A Computational Study on the Device Performance of Graphene Nanoribbon Heterojunction Tunneling FETs based on Bandgap Engineering

摘要

Novel device structures and electronic materials are required to further enhance the performance of digital circuits after the current MOSFET technology reaches its physical limits. While tunneling mechanism degrades the short channel MOSFET performance, it can be utilized as the major device operation in tunneling field-effect transistors (TFET) with promising features such as lower sub-threshold swing and OFF-state current (I_(OFF)). Furthermore, semiconducting graphene nanoribbon (GNR) has been proposed as a potential electronic material for TFET application due to its unique properties such as ultra-thin body structure and high carrier mobility. A small bandgap (E_G) material near the source-channel interface can be introduced to form heterojunction (HJ) which leads to a larger I_(ON) [1-3]. Therefore, in this work, we investigate the impact of the length and E_G of this HJ region on the device performance of graphene nanoribbon TFET. Firstly, GNR TFETs with uniform channel widths (W_C) of 1.2, 1.9 and 2.3 nm (with E_G = 1.22, 0.78 and 0.66 eV respectively [4]) were simulated. A channel length (L_C) of 16 nm was considered with a double-gated structure and silicon dioxide of 1 nm thickness was assumed for both gates. The source (drain) was heavily p-type (n-type) doped such that there was a shift of the Fermi level in the source (drain) by 0.12 eV below the valence band (above the conduction band), and the channel was assumed to be intrinsic. The quantum transport, namely the non-equilibrium Green's function formalism based on Dirac tight-binding approach developed in a previous work was implemented [5]. From the simulated results, the I_(DS)-V_(GS) exhibited the characteristic ambipolar behavior of TFET, with the minimum I_(DS) occurring at V_(GS)=V_(DS)/2 due to the symmetric doping concentrations at the source and drain [6]. Furthermore, it was observed that as the ribbon width increased from 1.2 to 2.3 nm, the ballistic I_(ON) increased from 2.1 to 7.3 mA/μm while I_(OFF) increased from 4.6×10~(-7) to 77 μA/μm. This was due to the decrease in E_G which resulted in a higher band-to-band (BTB) tunneling at the source-channel interface.