Liu Xingzhao

Improved Breakdown Voltage in AlGaN/GaN High Electron Mobility Transistors by Employing Polyimide/Chromium Composite Thin Films as Surface Passivation and High-Permittivity Field Plates

The breakdown voltage of AlGaN/GaN high electron mobility transistors (HEMTs) is enhanced by employing metal chromium (Cr) nanoparticle-embedded polyimide (PI) as a high-permittivity (high-K) dielectric covering both the source-gate and gate-drain regions. The PI/Cr composite high-K dielectrics acting as a field plate prevent the occurrence of strong electric fields produced at the drain side edge of the gate electrode to obtain an optimum lateral electric flux of HEMTs. The breakdown voltage is improved by approximately 35% when using the PI/Cr thin film dielectric field plate while maintaining high performance, a high transconductance value of 122.4 mS/mm, and a large saturated drain-current value of 748 mA/mm.

Sodium beta-alumina thin films as gate dielectrics for AlGaN/GaN metal?insulator?semiconductor high-electron-mobility transistors

Sodium beta-alumina (SBA) is deposited on AlGaN/GaN by using a co-deposition process with sodium and Al2O3 as the precursors. The X-ray diffraction (XRD) spectrum reveals that the deposited thin film is amorphous. The binding energy and composition of the deposited thin film, obtained from the X-ray photoelectron spectroscopy (XPS) measurement, are consistent with those of SBA. The dielectric constant of the SBA thin film is about 50. Each of the capacitance?voltage characteristics obtained at five different frequencies shows a high-quality interface between SBA and AlGaN. The interface trap density of metal?insulator?semiconductor high-electron-mobility transistor (MISHEMT) is measured to be (3.5~9.5)?1010 cm?2?eV?1 by the conductance method. The fixed charge density of SBA dielectric is on the order of 2.7?1012 cm?2. Compared with the AlGaN/GaN metal?semiconductor heterostructure high-electron-mobility transistor (MESHEMT), the AlGaN/GaN MISHEMT usually has a threshold voltage that shifts negatively. However, the threshold voltage of the AlGaN/GaN MISHEMT using SBA as the gate dielectric shifts positively from ?5.5 V to ?3.5 V. From XPS results, the surface valence-band maximum (VBM-EF) of AlGaN is found to decrease from 2.56 eV to 2.25 eV after the SBA thin film deposition. The possible reasons why the threshold voltage of AlGaN/GaN MISHEMT with the SBA gate dielectric shifts positively are the influence of SBA on surface valence-band maximum (VBM-EF), the reduction of interface traps and the effects of sodium ions, and/or the fixed charges in SBA on the two-dimensional electron gas (2DEG).

Breakdown voltage enhancement of AlGaN/GaN high electron mobility transistors by polyimide/chromium composite thin film passivation

A novel AlGaN/GaN high electric mobility transistor (HEMT) with polyimide (PI)/chromium (Cr) as the passivation layer is proposed for enhancing breakdown voltage and its DC performance is also investigated. The Cr nanoparticles firstly introduced in PI thin films by the co-evaporation can be used to increase the permittivity of PI film. The high-permittivity PI/Cr passivation acting as field plate can suppress the fringing electric field peak at the drain-side edge of the gate electrode. This mechanism is demonstrated in accord with measured results. The experimental results show that in comparison with the AlGaN/GaN HEMTs without passivation, the breakdown voltage of HEMTs with the PI/Cr composite thin films can be significantly improved, from 122 to 248 V.

