Improved modeling of Coulomb effects in nanoscale Schottky-barrier FETs

We employ a novel multi-configurational self-consistent Green's function approach (MCSCG) for the simulation of nanoscale Schottky-barrier field-effect transistors. This approach allows to calculate the electronic transport with a seamless transition from the single-electron regime to room temperature field-effect transistor operation. The particular improvement of the MCSCG stems from a division of the channel system into a small subsystem of resonantly trapped states for which a many-body Fock space becomes feasible and a strongly coupled rest which can be treated adequately on a conventional mean-field level. The Fock space description allows for the calculation of few-electron Coulomb charging effects beyond mean-field. We compare a conventional Hartree non-equilibrium Green's function calculation with the results of the MCSCG approach. Using the MCSCG method Coulomb blockade effects are demonstrated at low temperatures while under strong nonequilibrium and room temperature conditions the Hartree approximation is retained

Transportuntersuchungen und quantenmechanische Beschreibung von Einelektronenstrukturen in Halbleiterheterostrukturen

In this thesis the system of a laterally confined resonant tunneling diode in the single-electron transport regime is considered . Here the double barrier system is embedded in a semiconductor heterostructure . The following investigations focus mainly on the electronic transport and electronic structure inside such a quantum dot system, which is coupled to the two electron reservoirs via tunnel barriers . Experimental as well as theoretical results are presented . In particular the influence of an extern" al magnetic field an this system is discussed . The theoretical part gives a description of the magneto-transport by use of a quantum dot model system based on realtime Green's functions . Furthermore the consideration of a projected density matrix provides insight into the electronic structure of the quantum dot . As a result a capacitive quantum dot regime is defined employing the results of the numerical simulations . In the experimental part of the thesis a vertical field effect transistor with a lateral Schottky-Gate (MESFET) and an embedded double barrier structure is presented. Magneto-transport measurements in the sub-Kelvin regime are performed on such a quantum dot system with variable lateral quantization energy controlled by the applied gate voltage . The analysis of the observed typical step-current-voltage characteristics yields information about the electronic structure of the system for the given boundary conditions. A direct comparision of the simulated characteristics of the structure with the measurements shows a very good agreement within the error boundaries for a suitable choice of open structure parameters in the theoretical quantum dot model

Origin of Improved RF Performance of AlGaN/GaN MOSHFETs Compared to HFETs,

In this paper, the influence of a 10-nm-thick silicondioxide layer, as a passivation or as a gate insulation, on the performance of heterojunction field-effect transistors (HFETs) and metal–oxide–semiconductor HFETs (MOSHFETs), based on an undoped AlGaN/GaN heterostructure on a SiC substrate, was investigated. Channel-conductivity results yield a nearly 50% increase of mobility in the MOSHFET samples compared to the unpassivated HFETs. This increase of the transport properties of the MOSHFET channel is confirmed by a similar 45% increase of the cutoff frequency, from 16.5 to 24 GHz. Hall measurements, however, show a 10% decrease of the mobility in the heterostructure with a SiO2 top layer. In this paper, the superior performance of the MOSHFET transistor, in contradiction to the Hall results, is attributed to the screening of the Coulomb scattering of the charged surface defects by the gate-metallization layer