Redwan N. Sajjad

High efficiency switching using graphene based electron “optics”

We demonstrate a way to open a gate-tunable transmission gap across graphene p-n junction by introducing an additional barrier in the middle that replaces Klein tunneling with regular tunneling, allowing us to modulate current by several orders of magnitude. The gap arises by angularly sorting electrons by their longitudinal energy and filtering out the hottest, normally incident electrons with the tunnel barrier, and the rest through total internal reflection. Using analytical and atomistic numerical studies, we show that the barrier causes graphene p-n junction act as a metamaterial with metal-insulator transition and overcome the KTln10/decade limit for subthreshold conduction.

Electronic Properties and Orientation-Dependent Performance of InAs Nanowire Transistors

The electronic properties, namely, the band structures, the band gaps, and the electron effective masses of hydrogen-passivated InAs nanowires grown in 〈100〉 , 〈110〉, and 〈111〉 crystallographic directions are studied using sp³d⁵s* orbital-basis tight-binding model. We then parameterize the band gaps and electron effective masses to facilitate device simulation and to study the orientation-dependent performance of n-channel InAs nanowire transistors using a top-of-the-barrier model. The 〈111〉 and 〈110〉 wire transistors have better performance metrics. The quantum-confinement effect is largest in the 〈100〉 wire, which results in a higher band gap and a heavier effective mass for relatively smaller diameter wires. The consequence is lower current, higher density of states, higher quantum capacitance, and longer delay in the 〈100〉 wire transistors. The 〈110〉 and 〈111〉 wires have a very similar quantum-confinement effect, even for the smaller diameters, which results in similar band gaps, similar effective masses, and similar performance metrics.

Trap Assisted Tunneling and Its Effect on Subthreshold Swing of Tunnel FETs

We provide a detailed study of the oxide- semiconductor interface trap assisted tunneling (TAT) mechanism in tunnel FETs to show how it contributes a major leakage current path before the band-to-band tunneling (BTBT) is initiated. With a modified Shockley-Read-Hall formalism, we show that at room temperature, the phonon assisted TAT current always dominates and obscures the steep turn ON of the BTBT current for common densities of traps. Our results are applicable to top gate, double gate, and gate-all-around structures, where the traps are positioned between the source-channel tunneling regions. Since the TAT has strong dependence on electric field, any effort to increase the BTBT current by enhancing local electric field also increases the leakage current. Unless the BTBT current can be increased separately, calculations show that the trap density Dit has to be decreased by 40-100 times compared with the state of the art in order for the steep turn ON (for III-V materials) to be clearly observable at room temperature. We find that the combination of the intrinsic sharpness of the band edges (Urbach tail) and the surface trap density determines the subthreshold swing.

Manipulating chiral transmission by gate geometry: switching in graphene with transmission gaps.

We explore the chiral transmission of electrons across graphene heterojunctions for electronic switching using gate geometry alone. A sequence of gates is used to collimate and orthogonalize the chiral transmission lobes across multiple junctions, resulting in negligible overall current. The resistance of the device is enhanced by several orders of magnitude by biasing the gates into the bipolar npn doping regime, even as the ON state in the homogeneous nnn regime remains highly conductive. The mobility is preserved because the switching involves the suppression of transmission over a range of energy (transmission gap) instead of a structural band gap that would reduce the number of available channels of conduction. Under a different biasing scheme (npn to npp), this transmission gap can be made highly gate tunable, allowing a subthermal turn-on that beats the Landauer bound on switching energy, limiting present-day digital electronics.

Atomistic deconstruction of current flow in graphene based hetero-junctions

We describe the numerical modeling of current flow in graphene heterojunctions, within the Keldysh Landauer Non-equilibrium Green’s function (NEGF) formalism. By implementing a k-space approach along the transverse modes, coupled with partial matrix inversion using the Recursive Green’s function Algorithm (RGFA), we can simulate on an atomistic scale current flow across devices approaching experimental dimensions. We use the numerical platform to deconstruct current flow in graphene, compare with experimental results on conductance, conductivity and quantum Hall, and deconstruct the physics of electron ‘optics’ and pseudospintronics in graphene pn junctions. We also demonstrate how to impose exact open boundary conditions along the edges to minimize spurious edge reflections.

Performance benchmarking of p-type In 0.65 Ga 0.35 As/GaAs 0.4 Sb 0.6 and Ge/Ge 0.93 Sn 0.07 hetero-junction tunnel FETs

We experimentally demonstrate and benchmark the performance of p-channel TFETs (PTFETs) comparing Group III-V (In<inf>0.65</inf>Ga<inf>0.35</inf>As/GaAs<inf>0.4</inf>SW<inf>0.6</inf>) against Group IV (Ge/Ge<inf>0.93</inf>Sn<inf>0.07</inf>) semiconductor hetero-junctions. This is enabled via gate stack engineering with extremely scaled dielectrics achieving the highest accumulation capacitance density (≥3μF/cm²) on both GaAs<inf>0.4</inf>Sb<inf>0.6</inf> and Ge<inf>0.88</inf>Sn<inf>0.12</inf> channels, respectively. Temperature and electric field dependent I-V measurements coupled with first-principles density functional theory (DFT) based band-structure calculations and analytical modeling based on modified Shockley-Read-Hall formalism, are used to quantify contributions to carrier transport from band-to-band tunneling and trap-assisted tunneling (TAT). GeSn based PTFETs are found to outperform In<inf>0.65</inf>Ga<inf>0.35</inf>As/GaAs<inf>0.4</inf>Sb<inf>0.6</inf> PTFETs benefiting from band-gap engineering (higher I<inf>on</inf>) and reduced phonon assisted TAT current (lower D<inf>it</inf>).

