2016.11.11 SiC Power Device
In succession to our summary of SiC power devices, we move to explanations of specific devices. We begin with SiC Schottky barrier diodes.
SiC Schottky Barrier Diodes and Si Schottky Barrier Diodes
We begin with an explanation of the structure of SiC Schottky barrier diodes (hereafter "SBDs"). As indicated in the diagram below, a junction with a metal (a Schottky junction) is formed in order to obtain a Schottky barrier in the SiC, which is a semiconductor. The structure is essentially the same as that of a Si Schottky barrier diode, and fast operation is likewise an important feature.
The features of SiC SBDs are principally excellent high-speed operation combined with a high voltage. In order to raise the breakdown voltage of a Si SBD, the n-type layer, shown in the diagram, is made thicker, and the carrier concentration is lowered; but this means a high resistance value, a higher VF, and other changes resulting in large losses and characteristics not suited to practical use. For this reason, 200 V is the limit for Si SBD withstand voltages. On the other hand, SiC has a dielectric breakdown field intensity ten times greater than that of silicon, and so SiC devices can have high voltages while retaining characteristics well-suited for practical use.
ROHM mass-produces 650 V and 1200 V SiC SBDs, and is working on development of a 1700 V device.
SiC SBDs and Si PN-Junction Diodes
Si diodes having breakdown voltages comparable to or exceeding those of SBDs are PN-junction diodes (here abbreviated to "PNDs"). The diagram below shows the structure of a Si PN diode. In an SBD, only electrons move to cause current to flow, but in a PN-junction diode, electrons and holes cause current to flow. Through the accumulation of holes, which are the minority carrier, in the n layer, the resistance value falls. Because of this, a high withstand voltage and low resistance are achieved simultaneously, but the turn-off speed is slow.
When the operating speed of a PN-junction diode is raised, the result is a fast-recovery diode (FRD); even so, however, the trr (reverse recovery time) and other characteristics are inferior to those of SBDs. As a consequence, much effort is devoted to studying trr losses in Si PN-junction diodes with high withstand voltages, and the inability to handle fast switching frequencies in switching power supplies remains a problem.
The diagram on the upper-right shows the breakdown voltage coverage for Si SBDs, PNDs and FRDs and for SiC SBDs. We see that SiC SBDs cover a considerable part of the voltage ranges of PNDs and FRDs. SiC SBDs offer both fast operation and high voltages, and so can be used to greatly reduce the Err (recovery losses) compared with PNDs and FRDs, and also enable switching at high frequencies. As a result, more compact transformers and capacitors can be used, resulting in smaller equipment sizes.
Below is part of the data sheet of a SiC SBD with a 1200 V rated voltage. In the next section, we will explain principal characteristics.
・Features of SiC SBDs are excellent high-speed operation combined with a high voltage.
・Compared with Si PN diodes having high voltages, SiC devices afford excellent reverse recovery times and other high-speed characteristics, and so make possible lower losses and more compact equipment.