Leveraging SiC properties to improve power switches

An ideal power switch is able to carry large current with zero voltage drop in the on-state, block high voltage with zero leakage in the off-state and include zero energy loss when switching from off- to on-state and vice versa. With silicon, it is difficult to combine these characteristics, especially at high voltage and current.

Si power devices with higher breakdown voltages have considerably high on-resistance per unit area. IGBTs have been mainly used in devices with breakdown voltages of 600V or above. IGBTs achieve lower on-resistance than MOSFETs by injecting minority carriers into the drift region, a phenomenon called conductivity modulation. These minority carriers generate tail current when transistors are turned off, resulting in a significant switching loss. SiC devices do not need conductivity modulation to achieve low on-resistance since they have much lower drift-layer resistance than Si devices. MOSFETs generate no tail current in principle. As a result, SiC MOSFETs have much lower switching loss than IGBTs, which enables higher switching frequency, smaller passives, smaller and less expensive cooling system. Compared to 600 to 1500V silicon MOSFETs, SiC MOSFETs have smaller chip area (mountable on a compact package) and a low recovery loss of body diodes. Rohm’s current line-up includes 650 and 1,200V planar type MOSFETs and 1,700V MOSFETs are under development.

Since SiC has a dielectric breakdown field strength 10 times higher than that of Si, high breakdown voltage devices can be achieved with a thin drift layer with high doping concentration. This means, at the same breakdown voltage, SiC devices have low specific on-resistance (on-resistance per unit area). For example, 1200V SiC-MOSFETs can provide the same on-resistance as Si-MOSFETs and Si super junction MOSFETs with a chip size 35 times and 10 times smaller, respectively. Smaller chip size reduces gate charge Qg and capacitance. Existing Si super junction MOSFETs are only available for breakdown voltages up to around 900V. SiC-MOSFETs have breakdown voltages up to 1,700V or higher with low on-resistance. Since SiC-MOSFETs have no threshold voltage, they have a low conduction loss over wide current range. Si-MOSFETs’ on-resistance at 150°C is more than twice that at room temperature, whereas SiC-MOSFETs’ on-resistance increases only at a relatively low rate. This facilitates thermal design for SiC-MOSFETs and provides low on-resistance at high temperatures.

XHEAD Resistance values

Although SiC-MOSFETs have lower drift layer resistance than Si-MOSFETs, the lower carrier mobility in SiC means their channel resistance is higher. For this reason, the higher the gate voltage, the lower the on-resistance. Resistance becomes progressively saturated as Vgs gets higher than 18V. SiC-MOSFETs do not saturate with the gate voltage Vgs of 10 to 15V which is applied to typical IGBTs and Si-MOSFETs. It is recommended to drive SiC-MOFETs with Vgs set to 18V in order to obtain adequately low on-resistance. Using SiC-MOSFETs with Vgs below 13V may cause thermal runaway.

Figure 1: Comparison of gate resistance values

The threshold voltage of SiC-MOSFETs is about the same as Si-MOSFETs, i.e. approximately 3V at room temperature (normally ‘off’) at a few mA. However, since approximately 8V or more of gate voltage is required to conduct several amperes of current, SiC-MOSFET can be said to have higher noise immunity than IGBT to accidental turn-on. The threshold voltage decreases with increasing temperature.

The turn-on switching rate of SiC-MOSFET is several tens of nanoseconds, which is equivalent to that of Si-IGBT and Si-MOSFET. However, inductive load switching causes a recovery current from commutation to the upper arm diodes to pass through the lower arm. Si-FRDs and Si-MOSFET body diodes normally have exceedingly high recovery current, resulting in heavy losses, and these losses tend to worsen at high temperature. In contrast, SiC-SBDs have low recovery current and short recovery time which is reasonably independent of temperature. SiC-MOSFET’s body diode also has recovery performance equivalent to that of discrete SiC-SBDs. This fast recovery diode performance reduces turn on loss (Eon). The switching rate depends largely on the external gate resistance Rg. For fast switching, it is recommended to use a small external gate resistor of several ohms. The selection of appropriate gate resistance must take surge voltage into account (see figure 1).

A double-pulse clamped inductive load test setup underlines the switching advantages of a SiC-MOSFET ( in this case, Rohm’s SCH2080KE) co-packaged with a SiC-SBD compared to a Si-IGBT co-packaged with Si-FRD.

XHEAD Switching operations

With no tail currents, SiC MOSFETs can have turn off loss (Eoff) that is approximately 90% smaller than that of IGBTs. The lower Eoff allows SiC-MOSFETs to switch at 50kHz and higher, allowing the size of passives and/or cooling systems to be significantly reduced.

The internal gate resistance is dependent on the sheet resistance of gate electrode material and chip size; the smaller the chip, the higher the gate resistance. At the same rating, SiC-MOSFET die is smaller than Si die, therefore, SiC-MOSFETs tend to have lower junction capacitances but higher gate resistance. For fast switching operation, it is recommended to use low external gate resistor of several ohms while monitoring surge conditions.

SiC-MOSFETs are normally off voltage-controlled devices. Hence they are easy to drive and incur less gate drive loss. The off-on gate voltage swing is nominally 0 to 18V. If high noise tolerance and fast switching are required, negative voltage of approximately three to 5V can also be used.

Figure 2: Comparison of turn off loss

XHEAD Voltage values

Like Si-MOSFETs, SiC-MOSFETs contain a parasitic (body) diode formed in the P-N junction. However, the SiC MOSFETs body diode has high threshold voltage (around 2V) and relatively large forward voltage drop (Vf) due to the fact that the bandgap of SiC is three times larger than that of Si. When connecting an external anti-parallel freewheeling diode to Si-MOSFET, an additional low-voltage blocking diode should be connected to the MOSFET in series to prevent the conduction through the “slow” body diode. This is not needed with SiC MOSFETs since the Vf of their body diodes is sufficiently high compared to that of a typical external free-wheeling diode. The high Vf of the body diode can be reduced by turning on the gate voltage for reverse conducting like synchronous rectification (see figure 3).

The recovery performance of a SiC MOSFET is equivalent to that of a discrete SiC SBD. This enables a reduction in recovery loss to a fraction compared to a body diode of Si-MOSFET or Si-FRD used with IGBT as a freewheeling diode. Like SBD, the recovery time of the body diode is independent of forward current If and fixed for a given dI/dt.

In inverter applications, SiC-MOSFETs with or without anti-parallel SiC-SBD can achieve an exceptionally-low recovery loss and can be expected to reduce noises due to a very small reverse recovery current.

XHEAD Trench MOSFETs

Today, Rohm Semiconductor offers SiC MOSFETs, covering breakdown voltage from 400V to 1700V and current rating from 10A to 63A in through-hole packages as well as bare die.

There are plans to offer a trench MOSFET. This vertical architecture has extremely low specific on resistance, thanks to the elimination of the JFET resistance inherent in the planar architecture, and much higher channel mobility. Specific on-resistance of less than 1.25mΩ/cm² has been achieved for the 1200V device. Smaller die size mean trench MOSFETs will also cost less, but will allow switches with very high current rating - 60A or larger – to be cost effectively manufactured. This, in turn, enables the development of more cost-efficient SiC power modules that can handle very high current, e.g. 600A to 1kA. Such modules are not economically feasible today as they require a significantly larger number of individual die.