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Novel field-effect Schottky barrier transistors based on graphene-MoS2 heterojunctions.

Tian H, Tan Z, Wu C, Wang X, Mohammad MA, Xie D, Yang Y, Wang J, Li LJ, Xu J, Ren TL - Sci Rep (2014)

Bottom Line: Recently, two-dimensional materials such as molybdenum disulphide (MoS2) have been demonstrated to realize field effect transistors (FET) with a large current on-off ratio.Moreover, the field effective mobility of the FESBT is up to 58.7 cm(2)/V · s.Our theoretical analysis shows that if the thickness of oxide is further reduced, a subthreshold swing (SS) of 40 mV/decade can be maintained within three orders of drain current at room temperature.

View Article: PubMed Central - PubMed

Affiliation: 1] Institute of Microelectronics, Tsinghua University, Beijing 100084, China [2] Tsinghua National Laboratory for Information Science and Technology (TNList), Tsinghua University, Beijing 100084, China [3].

ABSTRACT
Recently, two-dimensional materials such as molybdenum disulphide (MoS2) have been demonstrated to realize field effect transistors (FET) with a large current on-off ratio. However, the carrier mobility in backgate MoS2 FET is rather low (typically 0.5-20 cm(2)/V · s). Here, we report a novel field-effect Schottky barrier transistors (FESBT) based on graphene-MoS2 heterojunction (GMH), where the characteristics of high mobility from graphene and high on-off ratio from MoS2 are properly balanced in the novel transistors. Large modulation on the device current (on/off ratio of 10(5)) is achieved by adjusting the backgate (through 300 nm SiO2) voltage to modulate the graphene-MoS2 Schottky barrier. Moreover, the field effective mobility of the FESBT is up to 58.7 cm(2)/V · s. Our theoretical analysis shows that if the thickness of oxide is further reduced, a subthreshold swing (SS) of 40 mV/decade can be maintained within three orders of drain current at room temperature. This provides an opportunity to overcome the limitation of 60 mV/decade for conventional CMOS devices. The FESBT implemented with a high on-off ratio, a relatively high mobility and a low subthreshold promises low-voltage and low-power applications for future electronics.

No MeSH data available.


Related in: MedlinePlus

The energy band diagrams for GMH and MGM.(a) Schematic view of the energy level alignment of the FESBT with Si/SiO2/MoS2/Graphene structure. (b) Schematic band diagrams of GMH with Vgate = 0. (c) Schematic band diagram of GMH with Vgate > 0. Applying a positive voltage on the gate induces electrons in graphene, decreasing its work function and the Schottky barrier height. (d) Schematic band diagrams of GMH with Vgate < 0. Applying a negative voltage on the gate induces holes in graphene, lowering its Fermi level and increasing the Schottky barrier height. Panels (e) and (f) show the energy diagram of the MGM under different gate voltages. When a higher gate voltage is applied, it induces more charges in graphene, due to which the Fermi level rises. Thus the energy barrier between graphene and MoS2 decreases and the conductance increases. In this way, the current flow is modulated by the gate voltage.
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f5: The energy band diagrams for GMH and MGM.(a) Schematic view of the energy level alignment of the FESBT with Si/SiO2/MoS2/Graphene structure. (b) Schematic band diagrams of GMH with Vgate = 0. (c) Schematic band diagram of GMH with Vgate > 0. Applying a positive voltage on the gate induces electrons in graphene, decreasing its work function and the Schottky barrier height. (d) Schematic band diagrams of GMH with Vgate < 0. Applying a negative voltage on the gate induces holes in graphene, lowering its Fermi level and increasing the Schottky barrier height. Panels (e) and (f) show the energy diagram of the MGM under different gate voltages. When a higher gate voltage is applied, it induces more charges in graphene, due to which the Fermi level rises. Thus the energy barrier between graphene and MoS2 decreases and the conductance increases. In this way, the current flow is modulated by the gate voltage.

Mentions: Based on the above analysis, a clearer working principle may be proposed for our FESBT. The energy level alignment of the FESBT with Si/SiO2/MoS2/Graphene structure is shown in Figure 5a. Since the affinity energy of MoS2 (4.15 eV) is smaller than the work function of the Graphene (4.5 eV), the Schottky junction can form. Figure 5b ~ 5d shows how electrons transport through the graphene-MoS2 Schottky junction in response to the modulation by the gate voltage. As shown in Figure 5b, there is a built-in voltage between Si and MoS2 after Fermi-level alignment, which could induce dipoles in the oxide. Applying a positive voltage to the gate induces electrons in graphene (Figure 5c), raising its Fermi level and thus decreasing the Schottky barrier height. When a negative voltage is applied to the gate (Figure 5d), it could induce holes in graphene and decrease its Fermi level, leading to the increase in Schottky barrier height. As for the working principle of the MGM-FESBT, the MoS2-graphene-MoS2 can be modeled as two Schottky junctions connected back to back. The band diagrams are illustrated in Figures 5e and 5f. When applying a low gate voltage, the Schottky barrier heights for both junctions are large. One of the Schottky junctions is positively biased, and the other is negatively biased. The current is suppressed by the negatively biased one. So the device behaves like a Schottky junction under reverse bias. When applying a higher gate voltage, the Fermi level of graphene rises and the energy barriers of two junctions decrease. Thus the conductance becomes much higher, which makes the device with “improved contact”.


