resonant gate transistor as in fin fet

7/30/2019 Resonant Gate Transistor as in Fin Fet

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RESONANT GATE TRANSISTOR AS AN

INDEPENDENT GATE FIN-FET

Jyotirmoy Deka

Department of Electronics & Communication Engineering

Don Bosco College of Engineering and Technology

Roll No - DC2010BTE4081

Fourth semester (sec-B)

[email protected]

ABSTRACT

A device is described which permits high- Q frequency selection to be incorporated into silicon integrated

circuits. It is essentially an electrostatically excited tuning forkemploying field effect transistor. The device,which is called the resonant gate transistor ( RGT), can be batch-fabricated in a manner consistent with silicontechnology. Experimental RGTs with gold vibrating beams operating in the frequency range 1 kHz


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Nathanson et al. demonstrated the Resonant GateTransistor (RGT), driving resonance in a goldcantilever with an air-gap capacitive electrode [1].The RGT cantilever functioned as the gate of anair-gap transistor, with output drain current

modulated by the cantilever resonant motion. Thisdevice achieved a resonance frequency of 30 kHzwith quality factor of ~70 despite the limited

processing capabilities of the time. Fabricationlimitations prevented the proliferation of these andother MEMS devices until the advent of silicon-

based surface micromachining.FET sensing has only recently regained momentumas a means of mechanical detection, and has beenimplemented in a variety of micromechanicaldevices. Resonant Gate Transistors similar to

Nathansons device have been demonstrated insilicon air-gap resonators up to 14 MHz [2,3].Mechanical resonators sensed through direct elasticmodulation of a transistor channel have also beendemonstrated. Such devices include air gapresonators with FETs embedded in the resonator

body up to 71 MHz [4], mechanical mixing insingle electron transistors up to 245 MHz [5], and

piezoelectric high electron mobility transistor(HEMT) channel modulation in GaN resonators upto 2 MHz [6].

Internal Dielectric Transduction

To improve electrostatic transduction efficiencyand scale MEM resonators into the GHz domain,we previously demonstrated longitudinal silicon

bar resonators using a novel method to drive andsense acoustic waves in the bar. This mechanism,termed internal dielectric transduction [7],incorporates thin dielectric film transducers insidethe resonator body for capacitive transduction.Internal dielectrically transduced resonators haveyielded acoustic resonance frequencies up to 6.2

GHz [8] and frequencyquality factor products(f.Q) up to 5.1x1013 [9]. Moreover, thesedielectrically transduced resonators demonstrateimproved efficiency as resonance frequencyincreases, providing a means of scaling MEMresonators to previously unattainable frequencies.However, at multi-GHz frequencies capacitivefeed-through becomes significant and preventscapacitive detection of MEM resonance withoutthree-port mixing measurements.

Unlike capacitive sensing employed in theseresonators, FET sensing can amplify the

mechanical signal prior to any feed-throughparasitics. Combining the benefits of FET sensingwith the frequency scaling and high-Q capabilitiesof internal dielectrically transduced resonators, theauthors recently demonstrated a Resonant Gate

Transistor (RGT) [10] operating at 11.7 GHz withQ of over 1800. This device incorporates a field-effect transistor into the resonator body for internalamplification of the resonant signal. The best RGTgeometry for optimal dielectric transduction at highfrequencies is an internal dielectrically transducedlongitudinal-mode resonator, with dielectric films

positioned at points of maximum strain. As wescale to higher frequency, the width of theresonator decreases, eventually converging to ageometry very similar to that of Independent-GateFin-FETs [11].

THE RESONANT GATE TRANSISTOR

Theory

The principle of operation of the internaldielectrically transduced RBT is shown in Fig. 1.The region in light grey represents the active regionof the resonator, while the blue region is highlydoped. The active region near the drive gate is

biased into accumulation (red), so that a largecapacitive force acts across the thin dielectric film(yellow), driving longitudinal vibrations in the

body. A gate voltage is applied to the opposinggate, generating an inversion channel (blue) whichresults in a DC drain current. At resonance, elasticwaves formed in the resonator modulate the draincurrent both by physically changing the gatecapacitance and by piezo resistive modulation ofcarrier mobility. The internally amplified RGT hassignificantly lower output impedance thancapacitive detection mechanisms, simplifyingimpedance matching with active circuits.


