on which an N-epitaxial layer with arsenic as a dopant is formed. The anode is made of materials such as molybdenum or aluminum to form a barrier layer. Using silicon dioxide (SiO2) to eliminate the electric field in the edge region and improve the withstand voltage of the tube. The N-type substrate has a very small on state resistance, and its doping concentration is 100% higher than that of the H-layer. An N+cathode layer is formed below the substrate 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 voltage is applied at 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 narrows and its internal resistance decreases; On the contrary, if a reverse bias is applied at both ends of the Schottky barrier, the Schottky barrier layer widens and its internal resistance increases.

In summary, the structural principle of Schottky rectifier tubes is significantly different from that of PN junction rectifiers. PN junction rectifiers are commonly referred to as junction rectifiers, while metal semi conductor rectifiers are referred to as Schottky rectifiers. Aluminum silicon Schottky diodes manufactured using silicon planar technology have also been introduced, which not only saves precious metals, significantly reduces costs, but also improves parameter consistency.

Schottky diode applications

The structure and characteristics of SBD make it suitable for high-frequency rectification in low voltage and high current output fields, detection and mixing at very high frequencies (such as X-band, C-band, S-band, and Ku band), and clamping in high-speed logic circuits. SBD is also commonly used in ICs, such as 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, the electrical parameters of SBD used for detection and mixing also include intermediate frequency impedance (referring to the impedance presented by SBD to the specified intermediate frequency when applying rated local oscillator power, generally between 200 Ω and 600 Ω), voltage standing wave ratio (generally ≤ 2), and noise coefficient.

Schottky diode effect

Schottky diode, also known as Schottky barrier diode (SBD), is a low-power, ultra-fast semiconductor device. The most significant feature is the extremely short reverse recovery time (which can be as small as a few nanoseconds), and the forward conduction voltage drop is only about 0.4V. It is commonly used as a high-frequency, low-voltage, high current rectifier diode, freewheeling diode, protective diode, and also useful as a rectifier diode and small signal detector diode in microwave communication circuits. It is common in communication power supplies, frequency converters, etc.

A typical application is in the switching circuit of a bipolar transistor BJT, where a Shockley diode is connected to the BJT to clamp it, resulting in the transistor being in a very close to cut-off state when in a conduction state, thereby improving the switching speed of the transistor. This method is a technique used in TTL internal circuits of typical digital ICs such as 74LS, 74ALS, and 74AS.

The biggest characteristic of Schottky diodes is that the forward voltage drop VF is relatively small. At the same current, its forward voltage drop is much smaller. Additionally, its recovery time is short. It also has some drawbacks: low voltage resistance and slightly higher leakage current. When selecting, comprehensive consideration should be given.