Date of Award
Open Access Dissertation
Ultra-wide Band Gap (UWBG) (EG > 3.4 eV) AlxGa1-xN channel devices are promising candidates for compact next-generation power electronics due to the scaling of the breakdown field with alloy composition. The huge opportunity in consumer electronics is the major driving force behind the recent developments of UWBG AlGaN-channel HEMTs. One of the most critical issues limiting the device performance at high power level is device degradation primarily caused by gate leakage current. Use of SiO2 gate insulator is a standard method to suppress gate leakage but it causes threshold voltage shift towards more negative requiring more external voltage to turn off the device. The threshold shift becomes significant with the increase of oxide thickness which is not desired in many power electronics application. One potential solution to both gate leakage and threshold shift issue is the use of high-k dielectric as a gate insulator. It has been seen that, these high-k oxides also introduce charges in the oxide and oxide/barrier interface which are positive in nature, thus shifting the threshold voltage to practically un-usable range. The threshold voltage is almost 2× for an oxide thickness of 20 nm compared to similar geometry HFET (no oxide under the gate). In this dissertation we develop a novel processing technique to control the polarity of the interfacial charges that in return controls the threshold voltage. It was done by high temperature gate oxide annealing. To demonstrate the threshold shift mechanism, ZrO2 MOSHFETs with different thicknesses were fabricated. Annealing enables excellent threshold voltage control thus minimizing the threshold voltage shift in these high-k MOSHFETs.
Gate insulators are expected to have higher bandgap (EG) to superior leakage characteristics. Material bandgap is inversely proportional to the dielectric constant, so higher the k-value lower is the bandgap. To study the effect of EG and k on UWBG AlGaN-channel MOSHFET performance, we choose three different dielectrics Al2O3 (k=9, EG=8.8 eV), ZrO2 (k=25, EG = 5.8 eV) and TiO2 (k=80, EG=3.2 eV) with three distinct set of k and EG values. Al2O3 with highest EG shows lowest gate leakage while TiO2 with lowest EG shows highest leakage in these set of oxides. TiO2 and ZrO2 shows maximum positive threshold shift, while Al2O3 shows lower positive shift compared to others. High temperature operation of these devices shows promising results, thus making these MOSHFETs ideal candidate for next generation power electronics. To maximize both EG and k value out of these materials we develop hybrid oxide stack by combining ZrO2 and Al2O3.
The drain current density in typical depletion mode AlGaN-channel devices is well below 1 A/mm where for GaN-channel devices it is 1-2 A/mm. In this dissertation, incorporating the hybrid oxide with perforated channel geometry we fabricated depletion (D-) and enhancement (E-) mode Al0.4Ga0.6N-channel MOSHFETs. Perforated channel shows significant reduction in access resistance and hybrid oxide allows positive gate bias application as high as +12 V, that enable realization of drain current density of 1.3 A/mm and 0.48 A/mm respectively, for depletion and enhancement mode MOSHFETs while maintaining extremely low gate leakage. The superior device performance discussed in this dissertation demonstrates that UWBG AlGaN channel D- and E-mode devices are promising for power electronics.
Mollah, M.(2021). Ultrawide Bandgap Algan-Channel Metal Oxide Semiconductor Heterostructure Field Effect Transistors With High-K Gate Dielectrics. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/6775