(Invited) Physics of GaN High Electron Mobility Transistors

Abstract

The device physics of GaN/AlN/InN High Electron Mobility Transistors (HEMTs) is affected by wurtzite (hexagonal) symmetry leading to strong spontaneous and piezoelectric polarization differences at AlGaN/GaN and AlGaInN/InGaN interfaces (polarization doping). The polarization doping enhances the electron mobility in the device channel and leads to the formation of two-dimensional (2D) electron gas with concentrations 10 to 20 times higher than that for more conventional field effect transistors (over 4.5x1013 cm-2 for AlGaN/GaN HEMTs). The resulting low channel resistance makes parasitic series resistances to be very important. The high current carrying capability due to a very high carrier concentration in the device channel and a very high breakdown voltage lead to high power densities and make the issues of heat dissipation to be a priority. Switching voltage is typically proportional to the gate-to-drain separation, which could be many microns for high voltage switches. The breakdown voltage is affected by the peaks in the electric field near the gate edge or the edges of the metal field plates introduced for boosting the breakdown voltage. The device characteristics strongly depend on the substrate that affects the dislocation and point defect densities in the device channel and determines heat dissipation. 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. Strain induced during growth and caused by a strong piezoelectric effect in the device channel is another crucial factor affecting the device reliability (linked to the transient and permanent detects induced under the voltage stress). Novel emerging designs, such as Perforated Channel HFET (PC-HFET), devices with low conducting passivation covering the gate-to-drain spacing and/or inserted between the gate and the barrier layer account for this device physics and might be especially beneficial for high temperature and radiation hard electronics applications.