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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 experimental results of the gate controlled GMH.(a) Current vs. bias voltage characteristic of a GMH (Vgate = 0), showing a Schottky diode characteristic with ~1.1 ideality factor. (b) The current vs. bias voltage characteristics of GMH at various Vgate. The black arrow indicates the direction of increasing Vgate. (c) The current vs. gate voltage characteristics of GMH at various Vbias. The black arrow indicates the direction of increasing Vbias. (d) The forward bias current as a function of Vgate. A unipolar control of forward current with the ratio of 105 is obtained.
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f2: The experimental results of the gate controlled GMH.(a) Current vs. bias voltage characteristic of a GMH (Vgate = 0), showing a Schottky diode characteristic with ~1.1 ideality factor. (b) The current vs. bias voltage characteristics of GMH at various Vgate. The black arrow indicates the direction of increasing Vgate. (c) The current vs. gate voltage characteristics of GMH at various Vbias. The black arrow indicates the direction of increasing Vbias. (d) The forward bias current as a function of Vgate. A unipolar control of forward current with the ratio of 105 is obtained.

Mentions: The forward characteristics at a low bias voltage show a diode ideality factor of ~1.1 (Figure 2a). When the bias voltage is ramped from 0 to 0.05 V, the current increases by three orders of magnitude. This indicates that the measured current may be due to tunneling between graphene and MoS2. Moreover, such a low threshold voltage (Vth ~ 0.14 V) of GMH is promising for low-voltage operation. Figure 2b shows the output characteristics (Id-Vg) at different gate voltages (Vg). These results show a pronounced increase in conductance with increasing Vg. The change of the current flow is due to the modification of Schottky barrier height by gate voltage. The transfer characteristics at different drain voltages is shown in Figure 2c. For the FESBT the largest current density is about 2,400 A/cm2 obtained with Vg = 40 V and Vd = 1 V. The transfer curve in Figure 2d, plotted on a log scale, shows that the FESBT exhibits a ~105 on-off ratio with the gate voltage ranging from −40 V to 40 V. Note that a 300-nm-thick SiO2 is used as back gate oxide in our device. As shown in figure S3, the on/off ratio of the GMH device (105) is much higher than that of the same MoS2 flake (944) used in our device. Accordingly, we conclude and emphasize that the Schottky junction is responsible for the higher on/off ratio. The ability of the gate control as well as the subthreshold swing (SS) should be further enhanced if a thinner oxide or high-κ dielectric is introduced, which will be discussed later in more detail. We estimate the field effective mobility of FESBT by , where L, W, Cox and gm are the channel length, width, gate capacitance per area, and the transconductance that is defined by at a constant Vd, respectively. Note that since both MoS2 and graphene were obtained by mechanical exfoliation, the shapes are irregular. As shown in Figure 1d, we evaluated the channel length (L) to be 1.3 μm and the width (W) to be 600 ~ 900 nm. The error bars are used for mobility extraction. To benchmark the performance of FESBTs, the electrical measurement results for the transistors based on pristine graphene and MoS2 flakes are shown in Figures S1 ~ S4 in the supporting information. Interestingly, the mobility values for the transistors based on pristine graphene and MoS2 flakes are around 179.7 and 6.5 cm2/V·s, respectively. Here the mobility of the transistors based on the MoS2 flakes is significantly lower than the 58.7 cm2/V·s mobility of the FESBT device. It has been widely reported that MoS2 is a n-type material, which is corroborated by the high threshold voltage ~18 V as shown in Figure S3, where the current has not saturated even at Vg = 40 V. This is unfavorable for the operation of logic electronics and the mobility extracted from the linear regime could be underestimated. We notice that the threshold voltage of a FESBT is shifted to a negative gate voltage. The core of our device is laid on the Schottky junction and the current modulation is implemented through the sufficient control of the Schottky barrier height. The shift of the threshold voltage is attributed to the new working principle of our device.