Novel field-effect Schottky barrier transistors based on graphene-MoS2 heterojunctions.

Tian H, Tan Z, Wu C, Wang X, Mohammad MA, Xie D, Yang Y, Wang J, Li LJ, Xu J, Ren TL - Sci Rep (2014)

The energy band diagrams for GMH and MGM.(a) Schematic view of the energy level alignment of the FESBT with Si/SiO2/MoS2/Graphene structure. (b) Schematic band diagrams of GMH with Vgate = 0. (c) Schematic band diagram of GMH with Vgate > 0. Applying a positive voltage on the gate induces electrons in graphene, decreasing its work function and the Schottky barrier height. (d) Schematic band diagrams of GMH with Vgate < 0. Applying a negative voltage on the gate induces holes in graphene, lowering its Fermi level and increasing the Schottky barrier height. Panels (e) and (f) show the energy diagram of the MGM under different gate voltages. When a higher gate voltage is applied, it induces more charges in graphene, due to which the Fermi level rises. Thus the energy barrier between graphene and MoS2 decreases and the conductance increases. In this way, the current flow is modulated by the gate voltage.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4127518&req=5

f5: The energy band diagrams for GMH and MGM.(a) Schematic view of the energy level alignment of the FESBT with Si/SiO2/MoS2/Graphene structure. (b) Schematic band diagrams of GMH with Vgate = 0. (c) Schematic band diagram of GMH with Vgate > 0. Applying a positive voltage on the gate induces electrons in graphene, decreasing its work function and the Schottky barrier height. (d) Schematic band diagrams of GMH with Vgate < 0. Applying a negative voltage on the gate induces holes in graphene, lowering its Fermi level and increasing the Schottky barrier height. Panels (e) and (f) show the energy diagram of the MGM under different gate voltages. When a higher gate voltage is applied, it induces more charges in graphene, due to which the Fermi level rises. Thus the energy barrier between graphene and MoS2 decreases and the conductance increases. In this way, the current flow is modulated by the gate voltage.
Mentions: Based on the above analysis, a clearer working principle may be proposed for our FESBT. The energy level alignment of the FESBT with Si/SiO2/MoS2/Graphene structure is shown in Figure 5a. Since the affinity energy of MoS2 (4.15 eV) is smaller than the work function of the Graphene (4.5 eV), the Schottky junction can form. Figure 5b ~ 5d shows how electrons transport through the graphene-MoS2 Schottky junction in response to the modulation by the gate voltage. As shown in Figure 5b, there is a built-in voltage between Si and MoS2 after Fermi-level alignment, which could induce dipoles in the oxide. Applying a positive voltage to the gate induces electrons in graphene (Figure 5c), raising its Fermi level and thus decreasing the Schottky barrier height. When a negative voltage is applied to the gate (Figure 5d), it could induce holes in graphene and decrease its Fermi level, leading to the increase in Schottky barrier height. As for the working principle of the MGM-FESBT, the MoS2-graphene-MoS2 can be modeled as two Schottky junctions connected back to back. The band diagrams are illustrated in Figures 5e and 5f. When applying a low gate voltage, the Schottky barrier heights for both junctions are large. One of the Schottky junctions is positively biased, and the other is negatively biased. The current is suppressed by the negatively biased one. So the device behaves like a Schottky junction under reverse bias. When applying a higher gate voltage, the Fermi level of graphene rises and the energy barriers of two junctions decrease. Thus the conductance becomes much higher, which makes the device with “improved contact”.

Bottom Line: Recently, two-dimensional materials such as molybdenum disulphide (MoS2) have been demonstrated to realize field effect transistors (FET) with a large current on-off ratio.Moreover, the field effective mobility of the FESBT is up to 58.7 cm(2)/V · s.Our theoretical analysis shows that if the thickness of oxide is further reduced, a subthreshold swing (SS) of 40 mV/decade can be maintained within three orders of drain current at room temperature.

View Article: PubMed Central - PubMed

Affiliation: 1] Institute of Microelectronics, Tsinghua University, Beijing 100084, China [2] Tsinghua National Laboratory for Information Science and Technology (TNList), Tsinghua University, Beijing 100084, China [3].

ABSTRACT
Recently, two-dimensional materials such as molybdenum disulphide (MoS2) have been demonstrated to realize field effect transistors (FET) with a large current on-off ratio. However, the carrier mobility in backgate MoS2 FET is rather low (typically 0.5-20 cm(2)/V · s). Here, we report a novel field-effect Schottky barrier transistors (FESBT) based on graphene-MoS2 heterojunction (GMH), where the characteristics of high mobility from graphene and high on-off ratio from MoS2 are properly balanced in the novel transistors. Large modulation on the device current (on/off ratio of 10(5)) is achieved by adjusting the backgate (through 300 nm SiO2) voltage to modulate the graphene-MoS2 Schottky barrier. Moreover, the field effective mobility of the FESBT is up to 58.7 cm(2)/V · s. Our theoretical analysis shows that if the thickness of oxide is further reduced, a subthreshold swing (SS) of 40 mV/decade can be maintained within three orders of drain current at room temperature. This provides an opportunity to overcome the limitation of 60 mV/decade for conventional CMOS devices. The FESBT implemented with a high on-off ratio, a relatively high mobility and a low subthreshold promises low-voltage and low-power applications for future electronics.

No MeSH data available.


Related in: MedlinePlus