Schottky diode is named after its inventor, Dr. Schottky (Schottky), and SBD is the abbreviation of Schottky Barrier Diode (Schottky Barrier Diode, abbreviated as SBD). SBD is not made by using the principle of contacting P-type semiconductor and N-type semiconductor to form PN junction, but using the principle of metal-semiconductor junction formed by contacting metal and semiconductor. Therefore, SBD is also called metal-semiconductor (contact) diode or surface barrier diode, which is a kind of hot carrier diode.
1 Principle of Schottky diode
A Schottky diode is a metal-semiconductor device made of a noble metal (gold, silver, aluminum, platinum, etc.) A as the positive electrode and an N-type semiconductor B as the negative electrode, and the potential barrier formed on the contact surface of the two has rectification characteristics. Because there are a large number of electrons in the N-type semiconductor and only a small amount of free electrons in the noble metal, the electrons diffuse from B with high concentration to A with low concentration. Obviously, there are no holes in metal A, and 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, thus forming a potential barrier, and its electric field direction is B→A. However, under the action of the electric field, the electrons in A will also produce a drift motion from A→B, thus weakening the electric field formed due to the diffusion motion. When a space charge region of a certain width is established, the electron drift movement caused by the electric field and the electron diffusion movement 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 substrate, on which an N-epitaxial layer with arsenic as a dopant is formed. The anode uses materials such as molybdenum or aluminum to make the barrier layer. Silicon dioxide (SiO2) is used to eliminate the electric field in the edge area and improve the withstand voltage value 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 under 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 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; 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.
To sum up, the structure principle of the Schottky rectifier is very different from that of the PN junction rectifier. Usually, the PN junction rectifier is called a junction rectifier, and the metal-semiconductor rectifier is called a Schottky rectifier. , Aluminum-silicon Schottky diodes manufactured by silicon planar technology have also come out, which can not only save precious metals, greatly reduce costs, but also improve the consistency of parameters.
2 Advantages of Schottky diodes
SBD has the advantages of high switching frequency and low forward voltage, but its reverse breakdown voltage is relatively low, mostly not higher than 60V, and the highest is only about 100V, which limits its application range. For example, freewheeling diodes of power switching devices in switching power supply (SMPS) and power factor correction (PFC) circuits, high-frequency rectifier diodes above 100V for transformer secondary, high-speed diodes of 600V ~ 1.2kV in RCD snubber circuits, and For 600V diodes used in PFC step-up, only fast recovery epitaxial diodes (FRED) and ultra-fast recovery diodes (UFRD) are used.
The reverse recovery time Trr of UFRD is also more than 20ns, which cannot meet the needs of SMPS with 1MHz to 3MHz in fields such as space stations. Even for an SMPS with hard switching at 100kHz, due to the large conduction loss and switching loss of UFRD, the case temperature is high, and a large heat sink is required, which increases the size and weight of the SMPS, which does not meet the miniaturization and thinning requirements. development trend. Therefore, the development of high-voltage SBDs above 100V has always been a research topic and a focus of attention. In recent years, SBD has made breakthrough progress, 150V and 200V high-voltage SBDs have been listed, and the SBD of more than 1kV made of new materials has also been successfully developed, thus injecting new vitality and vitality into its application.
3 Disadvantages of Schottky diodes
The biggest disadvantage of Schottky diodes is their low reverse bias voltage and large reverse leakage current. For Schottky diodes made of silicon and metal, their reverse bias rated withstand voltage is only up to 50V, while The reverse leakage current value has a positive temperature characteristic, and it tends to increase rapidly as the temperature rises. The physical design should pay attention to the hidden danger of thermal runaway. In order to avoid the above problems, the reverse bias voltage of Schottky diodes in actual use will be much smaller than its rated value. However, Schottky diode technology has also improved, and its reverse bias voltage rating can reach a maximum of 200V.
4 Differences between Schottky diodes and ordinary diodes
The biggest feature of Schottky diodes is small forward voltage drop and short reverse recovery time. Schottky diodes have low turn-on voltage and small charge storage effect, making them suitable for high-frequency operation. Under the same current condition, its forward voltage drop is much smaller than that of ordinary diodes. It also has the characteristics of low loss, low noise, high detection sensitivity, stability and reliability, etc. It is used in microwave communication and radar for frequency mixing, detection, modulation, frequency multiplication, ultra-high-speed switching, and low-noise amplification.
Therefore, the most obvious differences between Schottky diodes and ordinary diodes are as follows:
1. Schottky diodes have a lower forward voltage drop than ordinary diodes, so they consume less power.
2. The reverse recovery time of Schottky diodes is shorter than that of ordinary diodes, so the operating frequency is higher.
3. The reverse withstand voltage of Schottky diodes is lower than that of ordinary diodes, generally lower than 200V.
4. Schottky diodes pass a stronger current than ordinary diodes.
5. Schottky diodes have smaller junction capacitance than ordinary diodes.
6. Schottky diodes can pass high frequency current.