Venturing Through the Forbidden Band

摘要

Over the past three decades, industry and the U.S. Government have invested hundreds of millions of dollars in an emerging area called wide-bandgap materials. The technical significance of these materials is that they can be made into semiconductor devices capable of handling much higher voltages than silicon, while withstanding and operating at much higher temperatures. Such materials also have unique optoelectronic capabilities that allow them to emit blue and UV light. Thus, many aspects of our lives can be touched by transistors and diodes made from a new class of materials. What makes a semiconductor wide bandgap. The answer remains at the atomic level of the material. A range of energies called the forbidden band separates the valence band and the conduction band of a solid-state material. The valence and conduction bands hold electrons; however, no electrons may reside in the forbidden band. When the forbidden band is wider, more energy is required to promote an electron from the valence band into the conduction band. If a material has no forbidden band (i.e., the conduction band is the valence band), it behaves as metal. If it has a very wide band, it is a good insulator. Semiconductors lie somewhere in the middle. When we speak of wide-bandgap materials, we are referring to gallium nitride (GaN), silicon carbide (SiC), and other compound semiconductors that have a relatively wide forbidden band (on the order of between 1.7 and 7 electron volts) compared with silicon and gallium arsenide. More work is still needed for this technology to be available for many of the applications mentioned in this publication. Issues such as gate leakage and defect densities (which affect wafer size) need to be addressed.