(Electronics and Photonics Division Award) Physics of Wide Band Gap Semiconductor Devices

Abstract

Silicon carbide and nitride based devices have found important commercial applications in both optical and electronic devices. AlGaN deep ultraviolet Light Emitting Diodes (LEDs) are now used for water purification, sterilization, and prolonging food storage. Silicon carbide and gallium nitride based transistors compete with silicon for high power applications and with III-V based transistors for RF applications. The device physics of all these devices is determined by the wurtzite (hexagonal) symmetry leading to strong spontaneous and piezoelectric polarization and by a large polar optical phonon energy, which is the consequence of the strong bonding and small atomic masses of carbon and nitrogen. In AlGaN/GaN and AlGaInN/InGaN devices, the differences in the polarization at the heterointerfaces induce two-dimensional (2D) electron or hole gases at the interfaces (polarization doping) with the electron sheet concentrations up to 10 to 20 times higher than those for more conventional field effect transistors (reaching over 4.5x1013 cm-2 for AlGaN/GaN High Electron Mobility Transistors (HEMTs)). In deep UV LEDs, the polarization induced 2D hole gas is used for improved p-type contacts. A high 2D electron concentration in the III-N HEMT channels enables superior switching characteristics but makes parasitic series resistances and capacitances to be very important. The high current carrying capability and a very high breakdown voltage lead to high power densities and make the issues of heat dissipation to be a priority. The thermal conductivity of SiC (490 W/m K at room temperature) is larger than that for copper (385 W/m K) making it to be an excellent choice as a substrate for power devices. For AlGaN/GaN HEMTs, the research started from using sapphire substrates, but silicon substrates have become a commercial technology. SiC, diamond, or AlN bulk or template substrates could have significant advantages over silicon. The substrates also affect the dislocation and point defect densities in the device channel and strain induced during growth and caused by a strong piezoelectric effect. Such strain is a crucial factor affecting the device reliability (linked to the transient and permanent detects induced under the voltage stress). The large bandgap suppresses the subthreshold leakage making the surface leakage to be one of the dominant leakage mechanisms. Ballistic, overshoot, and electron runaway effects play a dominant role in short channel wide band gap devices now competing for applications in the terahertz (far infrared) range. Figure 1