Georges Pavlidis

High Resolution Thermal Characterization and Simulation of Power AlGaN/GaN HEMTs Using Micro-Raman Thermography and 800 Picosecond Transient Thermoreflectance Imaging

Self-heating in gallium nitride based high frequency, high electron mobility power transistors (GaN HEMTs) is inspected using micro-Raman thermography and 800 picosecond transient thermoreflectance imaging. The two methods provide complementary temperature information inside the semiconductor and on top metal layers of the GaN HEMT. Self heating is measured under both steady-state and ultra-fast pulsed transient operation with submicron spatial resolution, 50 milliKelvin temperature resolution, and nanosecond time resolution. Fine grain electrothermal modeling of the HEMT steady state and transient self-heating are presented alongside measurements. Large spatial and temporal temperature gradients are quantified. Deviations due to unknown parameters are discussed.

Thermal simulation of heterogeneous GaN/ InP/silicon 3DIC stacks

Integration of materials such as GaN, InP, SiGe, and Si is a natural extension of the 3D-IC perspective and provides a unique solution for high performance circuits. In this approach, application of a component is no longer dependent on semiconductor material selection. In this paper, preliminary results are presented which examine the thermal performance of the technology. A thermal analysis prototype solution in Mentor Graphics™ Calibre® provides surface heat maps based on IC layout, material property, and geometric configuration files. Chiplets are connected by heterogeneous interconnect (HIC). Differences in thermal performance of GaN and InP chiplets are explored by varying the number of HICs. Two methods for building up the model of a test chip are compared. One method uses custom scripts to place discrete blocks in the model to represent HICs, while the other uses thermal material properties extracted from the layout. Measurements presented confirm simulated results.

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...

Thermal characterization of GaN vertical p-i-n diodes

In this study, pioneering research was performed on GaN p-i-n diodes for the first ever assessment of surface temperature distribution by incorporating the use of infrared (IR) thermography, thermoreflectance thermal imaging, Raman thermometry, and thermal simulations. Each technique was advanced in order to obtain self-consistent results with higher accuracy. A two-temperature emissivity calibration procedure was utilized for IR measurements to acquire a temperature map of the p-contact metallization. Higher spatial resolution thermoreflectance thermal imaging was performed with a diverse range of illumination wavelengths including 470 nm and 530 nm. To confirm the results of thermoreflectance, TiO2 thermal nanoprobes were deposited on the device surface which enabled Raman thermometry to be performed on the p-contact metallization. Coherence of the techniques was then validated through thermal modeling. The results suggest that IR thermography, when using the two-temperature emissivity correction procedure, gives reasonable results at high power dissipating conditions. Thermoreflectance and nanopowder assisted Raman thermometry are viable options for GaN vertical device temperature assessment. However, results from Raman thermometry possess relatively large uncertainties and thermoreflectance measurements require multiple illumination wavelengths to ensure the validity of the measured temperatures that are derived from the thermoreflectance calibration coefficient.

Characterization of AlGaN/GaN HEMTs Using Gate Resistance Thermometry

In this paper, gate resistance thermometry (GRT) was used to determine the channel temperature of AlGaN/GaN high electron-mobility transistors. Raman thermometry has been used to verify GRT by comparing the channel temperatures measured by both techniques under various bias conditions. To further validate this technique, a thermal finite-element model has been developed to model the heat dissipation throughout the devices. Comparisons show that the GRT method averages the temperature over the gate width, yielding a slightly lower peak temperature than Raman thermography. Overall, this method provides a fast and simple technique to determine the average temperature under both steady-state and pulsed bias conditions.

Thermal raman and IR measurement of heterogeneous integration stacks

Thermal management and planning is important for heterogeneous integration due to the introduction of a complex thermal path. Thermal measurement of operating devices provides necessary data points for future design as well as validation of models. In this paper, two methods for measuring thermal performance of DAHI (Diverse Accessible Heterogeneous Integration) GaN HEMTs are presented and contrasted: IR microscopy and micro Raman spectroscopy. The QFI IR system uses a per-pixel material emissivity flat temperature calibration when the device is in an off-state, and then calculates operating temperatures by CCD exposure. Two separate QFI systems with differing CCD resolutions were used to collect thermal data and are compared. Raman Thermometry by contrast, is a laser point measurement of the frequency shift in scattered photons due to phonon vibrational modes whose frequencies are temperature dependent. Differences in measurements between the two methods arising from the stack of materials used in the DAHI process and their transparency are discussed. A method for measuring the surface temperature of the devices through Raman by the use of TiO2 nanoparticles is also presented in conjunction with a profile of the HEMT. Measurements are presented alongside thermal simulation results using prototype software Mentor Graphics™ Calibre®.

The thermal effects of substrate removal on GaN HEMTs using Raman Thermometry

The ability to fabricate AlGaN/GaN high electron mobility transistors (HEMTs) on Si substrates has enabled the production of low cost high power electronics. To further enhance the performance of GaN electronics for high power conversion, the ability to maintain high off-state breakdown voltages with large electron densities is necessary. The use of Si substrates, however, limits the devices capabilities due to its weak electrical field strength. This limitation has been identified as the main cause for breakdown in HEMTs when the high electric field reaches the silicon substrate underneath the region between the gate and drain. To overcome this obstacle, removal of the Si substrate between the gate and drain region has shown to increase the devices breakdown voltage up to 3000 V. While removing the Si substrate extends the capabilities of GaN HEMTs for high voltage applications, the effects of the Si removal on the thermal performance during operation has not yet been investigated. Raman Thermometry, a well-developed technique, is used to compare the maximum temperature rise between a Local Substrate Removed (LSR) device and a non-LSR device. The application of nanoparticles (TiO2 and ZnO) for measuring surface temperatures via Raman spectroscopy is also investigated and applied to determine a more accurate temperature of the gate junction temperature. The LSR device was found to have a much higher thermal resistance than its non-LSR device counterpart limiting the maximum power dissipation the LSR device can achieve before severe degradation. Volumetric averaged residual stress mapping was also measured via Raman Spectroscopy and suggests the removal of the Si relaxes the stress in the GaN buffer layer and AlGaN barrier which can be exploited in designs to improve reliability. Methods to improve the thermal reliability of LSR devices are key to implementing such devices as future power switches.