Interband Tunnel Transistors

Interband tunnel transistors have been attracting increasing attention because of their potential to achieve subthreshold swings below the 60 mV/decade thermionic limit, and realize high performance and low power dissipation simultaneously. This work explores the design and modeling of semiconducting and graphene nanoribbon-based tunnel transistors, to understand the performance measures and guide the experimental development. Experiments in the formation of Ge interband junctions are also described.

Analytic expressions for Zener tunneling in one-, two-, and three-dimensional semiconductors are derived to establish the guidelines for tunnel transistor design. An analytic expression is derived, showing that the subthreshold swing of interband tunnel transistors is a function of the gate-to-source voltage and can be less than the thermionic limit of 60 mV/decade in MOSFET. Based on this expression, a new fully-depleted interband tunnel transistor structure is proposed and designed. The low subthreshold swing is verified by Synopsys TCAD simulation. Germanium interband tunnel transistors are shown by simulation to exhibit improved on-state performance vs. Si, because of the smaller bandgap and effective mass. To realize the proposed Ge interband tunnel transistor, a rapid melt growth process was developed to form submicron p+n+ Ge tunnel junctions. Transmission electron microscopy (TEM) reveals the regrown film and a contact microstructure consistent with the Al-Ge phase diagram. Negative differential resistances are observed which indicate the junction was abrupt heavily-doped.

A graphene nanoribbon (GNR) tunnel transistor is first proposed and modeled analytically by quasi-1D Poisson equation. An improved numerical model is developed that treates energy-dependent transmission coefficients, direct source-to-drain tunneling, and self-consistent channel electrostatics. Graphene nanoribbons have a width-tunable bandgap and ultra-thin body layer, which is especially favorable for tunnel transistor applications. It is shown by simulation that the GNR tunnel transistors at the long channel limit can operate at 0.1 V with an ultra-low subthreshold swing of 2.8 mV/decade, but the subthreshold swing and off-state current are degraded at short channel length due to direct source-to-drain tunneling. Smaller ribbon widths (down to a certain limit) can significantly improve the off-state behavior without considerably affecting the on-state current density and speed. For 20 nm channel length, GNR tunnel transistors with ribbon width of 2 and 3 nm can achieve high performance and low operating power simultaneously, meeting 2012 ITRS targets.