No abstract

Based on the Parlinsk–Li–Kawasoe method and density function theory, the phonon properties of some phase change memory materials are calculated. Thermal conductivities are acquired from our calculation, and they are in reasonable agreement with our experimental measurements. Phonon dispersion curve, mismatch between phonon density of states (DOS), partial density of states (PDOS) at interfaces, and thermal conductivities are studied. The results show that the low phonon group velocity and the thermal resistance at interfaces result in the low thermal conductivity of the GeTe/Sb2Te3 superlattice-like material, making it the best choice for phase change memory.

No abstract

No abstract

GaSb/InAs heterojunction tunnel field-effect transistors are strong candidates in building future low-power integrated circuits, as they could provide both steep subthreshold swing and large ON-state current (I ON ). However, at short channel lengths they suffer from large tunneling leakage originating from the small band gap and small effective masses of the InAs channel. As proposed in this article, this problem can be significantly mitigated by reducing the channel thickness meanwhile retaining a thick source-channel tunnel junction, thus forming a design with a non-uniform body thickness. Because of the quantum confinement, the thin InAs channel offers a large band gap and large effective masses, reducing the ambipolar and source-to-drain tunneling leakage at OFF state. The thick GaSb/InAs tunnel junction, instead, offers a low tunnel barrier and small effective masses, allowing a large tunnel probability at ON state. In addition, the confinement induced band discontinuity enhances the tunnel electric field and creates a resonant state, further improving I ON . Atomistic quantum transport simulations show that ballistic I ON = 284A/m is obtained at 15nm channel length, I OFF = 1 × 10 −3 A/m, and V DD = 0.3V. While with uniform body thickness, the largest achievable I ON is only 25A/m. Simulations also indicate that this design is scalable to sub-10nm channel length.Tunnel field-effect transistor (TFET), a promising replacement of classical metal-oxide-semiconductor fieldeffect transistor (MOSFET) for future low-power integrated circuits, has been intensively studied over a decade. The advantages of TFET come from its steep subthreshold swing (SS) that overcomes the 60mV/dec limit of a conventional MOSFET, allowing substantial supply voltage (V DD ) scaling 1 . However, because of low tunnel probability the steep SS usually occurs at very low current level 2,3 . This leads to insufficient ON-state current (I ON ) and thus large switching delay (CV DD /I ON ). Various approaches have been proposed to improve the low I ON . In particular, GaSb/InAs heterojunction based TFETs can considerably boost I ON due to their broken/staggered-gap band alignment 4,5 .However, as the channel length scales to sub-20nm as projected by International Technology Roadmap for Semiconductors (ITRS) for the next technology nodes 6 , the GaSb/InAs n-type TFETs suffer from large ambipolar and source-to-drain tunneling leakage due to the small band gap and the small effective masses of the InAs channel. These leakage can be reduced by reducing the body thickness 7 , because the band gap and the effective masses of the InAs channel increase as the body thickness decreases. Meanwhile, the large band gap and large effective masses also reduce the tunneling probability across the tunnel junction. The resonant TFET with a reversed InAs/GaSb heterojunction can have a steep SS at short gate length 7 but the I ON is limited by the narrow resonant transmission peak 8 . The channel heterojunction design with a large band-gap AlInAsSb alloy as the...

A high performance triple-heterojunction (3HJ) design has been previously proposed for tunneling FETs (TFETs). Compared with single heterojunction (HJ) TFETs, the 3HJ TFETs have both shorter tunneling distance and two transmission resonances that significantly improve the ON-state current ($I_{\rm{ON}}$). Coherent quantum transport simulation predicts, that $I_{\rm{ON}}=460\rm{\mu A/\mu m}$ can be achieved at gate length $Lg=15\rm{nm}$, supply voltage $V_{\rm{DD}}=0.3\rm{V}$, and OFF-state current $I_{\rm{OFF}}=1\rm{nA/\mu m}$. However, strong electron-phonon and electron-electron scattering in the heavily doped leads implies, that the 3HJ devices operate far from the ideal coherent limit. In this study, such scattering effects are assessed by a newly developed multiscale transport model, which combines the ballistic non-equilibrium Green's function method for the channel and the drift-diffusion scattering method for the leads. Simulation results show that the thermalizing scattering in the leads both degrades the 3HJ TFET's subthreshold swing through scattering induced leakage and reduces the turn-on current through the access resistance. Assuming bulk scattering rates and carrier mobilities, the $I_{\rm{ON}}$ is dropped from $460\rm{\mu A/\mu m}$ down to $254\rm{\mu A/\mu m}$, which is still much larger than the single HJ TFET case

No abstract

A triple-heterojunction (3HJ) design is employed to improve p-type InAs/GaSb heterojunction (HJ) tunnel FETs. The added two HJs (AlInAsSb/InAs in the source and GaSb/AlSb in the channel) significantly shorten the tunnel distance and create two resonant states, greatly improving the ON state tunneling probability. Moreover, the source Fermi degeneracy is reduced by the increased source (AlInAsSb) density of states and the OFF state leakage is reduced by the heavier channel (AlSb) hole effective masses. Quantum ballistic transport simulations show, that with V_{DD} = 0.3V and I_{OFF} = 10^{-3}A/m, I_{ON} of 582A=m (488A=m) is obtained at 30nm (15nm) channel length, which is comparable to n-type 3HJ counterpart and significantly exceeding p-type silicon MOSFET. Simultaneously, the nonlinear turn on and delayed saturation in the output characteristics are also greatly improved