Minjong Lee, Tae Wook Kim, Chang Yong Park, Kimoon Lee, Takashi Taniguchi,Kenji Watanabe, Min-gu Kim,6?, Do Kyung Hwang,7?, Young Tack Lee,8?

ABSTRACT Two-dimensional van der Waals (2D vdW)material-based heterostructure devices have been widely studied for high-end electronic applications owing to their heterojunction properties. In this study, we demonstrate graphene (Gr)-bridge heterostructure devices consisting of laterally series-connected ambipolar semiconductor/Gr-bridge/n-type molybdenum disulfide as a channel material for field-effect transistors (FET). Unlike conventional FET operation, our Gr-bridge devices exhibit nonclassical transfer characteristics (humped transfer curve),thus possessing a negative differential transconductance.These phenomena are interpreted as the operating behavior in two series-connected FETs, and they result from the gate-tunable contact capacity of the Gr-bridge layer.Multi-value logic inverters and frequency tripler circuits are successfully demonstrated using ambipolar semiconductors with narrow- and wide-bandgap materials as more advanced circuit applications based on non-classical transfer characteristics. Thus, we believe that our innovative and straightforward device structure engineering will be a promising technique for future multi-functional circuit applications of 2D nanoelectronics.

KEYWORDS Graphene bridge; Heterostructure device; Non-classical transfer characteristics; Multi-value logic inverter; Frequency tripler

1 Introduction

Two-dimensional van der Waals (2D vdW) nanomaterials have provided intriguing opportunities for various applications in nanoelectronics. For example, graphene (Gr) is a versatile material owing to its excellent carrier mobility and compatibility with various applications; however,the absence of a bandgap limits its use as a semiconductor channel beyond silicon-based electronics [1, 2]. Thus, it can be considered as a conductor material in future electronic devices, and several research groups have used exfoliated Gr as source (S), drain (D), and gate (G) electrodes to achieve all-2D material-based field-effect transistor (FET) applications [3–6]. The Gr S/D electrodes can effectively overcome the Schottky barrier, generally observed between 2D vdW semiconductors and metal contacts [7–10], owing to its gatedependent Fermi level (EF) modulation [4, 11]. Because of this advantage, Gr electrodes are widely used in 2D vdW materials-based advanced electronic devices, thereby providing an innovative device structure and excellent device performance.

The 2D vdW semiconductors, such as transition metal dichalcogenides (TMDs) and 2D Xenes, have emerged because of their unique electrical properties since the discovery of Gr. Among these, tungsten diselenide (WSe2)and molybdenum ditelluride (MoTe2) possess strong gatedependent characteristics; essentially, they exhibit ambipolar properties in FET applications [12, 13]. The tunable bandgap of WSe2and MoTe2is investigated as 1.22 (bulk)to 1.64 eV (monolayer) [14, 15] and 0.9 (bulk) to 1.1 eV(monolayer) [16, 17], respectively. In contrast, black phosphorus (BP) and palladium diselenide (PdSe2), which have narrower bandgaps, exhibit stronger ambipolar properties than WSe2and MoTe2active channels; therefore, they allow the realization of high-performance device applications. The tunable bandgap of BP and PdSe2is reported as 0.3 (bulk) to 1.0 eV (monolayer) [18–20] and near-zero (bulk) to 1.3 eV(monolayer) [21–25], respectively.

These 2D vdW ambipolar semiconductors are expected to open new horizons for future nanoelectronics by developing new functionalized device applications, such as frequency doublers [26–30], reconfigurable homojunction diodes [31–34], and security circuits [35]. In advanced studies on 2D vdW semiconductors, heterostructure devices constructed using various 2D vdW materials as lego-like building blocks have been explored by numerous research groups to fabricate creative device architectures and investigate their unique junction properties [36–39]. A representative assembled device is a vertically stacked heterojunction diode consisting of two 2D vdW semiconductors for photodiodes [40–43], light-emitting diodes [44, 45], Esaki diodes[46–48], and extraordinary applications [49, 50].

In this study, a laterally series-connected ambipolar semiconductor/Gr/n-type molybdenum disulfide (MoS2)cascade channel device, called a Gr-bridge heterostructure,was studied beyond heterostructure junction device applications. Each active channel part exhibits distinct transport characteristics, such as ambipolar (PdSe2or WSe2), mostly metallic (Gr-bridge), and unipolar (MoS2) properties. However, the series-connected cascade channel combines the transport characteristics of each part, thereby obeying the largest resistance among the channel materials, i.e., the total resistance of the Gr-bridged cascade channel. The Gr-bridge reduces the potential barrier height between the ambipolar semiconductor andn-type MoS2junction region owing to its metallic and gapless energy band properties. Using this approach, the Gr-bridge allows the realization of unique switching devices and advanced application methods. We successfully demonstrated a multi-value logic inverter circuit and a frequency tripler for advanced electronic applications. Thus, we believe that the Gr-bridge heterojunction structure will open the gate for future electronics by designing device architectures and blending electrical properties toward high-end applications.

