- Post Doctoral
MIT Unit Affiliation:
- Electrical Engineering & Computer Science
Post Doc Sponsor / Advisor:
Date PhD Completed:
Top 3 Areas of Expertise:
Interested in Teaching and Research at a research University
Research interests: Physics, modeling and simulation of exploratory nanoelectronic devices for various applications. Expertise in quantum transport theory, interested in novel materials such 2D materials.
Expected End Date of Post Doctoral Position:
Post-doctoral program, (funded by NEEDS, E3S), Advisor: Prof. Dimitri Antoniadis, 2015-present
- § Tunnel Field-Effect Transistor (TFET)– developed a comprehensive model to understand experimental results on TFETs, specifically the lack of low subthreshold swing that theoretical models predict. The multi-phonon based modified Shockley-Read-Hall formalism for trap assisted tunneling explains TFET experiments and provides a general guideline for the required material quality to achieve low power switching with TFET.
- § Using quantum transport simulation, analyzed and interpreted experimental data of sub- 10 nm transistor based on MoS2 that was fabricated for the first time at MIT (Palacios group). Extracted material properties and projected intrinsic performance for MoS2.
§ Currently working towards a model for 2D heterostructure based tunneling and thermionic devices made with transition metal dichalcogenides, GaN, graphene etc.
PhD program, (funded by INDEX center, SRC-NIST), Advisor: Prof. Avik Ghosh, 2009-2014
- § Graphene based chiral tunnel transistor: Angular (chiral) tunneling across graphene based pn junctions resembles optical refraction. Using the angle dependent transmission, proposed the concept of transmission gap that is collapsible with voltage and thus avoid the Boltzmann switching limit. SRC-NIST sponsored NRI-INDEX center, adopted this concept as a major theme.
- § Numerical simulation: Developed Non-Equilibrium Green’s Function (NEGF) based quantum transport simulation platform for large scale graphene devices in experimental dimension (up to microns) and explained several recent experiments. Using a modified Dirac Hamiltonian and parallel computing, graphene transport in the diffusive regime is captured for the first time using NEGF matching closely with experimental results.
§ TI based spintronics, towards high efficiency spin current generation: Demonstrated high spin-charge current gain (~30) in topological insulator (TI), more than any other material (mostly <1) with a pn junction. This concept is inspired from graphene and based upon combining chiral tunneling and spin-momentum locked surface states in TI.
Master’s program 2007-2008
- § Studied electronic properties of Silicon and III-V nanowires with atomistic-NEGF simulation. Predicted strain effects (tunable bandgap and effective mass) in silicon nanowires, which were later demonstrated experimentally.
Graphene is considered to be a wonder material for its unique physical properties. In graphene, record-breaking numbers have been shown for the thermal and electrical conductivities, me- chanical strength, electronic mobility, chemical sensing, filtering and optoelectronic properties. Therefore, it has potential for various electronic, spintronic and photonic applications. In this dissertation, we investigate graphene’s potential as a channel material for digital logic applications using electro-statically built graphene pn junction (GPNJ). Despite graphene’s high electrical conductivity and other useful properties, the lack of bandgap makes it difficult to accomplish logic implementation, which requires a large amount of current modulation with gate voltage. In graphene pn junction, the linear, photon like energy dispersion combined with zero bandgap leads to an electron transport much like optical refraction and carrier trajectories are governed by an equivalent Snell’s law. Determined by the wave-function dynamics, we also have a unique angle (transverse mode) dependent transmission through GPNJ. This research aims to manipulate such angle dependent transport with gate geometry for switching. We show that such scheme is capable of switching without having to open a structural bandgap, but with what we call a ‘Transmission Gap’. We show that the gap is gate tunable, preserves the ON current and yields steep sub-threshold slope. Combined with graphene’s high current carrying capability, these properties make the switch energy efficient. The device designs are complemented with our benchmarking of recent experiments on angle dependent transport in GPNJ. We also show an intriguing implication of the pn junction based conductance control in another novel material: topological insulator (TI) which has similar bandstructure on its surface as graphene. A TI based pn junction is shown to produce highly spin polarized current with low charge current, following a very similar tunneling physics in graphene pn junction. Such gate controlled spin current can have implication in a spin based logic circuits, where a spin polarized current is needed to rotate a ferro-magnet with as low charge current as possible to decrease dissipation. Throughout the dissertation, we show simulation results from a sophisticated quantum mechanical numerical modeling platform based upon Non-Equilibrium Green’s Function (NEGF) formalism, developed to augment the analytical formalism. The numerical platform is optimized so that it can perform calculations that the analytical model cannot possibly do: model all kinds of transport (e.g. electron, spin) for small to large scale devices (up to experimental device size) for both ballistic and diffusive regime including non-idealities such as charge impurity scattering and edge effects.
Top 5 Awards and honors (name of award, date received):
5 Recent Papers:
Redwan N. Sajjad et. al., "Trap assisted tunneling and its impact on subthreshold swing in tunnel field effect transistors", accepted for publication in IEEE Transactions on Electron Devices.
Redwan N. Sajjad and Dimitri Antoniadis, "A compact model for Tunnel FET for all operation regimes including trap assisted tunneling", 74th Device Research Conference, Delaware, 2016.
A. Nourbakhsh, A. Zubair, A. Tavakkoli, R. Sajjad, et. al., “Monolayer MoS2 FETs with Sub-10 nm Channel Formed by Directed Self-Assembly”, Symposia on VLSI Technology, Honolulu, 2016.
K. M. Masum Habib, Redwan N. Sajjad and Avik Ghosh, “Chiral tunneling of topological states: towards the efficient generation of spin current using spin-momentum locking”, Physical Review Letters, vol. 114, 176801 (2015).
Redwan N. Sajjad, Frank Tseng, K. M. Masum Habib, Avik Ghosh, “Quantum transport at the Dirac point: Mapping out the minimum conductivity from pristine to disordered graphene”, Physical Review B, vol. 92, 205408 (2015).