R. Aubry

SThM Temperature Mapping and Nonlinear Thermal Resistance Evolution With Bias on AlGaN/GaN HEMT Devices

Channel temperature has a strong impact on the performance of a microwave power transistor. In particular, it has a strong influence on the power gain, energetic efficiency, and reliability of the device. The thermal optimization of device geometry is therefore a key issue, together with precise measurements of temperature within the channel area. In this paper, we have used scanning thermal microscopy to perform temperature mapping, at variable dc bias points, on an AlGaN/GaN high-electron mobility transistor made on epilayers grown on silicon carbide substrate. We have analyzed the variation of the thermal resistance values, which are deduced from these measurements, with bias conditions VGS and VDS. The observed nonlinear behavior is found to be in excellent agreement with physical simulations, strongly pointing out the large variability of the extension of the dissipation area with the dc bias conditions

Surface potential of n- and p-type GaN measured by Kelvin force microscopy

n- and p-type GaN epitaxial layers grown by metal-organic chemical vapor deposition with different doping levels have been characterized by Kelvin probe force microscopy (KFM). To investigate the surface states of GaN beyond instrumental and environmental fluctuations, a KFM calibration procedure using a gold-plated Ohmic contact as a reference has been introduced, and the reproducibility of the KFM measurements has been evaluated. Results show that the Fermi level is pinned for n- and p-type GaN over the available doping ranges, and found 1.34±0.15eV below the conduction band and 1.59±0.18eV above the valence band, respectively.

State of the Art 58W, 38% PAE X-Band AlGaN/GaN HEMTs Microstrip MMIC Amplifiers

This paper presents the results obtained on X-Band GaN MMICs developed in the frame of the Kerrigan project launched by the European Defense Agency. A new step was achieved, 58 W of output power with 38% PAE in X-Band were obtained using an 18 mm 2 2-stages amplifier. To our knowledge, these results present a new state-of-the-art of X-Band MMIC power amplifiers.

43W, 52% PAE X-Band AlGaN/GaN HEMTs MMIC Amplifiers

This paper presents the results obtained on X-Band GaN MMICs developed in the frame of the Korrigan project launched by the European Defense Agency. GaN has already demonstrated excellent output power levels, nevertheless demonstration of excellent PAE associated to very high power in MMIC technology is still challenging. In this work, we present State-of-the-Art results on AlGaN/GaN MMIC amplifiers. An output power of 43W with 52% of PAE was achieved at 10.5 GHz showing that high power associated with high PAE can be obtained at X-band using MMIC GaN technology.

High linearity performances of GaN HEMT devices on silicon substrate at 4 GHz

In this letter, we demonstrate that, for high linearity application, GaN devices benefit from their high drain-source bias voltages. An improvement up to 20 dB in intermodulation ratio can be observed at high power levels compared to usual GaAs PHEMT devices. This study demonstrates that the high bandgap GaN devices are ideal candidates for the applications requiring high power and linearity simultaneously.

Evaluation of Thermal Versus Plasma-Assisted ALD Al 2 O 3 as Passivation for InAlN/AlN/GaN HEMTs

Al2O3 films deposited by thermal and plasma-assisted atomic layer deposition (ALD) were evaluated as passivation layers for InAlN/AlN/GaN HEMTs. As a reference, a comparison was made with the more conventional plasma enhanced chemical vapor deposition deposited SiNx passivation. The difference in sheet charge density, threshold voltage, fT and fmax was moderate for the three samples. The gate leakage current differed by several orders of magnitude, in favor of Al2O3 passivation, regardless of the deposition method. Severe current slump was measured for the HEMT passivated by thermal ALD, whereas near-dispersion free operation was observed for the HEMT passivated by plasma-assisted ALD. This had a direct impact on the microwave output power. Large-signal measurements at 3 GHz revealed that HEMTs with Al2O3 passivation exhibited 77% higher output power using plasma-assisted ALD compared with thermal ALD.

Broadband hybrid flip-chip 6-18 GHz AlGaN/GaN HEMT amplifiers

GaN Based HEMTs have shown superior power-frequency performances than lower band-gap materials. In this paper, we present the design of broadband hybrid 6-18 GHz amplifiers based on AlGaN/GaN HEMT technology with a flip chip approach. Measurements of a single ended amplifier based on a 0.6mm gate width device allow us to achieve more than 1.8W in the [6.5-16] GHz bandwidth corresponding to a power density of 3W/mm. A Maximum output power is obtained at 8 GHz at 2.7W corresponding to 4.5W/mm. Average typical PAE values higher than 17% in the bandwidth with a maximum of 39% were obtained. A balanced amplifier based on two single ended amplifiers was also realized. The output power is above 2.8W in the [7-17] GHz bandwidth corresponding to a power density of 2.4W/mm. Maximum output power is obtained at 7.5 GHz at 4.5W corresponding to 3.8W/mm.

Electrical performances of AlInN/GaN HEMTs. A comparison with AlGaN/GaN HEMTs with similar technological process

A study of the electrical performances of AlInN/GaN High Electron Mobility Transistors (HEMTs) on SiC substrates is presented in this paper. Four different wafers with different technological and epitaxial processes were characterized. Thanks to intensive characterizations as pulsed-IV, [S]-parameters, and load-pull measurements from S to Ku bands, it is demonstrated here that AlInN/GaN HEMTs show excellent power performances and constitute a particularly interesting alternative to AlGaN/GaN HEMTs, especially for high-frequency applications beyond the X band. The measured transistors with 250 nm gate lengths from different wafers delivered in continuous wave (cw): 10.8 W/mm with 60% associated power added efficiency (PAE) at 3,5 GHz, 6.6 W/mm with 39% associated PAE at 10.24 GHz, and 4.2 W/mm with 43% associated PAE at 18 GHz.

Design of GaN-based balanced cascode cells for wide-band distributed power amplifier

This paper reports on the design of a cascode GaN HEMT cell dedicated to 4-18 GHz flip-chip distributed power amplifier. The active device is a 8x50 mum AlGaN/GaN HEMT grown on SiC substrate. The GaN-based die which integrates the active cascode cell and its matching elements is flip-chipped via electrical bumps onto an AIN substrate. The matching elements of the balanced cascode cell are composed of series capacitances on the gate of both transistors with additional resistances to insure stability and bias path. The series capacitor on the gate of the 1st transistor is added for the distributed amplifier optimisation while the series capacitor on the gate of the 2 nd transistor is dedicated

First demonstration of AlInN/GaN HEMTs amplifiers at K band

AlInN/GaN HEMTs have shown outstanding power performances for high frequency applications, due in particular to their high current densities and their thinner barrier layers than in AlGaN/GaN HEMTs that minimize short channel effects. In this paper, we present the first published power results of two K-band hybrid amplifier demonstrators at 20GHz and 26.5GHz using 0.25µm gate length devices. At these frequencies, respectively, cw RF output power of 4.5 Watts with 20% PAE and 1.65 W with 15.5 % of PAE were obtained. These state-of-the-art results confirm the potential of AlInN/GaN technology for high frequency applications.