Siddarth Sundaresan

Microwave dielectric heating of fluids in an integrated microfluidic device

The ability to selectively and precisely control the temperature of fluid volumes ranging from a few microliters to sub-nanoliters in microfluidic networks is vital for a wide range of applications in micro total analysis systems (μTAS). In this work, we characterize and model the performance of a thin film microwave transmission line integrated with a microfluidic channel to heat fluids with relevant buffer salt concentrations over a wide range of frequencies. A microchannel fabricated in poly(dimethylsiloxane) (PDMS) is aligned with a thin film microwave transmission line in a coplanar waveguide (CPW) configuration. The electromagnetic fields localized in the gap between the signal and ground lines of the transmission line dielectrically heat the fluid in the selected region of the microchannel. Microwave S-parameter measurements and optical fluorescence-based temperature measurements are used with a theoretical model developed based on classical microwave absorption theory to fully characterize the temperature rise of the fluid. We observe a 0.95 °C mW−1 temperature rise at 15 GHz and confirm that the temperature rise of the fluid is predominantly due to microwave dielectric heating.

Microwave annealing of Mg-implanted and in situ Be-doped GaN

An ultrafast microwave annealing method, different from conventional thermal annealing, is used to activate Mg-implants in GaN layer. The x-ray diffraction measurements indicated complete disappearance of the defect sublattice peak, introduced by the implantation process for single-energy Mg-implantation, when the annealing was performed at ≥1400 °C for 15 s. An increase in the intensity of Mg-acceptor related luminescence peak (at 3.26 eV) in the photoluminescence spectra confirms the Mg-acceptor activation in single-energy Mg-implanted GaN. In case of multiple-energy implantation, the implant generated defects persisted even after 1500 °C/15 s annealing, resulting in no net Mg-acceptor activation of the Mg-implant. The Mg-implant is relatively thermally stable and the sample surface roughness is 6 nm after 1500 °C/15 s annealing, using a 600 nm thick AlN cap. In situ Be-doped GaN films, after 1300 °C/5 s annealing have shown Be out-diffusion into the AlN layer and also in-diffusion toward the GaN/SiC in...

Ultrahigh-temperature microwave annealing of Al+- and P+-implanted 4H-SiC

The GMU work is supported by Army Research Of- fice Dr. Prater under Grant No. W911NF-04-1-0428 and a subcontract from LT Technologies under NSF SBIR Grant No. 0539321.

12.9 kV SiC PiN Diodes with Low On-State Drops and High Carrier Lifetimes

Sharp avalanche breakdown voltages of 12.9 kV are measured on PiN rectifiers fabricated on 100 µm thick, 3 x 1014 cm-3 doped n- epilayers grown on n+ 4H-SiC substrates. This equates to a record high 129 V/µm for a > 10 kV device. Optimized epilayer, device design and processing of the SiC PiN rectifiers result in a > 60% blocking yield at 10 kV, ultra-low on-state voltage drop and differential on-resistance of 3.75 V and 3.3 mΩ-cm2 at 100 A/cm2 respectively. Open circuit voltage decay (OCVD) measured carrier lifetimes in the range of 2-4 µs are obtained at room temperature, which increase to a record high 14 µs at 225 °C. Excellent stability of the forward bias characteristics within 10 mV is observed for a long-term forward biasing of the PiN rectifiers at 100 A/cm2. A PiN rectifier module consisting of five parallel large area 6.4 mm x 6.4 mm 10 kV PiN rectifiers is connected as a free-wheeling diode with a Si IGBT and 1100 V/100 A switching transients are recorded. Data on the current sharing capability of the PiN rectifiers is also presented.

Characteristics of in situ Mg-doped GaN epilayers subjected to ultra-high-temperature microwave annealing using protective caps

Different protective caps (AlN, MgO, graphite) are investigated for their feasibility for protecting GaN surfaces during ultra-high-temperature (>1300 °C) microwave annealing. Compared to other capping materials, pulsed-laser-deposited AlN is found to protect the GaN surface more effectively, during ultra-fast microwave annealing at temperatures as high as 1500 °C. The RMS surface roughness (0.6 nm) of the GaN sample annealed at 1500 °C with an AlN cap in place is similar to the value (0.3 nm) measured on the as-grown sample. The photoluminescence and electrical measurements have indicated a decrease in the compensating deep donor concentration for the increasing microwave annealing temperature, as long as the surface integrity of the GaN epilayer is preserved. These results indicate the attractiveness of the AlN cap for microwave annealing of ion-implanted GaN.

