M. Alomari

Barrier-Layer Scaling of InAlN/GaN HEMTs

We discuss the characteristics of high-electron mobility transistors with barrier thicknesses between 33 and 3 nm, which are grown on sapphire substrates by metal-organic chemical vapor deposition. The maximum drain current (at VG = 2.0 V) decreased with decreasing barrier thickness due to the gate forward drive limitation and residual surface-depletion effect. Full pinchoff and low leakage are observed. Even with 3-nm ultrathin barrier, the heterostructure and contacts are thermally highly stable (up to 1000degC).

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 MOSHEMT With Self-Aligned Thermally Generated Oxide Recess

We report on lattice-matched InAlN/GaN MOSHEMTs with an oxide-filled recess, self-aligned to the gate prepared by thermal oxidation at 800degC in oxygen atmosphere. The device delivered a maximum current density of 2.4 A/mm. Pulse measurements showed no apparent lag effects, indicating a high-quality native oxide. This was confirmed by monitoring the radio-frequency load lines in the time domain. The MOSHEMT yielded a power density of 6 W/mm at a drain voltage as low as 20 V and at 4 GHz, a power added efficiency of 32% and an ft and f max of 61 and 112 GHz, respectively, illustrating the capability of such MOSHEMT to operate at high frequencies.

Thermal conductivity of ultrathin nano-crystalline diamond films determined by Raman thermography assisted by silicon nanowires

The thermal transport in polycrystalline diamond films near its nucleation region is still not well understood. Here, a steady-state technique to determine the thermal transport within the nano-crystalline diamond present at their nucleation site has been demonstrated. Taking advantage of silicon nanowires as surface temperature nano-sensors, and using Raman Thermography, the in-plane and cross-plane components of the thermal conductivity of ultra-thin diamond layers and their thermal barrier to the Si substrate were determined. Both components of the thermal conductivity of the nano-crystalline diamond were found to be well below the values of polycrystalline bulk diamond, with a cross-plane thermal conductivity larger than the in-plane thermal conductivity. Also a depth dependence of the lateral thermal conductivity through the diamond layer was determined. The results impact the design and integration of diamond for thermal management of AlGaN/GaN high power transistors and also show the usefulness of the nanowires as accurate nano-thermometers.

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.

InAlN/GaN heterostructures for microwave power and beyond

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.

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

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).

Above 500 °C operation of InAlN/GaN HEMTs

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.

Thermal stability of 5 nm barrier InAlN/GaN HEMTs

In this paper, the properties of 5 nm barrier HEMT devices has been investigated in a temperature ramping experiment as used for AlGaN/GaN devices. The pinch-off voltage remains identical and the Schottky diode leakage only marginally increased. The ohmic contacts, which have been alloyed at 850 degC by RTA show also exceptional stability. The initial alloying cycle still kept the alloy front just out of reach for high tunnelling probability. It has been moved within this distance in a very controlled way, however without degrading the contact resistance by the 1000 degC post alloying cycle. In conclusion, the InAlN/GaN heterojunction is exceptionally stable even for barrier thicknesses below 10 nm. Experiments for barrier thicknesses of approx. 2.5 nm (verified by a pinch-off voltage of -0.8 V) are presently performed and results will be reported.

Thick nano-crystalline diamond overgrowth on InAlN/GaN devices for thermal management

The use of high quality diamond overlayers as heat sink has been studie d extensively in the past two decade s for high power devices. The outstanding diamond thermal conductivity would indeed enable to extract the extremely high power density of GaN-based devices, whose operation is often limited to pulse mode to prevent excessive device overheating [1].