Steep subthreshold swing Double - Gate tunnel FET using source pocket engineering: Design guidelines

In this work, we propose a promising source engineered Double - Gate (DG) Tunnel Field Effect Transistor (TFET) device capable of providing remarkably low value of Subthreshold Swing (SS) with sufficiently high drive current. Using Sentaurus TCAD simulations, we demonstrate that counter-doped horizontal pockets (doping of pocket is kept opposite to that of source) when placed in the source region introduces a built-in band bending at the source-pocket junction. This lowers the minima of Conduction Band (CB) in pocket region thereby reducing the barrier width as the pocket CB moves closer to source Valence Band (VB). As a result, stronger electric field is observed thereby reducing the threshold voltage (onset of band-to-band tunneling (BTBT)) and subsequent reduction in subthreshold swing. To boost the ON-current and suppress ambipolarity, high-k dielectric material along with low work-function gate material is introduced at source side and low-k gate dielectric along with high work-function gate material is introduced at drain side. Compared to point tunneling in conventional TFETs, the gate overlapped pockets in the proposed structure result in an increase in cross-section area available for BTBT thereby leading to line tunneling of carriers from the source to the pocket, resulting in higher ON-current. In this work, we discuss the role of source engineering in boosting the performance of Hetero-Dielectric (HD) Dual-Metal-Double-Gate (DMDG) TFET. We provide design guidelines to achieve steeper subthreshold swing while considering pocket doping and pocket thickness as the key parameters. A comparative study of conventional DG-TFET and HD-DG-TFET with the proposed Gate-over-Pockets (GoP) HD-DMDG-TFET structure is done. When compared to the conventional DG-TFET with same geometrical parameters, the proposed structure provides $\sim 33\times$ steeper SS, more than two order improved ON-current, two order lower ambipolar current and 133 folds better I_on/I_off thus becomes the perfect choice for low power applications.