Effect of biaxial strain on the performance of graphene/BN hetero bilayer based field effect transistor

Abstract

Recent simulation study shows that a considerable and stable band gap can be opened in 2D materials by applying biaxial strain. But a systematic study on the effect of biaxial strain on band gap and other important material electronic parameters are still missing in literature. In this thesis, the structural and electronic properties of various configurations of graphene/hexagonal Boron Nitride hetero bilayer (C/h-BN HBL) system has been investigated using density functional theory (DFT) under local density approximation (LDA).Biaxial strain has been applied on this material and its effect on electronic band structure and properties derived from band structure such band gap, carrier effective mass, Fermi velocity have also been simulated. This work reveals that irrespective of the application of biaxial strain it remains direct and hence can be exploited in optoelectronic applications. Also, it has been observed that the band gap increases monotonically (29.7meV-113.4meV) from compressive to tensile strain region. The electron effective mass (0.003732me-0.024902 me) and hole effective mass (0.003747 me-0.023888 me) also increases from compressive to tensile strain region. But Fermi velocity for both electron (1.257278×106 m/s-0.3866997×106 m/s) and hole (1.265725×106 m/s-0.3816452×106 m/s) shows an opposite trend. From density of states (DOS) it is clear that this material system shows a linear trend in lower energy region as observed in pristine graphene. Band structure calculation shows an almost linear dispersion relationship at Dirac point which predicts the Fermion like characteristics of electrons in this material system. Finally, biaxial strained materials were implemented in the channel of a top gate ballistic MOSFET and device performance was investigated using non-equilibrium Green’s function (NEGF) coupled with DFT. The results show that for lower band gap material the transport lacks saturation region which is crucial for stable operation of transistors in logic circuits. It is also obvious from the results that carrier effective mass and Fermi velocity has negligible effect on drive current. Ultimately, an optimum channel material (12% tensile strained B3 configuration) was proposed and implemented in the channel of MOSFET. Better device performance with current saturation has been observed with this proposed material.