1
Physical properties and characteristics of SiC materials
SiC (Silicon Carbide) is a compound semiconductor material composed of Si (silicon) and C (carbon). The critical breakdown field strength of SiC is 10 times that of Si, the band gap is 3 times that of Si, and the thermal conductivity is 3 times that of Si, so it is considered a power device material that exceeds the limit of Si. There are various crystal forms in SiC, and their physical properties are also different. Among them, 4H-SiC is most suitable for the production of power devices. In addition, SiC is the only compound semiconductor that can be thermally oxidized to form SiO2, so it is suitable for preparing MOS-type power devices.
2
Features of power devices
The critical breakdown field strength of SiC is 10 times that of Si. Therefore, compared with Si devices, a high-voltage power device can be made with a drift layer with a higher impurity concentration and a thinner thickness. The on-resistance of high withstand voltage power devices is mainly derived from the resistance of the drift layer. Therefore, high withstand voltage devices with very low on-resistance per unit area can be obtained by using SiC. Theoretically, for devices with the same withstand voltage, the drift layer resistance per unit area of ​​SiC can be reduced to 1/300 of that of Si. In Si materials, in order to improve the on-resistance increase caused by higher withstand voltage, minority carrier devices such as IGBT (Insulated Gate Bipolar Transistor: Insulated Gate Bipolar Transistor) are mainly used. Device), but there is a problem of large switching loss, and the result is that the resulting heat will limit the high-frequency drive of the IGBT. However, SiC materials can achieve high withstand voltage with majority carrier devices (Schottky barrier diodes and MOSFETs) of high-frequency device structure, thereby simultaneously achieving "high withstand voltage", "low on-resistance", and "high frequency" "These three characteristics. In addition, the band gap is wider, three times that of Si, so SiC power devices can work stably even at high temperatures.
3
SiC MOSFET features
a
Device structure and characteristics
The higher the withstand voltage device in the Si material, the larger the on-resistance per unit area (increased by the ratio of about 2 to 2.5 power of the withstand voltage value), so IGBT (Insulated Gate Bipolar) is mainly used for voltage above 600V. Type transistor). IGBT injects holes as minority carriers into the drift layer through conductivity modulation, so the on-resistance is smaller than MOSFET, but at the same time, due to the accumulation of minority carriers, tail current will be generated during Turn-off. This causes great switching losses. The drift layer resistance of SiC devices is lower than that of Si devices, and high withstand voltage and low on-resistance can be achieved with MOSFET without conducting conductance modulation. Moreover, MOSFET does not generate tail current in principle, so when SiC-MOSFET is used to replace IGBT, switching loss can be significantly reduced, and the miniaturization of heat dissipation components can be realized. In addition, SiC-MOSFETs can be driven under high-frequency conditions where IGBTs cannot work, thereby enabling miniaturization of passive devices. Compared with Si-MOSFETs of 600V to 900V, SiC-MOSFETs have the advantages of small chip area (small package can be realized) and the recovery loss of body diode is very small. Mainly used in industrial machine power supply, inverter or converter of high efficiency power conditioner.
b
Standardized on resistance
The dielectric breakdown field strength of SiC is 10 times that of Si, so it can achieve high withstand voltage with a low-impedance, thin drift layer. Therefore, under the same withstand voltage value, SiC can obtain devices with lower on-resistance per unit area. For example, at 900V, the chip size of SiC-MOSFET only needs one-third of Si-MOSFET and one-tenth of SJ-MOSFET to achieve the same on-resistance. Not only can low on-resistance be achieved in a small package, but also the gate charge Qg and junction capacitance can be reduced. SJ-MOSFET has only 900V products, but SiC can easily achieve a withstand voltage above 1700V with a very low on-resistance. Therefore, there is no need to adopt a bipolar device structure such as IGBT (the on-resistance becomes lower, the switching speed becomes slower), and a device with low on-resistance, high withstand voltage, and fast switching can be realized.
c
Vd-Id characteristics
Unlike IGBT, SiC-MOSFET has no turn-on voltage, so low conduction loss can be achieved in a wide current range from small current to large current. On the other hand, the on-resistance of Si-MOSFET rises to more than twice that at room temperature at 150°C. Unlike Si-MOSFET, SiC-MOSFET has a relatively low rise rate, so it is easy to thermally design, and the on-resistance at high temperatures is also very high. low.
d
Drive gate voltage and on-resistance
The drift layer impedance of SiC-MOSFET is lower than that of Si-MOSFET, but on the other hand, according to the current technology level, the mobility of the MOS channel part of SiC-MOSFET is relatively low, so the impedance of the channel part is higher than that of Si devices. Therefore, the higher the gate voltage, the lower the on-resistance can be obtained (Vgs=20V or more, then gradually saturated).
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