2 Experimental Section

2.1 Device Fabrication

The 285 nm SiO2/p+-Si substrate was ultrasonically cleaned in acetone, methyl alcohol, and isopropyl alcohol for 15 min each. To construct the Gr-bridge structure,polydimethylsiloxane (PDMS) stamps were used to exfoliate and transfer 2D vdW nanomaterials onto the substrate through the direct imprinting method. Next, a lift-off process was employed to form the top-gate electrode and the extended S/B/D pad electrodes. E-beam lithography and evaporation were, respectively, used for patterning and depositing metal electrodes at the 3D Convergence Center of Inha University.

2.2 Electrical Characterization

All transfer, output, and VTC characteristics were measured using a semiconductor parameter analyzer (4156B,Agilent) in a dark probe station at room temperature(300 K). To demonstrate the dynamic performance of the WGM-FET, sinusoidal and ramp waveform signals were generated from a function generator (AFG31022,Tektronix).

3 Results and Discussion

3.1 2D Ambipolar Semiconductor and Gr?Bridge Heterostructure Device

Figure 1a shows the transfer characteristics of PdSe2(narrow bandgap, ～ 0.01 eV) and WSe2(wide bandgap, ～ 1.3 eV) channel-based conventional FET devices[15, 24]. The hexagonal boron nitride (h-BN) sandwich structure and Gr S/D electrodes were adopted to investigate their fundamental ambipolar transfer characteristics(ID–VG) (see Fig. S1 for the cross-sectional device schematics and optical microscopy (OM) images of the two ambipolar-FET devices). The WSe2-FET exhibits both lower drain ON (ID,pandID,n) and OFFID(ID,off) current levels; however, the ON/OFFIDratio (ION/IOFF) is higher than that of the PdSe2-FET. The narrow-bandgap materials typically have good electron and hole carrier mobilities;therefore, they bring a higher ONIDvalue in FET devices but poor OFFIDin general. These distinct transfer characteristics are due to their energy bandgap properties, as shown in Fig. 1b.

In 2D vdW ambipolar semiconductors, the gate voltage (VG) can electrostatically control the carrier concentration in the active channel of FET devices, namely the electrostatic doping effect [51]. A positiveVGover the threshold voltage of then-type region (Vth,n) causesEFto reach the conduction-band edge (Ec), whereas a negativeVGnear theVthof thep-type region (Vth,p) enablesEFto reach the valence-band edge (Ev) of ambipolar semiconductor channel materials [24, 52, 53]. Consequently,a wider bandgap will result in a larger valley-like transfer characteristic curve because it needs a strongerVG(gate field) to modulate theEFfrom the intrinsic Fermi level(Ei) toEcfor then-type transition (or theEFfromEitoEvforp-type transition) with a higherION/IOFFratio.EGvalues of the ambipolar channel materials can be easily estimated by using theVthdifference (ΔVth=Vth,n-Vth,p)and the average subthreshold slopes [SSavg= (SSn+ SSp)/2]of then-type andp-type regions by Eq. (1), where q is the electron charge and SS60is the ideal SS value of 60 mV dec?1[24, 35, 52, 53].

Figure 1c shows the schematic of a typical top-gate transistor device and the expected valley-like transfer characteristic model of ambipolar FETs. The narrow bandgap of the active channel implies that even a small change inVGleads to a large number of carriers in the active channel withVth,nandVth,pnear the transition point (center of the valley-like transfer curve), thereby resulting in a highION, highIOFF, and lowION/IOFFratio. Although a narrow-bandgap ambipolar semiconductor typically exhibits high-performance FET behavior, the inevitably highIOFFrenders it difficult to use for digital logic circuit applications with better switching characteristics, as compared to wide-bandgap ambipolar semiconductors [35]. Based on the ambipolar properties of PdSe2(narrow bandgap) and WSe2(wide bandgap) active channel materials, Gr-bridge heterostructure devices have been studied to achieve advanced electronic applications, such as multi-value logic inverters and frequency tripler circuits.

