A high-electron-mobility transistor (HEMT) is a field effect transistor (FET) that incorporates a junction between two materials with different band gaps as the channel instead of a doped region as is typically the case for metal-oxide semiconductor FETs (MOSFETs).  HEMTs are characterized by low on-state resistance, high breakdown voltage, and low switching losses, making them excellent power devices (e.g., power amplifiers) in, for example, wireless communication systems. 
In particular, HEMTs using gallium nitride (GaN) and aluminum GaN (AlGaN) on silicon substrates are important for handling high voltages and currents at high frequencies in power electronics.  GaN-based HEMTs are used more in power switching applications than other types of HEMTs as their characteristics and cost structure are proving to be very suitable for a wide range of applications. 
Conventional HEMTs are planar with both their source and drain disposed at the upper surface.  When the device is in its on-state, the main current flow is in the lateral direction from source to drain.  The gate is also on the same surface as the source and drain.  Consequently, metallization and routing for a device that can handle high currents require at least two, or more likely three, levels of metal.  In addition to the loss of power due to the resistance of these metal interconnects, the interconnects create parasitic inductive and capacitive components.  The resistive, capacitive, and inductive parasitics all contribute to degradation of the high frequency performance of the device, and also make it difficult to prevent oscillations in the circuit in which the device is placed when the device is switching states. 
Thus, there is a need for a device that reduces parasitic interconnections due to the complicated routing of the source, gate, and drain terminals.