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CXXXX American Chemical Society

Contact Electrification Field-EffectTransistorChi Zhang,†,§ Wei Tang,†,§ Limin Zhang,† Changbao Han,† and Zhong Lin Wang†,‡,*

†Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China, and ‡School of Material Science and Engineering,Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States. §C.Z. and W.T. contributed equally to this work.

Afield-effect transistor (FET) is a de-vice that uses an electric field totune/control the charge carrier trans-

port in a semiconductor material, which hasa wide range of applications in integratedcircuits and flexible electronics.1,2 As the FEThas a structure of three terminals, an exter-nally supplied gate voltage is needed forcontrolling the drain�source current.3�6

Due to the lack of an interactionmechanismbetween the external environment andelectronic device, the mechanical trans-ducer based on the FET cannot directly con-vert a mechanical stimulation/triggeringinto a control electrical signal in the FETstructure.7,8 In order to establish the me-chanosensation of the semiconductor de-vices, piezoelectric materials can be used inFETs to generate an internal electric controlsignal due to the piezoelectric polarizationcharges created at the interface region byapplying a strain.9,10 For the piezoelectricsemiconductor materials such as ZnO, GaN,and CdS with wurtzite or zinc blende struc-tures, the coupling between piezoelectric po-larization and semiconductor properties has

resulted in the emerging field of piezotronicssince 2007,11�16 which has enormous appli-cations, including but not limited to artificialintelligence, human�computer interaction,biomedicine, and communication.17�22

Recently, the invention of a triboelectricnanogenerator (TENG) has provided an ef-fective approach to convert ambient me-chanical energy into electricity.23�28 Theworking principle of the TENG is based onthe coupling of contact electrification andelectrostatic induction. Contact-inducedcharge transfer between two materials withopposite tribopolarity results in a potentialdifference when they are separated. Thispotential difference is an inner electricalsignal created by the external mechanicalforce, which could be used as a gate signalto tune/control the carrier transport char-acteristics in FET as the same effect asapplying a gate voltage.Here in this work, we developed an ex-

ternal force-triggered contact electrificationfield-effect transistor (CE-FET), which con-sists of a metal oxide semiconductor field-effect transistor (MOSFET) and a mobile

* Address correspondence tozlwang@gatech.edu.

Received for review July 19, 2014and accepted August 13, 2014.

Published online10.1021/nn5039806

ABSTRACT Utilizing the coupled metal oxide semiconductor field-effect transistor and triboelectric

nanogenerator, we demonstrate an external force triggered/controlled contact electrification field-effect

transistor (CE-FET), in which an electrostatic potential across the gate and source is created by a vertical

contact electrification between the gate material and a “foreign” object, and the carrier transport between

drain and source can be tuned/controlled by the contact-induced electrostatic potential instead of the

traditional gate voltage. With the two contacted frictional layers vertically separated by 80 μm, the drain

current is decreased from 13.4 to 1.9 μA in depletion mode and increased from 2.4 to 12.1 μA in

enhancement mode at a drain voltage of 5 V. Compared with the piezotronic devices that are controlled by

the strain-induced piezoelectric polarization charged at an interface/junction, the CE-FET has greatly expanded the sensing range and choices of materials

in conjunction with semiconductors. The CE-FET is likely to have important applications in sensors, human�silicon technology interfacing, MEMS,

nanorobotics, and active flexible electronics. Based on the basic principle of the CE-FET, a field of tribotronics is proposed for devices fabricated using the

electrostatic potential created by triboelectrification as a “gate” voltage to tune/control charge carrier transport in conventional semiconductor devices. By

the three-way coupling among triboelectricity, semiconductor, and photoexcitation, plenty of potentially important research fields are expected to be

explored in the near future.

