Group-III group-V compound semiconductors (often referred to as III-V compound semiconductors), such as gallium nitride (GaN) and its related alloys, have been under intense research in recent years due to their promising applications in power electronic and optoelectronic devices.  The large band gap and high electron saturation velocity of many III-V compound semiconductors also make them excellent candidates for applications in high temperature, high voltage, and high-speed power electronics.  Particular examples of potential electronic devices employing III-V compound semiconductors include high electron mobility transistor (HEMT) and other heterojunction bipolar transistors. 
During operation, a HEMT forms a large surface electric field around a gate edge, which affects the depletion region curve in a drift region between a gate structure and a drain.  While large electric field is one of the benefits of HEMT for use in power applications, the distribution of the depletion region during operation can negatively affect the breakdown voltage for the HEMT.  When negative bias is applied to the gate of the HEMT, the depletion region curve is formed directly under the gate and causes high surface electric field concentration around the gate edge.  The electric field concentration around the gate decreases breakdown voltage. 
In order to improve breakdown voltage, a metallic field plate is sometimes added over or next to the gate structure over a passivation layer that is between the gate structure and the drain.  The field plate modulates the surface electric field distribution and enhances the breakdown voltage.  However, new structures with high breakdown voltage for III-V compound semiconductor based transistors and methods for forming them continue to be sought.