Nidhi Chaturvedi

Mechanism of ohmic contact formation in AlGaN/GaN high electron mobility transistors

We tested various ohmic contact metallization schemes on AlGaN/GaN HEMTs to achieve low contact resistance, good surface morphology and proper line edge definition. Mo, Ni and Pt intermediate layers in these schemes replaced the intermediate Ti layer interposed between Al and Au in Ti/Al/Ti/Au contacts. We recorded almost similar values of the contact resistance lying in the range of 0.3–0.5 Ω mm on all the metallization stacks, but a significant difference was observed in their surface morphology and line edge definition. The formation of particular intermetallic compounds due to different intermediate layers was found to be responsible for this difference. Ti/Al/Ti/Au/WSiN contacts possessed excellent surface morphology. Mo based contact not only delivered good surface morphology and low contact resistance but also proper line edge definition. We found that the formation of the Al–Mo phase and GaMo3 compound was the key factor responsible for its remarkable performance.

Highly robust X-band LNA with extremely short recovery time

GaN-based low-noise amplifiers (LNAs) recently were shown to provide high ruggedness together with low noise figure. These LNAs allow for simplified receiver architectures, e.g., since no limiter is required to protect the input. This paper for the first time presents an investigation of the recovery time of a highly rugged GaN LNA. The X-band LNA is shown to survive input overdrive powers up to 46 dBm under pulsed and 40 dB under cw conditions, with a noise figure of 2.8 dB. Extremely short recovery times below 10 ns were simulated and proved to be below the measurement resolution.

On the Recovery Time of Highly Robust Low-Noise Amplifiers

Recently, GaN-based low-noise amplifiers (LNAs) were shown to provide high ruggedness together with low noise figure. Since no limiter is required to protect the input, these LNAs allow for simplified receiver architectures. This paper presents an in-depth analysis of the recovery time of a highly rugged LNA. Recovery time is measured in the time domain, and an analytical approximation is developed that allows to estimate and optimize recovery. A new measurement setup is established in order to determine the impact of the overdrive pulse on LNA gain. An X-band LNA is shown as an example. It survives input overdrive powers of up to 46 dBm under pulsed and 40 dBm under continuous wave conditions, with a noise figure of 2.8 dB. Extremely short recovery times below were simulated and measured.

Highly Rugged 30 GHz GaN Low-Noise Amplifiers

GaN low-noise amplifiers (LNAs) operating at 27-31 GHz are presented in this letter. The monolithically integrated LNAs were fabricated using the process line of the Ferdinand-Braun-Institut. Noise figures of 3.7 to 3.9 dB were measured. The ruggedness of the LNAs was verified by noise measurements after stressing the LNA for up to 2 h with up to 33 dBm of input power. These conditions are among the most severe stress tests reported in literature. To the best of the authors knowledge, this is the first demonstration of a GaN LNA in this frequency region.

Large Area AlGaN/GaN HEMTs Grown on Insulating Silicon Carbide Substrates

Large periphery Al 0.25 Ga 0.75 N/GaN-HEMTs on SiC-substrates are fabricated on a 2-inch process line using stepper lithography. DC characteristics reveal current densities above 1.2 A/mm and intrinsic transconductances of 360 mS/mm. Depending on device size the maximum frequency of oscillation f max varies from 27-79 GHz. With these devices a power density of 5.2 W/mm and a power level of 13.8 W is achieved at 2 GHz.

Influence of the device geometry on the Schottky gate characteristics of AlGaN/GaN HEMTs

In this work, we investigate the relevance of device geometry to the Schottky gate characteristics of AlGaN/GaN high electron mobility transistors. Changes of three-terminal gate turn-on voltage and gate leakage current on the gate—drain spacing, source—gate spacing and recess depth have been observed. Further examinations comparing device simulations and measurements suggest that gate turn-on voltage is influenced by the distribution of electric potential under the gate region which is related to the geometry. By proper design of the device, high gate turn-on voltage can be obtained for both depletion-mode and recessed enhancement-mode devices.

A non-uniform GaN power TWA for 2 to 10 GHz suitable for square-wave operation

A broadband GaN monolithic power amplifier covering the 2 to 10 GHz band is presented. It is based on a non-uniform traveling-wave architecture using 4 transistor cells with 4×125 µm gate width each. The amplifier achieves 7 to 11 dB small signal gain in the frequency band between 2 and 10 GHz. The circuit delivers between 2.5 and 4.5 W over the bandwidth from 2 GHz up to 10 GHz. At maximum output power a PAE higher than 20% is achieved. In addition, large signal operation with square wave signals have been demonstrated at 1 and 2 Gbps, with 5 W output power at 1 Gbps.

WSiN Cap Layer for Improvement of Ohmic Contact Morphology in AlGaN/GaN High Electron Mobility Transistors

A new technique using WSiN film as a protective cap layer of the internal ohmic metallization scheme and the GaN surface was developed to improve the surface morphology of the contact of AlGaN/GaN high electron mobility transistors (HEMTs). After annealing, this layer was selectively removed by patterning and dry etching. Metal contact surfaces covered with WSiN preserved a good surface morphology and edge definition. Moreover, the devices have a saturation current of 1 A/mm and a maximum transconductance of 235 mS/mm. When biased at 30 V, the output power density is 5.8 W/mm at 2 GHz. These results indicate a damage-free process for the smooth ohmic contacts formation.