KEYWORDS: contact electrification . field-effect transistor . triboelectric nanogenerator . electrostatic potential . tribotronics

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layer for contact electrification. With the process ofcontact and separation between the mobile electrodeand gate area of the MOSFET, the working mechanismof the CE-FET was demonstrated in two differentmodes. This design can generate an inner electrostaticpotential in the device as a gate voltage to tune/controlthe carrier transport between the drain and sourceinstead of the traditional gate voltage in the FET.Owing to the direct interaction mechanism betweenthe external environment and electronic device, theCE-FET is likely to have important applications insensors, human�silicon technology interfacing, MEMS,nanorobotics, and active flexible electronics. By cou-pling the semiconductor with triboelectricity, a field oftribotronics is proposed.The basic structure of the CE-FET is composed of

a back gate silicon-on-insulator (SOI) MOSFET and amobile layer, as schematically illustrated in Figure 1a.In the fabrication of the CE-FET device, a SOI wafer asthe sandwich structure is selected. The top silicon layeris p-type, 2.2 μm thick, and is used as the conductionchannel of the CE-FET. The two 1.0 μm thick aluminumpads are deposited on the surface and annealed toform Ohmic contacts, serving as the drain and sourceelectrodes. The buried silicon dioxide layer is 0.2 μmthick andused as the gate oxide. The silicon substrate isp-type, 500 μm thick, and heavy doped with a resistiv-ity lower than 0.005 Ω 3 cm. An aluminum layer is thendeposited on the bottom of the silicon substrate withOhmic contact as the gate electrode. The mobile layeris assembled next to the gate electrode, which consistsof a piece of Kapton film with the aluminum electrodedeposited on the back side. It can vertically contactand separate with the gate electrode by the externalforce. The gate electrode is completely off an externalvoltage source, and the mobile aluminum electrode

is connected to the source electrode. The drain andsource electrodes are connected with a voltage source,and holes are transported from the drain electrode tothe source electrode in the conduction channel. TheSEM images of the CE-FET in the top view and partialcross-sectional view are shown in Figure 1b,c.The CE-FET in this work presents a different mechan-

ism and working principle from the conventionalFET configuration, which is based on the back gateMOSFET, triboelectrification, and electrostatic induc-tion. The depletion mode of the device is shown inFigure 2a. For the vertical contact separation of themobile layer, the aluminum layer of the gate electrodeand the Kapton film form a pair of frictional layers witha certain gap in the original state. When the mobilelayer is first contacted with the aluminum layer asdriven by an external force, the electrons are injectedfrom the aluminum layer into the surface of the Kaptonfilm since Kapton is much more triboelectrically nega-tive than aluminum according to the triboelectricseries,29 leaving net positive electrostatic charges onthe aluminum layer and net negative electrostaticcharges on the Kapton film. The gate voltage is notinfluenced at this moment, and the width of theconduction channel is the same as the width of thetop silicon layer. When the Kapton film is graduallyseparated from the aluminum layer as the externalforce is vertically released, the electric field formed bythe triboelectric charges in the vertical direction inducethe electron flow from the mobile electrode to thesource electrode, forming an inner electric field voltageacross the gate and source electrodes. When theexternal force is completely released and the gapbetween the two frictional layers returns to the originalstate, more electrons flow from the mobile electrodeto the source electrode and the inner electric field

Figure 1. Schematic illustration of the contact electrification field-effect transistor. (a) Structure of the CE-FET based on a SOIwafer with heavy-doped silicon substrate and a mobile layer for vertical contact separation. (b) Top view of the CE-FET.(c) Partial cross-sectional view of the CE-FET.

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reaches themaximum. In this process, the inner electricfield is equivalently applied on a metal insulatorsemiconductor (MIS) capacitor in the MOSFET as agate voltage due to the extremely low resistivity ofthe silicon substrate, which is illustrated in the energydiagram shown in Figure 2b. Therefore, an inner chargepolarization is generated in the top silicon layer, whichattracts electrons and repels the holes in the bottom ofthe silicon layer. As holes are themajority carriers in thep-type silicon, a depletion zone will be formed in thisarea, which will decrease the width of the conductionchannel and thus the drain current. When the externalforce is vertically applied again and the Kapton filmgradually approaches the aluminum layer, the elec-trons flow back from the source electrode to themobile electrode with decreasing inner gate voltage.When the two frictional layers are contacted again,all of the electrons flow back to the mobile electrodeand the inner gate voltage is decreased to zero. In thisprocess, the inner charge polarization is depressed,which will increase the width of the conduction chan-nel and the drain current. Therefore, the inner gatevoltage can be generated and controlled by the ex-ternal force, which has the same effect as applyinga gate voltage. The equivalent circuit of the CE-FETin depletion mode is shown in Figure 2c. As the gapbetween two frictional layers and the inner positive

gate voltage by the external force increases, the draincurrent decreases in the depletion mode. The relation-ship between the inner positive gate voltage VG andthe gap d is analyzed using Guass Theorem in theSupporting Information, which can bedescribed by thefollowing equation:

