INSULATED GATE III-N HETEROSTRUCTURE FIELD-EFFECT TRANSISTORS
Unique materials properties of GaN-based semiconductors that make them promising for high-power high-temperature applications are high electron mobility and saturation velocity, high sheet carrier concentration at heterojunction interfaces, high breakdown field, and low thermal impedance (when grown over SiC or bulk AlN substrates). The chemical inertness of nitrides is another key property. An AlGaN/GaN Heterostructure Field Effect Transistor (HFET) has been a topic of intensive investigations since the first report in 1991 [1]. Several groups demonstrated high power operation of AlGaN/GaN HFETs at microwave frequencies [2,3,4], including a 100 W output power single chip amplifier developed by Cree, Inc. and devices with 100 GHz cut-off frequency reported in [5]. However, in spite of impressive achievements, the potential of nitride based HFETs has not been fully realized as yet. The RF powers expected from the fundamental properties of nitride based materials significantly exceed the experimental data. One of the key problems limiting the HFETs RF characteristics is high gate leakage currents causing DC and RF parameter degradation. When the gate voltage goes positive the forward leakage current shunts the gate-channel capacitance and thus limits the maximum device current. When the gate voltage is negative, high voltage drop between the gate and the drain causes premature breakdown and thus limits maximum applied drain voltage. In addition, gate leakage currents increase the device sub-threshold currents, which decrease the achievable amplitude of the RF output. These limitations become even more severe at high ambient temperatures. The characteristics of III-N HFETs can be considerably improved by implementing a new approach, which results from the demonstration of good quality of SiO2/AlGaN and Si3N4/AlGaN interfaces. This approach opens up a way to fabricate insulated gate heterostructure field-effect transistors (IGHFETs), which have the gate leakage currents several orders of magnitude below those of regular HFETs, and exhibit better linearity and higher channel saturation currents. In this chapter, we describe design, characterization and applications of these novel devices.
