This article presents the results of modeling the heterostructure of a normally-off n-channel transistor with various designs of field plates on electrodes. The use of field plates makes to possible to effectively control the distribution of  the field in the channel and increases the breakdown voltage. The optimal design parameters of field plates to achieve maximum BV were determined by study of the current-voltage characteristics, the distribution of the field in the  channel and the concentration of the majority carriers in the channel.