University of Minnesota

EE 5164

Semiconductor Properties and Devices II

2015 Project Paper

Introduction to Two-Dimensional Field Effect Transistors

Yi Ren

“Pushing” by the Moore’s scaling law for more than 40 years, engineers have

reduced the gate length of the transistor below 30nm[1]. However, there will be an

ultimate limits of the size of the transistors and noted by Gordon Moore himself on 13

April 2005: “In terms of size (of transistors) you can see that we're approaching the

size of atoms which is a fundamental barrier”[2]. As the most popular two-

dimensional material, graphene successful demonstrated it’s extraordinary properties

in electrical, optical and thermo-mechanical. It creates a new era for 2-D materials

and technically provides an method to achieve 2-dimensional field effect transistors

by using 2-D material, which may reduce the size of the transistors in atomic levels

and finally reach the limitation of the Moore’s law in few decades. This paper will

first give an general description of 2-D FETs, which includes its general structural and

advantage compared with the conventional FETs. Then I will introduce some

properties of graphene and transition-metal dichalcogenides (TMD), which are the

majority components of nowadays 2-D FETs. Finally the paper will shown several 2-

D FETs that based on different 2-D TMD materials.

From conventional FET to 2-D FET

Field effect transistor is a transistor that uses an electrical field at gate terminal to

control the shape of the conductive channel from source to drain and achieve to

control the current flow from source terminal to drain terminal. The FET has a very

high switching current ratio and input resistance, hence, it is widely applied in the

digital circuit especially the integrated circuit. Conventional field-effect transistors

operate on the basis of energy filtering of electrons (or holes) flowing over a barrier.

The barrier is controlled with a voltage and there is a subthreshold swing (SS) limit

that the current can not be changed more than 60 mV/decade[3]. For most of 3-D

crystal material, they can not reach this limit because of the dangling bonds. Hence,

they need more voltage to achieve the high on/off current ratio which means they will

have high power consumption. One way to improve the performance of the

conventional transistors is reducing the dimension of the transistor. By reducing the

gate length , the inherent capacitances and resistances [Fig. 1]

Fig. 1 Inherent resistances and capacitances in the n-channel MOSFET structure.[4]

will reduce, which results in a high speed response and low power consumption,

shown in Fig. 2. And the thin channel can against short-channel effects down to very

short gate length [5].

In order to reach the limitation of gate length and thickness of the channel,

engineers finally decide to use 2-D material, which means the channel can be just one

atomic layer thick, to build the FET and this is how 2-D FET comes from.

a

b

Fig. 2 The performance of transistor versus dimensional. a, cut-off frequency of different

transistors versus gate length [5]. b, the active power performance of different Intel process

[6].

The general idea of the 2-D FETs is that inside of using doped semiconductor like

silicon and gallium arsenide as the channel, they use 2-D material as their channel.

One of the primary advantages of 2-D films is that instead of using chemical bond, the

layers materials are bonded by the van der Waal's (vdW) forces. The absence of

dangling bonds can reduce surface roughness scattering and also reduce the interface

traps resulting in high mobilities [7] and eliminate performance reduce due to

interface states [8]. On the other hand, the 2-D FET have potential to reduce the scale

of the transistor into atomic levels, which means we can fabricate more transistors on

one chip and finally reach the limitation of the Moore’s scaling Law.

As the milestone the of 2-D FET, 2-D material is necessary to be introduced. The

fabrication and performance of the 2-D FET are highly dependent on the property of

the 2-D materials. The next section will introduce two 2-D materials that are studied

extensively for a new generation of ultra thin electronics.

2-D Material

The successful produced and isolated of graphene in 2004 raise a revolution in

science. It shows that when the scale of the conventional material reduce to atomic

level, the special properties will show up. This discovery leads the scientists to

extensively study on 2-D material.

Graphene

Graphene is an allotrope of carbon and is a purely 2-D material. It has regular

hexagons lattice form with a carbon atom at each corner (Fig. 3). One of the property

of graphene that attracts the transistor engineer is its high carrier mobility at room

temperature. Mobilities of 10,000–15,000 cm2 V-1 s-1 are measured for exfoliated

graphene on SiO2-covered silicon wafers[9] and the recent research measured

mobilities of around 23,000 cm2 V-1 s-1 in top-gated graphene MOS channels [5].

Although these value seems attractive, they all measured the large-area graphene,

which is gapless. Large-area graphene is a zero-gap semimetal, because its conduction

and valence bands meet at the Dirac points on the edge of Brillouin zone (Fig. 3 b(i)).

