The Effects Of Charged Dielectric Thin Films On Performances Of AlGaN/GaN MISHEMTs

AlGaN/GaN-based heterostruture devices, such as AlGaN/GaN based high electron mobility transistors(HEMTs), are promising candidates for power electronic applications due to their attractive properties, including high breakdown field, high electron mobility, and high-temperature operation capability. GaN-based electronic devices are also considerably more radiation-hardness than their conventional GaAs-based counterparts due to the higher displacement energy of the nitride materials. However, high-density two-dimensional electron gas(2DEG) induced by spontaneous and piezoelectric polarization effects presents the conventional AlGaN/Ga N HEMTs as depletion-mode(D-mode) transistors with a threshold voltage(Vth) typically around-4 V.Enhancement-mode(E-mode) devices with a positive Vth are more desirable than conventional D-mode devices in electronic applications because they provide the advantages of a simple circuit design and fail-safe operation. Thus, in order to expand the range of these applications, the development of good radiation resistance E-mode devices have become essential as well.In this dissertation, the high performance normally-off AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors(MISHEMTs) had been achieved by using the fluorinated Al2O3 thin films as gate dielectrics. Comparing with the E-mode HEMT with the same Vth, the E-mode MISHEMT had better electrical performances and radiation resistance.The fluoride-based plasma treatment resulted in incorporation of negatively charged F ions into the surface layer of Al2O3 dielectrics and positively shifted the threshold voltage while diminishing the plasma-induced damage to the AlGaN/GaN heterostructure. The damages could be repaired and the mobility could be recovered by a post-gate annealing step at 400 oC for 10 min. At the time, the Vth of E-mode MISHEMT shown a good thermal stability.In order to realize E-mode devices with sufficiently high Vth which could be used as power switches, two methods were adopted: increased fluoride-based plasma treatment time and deposited a second Al2O3 thin film as a blocking oxide. The E-mode MISHEMT with high Vth by using the second method shown better electrical performances.The variations in binding-energy spectrum and valance-band spectrum in fluorinated-Al2O3/AlGaN/GaN were studied, providing insights to mechanism underlying drastic Vth modulation of MISHEMTs with fluorinated Al2O3 gate dielectrics. It was found that not the surface potential but rather the negative charges in Al2O3 gate dielectrics were the primary factor responsible for conversion from D-mode to E-mode MISHEMT by using fluorinated Al2O3 as gate dielectrics.E-mode HEMT and MISHEMT were exposed to 60 Co γ-rays at a dose of 3 Mrad(Si). The saturation drain current and maximal transconductance of E-mode HEMT decreased. Moreover, either forward or reverse gate bias current significantly increased, while the threshold voltage was relatively unaffected. The degradations of E-mode HEMT were caused by the creation of electronegative surface state charges in source–gate spacer and gate–drain spacer during irradiation. Because of protection of Al2O3, 60 Co γ-rays radiation affected the performances of E-mode MISHEMT slightly.E-mode HEMT and MISHEMT were exposed to 1.8 MeV high-energy electron radiation as well. The saturation drain current and maximal transconductance of E-mode HEMT increased after irradiation. However, almost no change of threshold voltage and gate leakage current were observed. The electron mobility decreased after fluorine plasma treatment and recovered after radiation. Conductance measurements in a frequency range from 10 kHz to 10 MHz were used to characterize the trapping effects in the devices. The fast density of the trap increased by fluoride-based plasma treatment while decreased by radiation. The electron radiation could recover the etch damage caused by F plasma treatment. The E-mode MISHEMT shown a good radiation resistance because of passivation.The E/D inverter with D-mode and E-mode MISHEMT was also fabricated. At a supply voltage of 3 V, the E/D inverter shown an output logic swing of 2.8 V, a logic-low noise margin of 0.34 V and a logic-high noise margin of 1.86 V at room temperature.