Uttiya Chowdhury

TEM Observation of Crack- and Pit-Shaped Defects in Electrically Degraded GaN HEMTs

AlGaN/GaN high-electron mobility transistors stressed under dc bias at various channel temperatures were studied using transmission electron microscopy for evidence of physical damage. Stressed devices consistently developed crack- and pit-shaped defects in the AlGaN/GaN crystal material under the drain-side edge of the gate, whereas side-by-side as-processed unstressed devices did not show these features. Furthermore, the amount of physical damage was found to correlate to the amount of electrical degradation as measured by the change in IDmax from before and after stress. The formation of these defects is consistent with the theory of damage from the inverse piezoelectric effect.

Physical degradation of GaN HEMT devices under high drain bias reliability testing

The AlGaN/GaN heterostructure HEMTs were epitaxially grown using MOCVD on semi-insulating SiC substrates. Standard III–V commercial production processing technology was used to fabricate the devices, which were then subjected to stress under accelerated DC life-tests with base-plate temperatures of 82, 112, and 142 C. Drain bias of 40 V and time-zero drain current of 250 mA/mm were applied. TEM samples were prepared via the lift-out technique using a focused ion beam (FIB). TEM analysis revealed that electrically degraded devices always contain a pit-like defect next to the drain in the top AlGaN layer. It has been found that the degree of the defect formation strongly correlates to drain current (IDmax) degradation.

Temperature assessment of AlGaN/GaN HEMTs: A comparative study by Raman, electrical and IR thermography

The accuracy of different thermography techniques for the determination of AlGaN/GaN HEMT channel temperature was investigated. Micro-Raman thermography, a novel electrical testing method, and IR thermography were applied to measure the temperature in the active region of AlGaN/GaN HEMTs with different device geometries. Due to its accepted accuracy, micro-Raman thermography was performed on different devices in order to validate thermal simulation results. When compared to the validated thermal model, pulsed I-V measurements underestimated channel temperature to some degree, while IR thermography determined unrealistically low device temperatures.

Correlation between RF and DC reliability in GaN high electron mobility transistors

Although RE life test is a more definitive technique for RE FET reliability estimation, DC life test is often preferred over RE life test due to its simplicity. In this work, we study how degradation of DC performance in GaN high electron mobility transistors correlates to RE performance degradation in both RE and DC life tests. We show that DC life tests can seriously underestimate device lifetime in GaN HEMTs due to lower instantaneous VDG. The impact of gate current degradation on output power degradation is also discussed.

Method for Estimation of the Channel Temperature of GaN High Electron Mobility Transistors

A quick and reliable method to estimate the channel temperature of GaN high electron mobility transistors is extremely important in order to understand the physical degradation mechanisms as well as to extract a meaningful life time of the device. In this work, we present a simple yet powerful method to electrically measure the channel temperature of GaN HEMTs with a synchronized pulsed I-V setup. To validate the technique, we extract thermal resistance a) on the same device, multiple times, b) on multiple identical devices on the same wafer, c) on devices with different geometries and d) on identical devices with different level of degradation.

Progress in GaN Devices Performances and Reliability

With the DARPA Wide Bandgap Semiconductor Technology RF Thrust Contract, TriQuint Semiconductor and its partners, BAE Systems, Lockheed Martin, IQE-RF, II-VI, Nitronex, M.I.T., and R.P.I. are achieving great progress towards the overall goal of making Gallium Nitride a revolutionary RF technology ready to be inserted in defense and commercial applications. Performance and reliability are two critical components of success (along with cost and manufacturability). In this paper we will discuss these two aspects. Our emphasis is now operation at 40 V bias voltage (we had been working at 28 V). 1250 µm devices have power densities in the 6 to 9 W/mm with associated efficiencies in the low- to mid 60 % and associated gain in the 12 to 12.5 dB at 10 GHz. We are using a dual field-plate structure to optimize these performances. Very good performances have also been achieved at 18 GHz with 400 µm devices. Excellent progress has been made in reliability. Our preliminary DC and RF reliability tests at 40 V indicate a MTTF of 1E6hrs with1.3 eV activation energy at 150 0C channel temperature. Jesus Del Alamo at MIT has greatly refined our initial findings leading to a strain related theory of degradation that is driven by electric fields. Degradation can occur on the drain edge of the gate due to excessive strain given by inverse piezoelectric effect.

Physical degradation of GaN HEMT device observed in TEM during reliability test

HRTEM observation was conducted to find evidence of physical degradation in AlGaN/GaN heterostructure HEMT devices after life time testing. A strong relationship between electrical degradation and defects formation near the gate edge was observed. Detailed features of physical damages correlated with the degradation mechanism were discussed.