Modified Dirac Hamiltonian for efficient quantum mechanical simulations of micron sized devices

Representing massless Dirac fermions on a spatial lattice poses a potential challenge known as the Fermion Doubling problem. Addition of a quadratic term to the Dirac Hamiltonian provides a possible way to circumvent this problem. We show that the modified Hamiltonian with the additional term results in a very small Hamiltonian matrix when discretized on a real space square lattice. The resulting Hamiltonian matrix is considerably more efficient for numerical simulations without sacrificing on accuracy and is several orders of magnitude faster than the atomistic tight binding model. Using this Hamiltonian and the non-equilibrium Greens function formalism, we show several transport phenomena in graphene, such as magnetic focusing, chiral tunneling in the ballistic limit, and conductivity in the diffusive limit in micron sized graphene devices. The modified Hamiltonian can be used for any system with massless Dirac fermions such as Topological Insulators, opening up a simulation domain that is not readily ...

A compact model for tunnel FET for all operation regimes including trap assisted tunneling

We present a rigorous compact model for Tunnel Field Effect Transistors (TFET) that captures all essential features including Trap Assisted Tunneling (TAT) originating from surface traps (Dit). Inclusion of the TAT accurately captures the subthreshold behavior matching well with experimental data (Fig. 1). With self-consistent channel potential, ψ and drain injection, we show that the TFET quantum capacitance, Cq and ψ are controlled by both gate and drain biases resulting in Negative Differential Resistance (NDR) for negative drain bias (Fig 2). A Landauer-formalism-based source-drain saturation function Fsd is derived that obtains the effective tunnel energy window based on ψ and also the superlinear current depending on source degeneracy. We apply the model to an In0 53Ga0 47As homojunction TFET but the model is sufficiently general to use for other device structures. The model can be used to assess TFET performance in presence of different amounts of Dit.

A tunnel FET compact model including non-idealities with verilog implementation

Abstract We present a compact model for Tunnel Field Effect Transistors (TFET), that captures several non-idealities such as the Trap Assisted Tunneling (TAT) originating from interface traps ( D it ), along with Verilog-A implementation. We show that the TAT, together with band edge non-abruptness known as the Urbach tail, sets the lower limit of the sub-threshold swing and the leakage current at a given temperature. Presence of charged trap states also contributes to reduced gate efficiency. We show that we can decouple the contribution of each of these processes and extract the intrinsic sub-threshold swing from a given experimental data. We derive closed form expressions of channel potential, electric field and effective tunnel energy window to accurately capture the essential device physics of TFETs. We test the model against recently published experimental data, and simulate simple TFET circuits using the Verilog-A model. The compact model provides a framework for TFET technology projections with improved device metrics such as better electrostatic design, reduced TAT, material with better transport properties etc.

Conductance of graphene: Role of metal contact, charge puddles and differential gating

We demonstrate how several experiments on graphene transport can be explained semi-quantitatively within a Non-Equilibrium Greens Function (NEGF) formalism. The key features are controlled by-(i) the boundary potential established at a metal-graphene interface,(ii) charge puddles that help the conductivity in the ballistic limit and hurt in the diffusive limit and (iii) alignment of the local Dirac points in a multiply gated segment. Simulations reveal that at the ballistic limit, the conductance depends on the aspect ratio which controls tunneling from source to drain and across the metal-graphene interface. We show that the boundary potential VB at the interface together with Metal Induced Doping (MID) are critical to graphene transport-specifically, the maximum conductance achievable with a given metal contact, the electron-hole asymmetry (EHA) and the peak device resistance. The boundary potential is formed due to in-plane charge transfer from metal covered graphene to graphene on substrate [1] and may produce an additional smooth pn junction, typically ignored in existing models. In the experiments however, the contact resistance heavily depends on the fabrication procedure [2], varying from hundreds of Ω- μm to several thousands of Ω- μm [3]. A rigorous model of the performance limits of several contacts and change of carrier transport from ballistic to diffusive regime is lacking. We report the upper limit of the performance of various metal-graphene contacts and compare with the best available experimental values. To reach experimental dimensions, we use tight-binding real space calculations as well as the powerful KSF-RGFA approach (combination of K Space Formalism (KSF) and Recursive Greens Function Algorithm (RGFA) [4]), which allows us to simulate devices as large as microns in size.