2017.12.07 SiC Power Device
The last time, we explained differences with IGBTs. This time, we will discuss the forward characteristics and reverse recovery characteristics of the body diodes of SiC-MOSFETs.
MOSFETs, whether SiC-MOSFETs or otherwise, have a body diode between the drain and the source, as indicated in the diagram. As a consequence of the MOSFET structure, the body diode is formed by the pn junction between the source and drain, and is also called a parasitic diode or an internal diode. The performance of the body diode is one important parameter of the MOSFET, and is important when using the MOSFET in an application.
Forward Characteristics of the Body Diode of a SiC-MOSFET
The graph below shows the Vds-Id characteristics of a SiC-MOSFET. With the source as reference, a negative voltage is applied to the drain, and the body diode is in a forward-biased state. In the graph, the green trace for which Vgs = 0 V shows what is essentially the Vf characteristics of the body diode. Vgs is 0 V, that is, the MOSFET is in the off state and no channel current is flowing, and so under these conditions the Vd-Id characteristics can be said to be the Vf-If characteristics of the body diode. As was explained in the section “What is silicon carbide?”, SiC has a large band gap, and Vf is extremely high compared with that of Si-MOSFETs.
On the other hand, when 18 V is applied across the gate and source so that the SiC MOSFET is turned on, the current flowing in the channel with lower resistance is dominant, instead of the body diode. Below are shown MOSFET cross-sectional diagrams to aid understanding of the structural aspects of these different states.
Reverse Recovery Characteristics of the Body Diode of a SiC-MOSFET
Another important characteristic of the body diode of a MOSFET is the reverse recovery time (trr). It was previously explained in the section on SiC Schottky Barrier Diodes that trr is an important parameter relating to the diode switching characteristics. Of course because the body diode of a MOSFET has a pn junction, there is a reverse recovery phenomenon, and this appears as the reverse recovery time (trr). The trr characteristics of a 1000 V rated Si-MOSFET and of the SCT2080KE SiC-MOSFET are compared below.
As we see, the trr of the Si-MOSFET in this example is long, and a large current Irr flows. In contrast, the body diode of the SCT2080KE SiC-MOSFET is extremely fast. Both trr and Irr are so small as to be negligible, and the recovery loss Err is greatly reduced.
・The forward characteristic Vf of the body diode of a SiC-MOSFET is high compared with that of an Si-MOSFET.
・The trr of a SiC-MOSFET body diode is fast, and the recovery loss can be reduced relative to that of an Si-MOSFET.