Increased electrical and thermal coupling between different functional blocks, such as power amplifiers and low noise amplifiers, is expected in future communication and sensor systems due to increased level of integration. Simple means to evaluate the electrothermal coupling for different heterostructures, buffer designs and substrates are therefore of interest. A new test structure and methodology is proposed to evaluate the electrical and thermal coupling in gallium nitride heterostructures. The structure consists of an array of differently sized semiconductor resistors, acting as heating and sensing elements. The structure enables measurements of lateral thermal coupling. When dissipating 1.5 W in the heaters, the elevation in sensor temperature ranged from 22 K to 36 K for separations ranging from 49 µm to 229 µm. The transfer-function of the thermal coupling versus frequency exhibits a low-pass characteristic. The limitation of the thermal sensor is demonstrated by measuring the in-pulse behavior of a pulsed IV-measurement.

GaN-on-SiC RF technology suffers from the limited heat extraction capability of the SiC substrate and so replacing SiC with the highest thermal conductivity material known to mankind, diamond, offers the promise of RF devices which can deliver >5x higher power density than current GaN-on-SiC devices. Latest results from the EPSRC Programme Grant GaN-DaME will be reported.

We report on a new floating buffer model to explain “kink”, an undesirable hysteresis in the output characteristics of AlGaN/GaN HEMTs observed at low drain bias. The presence of unintentionally incorporated background carbon (C) can make the intentionally iron (Fe) doped buffer p-type, allowing it to electrically float. The supply of holes to charge the buffer arises due to a band-to-band trap-assisted leakage path rather than via impact ionization. Simulations of the kink show that it can be enabled and modified by small changes in concentration of background carbon that are well below the Fe density. We show HEMT measurements using two wafers with different background carbon having dramatically different kink behaviour fully consistent with the simulated model.

In this work, we investigate on-wafer short term (24 hours) stress test carried out on two wafers of two generations (GH25 ITN2 and GH25 ITN2a) of AlGaN/GaN HEMTs, from UMS qualified GH25 technology, with two different Schottky gate metallization. In order to observe degradation in short time, different conditions have been adopted. The first with extremely DC power dissipation and junction temperature values (i.e. 35 W/mm , Tj=350°C) and the second with high electric field in ON-state and OFF-state conditions. Devices with different level of degradation were analysed by means of STEM and EDX techniques. Au interdiffusion and surface damage seem to be correlated, thus suggesting a common origin (temperature, current flux) for both. The new metallization scheme and surface passivation of GH25 ITN2a prevents both interdiffusion and semiconductor damage up to Tj = 400°C.

We present here our last results on HEMT realized on InAlGaN/GaN heterostructure. Especially, we will show our last development concerning the passivation step containing Al2O3. This latter increases a little the lag effects on one hand but on the other hand, the robustness is highly improved and the transistors can be biased at 35V while maintaining low leakage currents. The best performances obtained @30GHz (CW) with this process are 3W/mm and 42% of associated PAE. Thanks to the enhanced robustness, the power density can be increased up to 9W/mm at VDS=32.5V.
In parallele of these developments, MMICs were fabricated for 2 bandwidth : [27.5-31 GHz] and [37-39.5GHz]. Preliminary results will also be presented.