Fig. 1 a Atomic crystal structures and ID–VG transfer characteristic curves of the ambipolar PdSe2- and WSe2-FETs. b Schematic of the relationship between Eg, Vth, and ID in ambipolar semiconductors. c Valley-like estimated electrical transfer model of 2D ambipolar semiconductorbased FETs according to the different energy bandgap properties (narrow and wide bandgap). d Conceptual device schematics and suggested electronic component symbol of the Gr-bridge heterostructure device (Gr-bridge FET). e Device configuration and resistance-in-series models of Gr-bridge FET

Such devices were formed as a platform of sequentially connected Gr-S, ambipolar semiconductors, Gr-bridge layers, MoS2, and Gr-D, as shown in Fig. 1d. The Gr bridge layer allows for the inherent characteristics of each 2D vdW semiconductor because it can provide tunable contact properties to both 2D vdW active channels according to its gatedependent Fermi level modulation [4, 11]. Essentially, the Gr-bridge FET can be regarded as a series connection of the ambipolar-FET and MoS2-FET, with the same electrical characteristics. Figure 1e shows the simple resistance-inseries model of Gr-bridge FET. The total resistance (Rtot)depends on the sum of the resistance in each FET part owing to the series connection properties; therefore,Rtotwill follow the higher resistance of the two active channels during the device operation. Based on these operating properties, the connected ambipolar and MoS2active channel devices in series will exhibit synthetic transfer characteristic curves as a breakthrough for high-end device applications.

3.2 PdSe2?Gr?MoS2 Heterostructure FET

The first Gr-bridge-based high-end device consists of ambipolar PdSe2(narrow bandgap) andn-type MoS2active channel materials for multi-value logic applications, as shown in Fig. 2a. Figure 2b, c shows the OM images before and after metal patterning for the extended S/D and common gate(G) electrodes. For sample preparation, the bottom h-BN,Gr S/D, MoS2-Gr-PdSe2heterostructure, and h-BN gate insulator were sequentially exfoliated and transferred onto a 285 nm-thick silicon dioxide (SiO2)/p+-silicon (Si) substrate.Subsequently, Ti/Au (5 /50 nm) electrodes were patterned and deposited using a combination of e-beam lithography and e-beam evaporation systems. The detailed step-by-step fabrication flow and thickness information for each 2D vdW material are depicted in Fig. S2 and S3, respectively.

Fig. 2 a Cross-sectional 3D device schematic of the PdSe2-Gr-MoS2 heterostructure FET (PGM-FET) for multi-value logic applications. OM images of the PGM-FET b before and c after the extended metal electrode fabrication. d ID–VG transfer characteristic curves of the MoS2-FET(blue dashed line), PdSe2-FET (red dashed line), and PGM-FET (black solid line) at VD of 1 V. e Estimated transconductance of the PGM-FET obtained from ID–VD output characteristic curves of f MoS2-FET, g PdSe2-FET, and h PGM-FET. (Color figure online)

Figure 2d shows theID–VGtransfer curves of the MoS2-FET (blue dashed line), PdSe2-FET (red dashed line), and PdSe2-Gr-MoS2heterostructure devices (solid black line) at a drain voltage (VD) of 1 V. We named the Gr-bridge device (PdSe2-Gr-MoS2FET) “PGM-FET.”Because the h-BN sandwich structure was adopted to provide high-quality interfaces, hysteresis-less ideal transfer properties exist in all FET operations [54–57]. The MoS2-FET exhibits strongn-type transfer characteristics,and the PdSe2-FET shows ambipolar transfer characteristics using the Gr bridge layer as a source. Because the graphene interlayer has a gate-tunable contact capacity, it can act as a “bridge,” thereby reducing the Schottky junction properties between the ambipolar andn-type active channels. The detailed Raman spectrum analysis to confirm the clean and non-interactive interface properties of Gr-bridge and TMDC channels are shown in Fig. S4. Owing to the effect of the Gr bridge interconnection, the PGM-FET exhibits a tilde ( ～)symbol-like humped transfer characteristic curve: it traces the lowerIDof either the PdSe2or MoS2FETs according to the sweptVG(see the solid black line in Fig. 2d). TheIDof the PGM-FET follows theIOFF(region I) and subthresholdID(region II) of the MoS2-FET, while regions III and IV,respectively, denote the case in the transferID,pandID,nof PdSe2.

To analyze the detailed transport properties of the PGMFET, the transconductance (gm= dID/dVG) was calculated,and a negativegmwas observed in region III (humped curve), as shown in Fig. 2e. Figure 2f–h shows the output characteristic (ID–VD) curves of the MoS2-FET, PdSe2-FET,and PGM-FET obtained fromVGranging from ? 2 to 2 V in steps of 0.5 V. The Gr bridge provides tunable ohmic contact properties; therefore, the output curves of the MoS2and PdSe2active channels exhibit typicaln-type and ambipolar transport properties, respectively. Moreover, the PGMFET exhibits the composite output characteristics of the MoS2-FET (VGrange of ? 2 to ? 1 V) and PdSe2-FET (VGrange of ? 0.5 to 2 V).

