Modeling, fabrication and characterization of silicon tunnel field effect transistors

Over the last decades, the continuous down-scaling of metal-oxide-semiconductor field-effect transistors (MOSFETs) enabled faster and more complex chips while at the same time the space and power-consumption was kept under control. However, in the future, the further reduction of the power consumption per unit area will be restricted by a fundamental limit of the inverse subthreshold swing of MOSFETs, which relates its on/off-current-ratio to the operation voltage. Since logic devices operate at a given on/off-current-ratio, the limited subthreshold swing will prevent further reduction of the operation voltage, which is the main parameter to reduce the power consumption. In this thesis, the Tunnel-FET (TFET) is studied as an alternative switching device which could overcome the physical limit of the subthreshold slope in MOSFETs. After introducing the working principle of the TFET, device parameters are studied extensively in quantum simulations based on the non-equilibrium-Green’s-function method. It is found that the performance of a nanowire device geometry is superior to that of planar structures and that the gate dielectric should be as thin as possible. Moreover, the impact of doping concentration on the switching behavior is investigated. For very large doping concentrations, the subthreshold swing is expected to deteriorate while smaller doping concentrations lead to reduced on-currents. Therefore, the doping concentrations need to be tailored to a specific application. Finally, TFETs with different substrate materials are simulated and the influence of bandgap and effective masses is illustrated. A small bandgap improves band-to-band tunneling currents, therefore, the on-currents of the TFET increase. However, due to the ambipolar behavior of the TFET, the off-currents increase as well. Therefore, an optimal TFET is proposed, which is a heterostructure nanowire that utilizes a small bandgap material at the source/channel-junction and a large bandgap material at the drain/channel-junction. The extensive simulations are complemented by a study on different experimental realizations of the TFET: As a first step, planar silicon TFETs were fabricated on ultra-thin-body silicon-on-insulator substrates. The resulting TFETs exhibit minimal inverse subthreshold slopes of 325 mV/dec and on-currents of the order of 10^-2 µA/µm. Since these results are inferior to MOSFET performance, optimizations of the doping concentration and gate dielectric thickness are investigated and both parameters are found to impact the performance as predicted by the simulations. Furthermore, the lateral steepness of the source doping profile is identified as an important parameter, which limits the switching slope. To benefit from the improved electrostatics of nanowires, in a second step, silicon nanowire array TFETs with widths of < 20 nm were fabricated using a top-down approach. In order to optimize the slope of the doping profile, for the first time laser annealing was employed for dopant activation in TFETs. To find the optimum annealing conditions, the impact of different laser energies in combination with a thermal post-anneal treatment on the TFET performance is studied. The electrical characterization of the nanowire TFETs shows an improvement of the subthreshold swing by about 10% and of the on-currents by one order of magnitude when compared to the planar TFETs. To deepen the understanding of TFET operation, low temperature measurements have been performed and band-to-band tunneling is found to be the dominant conduction method. Moreover, for the first time possible parasitic recombination mechanisms are identified in a TFET which might limit the switching slope in silicon. Since small-band gap heterostructure nanowires might offer largely improved tunneling probabilities, in this thesis, a first experimental realization of InSb nanowire MOSFETs is presented. As the bandgap is the most important property for TFET applications, it is carefully extracted from the electrical characteristics and it is found to match the value known from bulk InSb very well. In summary, this thesis presents quantum simulations and two experimental realizations of TFETs in silicon are studied in detail. Variations of device parameters show a path for further optimizations of silicon TFETs. As a first step beyond silicon, InSb nanowire MOSFETs are fabricated successfully for the first time and the potential of InSb for TFET operation is discussed