Application of SiC Schottky diode
Source:未知Author:admin Addtime:2020-02-03 11:16:42 Click:
SiC Schottky diodes can be widely used in switching power supplies, power factor correction (PFC) circuits, uninterruptible power supplies (UPS), photovoltaic inverters and other medium and high power fields, which can significantly reduce circuit losses and increase the operating frequency of circuits . Using silicon carbide SBD to replace the original silicon FRD in the PFC circuit can make the circuit work above 300kHz, and the efficiency remains basically unchanged, while the efficiency of the circuit using silicon FRD above 100kHz drops sharply. With the increase of the operating frequency, the volume of passive components such as inductors has decreased accordingly, and the volume of the entire circuit board has decreased by more than 30%.
The band gap of silicon carbide is 2.8 times (wide band gap) of silicon, reaching 3.09 electron volts. Its dielectric breakdown field strength is 5.3 times that of silicon, up to 3.2 MV / cm. Its thermal conductivity is 3.3 times that of silicon, which is 49 w / cm.k. Schottky diodes and MOS field-effect transistors made of silicon carbide have a thickness that is an order of magnitude thinner than that of silicon devices with the same withstand voltage. Its impurity concentration can be two orders of magnitude of silicon. Therefore, the impedance per unit area of a silicon carbide device is only one-tenth of that of a silicon device. Its drift resistance is almost equal to the total resistance of the device. Therefore, the heat generation of the silicon carbide device is extremely low. This helps reduce conduction and switching losses, and the operating frequency is generally more than 10 times higher than that of silicon devices. In addition, silicon carbide semiconductors have inherently strong resistance to radiation.
In recent years, power devices such as IGBTs (Insulated Gate Bipolar Transistors) made of silicon carbide materials have been able to use processes such as minority carrier injection to reduce their on-state impedance to one-tenth of ordinary silicon devices. Coupled with the small amount of heat generated by the silicon carbide device itself, the thermal conductivity of the silicon carbide device is excellent. In addition, silicon carbide power devices can work normally at a high temperature of 400 ° C. It can use a small device to control a large current. The operating voltage is also much higher.
What is the current development of silicon carbide devices?
1. Technical parameters: For example, the Schottky diode voltage is increased from 250 volts to more than 1,000 volts, the chip area is small, but the current is only tens of amps. The working temperature is increased to 180 ℃, which is far from the introduction of 600 ℃. The voltage drop is even more unsatisfactory. There is no difference with silicon materials. The high forward voltage drop must reach 2V.
2. Market price: about 5 to 6 times of silicon material manufacturing.
Where are the challenges in the development of silicon carbide (SiC) devices?
According to various reports, the problem is not the chip's principle design, especially the chip structure design is not difficult to solve. It is difficult to realize the manufacturing process of the chip structure.
1. Microtube defect density of silicon carbide wafers. Microtubules are a kind of macroscopic defects that can be seen by the naked eye. Before the development of silicon carbide crystal growth technology can completely eliminate the defects of microtubules, it is difficult to manufacture high-power power electronic devices with silicon carbide. Although the microtube density of high-quality wafers has reached a level not exceeding 15 cm-2. However, device manufacturing requires silicon carbide crystals with a diameter of more than 100mm, and the microtube density is lower than 0.5cm-2.
2. The epitaxial process has low efficiency. The homogeneous epitaxy of silicon carbide is generally performed at a high temperature of 1500 ° C or higher. Due to the problem of sublimation, the temperature cannot be too high, and generally cannot exceed 1800 ° C, so the growth rate is low. The liquid phase epitaxial temperature is lower and the rate is higher, but the yield is lower.
3. The doping process has special requirements. If the diffusion method is used, the diffusion temperature of silicon carbide is much higher than that of silicon. At this time, the masking SiO2 layer has lost the masking effect, and the silicon carbide itself is unstable at such a high temperature, so it is not appropriate to use diffusion method for doping. And doping with ion implantation. If p-type ion implanted impurities use aluminum. Since the aluminum atoms are much larger than the carbon atoms, the damage to the crystal lattice and the impurities in an inactive state are more serious, and they are often performed at a relatively high substrate temperature and annealed at a higher temperature. This brings about the problems of silicon carbide decomposition and silicon sublimation on the wafer surface. At present, there are still many problems with p-type ion implantation, and a series of process parameters from impurity selection to annealing temperature still need to be optimized.
4. Production of ohmic contacts. Ohmic contact is a very important process for device electrodes. To manufacture metal electrodes on silicon carbide wafers, the contact resistance is required to be lower than 10-5 Ωcm2, and the electrode materials can be achieved with Ni and Al, but the thermal stability is poor at temperatures above 100 ° C. The Al / Ni / W / Au composite electrode can be used to improve the thermal stability to 600 ° C and 100 hours, but its contact specific resistance is as high as 10-3Ωcm2. Therefore, it is difficult to form a good ohmic contact with silicon carbide.
5. Temperature resistance of supporting materials. The silicon carbide chip can work at a temperature of 600 ° C, but the supporting materials may not be able to withstand this high temperature. For example, electrode materials, solder, housings, insulating materials, etc. all limit the increase in operating temperature.
The above are just a few examples, not all. There are still many process problems that do not have an ideal solution, such as the silicon carbide semiconductor surface trenching process, terminal passivation process, and the interface state of the gate oxygen layer on the long-term stability of silicon carbide MOSFET devices. Consistent conclusions, etc., have greatly hindered the rapid development of silicon carbide power devices.
Why are SIC devices not yet popular?
As early as the 1960s, the advantages of silicon carbide devices were well known. The reason why it has not yet been popularized is because there are many technical problems, including manufacturing. Until now, the industrial applications of SIC materials have been mainly used as abrasives (corundum).
SIC does not melt within the controllable pressure range, but directly changes to gaseous state at a sublimation point of about 2500 ° C. Therefore, the growth of SIC single crystal can only start from the gas phase. This process is much more complicated than the growth of SIC. SI melts at about 1400 ° C. The main obstacle preventing SIC technology from achieving commercial success is the lack of a suitable substrate material for the industrial production of power semiconductor devices. In the case of SI, a single crystal substrate is often referred to as a wafer, which is a prerequisite and guarantee for production. A method for growing a large area SIC substrate was successfully developed in the late 1970s. But a substrate grown using an improved method called Lely is plagued by a microtubule defect.
As long as a microtube passes through a high-voltage PN junction, the ability of the PN junction to block the voltage is destroyed. In the past three years, the density of such defects has decreased from tens of thousands per square millimeter to dozens. In addition to this improvement, when the maximum size of the device is limited to a few square millimeters, the production yield may be greater than a few percent, so that the maximum rated current of each device is several amps. Therefore, before the successful commercialization of SIC power devices, more technical improvements must be made to the substrate materials of SIC.