Sukwon Choi

Optically isotropic-nanostructured liquid crystal composite with high Kerr constant

The relationship between material parameters of host nematic liquid crystals (LCs) and Kerr constant of their nanostructured chiral LC composites was investigated. We made certain that the Kerr constant of nanostrutured chiral LC composites was closely related to the parameters of their host LCs, such as value of the difference of refractive index (Δn), the dielectric anisotropy (Δe), and bend to splay elastic constant ratio (K33∕K11).

Analysis of the residual stress distribution in AlGaN/GaN high electron mobility transistors

A comparative analysis of the residual stress distributions across the conductive channel of Ga-face AlGaN/GaN high electron mobility transistors (HEMTs) is presented. Stress was measured by means of micro-Raman spectroscopy and micro-photoluminescence (PL). Raman measurements probed the volume average of the stress through the GaN layer whereas the stress near the GaN surface (AlGaN/GaN heterointerface) was acquired via PL. By combining Raman, PL, and x-ray diffraction, a self-consistent method was developed to accurately determine the variation in magnitude of stress throughout the thickness of the GaN layer. Based on this framework, it is observed in AlGaN/GaN HEMTs that a depth variation in the GaN residual stress occurs near the gate and ohmic electrodes. At these regions, the stress near the AlGaN/GaN interface (or GaN surface) exhibits a tensile shift compared to the stress averaged through the entire thickness of GaN. Across the conductive channel (away from the metal pads), the bulk average stres...

Thermometry of AlGaN/GaN HEMTs Using Multispectral Raman Features

In this paper, we utilize micro-Raman spectroscopy to measure temperature and stress in state-of-the-art AlGaN/GaN HEMTs. A rigorous discussion on the physical accuracy, precision, and precautions for diverse Raman thermometry methods is developed. Thermometry techniques utilizing shifts in a single Raman Stokes peak position underpredict the channel temperature due to induction of operational thermoelastic stress in operating devices. Utilizing the change in phonon linewidth by employing a proper reference condition gives true temperature results. Making use of frequency shifts in both the E2(high) and A1(LO) phonon modes offers accurate and time-efficient means to determine the state of temperature and thermal stress in operating AlGaN/GaN HEMTs presuming that linear relations between phonon frequencies and temperature/stress are well determined. Useful applications of this method such as monitoring stress in GaN wafers between fabrication steps and Raman thermography on AlGaN/GaN HEMTs are demonstrated.

The Impact of Bias Conditions on Self-Heating in AlGaN/GaN HEMTs

The thermal response of AlGaN/GaN high electron mobility transistors directly correlates with the overall performance and reliability of these devices. In general, a hot spot develops near the drain end of the gate electrode during power dissipation. The device channel temperature was examined via micro-Raman spectroscopy under various bias conditions where power dissipation levels were identical. Under these bias conditions, difference in internal states (sheet carrier density and electric held distribution) within the device alters the heat generation profile across the channel. High Vds conditions lead to significantly higher channel temperature compared to that for low Vds conditions although the power dissipation is kept constant. Experimental results show ~13°C deviation between Vds = 45 V and Vds = 7 V cases when the power dissipation is 4.5 W/mm. This suggests that bias conditions may have a relatively signihcant impact on device reliability and that this effect must be considered when building thermal models of devices under operation or undergoing accelerated life testing.

Electrical and structural dependence of operating temperature of AlGaN/GaN HEMTs

Abstract Understanding the distribution of the considerable heat generated in the active region of high power AlGaN/GaN high electron mobility transistors (HEMTs) at the sub-micron length scales relevant to the failures being observed in these devices is crucial for understanding device performance and reliability. In addition, electrical bias conditions and structural characteristics such as field plates alter the electric field distribution and thermal path within the device leading to changes in the heat generation profile across the channel. This in turn influences the value and location of the device peak temperature and the channel to ambient (or case or base-plate) thermal resistance. The channel temperature distribution of AlGaN/GaN HEMT structures with and without source connected field plates were examined via micro-Raman spectroscopy and coupled electro-thermal simulation. For both type of structures, high V ds conditions lead to significantly higher channel temperature compared to that for low V ds conditions for the same power dissipation level . This is important because the industry standard Arrhenius relation assumes the total power is sufficient to describe the device channel temperature and that the bias condition is irrelevant [1] . We explore the level of agreement between modeling and experiment, and also the extent to which variability in input parameters for the modeling affects model results. We show that operating bias condition has a significant role in device reliability by altering value and location of the peak temperature, which then alters the type and rate of thermally induced degradation taking place at critical locations such as the drain side corner of the gate. Specifically, care must be taken when extrapolating results of an accelerated life test to usage conditions at dissimilar bias conditions to consider if the results will be applicable.

