In SiC BJTs the problem is relatively low current amplification, for which
powerful driver stages are required to control and operate the components.
The current amplification is caused by stack defects in the SiC crystal brought
about by recombination. These local defects occur inSiCbipolar components if
they are operated in forward direction and electron-hole recombinations take
place. The energy released can cause faults in the crystal structure and
influences factors like the lifetime of theminority carriers.
Enhancement SiCJFET (normally-off)
From an application point of view, the small threshold voltage of approximately
1V presents the biggest challenge. It poses design requirements for the driver
stage, which must prevent unintentional turning on by external influences.
Furthermore, unlike a normally-on SiC JFET, this component requires a
separate body diode for most applications with inductive load, because it does
not haveonebuilt in.
All these problems, which are specific to each component, never happen with a
normally-on SiC JFET, because of its design. However, the open channel at non-
controlled gate in standard applications (e.g. a motor control) does impose a limitation.
Normally-onSiC JFETs are therefore usually operated in combinationwith a low-voltage
MOSFET. This is how the "normally-off" characteristic comes into play in the non-
shows a combination of this type, known as a cascode.
SiC JFETand cascode
Below, themode of operation of the JFET/MOSFET cascodewill be considered inmore
detail, referring to
As long as the n-channel low-voltage MOSFET is turned
off, there is negative voltage between the JFET gate and the source. This voltagemust
be smaller than the pinch-off voltage of the JFET. Only then the high-voltage JFET will
be blocked and external voltage can build up. TheMOSFET itselfmust be designed to a
voltage that is usually within the range of 30V to 50V, whereas the JFET, depending on
its execution, can take voltages up to well over 1kV. As soon as the MOSFET is
controlled positively and the gate-source voltage U
of the MOSFET reaches
the threshold voltage, the voltageU
of the JFET rises.When the voltage reaches
the value of the pinch-off voltage, the JFET opens and the current I
commutate from the freewheeling diode D
to the cascode. At the end of the switching
operation theentire load is flowing through the cascode.