An in-depth analysis of Schottky diodes

What is a Schottky diode? What precautions does it take? The Schottky diode is named after its inventor, Dr. Schottky. SBD is the abbreviation of Schottky Barrier Diode (abbreviated as SBD). SBD is not made by using the principle of PN junction formed by contact between P-type semiconductor and N-type semiconductor, but by the principle of metal-semiconductor junction formed by contact between metal and semiconductor. Therefore, SBD is also called a metal-semiconductor (contact) diode or surface barrier diode, which is a hot carrier diode.

Advantage:

SBD has the advantages of high switching frequency and forward voltage drop, but its reverse breakdown voltage is relatively low, mostly no 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 supplies (SMPS) and power factor correction (PFC) circuits, high-frequency rectifier diodes above 100V for transformer secondary, high-speed diodes of 600V to 1.2kV used in RCD buffer circuits, and PFC boost uses 600V diodes, etc. 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 SMPS of 1MHz to 3MHz 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 radiator is required, which increases the volume and weight of the SMPS and is not suitable for miniaturization and thinning. development trend. Therefore, the development of high-voltage SBD 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 launched, and SBDs exceeding 1kV made of new materials have also been successfully developed, thus injecting new vitality into their applications.

Disadvantages:

The biggest disadvantages of Schottky diodes are 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 rated withstand voltage. to 50V, and the reverse leakage current value has positive temperature characteristics and is easy to increase rapidly as the temperature increases. In practical design, attention should be paid to the hidden concern of thermal runaway. In order to avoid the above 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 improved, and its reverse bias voltage rating can reach a maximum of 200V.

structure:

The structure and materials of the new high-pressure SBD are different from the traditional SBD. Traditional SBD is composed of metal and semiconductor contacts. Metal materials can be aluminum, gold, molybdenum, nickel and titanium, etc., and semiconductors are usually silicon (Si) or gallium arsenide (GaAs). Since the mobility of electrons is greater than that of 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 SBD and increase the reverse breakdown voltage without making the series resistance too large, a high-resistance N- thin layer is usually epitaxially grown on the N+ substrate.

Package:

Schottky diodes are divided into two packaging forms: leaded and surface mount (SMD). Schottky diodes in leaded packages are usually used as high-frequency and high-current rectifier diodes, freewheeling diodes or protection diodes. It has two packaging forms: single tube type and double tube (double diode) type. Schottky pairs of tubes have three pinout methods: common cathode (the cathodes of the two tubes are connected), common anode (the anodes of the two tubes are connected), and series connection (the anode of one diode is connected to the cathode of another diode).

Features:

The main advantages of SBD include two aspects:

1) Since the Schottky barrier height is lower than the PN junction barrier height, its forward conduction threshold voltage and forward voltage drop are both lower than that of the PN junction diode (about 0.2V lower).

2) Since SBD is a majority carrier conducting 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. Since the reverse recovery charge of SBD is very small, the switching speed is very fast and the switching loss is also very small, which is especially suitable for high-frequency applications.

However, since the reverse barrier of SBD is thin and breakdown easily occurs on its surface, the reverse breakdown voltage is relatively low. Since SBD is more susceptible to thermal breakdown than PN junction diodes, the reverse leakage current is larger than that of PN junction diodes.

effect:

Schottky diode Schottky diode, also known as Schottky barrier diode (SBD for short), is a low-power, ultra-high-speed semiconductor device. The most notable feature is that the reverse recovery time is extremely short (can be as small as a few nanoseconds) and the forward voltage drop is only about 0.4V. They are mostly used as high-frequency, low-voltage, high-current rectifier diodes, freewheeling diodes, and protection diodes. They are also used as rectifier diodes and small-signal detection diodes in microwave communication and other circuits. It is common in communication power supplies, frequency converters, etc.

A typical application is in the switching circuit of a bipolar transistor BJT. By connecting a Shockley diode to the BJT for clamping, the transistor is actually in a very close to off state when it is in the on state, thereby increasing the switching speed of the transistor. This method is the technology 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. Under the same current condition, its forward voltage drop is much smaller. Plus it has a short recovery time. It also has some disadvantages: lower withstand voltage and slightly larger leakage current. Consider all aspects when choosing.

working principle:

Schottky diode is a metal-semiconductor device made of noble metal (gold, silver, aluminum, platinum, etc.) A as the positive electrode and N-type semiconductor B as the negative electrode. The barrier formed on the contact surface between the two has rectifying characteristics. Because there are a large number of electrons in N-type semiconductors and only a very small amount of free electrons in noble metals, electrons diffuse from B with a high concentration to A with a low concentration. 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 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 to B, thereby weakening the electric field formed by 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 above is an in-depth analysis of Schottky diodes, I hope it will be helpful to everyone.