Cross-sectional potential distribution of AlGaN/GaN HFETs with and without surface passivation by silicon
nitride (SiN<sub>x</sub>) has been investigated by using Kelvin probe force microscopy to study the effect of the surface
passivation layer on an electric field under high operating bias conditions. The measured FETs exhibited DC
characteristics of the maximum drain current of 750 mA/mm, threshold voltage of -5 V, and the transconductance
of 150 mS/mm. For the bias condition of the gate voltage of -5 V and the drain voltage of 40 V, the electric field
is mainly concentrated at three areas without relation to the presence or absence of the surface passivation layer.
One is the mid-point between the gate and drain electrodes at FET surface. The others are the mid-depth of
GaN buffer layer under the drain electrode and the interface between GaN buffer and SiC substrates from drain
edge toward source electrode. Near the surface of SiN<sub>x</sub>-passivated AlGaN/GaN HFETs, it is confirmed that the
intensity of electric field concentration decreases compared to that of no-passivated AlGaN/GaN HFETs. It is
considered that this result originates in the decrease of the surface charge by SiN<sub>x</sub> passivation. In addition, It is
found that the electric field concentration near the GaN/SiC interface has a tendency to become stronger rather
than that between the drain and gate electrodes by SiN<sub>x </sub>passivation.