17 Décembre – Thesis defense - Florent Albany
10 h30 Amphi Jean-Paul Dom - Laboratory IMS (Building A31)
Robustness assessment of normally-off GaN HEMT technology with fluorine ions implantation co-integrated with normally-on GaN HEMT technology.
Thanks to the outstanding properties of GaN, such as its wide band gap, its high critical electric field, its ability to operate at high temperatures and its high frequency performances, GaN-based HEMTs are considered as the best candidates for the next generation of RF power circuits. Meeting the stringent requirements of 5G, multifunctional GaN MMICs (Monolithic Microwave Integrated Circuits) have already successfully replaced discrete Si and GaAs circuits in base stations with considerable gains in terms of integration and performances. However, the standard GaN HEMT is normally-on type, which means that it is naturally in conduction mode in the absence of gate polarisation (VGS = 0 V). This is due to the strong spontaneous and piezoelectric polarisation of the material. The lack of technology that co-integrates normally-off and normally-on GaN HEMTs is a considerable disadvantage for many applications in terms of safety, reliability, integration and power consumption. The fabrication of normally-off GaN HEMTs at industrial scale remains a challenge due to the high carrier density in the channel. The aims of this thesis work is to assess the robustness and the performances of normally-off GaN HEMTs fabricated by the fluorine ions implantation technique. These HEMTs are manufactured by adding the required steps to the manufacturing process of a commercial normally-on GaN HEMT technology. They will allow the fabrication of digital circuits for the control of high performances power amplifiers circuits, always based on normally-on GaN HEMTs, due to their higher performance. Studies have been carried out on the first and on the second attempts of the founder to co-integrate normally-on and normally-off GaN HEMTs. Automated measurement procedures and tools were developed to assess devices performances and robustness. DC characterisation campaigns showed the high performances and the low dispersion of the HEMTs of the second batch. The normally-off GaN HEMT provide a truly positive threshold voltage Vth = +0,35 V, a peak transconductance gm,max = 590 mS.mm 1 and a drain current density IDS,max over 1 A.mm-1. Accelerated ageing step-stress tests showed several degradation modes related to the presence of fluorine ions in normally-off GaN HEMTs heterostructure. A negative shift in the voltage Vth of the latter is observed from a drain voltage VDS = 10 V. This shift reaches down to -0,85 V in on-state condition at VGS = 1,5 V and VDS = 18 V. A field-assisted migration mechanism of fluorine ions was proposed to explain the normally-off GaN HEMT degradations. This work contributes to a better understanding of the degradation mechanisms affecting the reliability and the performances of normally-off GaN HEMTs fabricated by the fluorine ion implantation technique.