Chun-Hyung Cho

Low Turn-On Voltage AlGaN/GaN-on-Si Rectifier With Gated Ohmic Anode

A novel AlGaN/GaN-on-Si rectifier with a gated ohmic anode has been proposed to reduce the turn-on voltage without breakdown-voltage degradation. The combination of an ohmic anode and a recessed Schottky gate is responsible for the low turn-on voltage and thus increases the forward current. In comparison with conventional Schottky diodes, the forward current at 1.5 V was increased by 2 to 3 times, whereas no breakdown-voltage degradation was observed. The proposed rectifier with an anode-to-cathode distance of 18 μm exhibited a turn-on voltage of 0.37 V, a forward current density of 92 mA/mm at 1.5 V, and a breakdown voltage of 1440 V.

Dynamic on-resistance of normally-off recessed AlGaN/GaN-on-Si metal–oxide–semiconductor heterojunction field-effect transistor

In this study, we investigated the process-dependent dynamic on-resistance [RDS(on)] characteristics of recessed AlGaN/GaN-on-Si metal–oxide–semiconductor heterojunction field-effect transistors with a SiO2 gate oxide. In order to improve the dynamic RDS(on) characteristics, the processing technology was carefully optimized, including post-metallization annealing and field plate formation. A threshold voltage of 2.0 V was achieved with a breakdown voltage of 1070 V. The fabricated device exhibited a DC RDS(on) of 5.22 mΩcm2 with very stable dynamic RDS(on) characteristics, i.e., a less than 20% increase up to the drain voltage of 200 V.

Error analysis for piezoresistive stress sensors used in flip chip packaging

Multi-element resistor rosettes on silicon are widely utilized to measure integrated circuit die stress in electronic packages and other applications. Past studies of many sources of error have led to rosette optimization and demonstrated that temperature-compensated stress extraction should be used whenever possible. In this work, we extend the error analysis to include the inherent uncertainty in the measured values of the sensor resistances and the temperature at time of the measurement. The stresses in an under-filled flip chip package are calculated using finite element simulation and utilized to evaluate the stress dependent sensitivities across the die surface. Monte Carlo simulation results confirm that temperature compensated rosette configurations should be utilized whenever possible.

Effects of additives on the mechanical and thermal properties of epoxy-based nanocomposites produced using sonication

Epoxy nanocomposites were synthesized in the presence of hydroxyapatite with the aid of an ultrasonicator. In general, as the amount of hydroxyapatite increased from 0wt% to 10wt%, the mechanical properties of the hydroxyapatite-containing nanocomposite were enhanced. The mechanical properties of the nanocomposite were significantly enhanced by the simple addition of 10 wt% of hydroxyapatite. Specifically, the storage modulus of the 10 wt% hydroxyapatite-containing nanocompsote was 3.2GPa, which is 46% higher compared to that of the pristine epoxy nanocomposite. The glass transition temperature of hydroxyapatite-containing nanocomposites generally decreased by few degrees in Celsius. To investigate the effect of additives on the mechanical properties of the epoxy-based nanocomposite, nanocomposites were synthesized using both montmorillonite and tellurium dioxide instead of hydroxyapatite. Intrestingly, both additive-based nanocomposite materials resulted in an increase in the storage modulus while the glass transition temperature decreased. These results demonstrate that the addition of few wt% of all three additives (hydroxyapatite, montmorillonite, and tellurium dioxide) can enhance the mechanical properties of epoxy-based nanocomposites.

Design Parameter Optimization of a Silicon-Based Grating Waveguide for Performance Improvement in Biochemical Sensor Application

We performed numerical analysis and design parameter optimization of a silicon-based grating waveguide refractive index (RI) sensor. The performance of the grating waveguide RI sensor was determined by the full-width at half-maximum (FWHM) and the shift in the resonance wavelength in the transmission spectrum. The transmission extinction, a major figure-of-merit of an RI sensor that reflects both FWHM and resonance shift performance, could be significantly improved by the proper determination of three major grating waveguide parameters: duty ratio, grating period, and etching depth. We analyzed the transmission characteristics of the grating waveguide under various design parameter conditions using a finite-difference time domain method. We achieved a transmission extinction improvement of >26 dB under a given bioenvironmental target change by the proper choice of the design procedure and parameters. This design procedure and choice of appropriate parameters would enable the widespread application of silicon-based grating waveguide in high-performance RI biochemical sensor.

Diode Embedded AlGaN/GaN Heterojuction Field-Effect Transistor

Monolithically integrated devices are strongly desired in next generation power ICs to reduce the chip size and improve the efficiency and frequency response. Three examples of the embedment of different functional diode(s) into AlGaN/GaN heterojunction field-effect transistors are presented, which can minimize the parasitic effects caused by interconnection between devices.

Stress-Sensors with High-Sensitivity Using the Combined Meandering-Patterns

In this work, the combined meandering-pattern stress-sensors were presented in order to achieve the high sensitivity of stress sensors. Compared to the previous works, which have been using the single meandering-pattern stress-sensors, the sensitivity was approximately observed to increase by 30%~70%. Also, in this paper, more simple and convenient stress-measurement method was presented.