VG ¼ εK 3Q0 3 dε0 3 εK 3 S0 þ ε0 3 CMIS 3 dK þ εK 3 CMIS 3 d

(1)

where Q0 is the frictional surface charge quantity, S0 isthe frictional surface area, ε0 and εK are the dielectricconstant of vacuum and Kapton, respectively, dK isthe thickness of the Kapton film, and CMIS is the MIScapacitance.The working principle of the CE-FET in the enhance-

ment mode is illustrated in Figure 3a. Different fromthe structure in the depletion mode, the Kapton film isattached to the back side of the gate electrode andthemobile layer is only a piece of aluminum film, whichcan vertically contact with and separate from eachother by an external force and form a pair of frictionallayers with a certain gap in the original state. When themobile electrode is first contacted with the Kapton filmby the external force, net positive charges are left onthe aluminum film and net negative charges are lefton the Kapton film. The gate voltage and the channelcarrier concentration are still not influenced at this

Figure 2. Schematic working principle of the CE-FET in depletion mode. (a) Change of the gate voltage and channel width inthe CE-FET with the vertical distance of themobile layer. (b) Energy diagram illustrating theMIS capacitor in the CE-FET at thepositive gate voltage. (c) Equivalent circuit of the CE-FET in depletion mode.

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moment. When the mobile electrode is gradually andvertically separated from the Kapton film to the originalstate as the external force is released, the electronsflow from the source electrode to themobile electrodeand an inner negative gate voltage is generated.Figure 3b illustrates the energy diagram of the MIScapacitor at the negative gate voltage. In this process,an inner charge polarization generated in the topsilicon layer attracts the holes but repels the electronsin the bottom of the silicon layer. Therefore, an en-hancement zone will be formed in this area, which willincrease the conduction channel carrier concentrationand the drain current. When the external force isvertically applied again and the aluminum film isgradually approaching the Kapton film to the contactstate, the electrons flow back from the mobile elec-trode to the source electrode with decreasing innergate voltage. In this process, the inner charge polariza-tion is depressed, which will decrease the conductionchannel carrier concentration and drain current. There-fore, the inner gate voltage can be generated andcontrolled by the external force, as well, but with anopposite effect and tendency in thismode. The equiva-lent circuit of the CE-FET in enhancement mode isshown in Figure 3c. As the gap between two frictionallayers and the inner negative gate voltage by theexternal force increases, the drain current is increased

in the enhancement mode. The relationship betweenthe inner negative gate voltage VG and the gap d arealso analyzed in the Supporting Information, which canbe described by the following equation:

VG ¼ � εK 3Q0 3 dε0 3 εK 3 S0 þ ε0 3 CMIS 3 dK þ εK 3 CMIS 3 d

(2)

RESULTS AND DISCUSSION

The characteristics of the MOSFET are measured firstby applying an external voltage to the gate electrodeof the CE-FET, which is schematically illustrated inFigure 4a. Figure 4b shows the ID�VD output charac-teristics with different VG from�5 to 0 V, and Figure 4cshows the ID�VG transfer characteristics at a drainvoltage of 5 V. The measured results fit well to theback gate MOSFET theory,30 which indicate that thedrain current is decreasedwith increasing gate voltage.When the gate voltage is less than about �2 V, thedrain current can be significantly tuned/controlled bythe gate voltage.The device of the CE-FET is fixed in a positioning

system, and the vertical distance between the twofrictional layers can be controlled accurately by theexternal force. The characteristics of the CE-FET inthe depletion mode are measured, and the ID�VDoutput characteristics with different vertical distances

Figure 3. Schematic working principle of the CE-FET in enhancement mode. (a) Change of the gate voltage and channelcarrier concentration in the CE-FET with the vertical distance of the mobile layer. (b) Energy diagram illustrating the MIScapacitor in the CE-FET at the negative gate voltage. (c) Equivalent circuit of the CE-FET in enhancement mode.

