2. Operation principle for the 4H-SiC Power MESFETs

For the *n*w\$\*\*!(z
/\_z3\$%\$z%\*(1 %\*#z\*+.)((5w+\*zc !,(!0%+\*z)+ !dz\* z\*+.)(¥ ly-off (enhancement mode) devices, as are shown in Fig.1, an *n*-type channel connects the drain and source regions with the *n* + type doping. The depletion layer under the metalsemiconductor contact determines the current flow across the channel between the source and drain electrodes. The thickness of the channel conductivity is modulated by the gate bias-dependent depletion region.

Figure 1. Normally-on (a) and normally-off (b) MESFETs at zero gate bias.

The channel of normally-off MESFETs is totally depleted by the gate build-in potential even at zero gate bias, and its threshold voltage is positive. In contrast, the normally-on /z\$2!zz"%\*%0!z.+//w/!0%+\*z+"z+\* 10%\*#z\$\*\*!(z0z6!.+z#0!z%/z\* z0\$!z\*!#¥ tive threshold voltage.

The basic operation principle will be discussed in this section for the 4H-SiC power MESFETs.

For the normally-on MESFETs, usually, the source is grounded, and the gate and drain are biased negatively and positively, respectively. A schematic diagram of the depletion region under the gate of MESFETs for a finite drain-to-source voltage is shown in Fig. 2. On this condition, the electrons will flow from the source to the drain and a current flow (*I* ds) occurs in the channel. When the negative gate bias is changed, or an altering-current (AC) voltage /%#\*(z%/z/1,!.,+/! z+\*z0\$!z %.!0wz1..!\*0zcdz#0!z%/\_z0\$!z0\$%'\*!//z+"z0\$!z !,(!0%+\*z(5¥ er or the width of the conducting channel, which determining the resistance of the channel, will be modulated, and thus the current flow in the channel is regulated. So, the MESFETs is actually a voltage-controlled electric device by the means that the gate bias modulates the conducting channel resistance, and thus controls the current flow in the channel.

For the *p*-channel normally-on MESFETs, the gate is biased positively, so as to ensure that the gate is reverse biased. Note that it is the electrons for the *n*-channel MESFETs, but the holes for the *p*-channel MESFETs that transport as the carriers in the channel. However, whether in the *n*-channel or *p*-channel MESFETs, the unique majority carriers undertake transporting the current. So, the MESFETs is an unipolar device.
