The Impact of GaN-buffer C-doping Profile on Short-Channel Effects and Ohmic Contact Formation for Millimeter Wave III-Nitride HEMTs
Examensarbete för masterexamen
Wireless, photonics and space engineering (MPWPS), MSc
The InAlN/AlN/GaN HEMT has a high breakdown field which allows for excellent power performance. In order to increase frequency operation, a reduction in device size is required, inherently increasing the short-channel effects and limiting device efficiency and linearity. It is possible to mitigate DIBL and subthreshold swing by increasing the carrier confinement in the 2DEG channel of the HEMT through optimizing the buffer design with the use of GaN-buffer C-doping. The impact of different C-doping profiles on short-channel effects is investigated by the fabrication and characterization of two-finger U-shaped HEMTs with 2 x 25 μm gate widths and 50, 100 and 180 nm gate footprints. DC and S-parameter analysis show an increased immunity to short-channel effects for devices with constant levels of intentional C-doping to the buffer, but that the high frequency and power performance is impaired by a reduction of the on-state conductance. Best RF performance was achieved for a ramped C-doping profile with values of 𝑓𝑇 and 𝑓𝑚𝑎𝑥 of 66 and 180 GHz respectively. Although, these devices showed a deteriorated performance in terms of short-channel effects, including pinch-off, output conductance and current leakage. Consequently, there is a trade-off between the reduction of short-channel effects and an increase of 𝑓𝑇 and 𝑓𝑚𝑎𝑥. Ohmic contact formation and the reduction of contact resistance and specific contact resistivity is important to consider in order to increase the efficient operation of HEMTs. Ohmic contacts induce negligible voltage drop and heat generation compared to the bulk resistance of the semiconductor, but are difficult to realize in wide-bandgap materials. It is possible to assist the formation of ohmic contacts and lower the resistivity by contact recess etching down to the substrate channel and by altering the metallization. Ta-based contacts are considered for their low optimum anneal temperature, improving the sheet resistance of the heterostructure material. Ta/Al/Ta contacts to AlGaN/AlN/GaN heterostructures have been found to be reproducible and to have good performance with concern to contact resistance, specific contact resistivity, surface morphology and line edge acuity. The exclusion layer formed in between the contacts and the semiconductor during anneal, causes N-vacancies in the buffer and promotes tunnelling of carriers through the barrier, which in turn lowers the contact resistance. The lower resistivity of ZrN compared to TaN, yields an interest in Zr-based contacts. Zr/Al/Zr/Ta/Au contacts to AlGaN/AlN/GaN show low optimum anneal temperature and contact resistance but further research is required.
Annan elektroteknik och elektronik , Elektronik , Informations- och kommunikationsteknik , Other Electrical Engineering, Electronic Engineering, Information Engineering , Electronics , Information & Communication Technology