1200 V SiC “Super” Junction Transistors operating at 250 °C with extremely low energy losses for power conversion applications

The electrical performance of GeneSiCs 1200 V/7 A SiC Super Junction Transistor (SJT) is compared with three best-in-class commercial Si IGBTs in this paper. Low leakage currents of <;100 μA at 325 °C, turn-on and turn-off switching transients of <;15 ns at 250 °C, current gain as high as 72, on-resistance as low as 235 mΩ, second-breakdown-free square RBSOA, and short-circuit withstand time of 22 μs were measured on the SiC SJTs. For switching 7 A and 800 V at 100 kHz, the SiC SJT + GeneSiC SiC Schottky rectifier as Free Wheeling Diode (FWD) achieved a total power loss reduction of about 64% when compared to the best all-Si IGBT+FWD configuration and a power loss reduction of about 47 %, when compared to the best Si IGBT + SiC Schottky FWD.

10 kV SiC BJTs — Static, switching and reliability characteristics

Open-base breakdown voltages as high as 10.5 kV (91% of theoretical avalanche limit and 125 V/μm), on-resistance of 110 mΩ-cm2 close to the unipolar limit of 94 mΩ-cm2, and current gain as high as 75 are measured on 10 kV-class SiC BJTs. Monolithic Darlington-connected BJTs fabricated on the same wafer yield current gains as high as 3400, and show Si BJT-like output characteristics with a differential on-resistance as low as 44 mΩ-cm2 in the saturation region and a distinct quasi-saturation region. Switching measurements performed at a DC link voltage of 5 kV and collector current of 8 A feature a collector current rise time as low as 30 ns during turn-on and collector voltage recovery time as low as 100 ns during turn-off. Very low turn-on and turn-off switching energies of 4.2 mJ and 1.6 mJ, respectively, are extracted from the switching transients, which are 19 and 25 times smaller than the corresponding switching energies reported on 6.5 kV Si IGBTs. When turnedon to a short-circuited load at a collector bias of 4500 V, the 10 kV BJT shows a temperature-invariant, withstand time in excess of 20 μs. Leakage currents <; 1μA (system limit) are measured, even after 234 hours of operation under a DC collector bias of 5000 V at elevated temperatures.

Rapidly Maturing SiC Junction Transistors Featuring Current Gain (β) > 130, Blocking Voltages up to 2700 V and Stable Long-Term Operation

SiC npn Junction Transistors (SJTs) with current gains as high as 132, low on-resistance of 4 mΩ-cm2, and minimal emitter-size effect are demonstrated with blocking voltages > 600 V. 2400 V-class SJTs feature blocking voltages as high as 2700 V combined with on-resistance as low as 5.5 mΩ-cm2. A significant improvement in the current gain stability under long-term high current stress is achieved for the SJTs fabricated by the high gain process.

15 kV SiC PiN diodes achieve 95% of avalanche limit and stable long-term operation

This paper reports on ultra-high voltage, >15 kV SiC PiN rectifiers exhibiting >95% of the avalanche rating and 115 V/μm. This is one of a few reports on > 15 kV blocking voltages measured on any single semiconductor device, and the highest percentage of the avalanche limit ever reported on devices fabricated on > 100 μm thick SiC epilayers. Excellent stability of on-state voltage drop (VF) is displayed by 5.76 mm2 and large-area, 41 mm2 PiN rectifiers, when continually biased at high current densities for several days. The impact of carrier lifetime on the device performance for SiC bipolar devices with ultra-thick (≥100 μm) base layers is investigated by comparing I-V-T characteristics of SiC PiN rectifiers fabricated on 100 μm and 130 μm thick epilayers.

Integrated SiC Anode Switched Thyristor Modules for Smart-Grid Applications

Silicon Carbide Anode Switched Thyristors (ASTs) overcome major limitations of conventional Si and SiC IGBT and GTO Thyristor solutions by providing robust, latch-up free turn-off at high currents, current saturation in the output characteristics, and a wide safe operating area (SOA) through series current controlled device turn-off. In this work, detailed static and switching characteristics of 6.5 kV-class SiC ASTs are reported, which include a low on-state voltage drop of 4 V at 100 A/cm2, slight positive temperature co-efficient of Von, current saturation at > 100 A Cathode currents and fast turn-on and turn-off times of 500 ns while switching 1300 V and 20 A.