AlGaN/GaN HEMT's were grown on 6″ Si <111> substrates and passivated using IMEC's in-situ SiN technique. The epitaxial growth was optimized to minimize RF losses due to the substrate-nitride interface while at the same time maintaining high buffer resistivity and low trap density.
To assess RF losses of the epitaxial layer structure, coplanar waveguides were defined on places of the wafer where the top in-situ SiN and AlGaN has been etched away. The attenuation of the RF signals on the coplanar waveguides remained below 0.3dB/mm for frequencies up to 6GHz.
The processing of HEMT's included mesa etching, the formation of TiAlMoAu ohmic contacts, 500nm long gates with source-connected field plates and an airbridge process. The gate fingers are 250µm wide, yielding a total gate periphery of 1.5mm for 6-finger devices and 5mm for the 20-finger version.
The devices' RF power performance was characterised on-wafer. To avoid damage to the RF-probes, active load-pull measurements were performed in pulsed mode with a 100µs period under 10% and 30% duty cycle respectively. As there was no difference between the performance using different duty cycles, we conclude that the channel temperature reaches steady-state in less than 10µs.
The 6-finger devices were operated in deep class AB operation mode but showed clear self-biasing effects. The bias voltage was changed from 30V to 60V in 10V increments. Under 60V bias, the devices provide an output power density of 7.9W/mm at 2GHZ with a PAE of 46% and 6.3W/mm at 4GHz with a PAE of 41%. The output power scales linearly with the bias voltage ranging from 30V to 60V, showing that there is no drain lag in the devices. We attribute this to the efficient surface passivation with the in-situ Si3N4 as well as to high crystal quality and hence the low trap density of the buffer layers. At 2GHz, the linear and saturated power gain are 22dB and 16.4dB respectively. An interesting observation is that the maximum gate current, even at power density levels of 7.9W/mm, remains below 50µA/mm which should prove beneficial for reliability.
The larger 20-finger devices of 5mm total gate periphery reach a maximum absolute output power of 20W at a bias of 40V which represents the limit of our on-wafer measurement system.
These results prove that the use of large area Si substrates is the only viable route forward for AlGaN/GaN HEMT's.