Doping design of GaN-based heterostructure field-effect transistors with high electron density for high-power applications

Abstract

For fully exploiting high potentiality of AlGaN/GaN heterostructure field-effect transistors (HFETs) for high-power applications, back-doping (BD) and channel doping (CD) designs have been proposed that make it possible to obtain high two-dimensional electron gas (2DEG) densities even for the devices with thin AlGaN barrier layers. Furthermore, a novel metal–insulator–semiconductor (MIS) HFET using a bi-layer of Al2O3/Si3N4 thin film has been proposed to efficiently suppress the gate leakage current in the devices with thin barrier layers, by taking advantage of highly-resistive Al2O3 layers and excellent quality of Si3N4/AlGaN interface. In BD and CD designs where an asymmetric double-heterostructure and a single-heterostructure are employed, respectively, donor atoms are additionally doped in the backside region beneath the 2DEG position and electrons are supplied also from these doped regions. By using BD and CD designs, very high 2DEG densities around 3 × 1013 cm−2 have been achieved in the Al0.3Ga0.7N/GaN HFETs whose barrier layer (Al0.3Ga0.7N) is designed to be as thin as 120 Å. A BD-HFET with the gate-length (Lg) of 1.5 μm has exhibited a high current density (Id) of 1.2 A/mm and a high transconductance (gm) of 200 mS/mm, which is ascribed to high 2DEG densities and thin barrier layers in these devices. A novel MIS structure mentioned above has been applied to CD-HFET, and a fabricated MIS CD-HFET with Lg = 1.5 μm has exhibited a state-of-the-art current density of 1.6 A/mm and gm = 145 mS/mm, along with reduced gate leakage current. Thus, BD- and CD-HFETs, combined with Al2O3/Si3N4 MIS structure, are promising for high-power applications and should be inevitably required in the future for further improving the device performance. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)