Study of Temperature Effect on Performance of Ultrawide Bandgap Al0.4Ga0.6N-Channel Depletion and Enhancement Mode MOSHFETs with ZrO2/Al2O3 Gate Insulator

Abstract

Abstract Body: Abstract: Ultrawide Bandgap AlxGa1-xN channel metal oxide semiconductor heterostructure field effect transistors (MOSHFETs) are promising candidates for high-power, high-temperature harsh environment applications. In our recent reports on Al0.4Ga0.6N-channel MOSHFETs with high-k ZrO2 and Al2O3 gate-dielectrics we found that, these dielectrics introduce negative fixed charges (Qox) as high as 1-3×1013 cm-2 depleting 2DEG density of 2×1013 cm-2 causing a positive shift of threshold voltage (VTH) compared to that for HFET (no gate dielectric). This is an important step toward realizing enhanced (E-) mode MOSHFET. Our data show that, ZrO2 possesses higher Qox resulting in stronger positive VTH shift in devices, while devices with Al2O3 demonstrate lower gate leakage. In this work we use stack of ZrO2/Al2O3 to fabricate MOSHFETs. E-mode device was realized using ZrO2/Al2O3 combined with gate recess. To separate effects of dielectric charges and damage from recess process on device performance depletion (D-) mode and E-mode devices were fabricated on the same wafer. Temperature dependent VTH instability of these MOSHFETs were studied and potential mechanism for this VTH instability is discussed. Experimental: The pseudomorphic device structure was grown using metalorganic chemical vapor deposition on a AlN/sapphire template. A graded composition (AlxGa1-xN, x=1-0.4) back barrier was grown to reduce internal stress and improve gate control in the devices[i], [ii] while the top 20 nm graded n-AlxGa1-xN (x from 0.6 to 0.3) was grown to facilitate ohmic contacts. The contact resistance was as low as 1.7 Ω-mm and the 2DEG sheet resistance was ~1900 ohm/sq. Device processing details published elsewhere.[iii] The fixed gate-length LG ≈ 2.0 μm, source to drain spacing, LSD=6 μm, were used for regular devices with 15 μm channel width, while for precise C(V,T,f) measurements we use a test structure with gate area 200x80 μm2 . Results and discussion: The combination of gate recessing and ZrO2/Al2O3 stack resulted in VTH shift of +12.2 V from D to E-mode device. The peak DC currents for D and E-mode devices were found to be 1.1 A/mm and 0.48 A/mm respectively at gate voltage +12 V while in the pulse mode it was 1.3 A/mm and 0.53 A/mm. The gate leakage current in both D and E-mode MOSHFETs is suppressed by ~3 orders of magnitude compared to that of MOSHFETs using singe Al2O3 or ZrO2 layer. ON/OFF ratio as high as 3×108 was achieved which is higher than ~2 orders of magnitude than that for Al2O3 or ZrO2. Temperature dependent VTH and gate leakage study was performed on the fabricated MOSHFETs. It has been found that, from RT to 150 °C there is VTH shift of + 1.7 V and -2.9 V for D- and E-mode MOSHFETs respectively. The VTH shift for similar device having Schottky gate HFET was found to be significantly smaller, +0.2 V which indicates that the VTH shift in MOSHFETs is mainly due to the charges in dielectric or at dielectric-barrier interface. In the E-mode devices, the effective channel mobility (µ) and VTH was additionally affected by radiative defects introduced during gate-recess. µ in D-mode devices decreases with temperature while it increases for the E-mode devices. Temperature dependent interface state density (DIT(T)) and Qox (T) were estimated using frequency- dependent C-V measurements. Our analysis show that in D-mode device, Qox(T) dominates over DIT(T). These fixed negative charges (Qox) deplete the channel giving a total VTH shift of +1.7 V. The extracted SS values of 99 mV/decade (D-mode) and 134 mV/decade (E-mode) indicate an increased density of DIT at the recessed interface in E-mode device. In E-mode, the DIT value is comparable to Qox, thus compensating the temperature effect on Qox and making the VTH more negative. [i]) G. Simin et al., Jpn. J. Appl. Phys., 40, L1142 (2001). [ii]) C. Ren et al., J. Semicond. 36, 014008-1 (2015). [iii]) S. Mollah et al., Appl. Phys. Express. 14, 014003 (2021).