Semiconductor device with improved field plate

A transistor device includes a semiconductor body, a spacer layer, and a field plate. The spacer layer is over at least a portion of a surface of the semiconductor body. The field plate is over at least a portion of the spacer layer, and includes a semiconductor layer between a first refractory metal interposer layer and a second refractory metal interposer layer. By including the semiconductor layer between the first refractory metal interposer layer and the second refractory metal interposer layer, the electromigration of metals in the field plate is significantly reduced. Since electromigration of metals in the field plate is a common cause of transistor device failures, reducing the electromigration of metals in the field plate improves the reliability and lifetime of the transistor device.