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The amplitude of vibrations of the internaldielectrically transduced RGT 0|RGT can be foundin an analysis similar to [7] and has beeninvestigated in [10]. The strain induced in theresonator piezoresistively modulates the drain

current running through the inversion layer.Assuming a piezoresistive coefficient of 110 forcurrent traveling perpendicular to the normal ofelastic wave fronts along , the change inmobility is given by

wheregis the dielectric thickness, Yis the Youngsmodulus and kn is the wave number of the nthharmonic. The piezoresistive mobility modulationof Eqn. 1 generates an AC current linearlydependent on the drain current:

The electromechanical transconductance is definedby

One of the fundamental obstacles of scaling MEMS

resonators to multi-GHz frequencies is the need fora sense transducer that overcomes feed-through and

parasitic capacitance and provides adequatesensitivity to measure mechanical motion at thosefrequencies. Fig. 2(a) shows a standardButterworth-Van Dyke (BVD) model of acapacitively transduced resonator whose output isfed into a common source amplifier. Aselectrostatically transduced resonators shrink insize to scale to GHz frequencies, the feed-throughcapacitance CFT limits the minimum detectable

electromechanical signal through the device. InFig. 2(a), both the feed-through current andelectromechanical current are amplified in theamplifier following the resonator, making 2-portdetection impossible at high frequencies.

The linear equivalent circuit (LEC) of adielectrically driven RBT is given in Fig. 2(b). Thismodel differs from that of a standard MOSFET intwo ways. First, the AC electrical signal is appliedto the back gate (B) biased in accumulation,leaving the inversion gate (G) at a constant biasvoltage. This biasing prevents the input signal fromelectrically modulating the inversion channel.

Second, the small signal current source is not aconstant function of frequency. In this model, theelectromechanical trans conductance gm,em has aLorentzian frequency dependence with peakamplitude defined by Eqn. 3 and a mechanical

quality factorm=1 /x LxCx

as in the case ofthe standard BVD model. The capacitive feed-through in the RBT is just the parasitic

Cbg, Cbs, and Cbd. Due to the symmetry of the RGT,Cbg and Cbd are equivalent to Cgs and Cgd found inthe amplifier of Fig. 2(a). Moreover, the feed-through component Cbg in the fully depleted RGT,equal in magnitude to CFT, is not amplified in anyway. Therefore, integration of FET sensing into theacoustic resonator enables amplification of theelectromechanical signal without amplification ofelectrical feed-through.

Independent-Gate Fin FET Resonance

Many three-dimensional transistors such asnanowire FET and tri-gate and independent gateFinFETs are currently under development forfuture CMOS nodes. As the RBT realized in [10]scales to higher frequencies, the length direction(perpendicular to the drain current in device)decreases resulting in a structure very similar to anIndependent-Gate (IG) FinFET. However, resonant

transistors are not limited to this topology; thedrive and sense mechanisms in the RGT can be


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extended to acoustically resonate transistors ofvarious geometries and materials.

In this paper, we demonstrate the acousticresonance of the released IG-FinFET similar to thatshown in Fig. 3, with a single-crystal silicon fin of114 nm width and 220 nm thickness and two

polysilicon gates. The gate dielectrics arecomposed of 15 nm silicon nitride on either side ofthe fin. As in the case of the RGT, one gate of theIG-FinFET is biased into accumulation to driveacoustic waves in the device. The other gate biasesthe fin into inversion to generate a drain current forFET sensing of the elastic waves. The active areaof this n-type device is lightly p-doped (NA = 1014

cm-3) while gates, source, and drain are highlydoped with As (ND ~ 1018 cm-3).

After the HF release of the device, both fin andgates are freely suspended and can resonate incontour plate modes defined by the compositegeometry of the gates and fin. Each gate of thedemonstrated device measured 500 nm (gate lengthalong the fin) by 400 nm. While many plate modesmay be possible in this structure, the longitudinalwaves generated perpendicular to the dielectricfilm reduces the number of modes excited bydielectric transduction of the device. Moreover, the

FET sensing averages the distributed piezoresistivecontribution of strain to the total AC drain currentalong a small region of the device (along the FETchannel) resulting in a further reduction ofundesired modes detected at the output.

A harmonic analysis of the IG-FinFET wasperformed in COMSOL Multiphysics to determinethe resonance modes which could be both excitedand detected in the device. A harmonic force of2.7x105 N/m2 was applied along both interfaces of

one dielectric film to simulate the electrostaticforce on the accumulation side of the fin. Thiscorresponds to an accumulation gate voltageACC=1 , a drain voltage D=4 , a groundedsource, and an AC excitation at the accumulationgate of=0.4 . There is a roughly linearvoltage drop from drain to source along the fin,resulting in a electrostatic force gradient along thedielectric. In the harmonic analysis, this isapproximated as an averaged voltage evenlydistributed along the length of the capacitor. Thedirection of the harmonic forces applied is shownin Fig. 4.

