Electric-field-dependent anisotropy of electron drift velocity in 4H-SiC

The electron drift velocities along the [0001] and [11¯00] directions in 4H-SiC were investigated over a wide range of electric field strength up to 200 kV/cm, employing a conductance method to test structures fabricated on n-type 4H-SiC(112¯0) epilayers. By carefully designing the test structures and measurement setup, interfering transient current and self-heating issues were effectively suppressed. As a result, observation of a proportional region at low fields (≤5 kV/cm), a saturation region at high fields (≥100 kV/cm), and an intermediate nonlinear region in the drift velocity–electric field relationship was achieved. The obtained low-field mobilities and their anisotropy were in agreement with the literature values, and the saturated velocities were extracted as 1.7×107 cm/s for both directions. The higher saturation velocity compared with Si was explained by the well-known higher values of optical phonon energy in 4H-SiC, which also contributes to the higher electric field required for velocity saturation in the material. The diminishing anisotropy in electron drift velocity with increasing the electric field can be qualitatively explained by the change in the energy distribution of electrons under higher electric fields together with the energy-dependent effective mass of electrons.