Polarization-reduced quaternary InAlGaN/GaN HFET and MISHFET devices

The reduction of the polarization charge in a GaN-based single-heterostructure field-effect transistor (HFET) by polarization engineering is proposed as a method for achieving normally off operation. The concept minimizes the dependence of the threshold voltage on the barrier layer thickness. Therefore, thicker gate dielectrics for suppression of gate leakage currents can be applied without a shift in threshold voltage. A polarization-reduced enhancement-mode (E-mode) InAlGaN/GaN HFET is presented and demonstrates the basic working principle. Also an insulated-gate device with only minor shift in threshold voltage compared to the HFET validates the new concept and demonstrates the advantages compared to commonly applied concepts for E-mode operation

Polarization-Engineered Enhancement-Mode High-Electron-Mobility Transistors Using Quaternary AlInGaN Barrier Layers

Group III nitride heterostructures with low polarization difference recently moved into the focus of research for realization of enhancement-mode (e-mode) transistors. Quaternary AlInGaN layers as barriers in GaN-based high-electron-mobility transistors (HEMTs) offer the possibility to perform polarization engineering, which allows control of the threshold voltage over a wide range from negative to positive values by changing the composition and strain state of the barrier. Tensile-strained AlInGaN layers with high Al contents generate high two-dimensional electron gas (2DEG) densities, due to the large spontaneous polarization and the contributing piezoelectric polarization. To lower the 2DEG density for e-mode HEMT operation, the polarization difference between the barrier and the GaN buffer has to be reduced. Here, two different concepts are discussed. The first is to generate compressive strain with layers having high In contents in order to induce a positive piezoelectric polarization compensating the large negative spontaneous polarization. Another novel approach is a lattice-matched Ga-rich AlInGaN/GaN heterostructure with low spontaneous polarization and improved crystal quality as strain-related effects are eliminated. Both concepts for e-mode HEMTs are presented and compared in terms of electrical performance and structural properties

Quanternary Enhancement-Mode HFET With In Situ SiN Passivation

A lattice-matched InAlGaN/GaN heterostructure with a barrier-layer thickness of 4 nm has been grown and passivated in situ with a 63-nm SiN by metal-organic chemical vapor deposition. Enhancement-mode heterostructure field-effect transistors have been realized by a fluorine-based surface treatment after the local removal of the SiN. The threshold voltage and transconductance were 0.65 V and 250 mS/mm, respectively, for a 1-mu m gate-length device. The benefits of an in situ SiN passivation are demonstrated: first, the stabilization of the barrier material and prevention from oxidation and second, the improvement of the device characteristics by reduced source resistance and reduced trapping effects