The effect of Radiation on the semiconductor-oxide interface, inducing interface trap states, has generally only been experimentally measured, which makes it difficult to quantify the impact of this radiation on device electrostatics. For an Ultra-Thin-Body (UTB) MOS device, the 1-D Band structure along the direction of confinement, if solved selfconsistently with the 1-D Poisson’s equation, while varying the band edge energy (∆Eedge) at the Si−SiO2 interface, can enable the quantification of the effect of interface trap states on channel electrostatics. In this work, we present an approach to correlate the radiation dose to the band edge energy (∆Eedge), thus enabling the band structure-based approach to be used to quantify the effect of these radiation-induced traps on the device electrostatics. We show a methodology that co-relates the interface charge induced by ∆Eedge and the charge yield, due to different radiating particles, on the Si − SiO2 interface. After identifying appropriate values of ∆Eedge for different particles and doses, the degradation due to radiation on the channel electrostatics can be accurately simulated with the atomistic band structure-based methodology. We also show an approach to extend this methodology to lower device temperatures, thus effectively quantifying the effect of radiation dose on UTB device electrostatics for a wide range of device temperatures.

Accurate determination of key device electrostatics parameters such as threshold voltage, Sub-Threshold Swing in Ultra-Thin Body (UTB) MOS transistors is made possible through accurate treatment of Quantum Confinement Effects (QCEs), along the channel thickness direction and Short-Channel Effects (SCEs), along the direction of the channel length. In this work, we propose a methodology to simulate the 2-D channel electrostatics of a UTB Double Gate (DG) MOS transistor structure, at room temperature, by solving the 2-D Poisson’s equation self-consistently with the 1-D band structure, thus enabling this approach to accurately take Quantum Confinement and short channel effects (SCEs) into account. By solving the 1-D band structure only once at the source-channel junction and updating the eigen energy (in the lateral direction) in the self-consistent loop, we show that this approach is able to obtain the 2-D channel electrostatics in a computationally efficient and accurate manner. By also extending this band structure-based approach to lower device temperatures, where carrier freeze-out effects manifest, we show the versatility of this approach in calculating the channel electrostatics over a wide range of device temperatures, while considering QCEs and SCEs.

UTSOI MOS devices enable the scaling of the channel length of MOS Transistors through scaling of the channel thickness and oxide thickness. Additionally, as they have a planar architecture similar to the conventional Bulk MOSFETs, these UTSOI MOSFETs have a simpler fabrication process compared to 3-D Transistors like FinFETs. Also, their performance can be efficiently tuned by applying a bias through the BOX. This allows a significant dynamic modulation of the speed/power trade-off, which is a powerful knob at the circuit level.

Ultra-thin body (UTB) devices are being used in many electronic applications operating over a wide range of temperatures. The electrostatics of these devices depends on the band structure of the channel material, which varies with temperature as well as channel thickness. The semi-empirical tight binding (TB) approach is widely used for calculating channel thickness dependent band structure of any material, at a particular temperature, where TB parameters are defined. For elementary semiconductors like Si, Ge and compound semiconductors like GaAs, these TB parameters are generally defined at only 0 K and 300 K. This limits the ability of the TB approach to simulate the electrostatics of these devices at any other intermediate temperatures. In this work, we analyze the variation of band structure for Si, Ge and GaAs over different channel thicknesses at 0 K and 300 K (for which TB parameters are available), and show that the band curvature at the band minima has minor variation with temperature, whereas the change of band gap significantly affects the channel electrostatics. Based on this finding, we propose an approach to simulate the electrostatics of UTB devices, at any temperature between 0 K and 300 K, using TB parameters defined at 0 K, along with a suitable channel thickness and temperature dependent band gap correction.

In Ultra-thin Body (UTB) devices, besides the Ultra-thin (UT) nature of the channel, which manifests in terms of Quantum Confinement Effects (QCEs), the Band-offsets between the oxide and channel materials at their interface, also tends to strongly impact the channel electrostatics. Despite being very accurate in calculating the band-structure and hence considering QCEs for a given channel material, the Tight-Binding (TB) method tends to be more complicated to use at the channel/oxide interface of MOS devices, while on the other hand the Effective Mass Approximation (EMA) in spite of being less accurate, is a simpler approach to consider the effects of band-offsets at the interface. Given its accuracy, we firstly use the $sp^3d^5s^*$ TB method to calculate the Band-structure and then by considering significant k-points, efficiently incorporate the QCE into the electrostatics of Double-Gate (DG) Silicon-on-Insulator (SOI) MOS devices. Considering these results as a reference, with the assumption of an infinite potential well, we propose a modified Effective Mass Approximation (mEMA) approach, whereby introducing energy correction parameters, along with the effective mass parameters, all of which are shown to be gate bias, channel and oxide thickness dependent, the results obtained from the proposed approach are shown to have good agreement with the results from TB method. In order to analyze the effect of Conduction-Band Offset variations on the channel electrostatics parameters, we consider an $SiO_2$ layer of thickness of $1$ $nm$ and show the effect of different Band-offsets on the integrated charge density and gate capacitance, using the mEMA approach.

The accurate calculation of channel electrostatics parameters, such as charge density and potential, in ultra-thin body (UTB) devices requires self-consistent solution of the Poisson’s equation and the full band structure, which is channel material and thickness dependent. For cubic crystals like silicon, the semi-empirical sp3d5s* tight-binding (TB) model is preferred in device simulations, over the density functional theory, to obtain the full band structure because of being computationally less intensive and equally accurate. However, the computational time of the TB model scales non-linearly with the channel thickness and becomes cumbersome for silicon, beyond 5 nm, primarily because of the increasing size of the TB hamiltonian that needs to be solved over the entire k-space, in the irreducible Brillouin zone. In this work, we precisely identify those k-points corresponding to the energies close to the band minima, where the Fermi-Dirac probability significantly affects electrostatics parameters. This enables us to demonstrate a computationally efficient approach based on solving the hamiltonian only on those reduced number of k-points. The rigorous benchmarking of the channel electrostatics parameters obtained from this approach is performed with results from accurate full band structure simulations showing excellent agreement over a wide range of channel thicknesses, oxide thicknesses, device temperatures and different channel orientations. By showing that the approach presented in this work is computationally efficient, besides being accurate, regardless of the number of atomic layers, we demonstrate its applicability for simulating UTB devices.