Kartik Ganapathi

How Good Can Monolayer MoS2 Transistors Be

Monolayer molybdenum disulfide (MoS(2)), unlike its bulk form, is a direct band gap semiconductor with a band gap of 1.8 eV. Recently, field-effect transistors have been demonstrated experimentally using a mechanically exfoliated MoS(2) monolayer, showing promising potential for next generation electronics. Here we project the ultimate performance limit of MoS(2) transistors by using nonequilibrium Greens function based quantum transport simulations. Our simulation results show that the strength of MoS(2) transistors lies in large ON-OFF current ratio (>10(10)), immunity to short channel effects (drain-induced barrier lowering ∼10 mV/V), and abrupt switching (subthreshold swing as low as 60 mV/decade). Our comparison of monolayer MoS(2) transistors to the state-of-the-art III-V materials based transistors, reveals that while MoS(2) transistors may not be ideal for high-performance applications due to heavier electron effective mass (m = 0.45 m(0)) and a lower mobility, they can be an attractive alternative for low power applications thanks to the large band gap and the excellent electrostatic integrity inherent in a two-dimensional system.

Heterojunction Vertical Band-to-Band Tunneling Transistors for Steep Subthreshold Swing and High on Current

We propose a heterojunction vertical tunneling field-effect transistor and show using self-consistent ballistic quantum transport simulations that it can provide very steep subthreshold swings and high on current, thereby improving the scalability of TFETs for high performance. The turn-on in the pocket region of the device is dictated by the modulation of heterojunction barrier height. The steepness of turn-on is increased because of simultaneous onset of tunneling in the pocket and the region underneath and also due to the contribution of vertical tunneling in the pocket to the current. These factors can be engineered by tuning heterojunction band offsets.

Analysis of InAs vertical and lateral band-to-band tunneling transistors: Leveraging vertical tunneling for improved performance

Using self-consistent quantum transport simulation on realistic devices, we show that InAs band-to-band Tunneling Field Effect Transistors (TFET) with a heavily doped pocket in the gate-source overlap region can offer larger ON current and steeper subthreshold swing as compared to conventional tunneling transistors. This is due to an additional tunneling contribution to current stemming from band overlap along the body thickness. However, a critical thickness is necessary to obtain this advantage derived from “vertical” tunneling. In addition, in ultra small InAs TFET devices, the subthreshold swing could be severely affected by direct source-to-drain tunneling through the body.

Dependence of intrinsic performance of transition metal dichalcogenide transistors on materials and number of layers at the 5 nm channel-length limit

The optimal performance of ultimately scaled transition metal dichalcogenide (TMD) FETs for four different materials and one to five layers is investigated using ballistic quantum transport calculations with material properties derived from first-principles. Large bandgaps and effective masses are shown to result in excellent switching performance at 5 nm gate lengths, thus showing potential for low-power applications. However, achievable current falls short of ITRS low operating power specifications.

Ballistic I-V Characteristics of Short-Channel Graphene Field-Effect Transistors: Analysis and Optimization for Analog and RF Applications

With the recent upsurge in experimental efforts toward fabrication of short-channel graphene field-effect transistors (GFETs) for analog and high-frequency RF applications-where the advantages of distinctive intrinsic properties of gapless graphene are expected to be leveraged-a critical understanding of the factors affecting both output and transfer characteristics is necessary for device optimization. Analyzing the device characteristics through ballistic electronic transport simulations within the nonequilibrium Greens function formalism, we show that a doping in the drain underlap region can significantly improve the quasi-saturation behavior in the GFET output characteristics and, hence, the output resistance and intrinsic gain. From this understanding, we provide a unified and coherent explanation for seemingly disparate phenomena-quasi-saturation and the recently reported three-terminal negative differential resistance in GFETs. We also investigate the scaling behavior of cutoff frequency and comment on some of the observed scaling trends in recent experiments.

Zener tunneling: Congruence between semi-classical and quantum ballistic formalisms

We compare the results of self-consistent ballistic quantum transport simulation of Zener tunneling in InAs, a direct bandgap semiconductor, with corresponding semi-classical solutions using the well-known Kane’s model and the Wentzel-Kramers-Brillouin (WKB) approximation. We find a qualitative difference between solutions obtained from Kane’s formulation and rigorous quantum-mechanical formalism. However, the WKB solution, with evaluation of action integral along the tunneling paths determined from the nearest neighbor sp3s* Hamiltonian, is shown to provide qualitative agreement. We discuss the issues involved in fitting semi-classical solutions with their quantum counterpart and also present a brief comparison of our results with experimental data.

Can quasi-saturation in the output characteristics of short-channel graphene field-effect transistors be engineered?

To summarize, we propose that quasi-saturation in short-channel GFET output characteristics can be effectively engineered by doping in the drain-underlap region and show using self-consistent NEGF simulations that a 0.2% p-type doping can enhance output resistance by 13x and intrinsic gain by 4x in 20 nm gate-length GFETs.

Monolayer MoS 2 transistors - ballistic performance limit analysis

To summarize, using ballistic NEGF-based transport simulations, we project the maximum performance achievable with monolayer MoS2 transistors. Our simulations show that these devices can provide (i) excellent switching behavior with very high ON current, (ii) a gm of about 3 mS/µm, and (iii) immunity to short channel effects thanks to the electrostatistically efficient 2-D geometry. We have also investigated the effect of underlap, barrier height and contact resistance on the device performance. We note that while these numbers are representative of the best performance MoS2 transistors can offer, the fact that they are significantly better than those for either state-of-the-art silicon, III–V or graphene makes MoS2 devices promising for future electronic applications.

Comparative analysis of the performance of InAs lateral and vertical band-to-band tunneling transistors

To summarize, we have shown that in comparison to a lateral device, a vertical structure may provide a larger ON current for similar OFF current. However, the subthreshold swing is degraded due to weaker gate control. We also show that there is a critical body thickness below which the vertical tunneling is greatly minimized. We find that significant vertical tunneling only starts at a large gate voltage and lateral tunneling almost acts as a leakage mechanism until this point. These facts indicate that (i) with a shallow pocket (ii) with right choice of doping densities in the source and pocket (iii) by effectively controlling the transport in lateral and vertical directions, e.g. by strain or by heterostructures, large ON currents with reasonable substreshold may be achieved.