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 experimental results of the gate controlled GMH.(a) Current vs. bias voltage characteristic of a GMH (Vgate = 0), showing a Schottky diode characteristic with ~1.1 ideality factor. (b) The current vs. bias voltage characteristics of GMH at various Vgate. The black arrow indicates the direction of increasing Vgate. (c) The current vs. gate voltage characteristics of GMH at various Vbias. The black arrow indicates the direction of increasing Vbias. (d) The forward bias current as a function of Vgate. A unipolar control of forward current with the ratio of 105 is obtained.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f2: The experimental results of the gate controlled GMH.(a) Current vs. bias voltage characteristic of a GMH (Vgate = 0), showing a Schottky diode characteristic with ~1.1 ideality factor. (b) The current vs. bias voltage characteristics of GMH at various Vgate. The black arrow indicates the direction of increasing Vgate. (c) The current vs. gate voltage characteristics of GMH at various Vbias. The black arrow indicates the direction of increasing Vbias. (d) The forward bias current as a function of Vgate. A unipolar control of forward current with the ratio of 105 is obtained.
Mentions: The forward characteristics at a low bias voltage show a diode ideality factor of ~1.1 (Figure 2a). When the bias voltage is ramped from 0 to 0.05 V, the current increases by three orders of magnitude. This indicates that the measured current may be due to tunneling between graphene and MoS2. Moreover, such a low threshold voltage (Vth ~ 0.14 V) of GMH is promising for low-voltage operation. Figure 2b shows the output characteristics (Id-Vg) at different gate voltages (Vg). These results show a pronounced increase in conductance with increasing Vg. The change of the current flow is due to the modification of Schottky barrier height by gate voltage. The transfer characteristics at different drain voltages is shown in Figure 2c. For the FESBT the largest current density is about 2,400 A/cm2 obtained with Vg = 40 V and Vd = 1 V. The transfer curve in Figure 2d, plotted on a log scale, shows that the FESBT exhibits a ~105 on-off ratio with the gate voltage ranging from −40 V to 40 V. Note that a 300-nm-thick SiO2 is used as back gate oxide in our device. As shown in figure S3, the on/off ratio of the GMH device (105) is much higher than that of the same MoS2 flake (944) used in our device. Accordingly, we conclude and emphasize that the Schottky junction is responsible for the higher on/off ratio. The ability of the gate control as well as the subthreshold swing (SS) should be further enhanced if a thinner oxide or high-κ dielectric is introduced, which will be discussed later in more detail. We estimate the field effective mobility of FESBT by , where L, W, Cox and gm are the channel length, width, gate capacitance per area, and the transconductance that is defined by at a constant Vd, respectively. Note that since both MoS2 and graphene were obtained by mechanical exfoliation, the shapes are irregular. As shown in Figure 1d, we evaluated the channel length (L) to be 1.3 μm and the width (W) to be 600 ~ 900 nm. The error bars are used for mobility extraction. To benchmark the performance of FESBTs, the electrical measurement results for the transistors based on pristine graphene and MoS2 flakes are shown in Figures S1 ~ S4 in the supporting information. Interestingly, the mobility values for the transistors based on pristine graphene and MoS2 flakes are around 179.7 and 6.5 cm2/V·s, respectively. Here the mobility of the transistors based on the MoS2 flakes is significantly lower than the 58.7 cm2/V·s mobility of the FESBT device. It has been widely reported that MoS2 is a n-type material, which is corroborated by the high threshold voltage ~18 V as shown in Figure S3, where the current has not saturated even at Vg = 40 V. This is unfavorable for the operation of logic electronics and the mobility extracted from the linear regime could be underestimated. We notice that the threshold voltage of a FESBT is shifted to a negative gate voltage. The core of our device is laid on the Schottky junction and the current modulation is implemented through the sufficient control of the Schottky barrier height. The shift of the threshold voltage is attributed to the new working principle of our device.

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