3.3 PGM?FET?Based Multi?value Logic Circuit Applications

Based on the non-classical humpedID–VGtransfer properties, we extended our PGM-FET study to multi-value logic circuit applications as a first-approach method. An external resistor of 10 M? was chosen for the resistive-load inverter circuit near the humpedIDcurve, as shown in Fig. 3a. Figure 3b shows the voltage transfer characteristics (VTC) of the PGM-FET-based resistive-load inverter circuit under different supply voltage (VDD) conditions. The inset shows three distinct levels of ternary inverted output voltage (Vout)responses. Input voltages (Vin) of ? 2, 0, and 2 V generateVoutofVDD,VDD/2, and approximately 0 V, respectively. Figure 3c shows the absolute voltage gain (|dVout/dVin|) of the ternary inverter logic circuit under differentVDDconditions.The voltage gain is approximately 4 (first voltage drop) and approximately 1.6 (second voltage drop) at aVDDof 2 V.The inset shows the dynamicVoutresponses obtained from a sinusoidal waveform ofVin, which can identify the dynamic ternary levels of the demonstrated ternary logic circuit. The peak-to-peak voltage (Vp-p) and periodic time (T) were 4 V and 1 s, respectively.

Fig. 3 a Circuit diagram of PGM-FET-based resistive-load inverter for multi-value logic circuit applications. b VTC characteristic curves of PGM-FET-based multi-value logic operations for VDD ranging from 0.1 to 2 V. The inset shows three distinguished output states (ternary logic states). c Voltage gain of the demonstrated ternary inverter logic circuit for VDD ranging from 0.1 to 2 V. The inset shows the dynamic Vout responses (red) from the sinusoidal waveform Vin (black). (Color figure online)

We developed inverted ternary logic for the Gr-bridge structure, which simply employs a narrow-bandgap ambipolar semiconductor (PdSe2) and ann-type semiconductor(MoS2). Based on the above understanding of the PGMFET-based ternary logic circuit, we attempted to investigate a direct PdSe2-MoS2junction FET (PMJ-FET) for a more advanced and practical device model, as shown in Fig. S5. Although the PMJ-FET also exhibits a non-classical humpedID–VGtransfer curve and ternary logic circuit operation, the absence of the Gr-bridge interlayer results in rectifying properties at the PdSe2/MoS2junction [58].Consequently, the PMJ-FET exhibits asymmetric (having direction) device characteristics and a limitation in dynamic operation, whereas the PGM-FET exhibits symmetric (bidirectional) logic circuit operation properties. Furthermore,reliability problems remain in the PMJ-FET because controlling the optimized junction properties between the two different semiconductor active channels is difficult. As shown in Fig. S4d, the PMJ-FET does not trace the transfer curves of the MoS2-FET and PdSe2-FET because of the rectifying junction properties of MoS2/PdSe2[42]. Therefore, adopting the Gr-bridge device structure will provide reliability and convenience for further study of high-end multi-value logic applications.

3.4 WSe2?Gr?MoS2 Heterostructure FET Device

As a second approach toward advanced electronic applications, WSe2, a wide-bandgap ambipolar active channel,was chosen to realize the Gr-bridge heterostructure device instead of the PdSe2-based ternary logic circuit. Figure 4a shows the 3D device schematic of WSe2-Gr-MoS2FET,named “WGM-FET.” Figure 4b, c shows the OM images before and after patterning of the extended S/D and G electrodes, respectively, through the same device fabrication processes of PGM-FET. The detailed process flow and thickness information for each layer are shown in Figs. S6 and S7,respectively. Figure 4d shows theID–VGtransfer curves of the MoS2-FET (blue dashed line), WSe2-FET (red dashed line), and WGM-FET (solid black line) atVDof 2 V. In this case, the transfer curve of WGM-FET follows that of the MoS2-FET in regions I and II and that of WSe2-FET in regions III and IV. However, unlike the case of PGMFET, the wider bandgap of the WSe2channel allows a lowIOFFlevel of WGM-FET (wider region III); therefore, the transfer characteristic curves resemble an uppercase letter“N”. Figure 4e shows thegm–VGcurve of the WGM-FET; a negativegmwas observed in operation region III owing to thep-type transition properties of the WSe2active channel.