Fig.3 Properties of graphene. a, structure of graphene. b, Band structure around the K point

of (i) large-area graphene, (ii) graphene nanoribbons, (iii) unbiased bilayer graphene, and

(iv) bilayer graphene with an applied perpendicular field [5].

Zero bandgap means large-area graphene can not be used as the channel because

it can not be switched off. However, there are three possible ways to open a bandgap

on graphene: by reducing the scale of large-area graphene in one dimension to form

graphene nanoribbons, by using electric field to bias bilayer graphene and by applying

strain to graphene. For the first method, when the width of the nanoribbons reduce

below 20nm, the bandgap is in excess of 200 meV [5]. But this value can be affect by

the roughness of the edge and even though the edge is perfect, this bandgap is still too

small to have a high on/off current ratio. In addition to that, as the bandgap increasing,

the mobility of the graphene will decrease extremely (200 cm2 V-1 s-1 for a 150 meV

bandgap)[10]. The second method needs very high voltage. Theoretically, in order to

reach values of 200–250 meV, we need fields about (1–3)×107 Vcm-1 and third

methods is hard to achieve in practice [5].

Although graphene has extremely high carrier mobility, it is constrained by its

zero-bandgap. Hence, in stead of use graphene as the channel, it can be a good

terminal material of the 2-D FET with very high electron conductivity.

Transition metal dichalcogenides monolayers

The transition metal dichalcogenides monolayers are atomically thin film of a

class of materials with the formula MX2, where M is a transition metal element(Mo,

W, and so on), and X is a chalcogen (S, Se, or Te). One layer of M atoms is

sandwiched between two layers of X atoms by van der Walls interaction (Fig.4).

Compared to graphene, TMD monolayers has good bandgap property such as MoS2

and WSe2 have direct bandgap in the range of 1.2−1.8eV.

Fig.4 Three-dimensional schematic representation and top view of a typical MX2 structure,

with the chalcogen atoms (X) in yellow and the metal atoms (M) in blue [11].

Compared to traditional semiconducting materials such as silicon, Ge, monolayer

TMD films have less surface roughness scattering without dangling bonds and also

reduce interface traps resulting in low density of interface states on the

semiconductor−dielectric interface. Another important feature of 2D TMD films is

their atomic thickness that allows easier control of channel charge by gate voltage and

high degree of vertical scaling that can reduce the short channel effects [7].

On of the disadvantage of monolayer TMD materials is the low carrier mobility

compared to conventional material like silicon. In order to increase mobility and drive

current performance, high dielectric constant dielectric material, thin nanosheet-

thickness and right contact metal should be chosen [7][12]. The next section will

introduce several 2-D FETs that have different solutions

Several 2-D FET

MoS2 FET

The basic structure of the MoS2 FET that is fabricated by B. Radisavljevic et al. In

2011 is shown in Fig.5. As you can see,

Fig.5 MoS2 monolayer transistors. a, optical image of a single layer of MoS2. b, optical

image of two FET transistors connect in series. c, three-dimensional schematic view of one of

the transistors shown in b [10].

compared to conventional MOSFET, the main difference is that the channel is

replaced by 2-D TMD material. They use monolayer MoS2 and HfO2, which

dielectric constant is 25, as the gate dielectric. The mobility of this single layer MoS2

transistor is at least 200 cm2 V-1 s-1 and the on/off current ratio is 1×108 at room

temperature [10]. At the bias voltage Vds =500 mV, the maximal measured on current

is 2.5 μA/μm by using Au as the contact metal. This transistor uses monolayer and

high dielectric constant material as gate dielectric to increase the mobility and its

similar to the gaphene nanoribbons channel transistor but has much higher on/off

current ratio. However, the on current is still too small. In order to improve that driven

current, people should find another contact material that has low contact resistance

with MoS2.

A lot of efforts have been devoted toward optimized metal contacts to the

monolayer MoS2 but still can not reach the optimized device performance. By

addressing this challenge in a new strategy, engineers decide to use graphene as

electrodes to get a nearly perfect Fermi level match with MoS2 when in the on-state

[13]. The structure of this MoS2 FET with graphene electrodes shown that is

fabricated by Yuan Liu et al. in 2014 is shown in Fig. 6. According to the property of

the

Fig. 6 Schematics of a BN/graphene/MoS2/BN sandwich structure with edge graphene

contacts [13].

Graphene, its fermi level can be modified by a gate potential [7]. By using this

method, they get the mobility up to 1300 cm2 V-1 s-1 at very low temperature (less than

10K) and they get the zero contact barrier with MoS2 at 1.9K. This method give the

engineers a hint of finding new contact material with atomic 2-D material.