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from 0 to 80 μm are shown in Figure 5a. Figure 5bshows the ID output characteristics at a drain voltageof 5 Vwith the different vertical distances, and the insetplots ID�d transfer characteristics. The experimental

results indicate that the drain current is decreasedwith increasing vertical distance, which shows a goodagreement with the theoretical analysis in Figure 2a.When the vertical distance is less than about 60 μm,the drain current can be significantly tuned/controlledby the external force. Compared with the MOSFETcharacteristics in Figure 4c, the variable distance bythe external force has produced a similar transfer curvefor the CE-FET, which has successfully replaced the roleplayed by the external gate voltage. It is worth notingthat the inner gate electrode suffers from a negativevoltage in the state when the vertical distance is zero,which might be due to the self-owned initial negativecharges on the Kapton film surface that might existwhen the device was fabricated.The characteristics of the CE-FET in the enhance-

ment mode are measured, as well, and the ID�VDoutput characteristics with different vertical distancesfrom 0 to 80 μm are shown in Figure 6a. Figure 6bshows the ID output characteristics at a drain voltage of5 V with the different vertical distances, and the insetplots ID�d transfer characteristics. The experimentalresults indicate that the drain current is increased withincreasing vertical distance, which is consistent withthe theoretical analysis in Figure 3a. When the verticaldistance is more than about 40 μm, the drain currentcan be significantly tuned/controlled by the externalforce. Compared with the characteristics in the deple-tion mode, the variable distance by the external forcehas also produced a similar transfer curve for theCE-FET but with an opposite tendency in this mode.The experimental results in bothmodes validate that

an inner gate voltage can be created in the CE-FET byan external force and contact electrification, which actsas a gate voltage for tuning/controlling the transportof charge carriers through the conduction channel.The direction of the inner gate voltage depends on thedifferent mode, and the value depends on the verticaldistance between the two frictional layers. By applyingan external force to the device to gate the carriertransport, a direct interaction mechanism betweenthe external environment and electronic device has

Figure 4. Characteristics of MOSFET based on a SOI waferwith 2.2 μm channel width and 200 μm channel length. (a)Schematic circuit diagram of the MOSFET. (b) ID�VD outputcharacteristics with different VG. (c) ID�VG transfer charac-teristics at a drain voltage of 5 V.

Figure 5. Characteristics of CE-FET in depletion mode. (a) ID�VD output characteristics with different vertical distances. (b) IDoutput characteristics at a drain voltage of 5 V with different vertical distances. The inset is the ID�d transfer characteristics.

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been established, which provides a new approach forsensors, human�silicon technology interfacing, MEMS,nanorobotics, and active flexible electronics.A comparison of traditional FET, piezotronic tran-

sistor, and CE-FET is presented in Table 1. Like thepiezotronic transistor, the CE-FET is a two-terminalsemiconductor device in which the external mechan-ical force can replace the traditional third terminal ofFET and directly create an inner electric field to tune/control the current carrier transport characteristics. Thedifference is that the self-generated inner piezopoten-tial controls the barrier height of the piezotronic tran-sistor, while the self-generated inner electrostaticpotential controls the channel width of the CE-FET asin the conventional FET. Compared with the piezo-tronic transistor with a limited strain sensing range,17,18

the tuning/controlling performance of the CE-FET de-pends on the vertical distance, triboelectric chargedensity, and MOSFET characteristics, which can havea larger sensing range for the external environment.Furthermore, the selected materials for the CE-FETcan be expanded to general semiconductors and anymaterials for contact electrification, far more thanthe choice of materials for piezotronic transistors.Therefore, the CE-FET could have wider applicationprospects in conjunction with the conventional semi-conductor devices.

The CE-FET is based on the MOSFET and contactelectrification effect, which can be used as fundamen-tal components in novel triboelectronic devicesand systems. We call this tribotronics, a new field ofresearch and applications in flexible electronics. Tribo-tronics is about the devices fabricated using the elec-trostatic potential created by triboelectrification as a“gate” voltage to tune/control charge carrier transportin the semiconductor. Tribotronics is a field by couplingthe semiconductor with triboelectricity, which is anextension of piezotronics first proposed in 2007,11 andalso a profound application of TENG first proposedin 2012.23

Figure 7 schematically shows the three-way couplingamong triboelectricity, semiconductor, and photo-excitation, which is the basis of tribotronics (tribo-electricity�semiconductor coupling), tribophotonics(triboelectricity�photon excitation coupling), opto-electronics, and tribo-phototronics (triboelectricity�semiconductor�photoexcitation). Besides the well-known field of optoelectronics, tribophotonics is aresearch field of light-emitting phenomenon on thematerials of triboelectrification,31,32 and tribo-phototro-nics is a research field further coupled with tribotronicsand tribophotonics. These new fields can derive plentyof potentially important directions and applicationsin various disciplines such as material, mechatronics,

Figure 6. Characteristics of the CE-FET in enhancement mode. (a) ID�VD output characteristics with different verticaldistances. (b) ID output characteristics at a drain voltage of 5 V with different vertical distances. The inset is ID�d transfercharacteristics.

