Schottky diodes use Schottky to block the reverse voltage on the metal or semiconductor contact surface, so that the current can conduct unidirectionally. Different from the traditional diode, the structure of Schottky and PN junction is very different. Fast recovery diode, as the name implies, is a semiconductor diode that can quickly recover the reverse time. This paper will introduce the differences between Schottky diode and fast recovery diode from the aspects of structure and performance characteristics.
Schottky diode
It is a diode of "metal semiconductor junction" with Schottky characteristics. Its forward start voltage is low. In addition to tungsten, the metal layer can also be made of gold, molybdenum, nickel, titanium and other materials. The semiconductor material is silicon or gallium arsenide. This kind of device is conductive by most carriers, so its reverse saturation current is much larger than that of PN junction conductive by few carriers. Because the storage effect of minority carriers in Schottky diode is very small, its frequency response is only limited by RC time constant, so it is an ideal device for high frequency and fast switching. Its working frequency can reach 100GHz. In addition, MIS (metal insulator semiconductor) Schottky diodes can be used to make solar cells or light emitting diodes.
Structural principle
To sum up, the structural principle of Schottky rectifier is very different from that of PN Junction Rectifier. PN junction rectifier is usually called Junction Rectifier, while metal half tube rectifier is called Schottky rectifier. In recent years, Aluminum Silicon Schottky diodes manufactured by silicon planar technology have also been developed, which not only saves precious metals, greatly reduces costs, but also improves the consistency of parameters.
The Schottky rectifier uses only one carrier (electron) to transport charge, and there is no accumulation of excess minority carriers outside the potential barrier. Therefore, there is no charge storage problem (qrr → 0), and the switching characteristics are significantly improved. The reverse recovery time can be shortened to less than 10ns. However, its reverse withstand voltage value is relatively low, generally not more than 100V. Therefore, it is suitable to work under low voltage and high current. The efficiency of low-voltage and large current rectifying (or freewheeling) circuit can be improved by using the characteristics of low-voltage drop.
Fast recovery diode
Fast recovery diodes refer to diodes with short reverse recovery time (below 5us). Gold doping measures are adopted in the process. Some of the structures adopt PN junction structure and some adopt improved pin structure. Its forward voltage drop is higher than that of ordinary diodes (1-2v), and the reverse voltage withstand is mostly below 1200V. In terms of performance, it can be divided into two levels: fast recovery and ultra fast recovery. The reverse recovery time of the former is hundreds of nanoseconds or longer, while the latter is less than 100 nanoseconds.
Schottky diode is a diode based on the potential barrier formed by the contact between metal and semiconductor. It is referred to as Schottky diode for short. It has a forward voltage reduction (0.4-1.0v), a reverse recovery time (0-10 nanoseconds), a large reverse leakage current and a low withstand voltage, generally lower than 150V. It is mainly used in low-voltage occasions.
Schottky diodes and fast recovery diodes are commonly used in switching power supplies. The difference is that the recovery time of the former is about 100 times smaller than that of the latter, and the reverse recovery time of the former is about several nanoseconds. The former has the advantages of low power consumption, high current and ultra-high speed.
In the manufacturing process of the fast recovery diode, gold doping, simple diffusion and other processes are adopted, which can obtain high switching speed and high withstand voltage. At present, fast recovery diodes are mainly used as rectifying elements in inverter power supply.
Reverse recovery time
What is reverse recovery time? When the voltage of the external diode transients from the forward direction to the reverse direction, the current flowing through the device cannot be transiently converted from the forward current to the reverse current. At this time, the minority carriers (holes) injected in the forward direction are extracted by the strong electric field of the space charge region. Since the density of these holes is higher than the balanced hole density of the base region, a reverse current much larger than the reverse leakage current will be generated at the reverse bias moment, that is, the reverse recovery current IRM. At the same time, the enhancement of the coincidence process also accelerates the decrease of these additional carriers density. Until the additional carriers accumulated in the base region completely disappear, the reverse current decreases and stabilizes to the reverse leakage current. The time taken by the whole process is the reverse recovery time.
The reverse recovery time TRR is defined as the time interval during which the current passes through the zero point from the forward direction to the specified low value. It is an important technical index to measure the performance of high-frequency freewheeling and rectifier devices.
Structure characteristics of fast recovery and ultra fast recovery diodes
The internal structure of the fast recovery diode is different from that of the ordinary diode. It adds the base region I between the p-type and n-type silicon materials to form the p-i-n silicon chip. Because the base region is very thin and the reverse recovery charge is very small, not only the TRR value is greatly reduced, but also the transient forward voltage drop is reduced, so that the tube can withstand a high reverse working voltage. The reverse recovery time of the fast recovery diode is generally several hundred nanoseconds, the forward voltage drop is about 0.6V, the forward current is several amperes to several thousand amperes, and the reverse peak voltage can reach several hundred to several thousand volts. The reverse recovery charge of the ultrafast recovery diode is further reduced, making its TRR as low as several tens of nanoseconds.
Most of the fast recovery and ultra fast recovery diodes below 20A are in the form of TO-220 package. From the perspective of internal structure, it can be divided into single pipe and opposite pipe (also known as double pipe). There are two fast recovery diodes inside the pair of tubes. According to the different connection methods of the two diodes, there are two types of common cathode to tube and common anode to tube. The fast recovery diodes with tens of AMPs are generally packaged in to-3p metal case. The tubes with larger capacity (several hundred a to several thousand a) are packaged in the form of bolt type or plate type.
test method
Routine test method
Under amateur conditions, the multimeter can detect the unidirectional conductivity of fast recovery and ultra fast recovery diodes, as well as whether there are open circuit and short circuit faults inside, and can measure the forward conduction voltage drop. If equipped with a megger, it can also measure the reverse breakdown voltage.
Example: measure an ultrafast recovery diode, and its main parameters are: TRR = 35ns, if = 5a, IFSM = 50a, VRM = 700V. Turn the multimeter to R × In gear 1, the read forward resistance is 6.4l, n = 19.5l. The reverse resistance is infinite. Further, VF = 0.03V is obtained/ × 19.5=0.585V。 Prove that the pipe is good.
matters needing attention:
Some single tubes have three pins in total, and the middle pin is an empty pin, which is usually cut off when leaving the factory, but some are not cut off;
If one of the pipes is damaged, it can be used as a single pipe;
R must be used when measuring the through pressure drop × 1st gear. If R is used × At 1K gear, because the test current is too small, which is far lower than the normal working current of the pipe, the measured VF value will be significantly lower. In the above example, if R is selected × Measured at 1K gear, the forward resistance is equal to 2.2k, and at this time, n = 9 grid. The calculated VF value is only 0.27v, which is far lower than the normal value (0.6V);
The recovery time of the fast recovery diode is 200-500ns;
The recovery time of the ultrafast diode is 30-100ns;
The recovery time of Schottky diode is about 10ns;
Moreover, their forward on voltage is also different. Schottky < fast recovery < high efficiency.
