Shinya Takashima

Sidewall Dominated Characteristics on Fin-Gate AlGaN/GaN MOS-Channel-HEMTs

The fin-gate structure was fabricated onto AlGaN/GaN MOS channel-high electron mobility transistors (MOSC-HEMTs), and the fin sidewall contribution to the MOS channel characteristics was investigated. In fin-gate MOSC-HEMTs (Fin-MOSC-HEMTs) with 120-nm fin width, significant suppression of short channel effect is obtained, but the threshold voltage (Vth) becomes lower than that of conventional MOSC-HEMTs. The fin width dependence shows continuous Vth decrease by decreasing the fin width, indicating that the narrower fin-gate channel is dominated by the fin sidewalls that are depletion mode nature. The channel sheet resistance extracted from channel length dependence and device transconductance for Fin-MOSC-HEMTs are superior to those of conventional MOSC-HEMTs when they are normalized by effective gate width as the summation of fin width, indicating contribution from sidewalls to channel conduction. Temperature dependence of Vth shows clearly different behavior between the MOSC-HEMT and Fin-MOSC-HEMT. These results demonstrate sidewall-dominated characteristics of these Fin-MOSC-HEMTs.

Metal?Oxide?Semiconductor Interface and Dielectric Properties of Atomic Layer Deposited SiO2 on GaN

The dielectric and MOS interface properties of SiO2 deposited with atomic layer deposition (ALD) on GaN with different surface treatments have been investigated with DC current–voltage (I–V) measurements and UV-assisted capacitance–voltage (C–V) measurements. Dielectric breakdown characteristics and leakage conduction mechanism for ALD SiO2 depend on surface conditions. Dry etch with NaOH post-etch GaN surface exhibited high oxide breakdown voltage with small distribution, larger barrier height characteristics, and higher charge to breakdown characteristics when compared with un-etched surface condition and dry etch with tetramethylammonium hydroxide (TMAH) post-etch surface condition. Moreover, fixed charge density and interface trap density at MOS interface extracted by UV-assisted C–V were comparable between un-etched surface sample and dry etch with NaOH post-etch surface sample, indicating dry etching damage recovery and demonstrating the usability of NaOH post-etching treatment. Comparison has also been made to a composite oxide of SiO2/Al2O3/SiO2, showing possibility of oxide charge engineering toward positive threshold voltage but carrier trapping by insertion of Al2O3.

Electron microscopy studies of the intermediate layers at the SiO2/GaN interface

As the first step toward understanding the electrical properties of SiO2/GaN systems, the interface was characterized using high-resolution scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS). An epitaxial crystalline intermediate layer with a thickness of ~1.5 nm was observed at the SiO2/GaN interface. STEM-EDS analyses revealed that this intermediate layer contained gallium and oxygen and mostly comprised the e-Ga2O3 phase. The e-Ga2O3/GaN interface was atomically smooth and free from misfit dislocations despite lattice mismatch of ~8.0%, suggesting that the initial oxidation of GaN surfaces is crucial to achieve good interfacial properties.

Control of the inversion-channel MOS properties by Mg doping in homoepitaxial p-GaN layers

Lateral GaN MOSFETs on homoepitaxial p-GaN layers with different Mg doping concentrations ([Mg]) have been evaluated to investigate the impact of [Mg] on MOS channel properties. It is demonstrated that the threshold voltage (V th) can be controlled by [Mg] along with the theoretical curve. The field effect mobility also shows [Mg] dependence and a maximum field effect mobility of 123 cm2 V−1 s−1 is achieved on [Mg] = 6.5 × 1016 cm−3 layer with V th = 3.0 V. The obtained results indicate that GaN MOSFETs can be designed on the basis of the doping concentration of the p-GaN layer with promising characteristics for the realization of power MOSFETs.