The field-effect transistor (FET) is a type of transistor that relies on an electric field to control the shape and hence the conductivity of a ‘channel’ in a semiconductor material. In junction field effect transistors (JFETs), the conductivity of the channel is controlled by the application of a voltage to a p-n junction. JFETs may be constructed as p-channel or n-channel and may be operated as enhancement mode devices or depletion mode devices. Similar to the JFET is the metal semiconductor field effect transistor (MESFET). The MESFET is quite similar to a JFET in construction and terminology. The difference is that instead of using a p-n junction for a gate, a Schottky metal-semiconductor junction is used in MESFETs.
JFETs and MESFETs are widely used electronic devices. For optimum device performance, it is important to obtain a device breakdown voltage as close as possible to the intrinsic capability of the underlying semiconductor material. However, the breakdown voltage of practical devices is reduced by the occurrence of high electric fields at the edges of the device. In particular, electric field crowding at the edges of the device leads to premature voltage breakdown. To minimize premature voltage breakdown, specialized edge termination structures must be implemented in order to obtain maximum breakdown voltage with relatively low associated on-state resistance. The multiple floating guard ring (MFGR) edge termination structure is used to alter the charge distribution and electric field at surfaces and material interfaces of semiconductor devices. The interface between the guard ring and the substrate in which it is embedded forms a depletion region that enhances resistance to voltage breakdown in an applied field. The MFGR also provides a cost-effective method of edge termination because it may use fewer fabrication steps than the Junction Termination Extension technique, another technique for edge termination. The MFGR, however, is very sensitive to surface charges in the dielectric-semiconductor interface. Positive charge at the dielectric-semiconductor interface may reduce the effectiveness of the floating guard rings and result in a reduction of blocking voltage for the devices. In addition, the charges near the dielectric-semiconductor interface, mostly positive, can move towards or away from the dielectric-semiconductor interface, causing time dependent breakdown voltage, or breakdown walk-out.