A frequency sweep from 30 to 50 GHz resulted inabout 8 resonance modes excited from the force onthe dielectric film. To determine the subset of thesemodes that would be detected with piezoresistiveFET sensing, the strain was integrated along the finat the region of inversion. As illustrated in Fig. 1,

the inversion region exists inside the fin at theinterface to the second dielectric film not used for


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driving resonance. Integration along this interfacereduced the number of detectable eigenmodes tothree in the simulation range of 20 GHz. Fig. 4shows a contour plot of the y-axis strain of acontour mode resonance with acoustic half-

wavelength corresponding to the width of the fin.This simulated contour mode of the IG-FinFET at35 GHz in Fig. 4 shows a 10th harmoniclongitudinal mode in the y-direction coupled to a7rd harmonic in the x-direction. Fig. 5 providesdisplacement and strain plots extracted from thismode across the fin. An AC drain current throughthe fin can result from both transverse (y-direction)and longitudinal (x-direction) strain acting

piezoresistively on the drain current flowing in thex-direction. The simulated mode exhibits both

positive and negative strain in the x-direction,resulting in cancelation of piezoresistivecontribution to the drain current from longitudinalstrain. The transverse strain inside the fin generated

by its /2 resonance is both tensile andcompressive. However, the current flowing throughthe fin is confined to the inversion region near oneof the dielectrics, where the transverse strain islargely uniform along the length of the fin. Thiscontributes to a net piezoresistive change in draincurrent at the resonance frequency.

In accordance with the theory of internal dielectrictransduction, the harmonic response of the structureresults in maximum strain and correspondingdisplacement nodes at the dielectric transducers.This configuration of drive (capacitive) and sense

(piezoresistive) transducers yields the highestefficiency conversion from the electrical tomechanical domain and back into an electricalsignal.

MEASUREMENT

The devices were tested in a two-port configurationat room temperature in a vacuum probe station, asshown in Fig 6. The resonating IG-FinFET isrepresented by a new RBT symbol. This symbol ofthe 4-terminal device illustrates the contribution of

mechanical resonance from the back gate to thecharacteristics of the transistor channel.

The devices were tested in vacuum to preventionization of air in the fringe fields near thedielectric, where up to 5 V were applied across a15 nm gap. The vacuum also prevented adsorptionof molecules onto the surface of the resonator overtime, which can degrade the quality factor. Afterde-embedding the device from the probe pads androuting, the transconductance is obtained from theY-parameters, m=2112, as in the case of conventional transistor measurements. A detailedexplanation of the de-embedding structures andalgorithm is provided in the supplementarymaterial of [10].


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CONCLUSION

The electromechanical resonance of transistorsenables internal amplification of the desiredmechanical signal, greatly improving the dynamicrange of GHz-frequency MEMS devices above the

parasitic capacitance floor. This sensingmechanism allows for detection of acousticresonance at previously inaccessible frequencies. Afreely-suspended IG-FinFET was implemented as aResonant Body Transistor, demonstrating anacoustic resonance frequency of 37.1 GHz andquality factor of 560. The hybrid NEMS-CMOSRBT will provide a small-footprint, low-power,high sensitivity building block for many RF andmm-wave applications.

REFERENCES

[1] H.C. Nathanson, W.E. Newell, R.A.Wickstrom, J.R. Davis Jr., The resonant gatetransistor,IEEE Transactions on Electron Devices1967, 14 (3), 117-133.

[2] C. Durand, F. Casset, P. Renaux, N. Abele, B.Legrand, D. Renaud, E. Ollier, P. Ancey, A.M.Ionescu, L. Buchaillot, In-plane silicon-on-nothing nanometer-scale resonant suspended gateMOSFET for in-IC integration perspectives,

Electron Device Letters 2008, 29 (5), 494-496.

[3] E. Colinet, C. Durand, L. Duraffourg, P.Audebert, G. Dumas, F. Casset, E. Ollier, P.Ancey, J.-F. Carpentier, L. Buchaillot, A.M.

Ionescu, Ultra-sensitive capacitive detection basedon SGMOSFET compatible with front-end CMOS

process, Journal of Solid-State Circuits 2009, 44(1), 247-257.

[4] D. Grogg, M. Mazza, D. Tsamados, A.M.Ionescu, Multi-gate vibrating-body field-effecttransistors (VB-FETs),IEEE IEDM2008, 1-4.

[5] Kim, H.S.; Qin, H.; Blick, R.H., Directmechanical mixing in a nanoelectromechanicaldiode,Applied Physics Letters 2007, 91, 143101.

[7] D. Weinstein, S.A. Bhave, "Internal dielectrictransduction: optimal position and frequencyscaling,"IEEE TUFFC2007, 54(12), pp. 2696-98.

[8] D. Weinstein, S.A. Bhave, S. Morita, S.Mitarai, K. Ikeda, "Frequency scaling andtransducer efficiency in internal dielectricallytransduced silicon bar resonators," Transducers2009, pp. 708-711.

[9] D. Weinstein, S.A. Bhave, "Internal dielectrictransduction of a 4.5 GHz silicon bar resonator,"

IEEE IEDM2007, pp.415-418.

[10] D. Weinstein, S.A. Bhave, "The resonant bodytransistor" Nano Letters ASAP 2010. DOI:

10.1021/nl9037517

[11] D.M. Fried, J.S. Duster, K.T. Kornegay,High-performance p-type independent-gateFinFETs, IEEE Electron Device Letters 2004,25(4), pp. 199-201.

resonant gate transistor as in fin fet

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