David Maier

Testing the Temperature Limits of GaN-Based HEMT Devices

The high temperature stability of AlGaN/GaN and lattice-matched InAlN/GaN heterostructure FETs has been evaluated by a stepped temperature test routine under large-signal operation. While AlGaN/GaN high-electron mobility transistors (HEMTs) have failed in an operating temperature range of 500°C, InAlN/GaN HEMTs have been operated up to 900°C for 50 h (in vacuum). Failure is thought to be still contact metallization stability related, indicating an extremely robust InAlN/GaN heterostructure configuration.

InAlN/GaN HEMTs for Operation in the 1000 Regime: A First Experiment

GaN-based heterostructures, and here, particularly, the lattice matched InAlN/GaN configuration, possess high chemical and thermal stability. Concentrating on refractory metal contact schemes, HEMT devices have been fabricated allowing high-temperature 1-MHz large-signal operation at 1000°C (in vacuum) for 25 h. Despite slow gate contact degradation, major degradation of the heterostructure could not be observed. Extrapolation of the RF characteristics suggests that operation up to gigahertz frequencies at this temperature may be feasible.

Ultrathin Body InAlN/GaN HEMTs for High-Temperature (600

Lattice matched 0.25-μm gatelength InAlN/GaN high electron mobility transistors are realized in an ultrathin body mesa technology (50-nm AlN nucleation layer/50-nm GaN buffer) on sapphire. At room temperature, the maximum output current density is I_DS=0.4A/mm, the threshold voltage V_th=-1.4 V with an associated subthreshold voltage swing of 73 mV/dec and a leakage current ≈ 1 pA (for W_G=50 μm) and thus a current on/off ratio of 10¹⁰. At 600°C, the maximum drain current, threshold voltage, and transconductance are nearly unchanged. The current on/off ratio is still approximately 10⁶. First 1-MHz class A measurements with ±2.0 V peak-to-peak signal amplitude have resulted in 109-mW/mm output power at V_DS=8.75 V.

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Lattice-matched InAlN/GaN high electron mobility transistors (HEMTs) have been prepared in a silicon-on-insulator (SOI)-like configuration. Here, this implies an ultrathin body 50 nm GaN channel/50 nm AlN nucleation layer material structure on sapphire with the active areas confined by mesa etching, resulting in semi-enhancement mode device characteristics. In contrast to conventional technologies, the device characteristics (maximum drain current, threshold voltage and 1 MHz large signal operation) change only within less than approx. 10% up to 600 degrees C compared to room temperature (RT). The current on/off ratio decreases from 10(10) at RT to 10(6) at 600 degrees C, due to residual defect activation. These first results of ultrathin body GaN-on-sapphire-based materials and device technology may indicate that essential improvements in the temperature-handling capability of electronic device structures beyond what is common at present may be possible with only limited sacrifice of device performance.

) Electronics

InAlN/GaN is indeed an alternative to the common AlGaN/GaN heterostructure in electronics and sensing. It enables operation at extremely high temperature once problems with contact metallization and passivation have been solved. It is the only heterostructure known presently, which allows overgrowth of high quality diamond films to combine two of the most stable semiconductors. Thus, applications reach from high power microwaves systems and high temperature electronics to sensing in harsh environment.

GaN-on-insulator technology for high-temperature electronics beyond 400 °C

The ceramic-like thermal/chemical stability of InAlN/GaN HEMT’s was illustrated by operation at 1000 °C for short periods of time [1] and continuous tests operating the devices under 1 MHz large signal conditions at 700 °C in vacuum [2]. The heterostructure itself does not degrade even if subjected to harsh environment processes, like nano-crystalline diamond overgrowth in hydrogen atmosphere at 700 °C over several hours [3]. The degradation mechanism at high temperatures is thus attributed to the degradation of the ohmic contact and gate metallizations. In the devices used for ohmic contact was a stack of Ti/Al/Ni/Au annealed at 800 °C, and the dominating failure mechanism visible was the overflow of Au between the source and drain and thus short-circuiting the entire device (see Fig. 1).

InAlN/GaN heterostructures for microwave power and beyond

This paper presents a thermally actuated nanocrystalline diamond micro-bridge for RF and high power applications. The diamond bridges are integrated on a CPW transmission line and small signal measurements of the actuator working as a switch are presented. The bridges are actuated at 2 volts drawing a current of 30 mA. Tunable inductors with an inductance ratio of 2.2 at 30 GHz are also presented. High power measurements in the range of 24-47 dBm for the diamond actuator in a microstrip topology are presented.

Au Free Ohmic Contacts for High Temperature InAlN/GaN HEMT's

Due to their ceramic-like thermal/chemical stability GaN-based HEMTs are expected to be of high robustness and may also be a prime candidate for reliable high temperature operation. In gas sensing AlGaN/GaN heterostructures have been investigated up to 800¿C. In a simple proof-of-concept experiment InAlN/GaN HEMTs have been operated at 1000¿C for a short period of time in vacuum. However in respect to continuous operation most tests have been limited to a temperature range below 500¿C. Here a continuous test is described operating devices under 1 MHz large signal conditions for 250 hrs at a given temperature increased in steps of 100¿C (in vacuum), concentrating on the temperature range above 500¿C, until failure.

Thermally Actuated Nanocrystalline Diamond Micro-Bridges for Microwave and High Power RF Applications

In this paper, a compressively stressed nanocrystalline diamond (NCD) actuator utilizing a thermal actuation scheme is presented. The growth chemistry of NCD films along with the mechanical properties of diamond are discussed in detail. Stress engineering is performed to realize compressively stressed diamond films for RF-MEMS applications. A NCD based bi-stable actuator is designed and fabricated on a low resistive silicon substrate. Tunable switches are implemented in CPW and microstrip topologies and small signal measurements are performed in the frequency range of 5-30 GHz. High power measurements are performed in the power spectrum of 24-47 dBm on the switches integrated in the microstrip topology.

Above 500 °C operation of InAlN/GaN HEMTs

GaN-based heterostructures, and here, particularly, the lattice matched InAlN/GaN configuration, possess high chemical and thermal stability. Concentrating on refractory metal contact schemes, HEMT devices have been fabricated allowing high-temperature 1-MHz large-signal operation at 1000°C (in vacuum) for 25 h. Despite slow gate contact degradation, major degradation of the heterostructure could not be observed. Extrapolation of the RF characteristics suggests that operation up to gigahertz frequencies at this temperature may be feasible.