WSe2 FET

The structure of back-gated WSe2 FET that is fabricated by Wei Liu et al. in 2012

is shown in Fig. 7. In their experiment, they use the ab initio density functional

a

b

Fig. 7 Back-gate WSe2 monolayer transistor. a, schematic of back-gated WSe2 monolayer

FET, highly n-doped silicon serves as back gate. b, optical image of fabricated WSe2

monolayer FETs. [7]

theory (DFT) calculations indicate that the d-orbitals of the contact metal is the key

point to form low resistance ohmic contacts with monolayer WSe2 [7]. Based on this

theory, they found the indium (In) results in a small contact resistance with WSe2.

Their back-gated In-WSe2 FET has a record on current of 210 μA/μm and the

mobility of 142 cm2/V·s with an on/off current ratio exceeding 106 [7].

Completely 2-D FET

The 2-D FET that I introduced above only have 2-D materials in their channel,

but this completely 2-D FET that is fabricated by Tania Roy et al. in 2014 shown in

Fig. 8 is built from all 2-D material components. They use large area CVD graphene

to contact MoS2 crystals, exfoliated boron nitride as the gate dielectric and exfoliated

graphene as the gate terminal[8].

Fig. 8 schematic of All-2D MoS2 FET with few-layer h-BN gate dielectric, and bilayer

graphene source/drain and multilayer graphene top-gate electrodes [8].

Although its mobility is only 33 cm2/V·s with on/off current ratio of 106 [8], it

demonstrates how the ultimate 2-D FET looks like in the future.

In summery, as the conventional transistors become smaller and smaller, the short

channel effects become an important problems that restricted the performance of the

ultra small transistors. However, the 2-D FET gives engineers a solution to improve

the performance of conventional transistor by using 2-D materials as their channel. So

far, 2-D FET still have a lot of problems such as high contact resistance between the

2-D material and contact material, low carrier mobility and complex fabrication

process to be solved, but I am sure that 2-D FET will finally replace the conventional

FET in the market and lead our life into a new era.

References

[1]. Luisier, M.; Lundstrom, Mark; Antoniadis, D.A.; Bokor, J., "Ultimate device

scaling: Intrinsic performance comparisons of carbon-based, InGaAs, and Si

field-effect transistors for 5 nm gate length," Electron Devices Meeting (IEDM),

2011 IEEE International , vol., no., pp.11.2.1,11.2.4, 5-7 Dec. 2011.

[2]. Manek Dubash (2005, April 13). "Moore's Law is dead, says Gordon

Moore". [Online]. Available: http://www.techworld.com/news/operating-

systems/moores-law-is-dead-says-gordon-moore-3576581/

[3]. Jena, D., "Tunneling Transistors Based on Graphene and 2-D

Crystals," Proceedings of the IEEE , vol.101, no.7, pp.1585,1602, July 2013.

[4]. Donald A. Neamen, "The Basic MOSFET Operation", in Semiconductor

Physics and Devices Basic Principles, 4th ed. New York: McGraw-Hill, 2010, pp.

423.

[5]. Schwierz, Frank,"Graphene Transistors," Nature Nanotechnology, July

2010, Vol.5(7), pp.487-496.

[6]. Ryan Smith (2014, August 11). "Intel’s 14nm Technology in detail". {Online}.

Available: http://www.anandtech.com/show/8367/intels-14nm-technology-in-

detail

[7]. Liu, W. et al., "Role of Metal Contacts in Designing High-Performance

Monolayer n-Type WSe2 Field Effect Transistors," Nano Letters, 2013 May,

Vol.13(5), pp.1983-1990.

[8]. Roy, T. et al., "Field-Effect Transistors Built from All Two-Dimensional

Material Components," Acs Nano, 2014 Jun, Vol.8(6), pp.6259-6264.

[9]. Novoselov, KS. et al., "Electric Field Effect in Atomically Thin Carbon Films

" Science, 2004 Oct 22, Vol.306(5696), pp.666-669.

[10]. Radisavljevic1, B. et al., "Single-layer MoS2 transistors," Nature

Nanotechnology, 2011 Mar, Vol.6(3), pp.147-150.

[11]. Zhiming M. Wang, "Progress on the Theoretical Study of Two-Dimensional

MoS2 Monolayer and Nanoribbon", in MoS2 Materials, Physics, and Devices.

Switzerland: Springer, 2014, pp. 3.

[12]. Min, Sung-wook et al., "Nanosheet thickness-modulated MoS2 dielectric

property evidenced by field-effect transistor performance,"Nanoscale, 2012,

Vol.5(2), pp.548.

[13]. Liu, Yuan et al., "Toward Barrier Free Contact to Molybdenum Disulfide

Using Graphene Electrodes," Nano Letters, 2015 April.