We report investigations on the fabrication and electrical characterization in the range 27°C -290 °C of normally off 4H-SiC circular MOSFET devices manufactured on p-type semiconductor. An high quality SiO2/SiC interface is obtained by nitrogen ion implantation conducted before the thermal oxidation of SiC. Two samples with different nitrogen concentration at the SiO2/SiC interface and one un-implanted have been manufactured. The sample with the highest N concentration at the interface presents the highest channel mobility and the lowest threshold voltage. With increasing temperature, in all the samples the threshold voltage decreases and the electron channel mobility increases, reaching the maximum value of about 40 cm2/Vs at 290 °C for the sample with the highest N concentration. The observed improvement of the mobility is related to the beneficial effect of the N presence at the SiO2/SiC interface, which leads to the reduction of the interface trap density with energy close to the conduction band. Our results demonstrate that N implantation can effectively be used to improve the electrical performance of surface n-channel 4H-SiC MOSFETs.