AlGaN/GaN MISHEMTs with Sodium-Beta-Alumina as the Gate Dielectrics

AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MISHEMTs) with sodium beta-alumina (SBA) thin films as the gate dielectrics are studied. AlGaN/GaN metal-semiconductor high-electron-mobility transistors (MESHEMTs) and MISHEMTs with Al2O3 thin-film gate dielectrics are also fabricated for comparative study. The MISHEMTs with SBA gate dielectrics show nearly four orders of magnitude lower gate leakage current and an approximately 60% increase in maximum transconductance, indicating that SBA can serve as an effective dielectric for AlGaN/GaN MISHEMTs. However, SBA gate dielectrics result in threshold voltage modulation of AlGaN/GaN MISHEMTs. Compared with those of AlGaN/GaN MESHEMTs, the threshold voltages of AlGaN/GaN MISHEMTs with Al2O3 gate dielectrics shift negatively from −5.5 V to −7.5 V. In contrast with the normally used gate dielectrics, the threshold voltages of MISHEMTs with SBA gate dielectrics shift positively from −5.5 V to −3.5 V Based on an x-ray photoelectron spectrum study and energy band spectrum calculation, the primary mechanism of the threshold voltage modulation is attributed to the decrease in the surface valence-band maximum and the increase in the barrier height by SBA gate dielectrics.

Aluminum Nitride Thin Films on Molybdenum/Polyimide Heterostructure for Bulk Acoustic Resonators

Abstract The c-axis textured aluminum nitride (AlN) thin films with columnar grains perpendicular to Molybdenum(Mo)/ Polyimide(PI)/Si(111) substrate could be obtained through reactive magnetron sputtering at room temperature. The full width at half maximum of the X-ray diffraction rocking curves and E2 (high) peak of Raman spectrum of the AlN thin films were 2.2° and 18.6 cm−1, respectively. The thin film bulk acoustic resonators (FBARs) with Mo/AlN/Mo/PI/Si (111) configuration were fabricated, and a PI/Mo heterostructure was used as acoustic isolation layer for the FBARs. The resonant frequency response of the FBARs was measured using a vector network analyzer, and an effective coupling coefficient of 5.4% was achieved.

Performance of an AlGaN/GaN MISHEMT with sodium beta-alumina for gate insulation and surface passivation

An AlGaN/GaN metal—insulator—semiconductor high-electron-mobility transistor (MISHEMT), with sodium beta-alumina (SBA) for both gate insulation and surface passivation, was investigated and compared with a conventional metal—semiconductor high-electron-mobility transistor (MESHEMT). The measured gate leakage current of the MISHEMT was reduced by approximately one order of magnitude as compared with that of the conventional MESHEMT. The saturation drain current of the AlGaN/GaN MISHEMT reached 830 mA/mm, which was about 43% higher than that of a conventional MESHEMT. The peak extrinsic transconductance of the MISHEMT was 103 mS/mm, which was similarly higher than that of the MESHEMT. The results suggested that the SBA thin film is an effective candidate gate dielectric for AlGaN/GaN MISHEMTs.

The effects of 60Co γ-ray irradiation on the DC characteristics of enhancement-mode AlGaN/GaN high-electron-mobility transistors

The effects of 60Co γ-ray irradiation on the DC characteristics of AlGaN/GaN enhancement-mode high-electron-mobility transistors (E-mode HEMTs) are investigated. The results show that having been irradiated by 60Co γ-rays at a dose of 3 Mrad (Si), the E-mode HEMT reduces its saturation drain current and maximal transconductance by 6% and 5%, respectively, and significantly increases both forward and reverse gate currents, while its threshold voltage is affected only slightly. The obvious performance degradation of E-mode AlGaN/GaN HEMTs is consistent with the creation of electronegative surface state charges in the source-gate spacer and gate-drain spacer after being irradiated.

Electrical Characteristics of Metal Contacts to Boron Doped Polycrystalline Semiconducting Diamond Thin Films

Low resistance ohmic contacts have been fabricated on chemical vapor deposition grown polycrystalline B doped semiconducting diamond thin films by thermally activated solid state reaction process. A bilayer metallization of Ti/Au was employed. After annealing at 820°C for 10 min in Ar atmosphere, the specific contact resistance of 2 × 10-4 Ω·cm2 has been obtained. The properties of the diamond/Ti interfaces have been characterized by using x-rays diffraction and scanning electron microscopy. The performance of semiconducting diamond thin film thermistors has been reported.