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Design And Fabrication Of A Monolithically Integrated Thermal Self-Protection Circuit With A High Voltage GaN-on-Si Power Hemt

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Abstract

Abstract

Dilip M. Risbud

DESIGN AND FABRICATION OF A MONOLITHICALLY INTEGRATED

THERMAL SELF-PROTECTION CIRCUIT WITH A HIGH VOLTAGE

GaN-on-Si POWER HEMT

In recent years, significant progress has been made in developing heterojunction GaN power high electron mobility transistors (HEMTs) on silicon substrates. While GaN-on-Si HEMTs have several performance advantages over silicon MOSFETs due to the material properties, the self-heating effect under high power dissipation conditions has a serious impact on device performance and reliability due to material degradation such as breakdown of the nitride or other dielectric layers. Instantaneous di/dt and dv/dt transients may cause rapid heating of the power HEMT during switching in applications such as automotive, power conversion and motor drive. Compared to the RF and microwave devices, high voltage power HEMTs (600 – 1200V) use a large number of gate fingers (or anode and cathode fingers in case of diodes) to achieve high voltage and current operation. However, such large designs using a compact layout to save die area run the risk of thermal crosstalk between the fingers. The resulting enhanced self-heating effect can lead to damage or destruction of the device. The assumption of uniform temperature distribution throughout the channel holds well for smaller power devices, but for the substantially larger high voltage power diodes the temperature distribution is not uniform from the center of the die to the outer edge. This non-uniformity along with self-heating effects poses serious challenges for device reliability and highlights the need to design self-protection features to prevent the device from destruction during operation.

The purpose of this thesis is two-fold. First, the non-uniform temperature distribution across the die is modeled and measured to confirm that the fingers at the center are the hottest and there is a gradual decrease in temperature away from the center towards the edge. This work reports the thermal characterization results of various large area multi-finger AlGaN/GaN Schottky Barrier Diodes (SBDs) fabricated in high voltage GaN-on-Si power semiconductor technology. An accurate thermal model was developed and thermal simulations were performed for the device structure to estimate the device temperature near the 2-dimensional electron gas (2-DEG) in SBDs and HEMT switches for various power densities. Micro-Raman thermography and infrared imaging techniques were used for experimental measurements to validate simulation results. The rise in channel temperature under various power densities and mapping of the thermal gradient across the device during operation is reported.

The second purpose is to utilize the detailed knowledge obtained from thermal characterization and design a novel thermal shutdown scheme that can be monolithically integrated into a high voltage power HEMT, thus providing a self-protection feature. A cell library of individual analog and digital building block sub-circuits was developed starting with modeling of the diode and the HEMT. A test chip of all individual cell designs was fabricated and each cell was tested at wafer level for functionality. This thesis describes the design, fabrication and test results of each building block sub-circuit. It was demonstrated that monolithic integration of smart control features is feasible in the emerging GaN-on-Si power semiconductor technology.

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