PhD Defense of Miltiadis ALEPIDIS

This thesis focuses on the development of a (bio)-chemical sensor based on an innovative detection paradigm, the out-of-equilibrium body potential in a silicon-on-insulator (SOI) device. The phenomenon belongs to the family of floating body effects, which are generally considered as parasitic in fully depleted SOI metal oxide semiconductor field effect transistors (MOSFET), but it was exploited for memory applications. The starting point of the thesis was the pseudo-MOSFET configuration with pressure controlled probes, where the bulk serves as gate and the two probes are source and drain. In this structure, typically used for electrical characterisation of SOI wafers, the transport at the interface between the Si film and the buried oxide is influenced by the charges placed on the top, similar to the working principle of an ISFET (ion sensing FET). DNA sensing in dry conditions, based on the out-of-equilibrium body potential was proven previously utilising this structure. In this context, the thesis focuses on three key elements concerning the technological developments needed to obtain a simple SOI sensor with metal contacts based on out-of-equilibrium body potential: (1) implementing dynamic measurements with triangular signals applied to the back-gate, (2) replacing the pressure controlled probes of the pseudo-MOSFET with deposited metal contacts and (3) proving the use of out-of-equilibrium body potential for “in-liquid” sensing. In parallel with the technological developments, significant progress was made on the understanding of the out-of-equilibrium body potential through TCAD simulations and modeling. A new TCAD simulation architecture, able to recreate the out-of-equilibrium body potential for pseudo-MOSFET, showed that its origin relates to the potential barrier created under the contacts. For SOI devices with deposited metal pads this behaviour is simply due to the Schottky junction between the metal and the low doped silicon film. This was simply reproduced by solving numerically the Poisson and continuity equations. Based on the above results, the device was be modeled by an equivalent circuit made of a Schottky diode in series with a MOS capacitor, where the inversion charge imposed by the back-gate in the capacitor must be provided by the junction. The proof-of-concept of sensing with body potential in our device was done for in-liquid pH detection. Further TCAD simulations revealed optimization paths and possible sensitivity enhancement through the out-of-equilibrium body potential reading with respect to the conductance shifts.
 

• Irina Ionica, MCF HDR Grenoble INP : Supervisor
• Francis Calmon, Pr INSA Lyon : Reviewer
• Jérôme Launay,  MCF HDR LAAS Toulouse : Reviewer
• Adrian Ionescu, Pr. EPFL : Examiner
• Francisco Gamiz, Pr. Université de Granada : Examiner
• Anne Kaminski, Pr. Grenoble INP : Examiner
• Stéphane Monfray, ingénieur-docteur STMicroelectronics : Examiner
• Maryline Bawedin, MCF Grenoble INP : Guest
• Gérard Ghibaudo, Dr CNRS, IMEP-LAHC: Guest