The impact of mechanical stress on the degradation of AlGaN/GaN high electron mobility transistors

Coupled electro-thermo-mechanical simulation and Raman thermometry were utilized to analyze the evolution of mechanical stress in AlGaN/GaN high electron mobility transistors (HEMTs). This combined analysis was correlated with electrical step stress tests to determine the influence of mechanical stress on the degradation of actual devices under diverse bias conditions. It was found that the total stress as opposed to one dominant stress component correlated the best with the degradation of the HEMT devices. These results suggest that minimizing the total stress as opposed to the inverse piezoelectric stress in the device is necessary in order to avoid device degradation which can be accomplished through various growth methods.

Thermal Design and Characterization of Heterogeneously Integrated InGaP/GaAs HBTs

Flip-chip heterogeneously integrated n-p-n InGaP/GaAs heterojunction bipolar transistors (HBTs) with integrated thermal management on wide-bandgap AlN substrates followed by GaAs substrate removal are demonstrated. Without thermal management, substrate removal after integration significantly aggravates self-heating effects, causing poor I - V characteristics due to excessive device self-heating. An electrothermal codesign scheme is demonstrated that involves simulation (design), thermal characterization, fabrication, and evaluation. Thermoreflectance thermal imaging, electrical-temperature sensitive parameter-based thermometry, and infrared thermography were utilized to assess the junction temperature rise in HBTs under diverse configurations. In order to reduce the thermal resistance of integrated devices, passive cooling schemes assisted by structural modification, i.e., positioning indium bump heat sinks between the devices and the carrier, were employed. By implementing thermal heat sinks in close proximity to the active region of flip-chip integrated HBTs, the junction-to-baseplate thermal resistance was reduced over a factor of two, as revealed by junction temperature measurements and improvement of electrical performance. The suggested heterogeneous integration method accounts for not only electrical but also thermal requirements providing insight into realization of advanced and robust III-V/Si heterogeneously integrated electronics.

MEMS switching of contour-mode aluminum nitride resonators for switchable and reconfigurable radio frequency filters

Switching of transducer coupling in aluminum nitride contour-mode resonators provides an enabling technology for future tunable and reconfigurable filters for multi-function RF systems. By using microelectromechanical capacitive switches to realize the transducer electrode fingers, coupling between the metal electrode finger and the piezoelectric material is modulated to change the response of the device. On/off switched width extensional resonators with an area of 24 dB switching ratio at a resonator center frequency of 635 MHz. Other device examples include a 63 MHz resonator with switchable impedance and a 470 MHz resonator with 127 kHz of fine center frequency tuning accomplished by mass loading of the resonator with the MEMS switches.

Thermal characterization of gallium nitride p-i-n diodes

In this study, various thermal characterization techniques and multi-physics modeling were applied to understand the thermal characteristics of GaN vertical and quasi-vertical power diodes. Optical thermography techniques typically used for lateral GaN device temperature assessment including infrared thermography, thermoreflectance thermal imaging, and Raman thermometry were applied to GaN p-i-n diodes to determine if each technique is capable of providing insight into the thermal characteristics of vertical devices. Of these techniques, thermoreflectance thermal imaging and nanoparticle assisted Raman thermometry proved to yield accurate results and are the preferred methods of thermal characterization of vertical GaN diodes. Along with this, steady state and transient thermoreflectance measurements were performed on vertical and quasi-vertical GaN p-i-n diodes employing GaN and Sapphire substrates, respectively. Electro-thermal modeling was performed to validate measurement results and to demonstrate th...

High-power flexible AlGaN/GaN heterostructure field-effect transistors with suppression of negative differential conductance

We investigate thermo-electronic behaviors of flexible AlGaN/GaN heterostructure field-effect transistors (HFETs) for high-power operation of the devices using Raman thermometry, infrared imaging, and current-voltage characteristics. A large negative differential conductance observed in HFETs on polymeric flexible substrates is confirmed to originate from the decreasing mobility of the two-dimensional electron gas channel caused by the self-heating effect. We develop high-power transistors by suppressing the negative differential conductance in the flexible HFETs using chemical lift-off and modified Ti/Au/In metal bonding processes with copper (Cu) tapes for high thermal conductivity and low thermal interfacial resistance in the flexible hybrid structures. Among different flexible HFETs, the ID of the HFETs on Cu with Ni/Au/In structures decreases only by 11.3% with increasing drain bias from the peak current to the current at VDS = 20 V, which is close to that of the HFETs on Si (9.6%), solving the probl...