Simulation bidimensionnelle de l'effet des pièges profonds dans le substrat sur les caractéristiques des transistors a effet de champ en Arséniure de Gallium (GaAs FETs)

摘要

In thisthesis, the reduction of the conductance of GaAs FETs by a negative voltage applied to the substrate, termed backgating or sidegating, was numerically modeled to determine which type of traps is responsible of this phenomenon. Drift diffusion Modelling wascarried out for several sets of deep levels in the substrate. It has been observed that deep acceptors are mainly responsible for backgating, independently of the shallow level type in the substrate. In this case, there is no threshold. However, when deep donors are present in the substrate, backgating is again reduced but with a threshold. The presence of a buffer layer between the channel and the semi-insulating substrate also helps reducing backgating.udA two dimensional-hydrodynamic model was carried out to predict the performance of short-gate length power III–V field effect transistors. The model is based on the conservation equations, deduced from the Boltzmann transport equation and solved in their whole form. This model is also well suited to study the effect of substrate deep levels on the device. The results of hydrodynamic model (physical model) were compared to those of the fully distributed model (electrical model), especially, as for high frequency operating. In fact, at high frequencies, the dimensions of the electrodes of microwave transistors such as FETs become comparable to the wavelength, highlighting the parasitic effect of wave propagation. Thus, this effect needs to be accurately evaluated in the device model to assure a reliable design.udIn the electrical model, the device width was then divided into an infinity number of segments, while each segment was considered as a combination of three coupled lines and a conventional FET equivalent circuit.By solving a set of multi-conductor transmission line equations using the Finite-Difference Time-Domain (FDTD) technique, an accurate and efficient transistor modeling approach was proposed.udFurthermore, thetwo dimensional hydrodynamic model had been shown to provide a valuable insight into theoperation devices and confirm in many cases the measurements.Thus,the HDM model was used to study the effect of the gate length and its the recess depth on the recessed gate MESFET and pHEMT. It was found that, the performance of these devices is improuved by shrinking the gate length and deepening the recess. The effect of increasing the delta doped densityon the pHEMT performance was also studied. It is a way to improve the transfer efficiency of electrons from the delta-doped AlGaAs layer to the InGaAs channel. For the deep levels, it was found that deep acceptors improve the transistor performance while deep donors degraded it.