科研成果 by Type: Conference Paper

In this letter, we have demonstrated that the circular transmission linear model (Marlow's CTLM) is unsuitable for GaN HEMTs structure alloyed ohmic contact resistance evaluation. Very large spread is found in the extracted ohmic resistance values from measured data using the commonly used CTLM test patterns, and some of the contact resistances are found to be negative. We suspect that the stress induced by ohmic contact formation process is the culprit, preventing the use of CTLM test pattern for GaN HEMTs structure ohmic contact resistance evaluation, because of the strong piezoelectric induced polarization property of the hexagonal Ill-nitride heterojunction device structure. Meanwhile, measured ohmic contact resistance (R-c) and sheet resistance (R-sq) are found to exhibit good uniformity using a properly prepared linear transmission line model (LTLM) test pattern in which all the Gallium nitride material extended beyond the gaps between the ohmic contact electrodes are removed.

Presence of atmospheric PAHs in urban and suburban region (Beijing, China) was studied in April and July 2011. Forty-four pairs of gas and particle (TSP) phase samples were collected every six day by high volume (Hi-Vol) air samplers at four sampling sites, and determined separately by GC/MS based on USEPA Method TO-13A. Average total concentration (gas + particles) of PAHs (T-PAHs) was 135.1±49.0 ng/m3 and 181.2±40.9 ng/m3 in April and July, respectively. Gas phase PAHs (G-PAHs) was the major fraction, comprising 63–92% of T-PAHs. Lighter (2-, 3-, 4-ring) and heavier (5-,6-ring) PAHs were found predominantly in gas and particle phase, respectively. 2- to 6- ring PAHs contributed 10%, 53%, 26%, 7% and 4% of T-PAHs, respectively. Five major PAHs, naphthalene (NAP), fluorene (FLU), PHE, fluoranthene (FLA), and pyrene (PYR) contributed 70 – 90% of T-PAHs. G-PAHs increased significantly while PAHs in particle phase (P-PAHs) decreased from April to July. Volatilization from soil and more emission from power generation increase might explain the increase of G-PAHs, and the washout of P-PAHs along with particles might explain the decrease of P-PAHs. Given particulate organic carbon (OC) and elemental carbon (EC) being well correlated, P-PAHs was moderately correlated with OC and EC, suggesting that there were other mechanisms contributing to P-PAHs different from those of OC/EC. Significant correlation between P-PAHs with SO2 and NO2 suggested coal combustion and automobile exhaust to be contamination contributors.

AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors (MOSHEMTs) with thick (>30 nm), high-kappa (TiO2/NiO), submicron-footprint (0.4 mu m) gate dielectric on SiC substrate are demonstrated, which are found to exhibit low gate leakage current (similar to 1 nA/mm of gate periphery), high I-MAX (1 A/mm), and high drain breakdown voltage (188 V). The derived current gain cutoff frequency is 30 GHz (from S-parameter measurements). The output power density is 6.6 W/mm, and the associated power-add ed-efficiency is 46% at 2.5 GHz frequency and 50 V drain bias. This high performance submicron-footprint MOSHEMT is highly promising for microwave power amplifier applications in communication and radar systems.