TABLE 1. Comparison of Traditional FET, Piezotronic Transistor, and Contact Electrification Field-Effect Transistor

field-effect transistor piezotronic transistor CE-FET

control external applied voltagecontrolling channel width

self-generated inner piezopotential controllinginterface/junction

self-generated inner electrostatic potentialcontrolling channel width

structure 3-terminal (S, D, G) 2-terminal 2-terminalgate voltage strain/mechanical deformation contact/friction/force/mechanical displacementsensing range none small largespeed fast (GHz) slower (kHz, MHz) slower (kHz, MHz)materials general semiconductor (Si, Ge...) piezoelectric semiconductor (ZnO, GaN...) general semiconductor (Si, Ge...)applications amplification, variable resistor,

electronic switchhuman/environmental interfacing, sensors, MEMS,flexible electronics, piezotronics, piezophotonics,piezo-phototronics, piezotromagnetism

human/environmental interfacing, sensors, MEMS,flexible electronics, tribotronics, tribophotonics,tribo-phototronics, tribotromagnetism

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information, automation, environment, chemistry, bio-medicine, and so on, which are projected in Figure 7and are expected to be explored in the near future.

CONCLUSIONS

By coupling the FET and contact electrification effect,an external force-triggered CE-FET composed of a

back gate SOI MOSFET and amobile layer is developed.With the triboelectrification and electrostatic induction,an inner gate voltage is created by the external forceand used for controlling the charge carrier transportthrough the conduction channel, which can replace thetraditonal external gate voltage. In both depletion andenhancement modes, it is validated that the chargecarrier transport process in CE-FET can be tuned/gatedby applying an external force to the device. The CE-FEThas established a direct interaction mechanism be-tween the external environment and electronic device,which is likely to have important applications in sensors,human�silicon technology interfacing, MEMS, nanor-obotics, and active flexible electronics.Based on the CE-FET as a fundamental component,

tribotronics is proposed as a field that couples triboe-lectricity with semiconductors, which is about thedevices fabricated using the external force to gatethe charge carrier transportation in the semiconductor.Compared to piezotronics, tribotronics has greatlyexpanded the sensing range and choices of materialsin conjunction with a general semiconductor, which isan extension of piezotronics. By the three-way cou-pling among triboelectricity, semiconductor, andphotoexcitation, plenty of important directions andapplications are expected to be explored in the nearfuture.

METHODSFabrication of the Contact Electrification Field-Effect Transistor. First,

a SOI wafer as a sandwich structure was prepared and selected.The top silicon layer of the SOI wafer was p-type and 2.2 μm inthickness. The buried silicon dioxide layer was 0.2 μm thick, andthe silicon substrate was p-type, 500 μm thick and heavy dopedwith a resistivity lower than 0.005 Ω 3 cm. Then, part of thetop silicon layer was masked, and two 1.0 μm thick aluminumpads were deposited by RF sputtering on the surface as drainand source electrodes. Similarly, a 1.0 μm thick aluminum layerwas deposited by RF sputtering on the bottom of the siliconsubstrate as the gate electrode. After that, the device wasannealed to form Ohmic contacts between the aluminumelectrodes with both the top silicon and substrate. The mobilelayer was fabricated based on a piece of aluminum film, whichwas vertically forced by the external force and resilience force.The Kapton film was attached to the surface of the mobilealuminum film in the depletionmode, while the back side of thegate electrode was attached in the enhancement mode.

Conflict of Interest: The authors declare no competingfinancial interest.

Acknowledgment. Support from the “thousands talents”program for the pioneer researcher and his innovation team,China, and the Beijing Municipal Science & TechnologyCommission (Z131100006013004, Z131100006013005) areacknowledged. We also thank Zhaohua Zhang, MingzengPeng, and Weiming Du for the discussions, and Xiang Yangand Yunxia Bai for their assistance in the preparation of thedevice.

Supporting Information Available: Analysis of the relation-ship between the inner gate voltage and the gap is included.This material is available free of charge via the Internet athttp://pubs.acs.org.

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