Fig. 4 a Cross-sectional 3D device schematic of the WSe2-Gr-MoS2 heterostructure FET (WGM-FET) for frequency tripler circuit applications. OM images of the WGM-FET b before and c after the extended metal electrode fabrication. d ID–VG transfer characteristic curves of the MoS2-FET (blue dashed line), WSe2-FET (red dashed line), and WGM-FET (black solid line). e Estimated transconductances of the WGM-FET obtained from d. (Color figure online)

3.5 WGM?FET?Based Frequency Tripler Circuit Applications

These non-classical transfer characteristics of the WGMFET bring unique VTC characteristics of an upside-down letter “N”-like curve in inverter logic circuit applications, as shown in Fig. 5a. To achieve a resistive-load inverter circuit,an external resistor of 100 M? was connected to the WGMFET. Figure 5b shows the VTC curve, and the four transition states were observed as sequentially “High”, “Low”, “High”,and “Low” output states according to the voltage sweep ofVinatVDDof 2 V. The inset shows the four distinguished logic states atVinof ? 1, ? 0.5, 1, and 2 V. Figure 5c shows the voltage gain of our demonstrated inverter circuit, which has both negative and positive values.

Fig. 5 a Circuit diagram of the WGM-FET-based resistive-load inverter circuit applications. b Upside-down letter “N”-like VTC curve of WGM-FET-based frequency tripler circuit operations at VDD of 2 V. The inset shows four distinguished output states, that is, sequentially repeated “High” and “Low” states. c Voltage gain of the demonstrated WGM-FET-based frequency tripler circuit. d Expected frequency response of frequency tripler (three cycles of Vout) converted from a single cycle of Vin. e Real-time Vout responses (red) of the frequency tripler circuit for 0.1, 0.5, and 1 Hz input sinusoidal waveform Vin (black). (Color figure online)

TheVinsweep from “Low (? 1 V)” to “High(2 V)” should produce sequential output states of“High–Low–High–Low” and the backwardVinsweep from“High” to “Low” generated the reversed output states of“Low–High–Low–High.” That is, a double sweep ofVinof “Low–High–Low,” similar to a single waveform, will produce the “High–Low–High–Low–High–Low–High”sequential output states. The repeatable “High” and “Low”output logic states enable an advanced frequency response application with respect to the sinusoidal waveform ofVin, as shown in Fig. 5d. As a result, a single cycle of sinusoidal waveformVinshould produce three cycles of the waveformVout. This circuit application method can be regarded as a “frequency tripler,” which can generate the output frequency (fout) to triple the value from the input frequency (fin). Finally, for the first time, we successfully demonstrated a frequency tripler application with a single Gr-bridge heterostructure FET consisting of a wide-bandgap ambipolar semiconductor. Figure 5e shows the real-timeVoutresponses for the 0.1, 0.5, and 1 Hz sinusoidal waveforms ofVin(see Fig. S8 for the realtimeVoutresponse for the 1 Hz ramp waveformVin). TheVoutresponses were analyzed using fast Fourier transform(FFT), as shown in Fig. S9. Our frequency response application should be economical to generate triple frequency toward low-power (frequency) and low-cost, more effective than the conventional frequency multiplier circuit for future advanced electronics.

4 Conclusions

In this study, Gr-bridge FETs consisting of laterally series-connected ambipolar semiconductor/Gr-bridge/ntype MoS2cascade channels were studied for high-end switching device applications based on their non-classical negative differential transconductance characteristics.The Gr-bridge layer could eliminate the Schottky junction properties between two semiconductor channels; therefore,the Gr-bridge FETs showed synthetic transfer characteristics, perfectly tracing the lowerIDof each channel material based on the simple resistance-in-series model, unlike the heterojunction devices without the Gr-bridge layer. Moreover, we successfully implemented two types of advanced electronic applications based on the bandgap properties of PdSe2(narrow-bandgap) and WSe2(wide-bandgap)ambipolar semiconductors for a multi-value logic inverter(PGM-FET) and frequency tripler (WGM-FET) circuits,respectively. Thus, we believe that the results of our Gr-bridge heterostructure devices and multi-functional circuit applications will provide reliability and convenience to open up a breakthrough toward future advanced electronics.

AcknowledgementsY. T. L. acknowledges the financial support from the National Research Foundation of Korea (NRF) (No. NRF-2021R1C1C1005235). D. K. H. acknowledges the financial support from the Korea Institute of Science and Technology (KIST) Institution Program (No. 2E31532).

FundingOpen access funding provided by Shanghai Jiao Tong University.

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Supplementary InformationThe online version contains supplementary material available at https:// doi. org/ 10. 1007/s40820- 022- 01001-5.