Temperature Effect on Performance of Enhancement Mode Al0.4Ga0.6n-Channel Moshfets with Hybrid Oxide

The metal oxide semiconductor heterostructure field effect transistors (MOSHFETs) based on Al x Ga 1-x N materials with the high aluminum composition is a promising choice for high-power, high-temperature harsh environment applications. In the present work the temperature stability of MOSHFETS with high- k ZrO 2 , Al 2 O 3 gate dielectrics has been studied. Our data show these high- k dielectrics introduce negative fixed charges (Q ox ) as high as 1-3×10 13 cm -2 depleting 2DEG density of 2×10 13 cm -2 causing a positive shift of threshold voltage (V TH ) compared to that for HFET which is an important feature for realizing enhanced (E-) mode MOSHFET. ZrO 2 possesses higher Q ox resulting in stronger positive V TH shift in devices, while devices with Al 2 O 3 demonstrate lower gate leakage. To take advantages of both oxides, UWBG Al 0.4 Ga 0.6 N channel E-mode MOSHFET has been fabricated. E-mode device was realized using the hybrid oxide (ZrO 2 /Al2O 3 ) 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 simultaneously. D-mode devices were protected during gate recess step to avoid additional damage. Thermally induced V TH instability of these MOSHFETs were studied and potential mechanism for this V TH instability is discussed. Experimental: Figure 1(a) shows the pseudomorphic device structure that was grown using metalorganic chemical vapor deposition on AlN/sapphire templates. A graded composition (Al x Ga 1-x N, x=1-0.4) back barrier reduces internal stress and improves gate control in the devices. [i] , [ii] The top 200 Å thick graded n-Al x Ga 1-x N (x from 0.6 to 0.3) layer assists with the formation of ohmic contacts resistance as low as 1.7 Ω-mm. The n -doping of this layer compensates the positive charges resulting from the reverse composition grading. [iii] The 2DEG sheet resistance was ~1900 ohm/□. Device processing details published elsewhere.[iv] The fixed gate-length L G ≈ 2.0 μm, source to drain spacing, L SD =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 μm 2 . Results and discussion: The combination of gate recessing and hybrid oxide resulted in threshold-voltage (V TH ) shift of +12.2 V from D to E-mode device (Figure 1(b)). The gate leakage current in both D and E-mode MOSHFETs is ~10 3 smaller than that of MOSHFETs using singe Al 2 O 3 or ZrO 2 layer (Figure 2(a)) which allows us to apply gate voltage as high as +12 V. The peak DC currents for D and E-mode devices were found to be 1.1 A/mm and 0.48 A/mm respectively while in the pulse mode it was 1.3 A/mm and 0.53 A/mm. ON/OFF ratio as high as 3×10 8 was achieved which is higher than ~2 orders of magnitude than that for Al 2 O 3 or ZrO 2 . We then performed temperature dependent threshold and gate leakage study of our fabricated MOSHFETs. Figure 2(b) shows the temperature dependent gate leakage characteristics of MOSHFETs of this study. It has been found that, in the D-mode MOSHFET, the V TH experiences positive V TH shift of + 1.7 V from RT to 150 °C; for the E-mode MOSHFET the shift is negative: -2.9 V (Figure 3(a)). The V TH shift for similar device having Schottky gate (no dielectric) (HFET) is significantly smaller, +0.2 V. This shows that the V TH shift is mainly due to the charges in dielectric or at dielectric-barrier interface. In the E-mode devices, the effective channel mobility and V TH was additionally affected by radiative defects introduced during gate-recess step. The mobility (µ) in D-mode devices decreases with temperature while it increases for the E-mode devices. We estimated temperature dependent interface state density (D IT (T)) and Q ox (T) using frequency- dependent C-V measurements as shown in Figure 3(b). Our analysis show that in D-mode device, Q ox (T) dominates over interface charges. These are fixed negative charges which deplete the channel giving a total V TH shift of +1.7 V from RT to 150°C. The extracted SS value for D and E-mode devices were 99 mV/decade and 134 mV/decade, indicating an increased density of interface traps (D IT ) at the recessed interface in the E-mode device. Larger D IT value in E-mode devices caused by a radiation damage from the barrier recessing process and becomes comparable with Q ox . Thus the temperature effect on Q ox is compensated by D IT changes, increasing SS and making the V TH (T) 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. Bajaj et al., Appl. Phys. Lett. 109 , 133508-1 (2016). [iv] ) S. Mollah et al., Appl. Phys. Lett. 117 , 232105 (2020). Figure 1