Hints, Tips and Solutions


How to examine the scattering mechanisms that are contributing to the reduced channel mobility in 4H-SiC MOSFETs?

 

In a MOSFET structure, silicon carbide, 4H-SiC in particular, is known to exhibit lower channel mobility than Si, mainly due to Coulomb scattering at trapped charge at the SiO2/4H-SiC interface, where a high interface trap density exists. Atlas provides an alternative inversion layer mobility model specifically intended for 4H-SiC. The model enabled by specifying the ALTCVT.N parameter for electrons or the ALTCVT.P parameter for holes on the MOBILITY statement takes into account four scattering mechanisms. These comprise the ionized impurity scattering in the bulk semiconductor, the surface roughness scattering, the acoustic surface phonon scattering, and the Coulomb scattering at trapped charge at the SiO2/4H-SiC interface. Using Matthiessen’s rule, the ALTCVT.N and ALTCVT.P model combines four mobility components related to their respective carrier scattering to form the total inversion layer mobility in 4H-SiC.

The bulk mobility component cannot be explicitly excluded from the ALTCVT.N and ALTCVT.P model, but a judicious choice of parameters will force it to be constant.

The surface roughness contribution is enabled by default, and can be disabled by clearing the flags ALT.SR.N or ALT.SR.P on the MOBILITY statement, e.g., MOBILITY ALTCVT.N ^ALT.SR.N

The surface phonon contribution is enabled by default, and can be disabled by clearing the flags ALT.SP.N or ALT.SP.P on the MOBILITY statement, e.g., MOBILITY ALTCVT.N ^ALT.SP.N

The Coulomb scattering mobility is automatically enabled for the ALTCVT.N and ALTCVT.P model. It can be disabled for electrons by setting the flag COULOMB.N on the MOBILITY statement or for holes by setting the flag COULOMB.P on the MOBILITY statement, e.g., MOBILITY ALTCVT.N ^COULOMB.N

The contributions of each of the four mobility components to the total channel mobility can be examined by simulating drain current (ID) versus gate voltage (VG) at a fixed drain voltage (VD).

Figure 1 shows a simple n-channel 4H-SiC MOSFET, along with acceptor and donor state density distributions in the band gap at the SiO2/4H-SiC interface as a function of energy referenced from the valence band edge. Based on this MOSFET structure, while VG is being swept from 0 to 10V at VD=0.1V, the electron mobility and the perpendicular electric field are extracted at the SiO2/4H-SiC interface in the middle of the channel region with the help of PROBE statements. Figure 2 illustrates the electron mobility mechanisms in the channel as a function of electric field perpendicular to the SiO2/4H-SiC interface. From this it follows that at low electric fields the total inversion layer mobility is substantially dominated by Coulomb scattering at interface charges and at moderate electric fields by surface phonon scattering. Surface roughness scattering becomes dominant only at high electric fields, which can also be seen clearly from a schematic plot of the channel depth dependence of electron mobility at VG=10V and VD=0.1V (Figure 3).

 

                 

Figure 1. A 4H-SiC n-channel MOSFET (top) with density of interface traps at the interface between 4HSiC and SiO2 (bottom).

 

                 
Figure 2. Electron mobility in an n-channel MOSFET as a function of perpendicular electric field.

 

                 

Figure 3. Variation of electron mobility profile across the device (top) and along the channel depth (bottom) in the high electric field regime
(VG=10V and VD=0.1V).