COMPARATIVE Ashishkumar Jaiswal Electronics and Telecommunication Thakur College

 COMPARATIVE ANALYSIS AND DESIGN OF OSCILLATOR USINGMESFET      Ashishkumar Jaiswal Electronics and Telecommunication Thakur College of Engineering and Technology,Mumbai 2.ashish95@gmail.

com Suryaprakash patel Electronics and Telecommunication Thakur College of Engineering and Technology,Mumbai [email protected]             Shubham Singh Electronics and Telecommunication Thakur College of Engineering and Technology,Mumbai [email protected] Mrs.

Anvita Birje Assistant Professor Electronics and Telecommunication Thakur College of Engineering and Technology,Mumbai [email protected]   ABSTRACT-MESFET stands for metal–semiconductor field-effect transistor. It is quite similarto a JFET in constructionand terminology.

The difference is that instead of using a p-n junction for agate, a Schottky (metal-semiconductor) junction is used.MESFETs are usually constructed in compound semiconductor technologies lackinghigh quality surface passivation such as GaAs, InP, or SiC, and are faster butmore expensive than silicon-based JFETs or MOSFETs. Production MESFETs are operated up to approximately45 G commonly used for microwave frequency communications and radar. The first MESFETs were developed in 1966, and a yearlater their extremely high frequency RF microwaveperformance was demonstrated.                                    INTRODUCTIONThe technology of metal–semiconductor field effecttransistor (MESFET) based on GaAs substrate has undergone a rapid developmentin recent years. The use of GaAs substrates for IC circuits offers someoutstanding advantages over the traditional Si substrate, especially in thearea of high frequency and high speed microwave circuits. Its applicationextends to low noise preamplifiers and linear amplifiers, oscillators andmixers in communication networks due to some of its outstanding physical properties,such as high cutoff frequency (more than 10 GHz for 1 ?m gate length) and highelectron mobility. Furthermore, GaAs circuits are suited for optoelectronicapplications due to its direct-bandgap property which can be grown with semi-insulatingbulk conductivity.

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rapid thermal annealing equipment have made the fabricationfor advanced GaAs MESFET simpler and lower cost.In the operation of a MESFET,theelectrical conduction between the source and drain is modulated by a biasapplied to the gate. The Schottky barrier formed by metal-semiconductorjunction makes the gate isolated to channel. Thus, by controlling the dopinglevel of the channel it is possible to obtain enhancement- or depletion-modetransistors. The switching characteristics of such digital inverters made fromGaAs MESFET’s are comparable, even much superior than those the Si-baseddigital inverters. The typical intrinsic time delay for such kind of inverterscan be lower than 100 ps for 1-?m gate length devices.

Conventional amorphoussilicon (a-Si) has few drawbacks, such as low pixel brightness and requireshigh field effect mobility for high drive currents. Recently, ZnO has developedas a substitute material for TFTs under wide area operations 7. It is a direct band gap semiconductor of II-VIsemiconductor group (~3.3-3.

4 eV), which possesses high electron mobility 8,high exciton binding energy~60meV 9 and can easily be n-doped. Due to largebandgap, it has higher breakdown voltage and can sustain large electric fieldsand hence, can be used for high temperature and high power operations.Moreover, ZnO based TFTs have been intensively studied due to their hightransparency, non-toxicity as well as their consistency with glass and plasticsubstrates10.

The three main parameters that determine the performance ofTFTs are on-off current ratio, field effect mobility and threshold voltage.Significant efforts have been made to achieve good performance through theseparameters in this paper. The process of scaling in thin film transistor hasoften resulted in higher device density, smaller device configuration andbetter performance. The industry roadmap estimates the barriers of continuousscaling will be due to practical technology as well as physical limitations11. So, the need arises for alternative device structures. Thus,     this paper aims to study the effect of varying channellengths 50nm down to 30nm on the I-V characteristics by means of simulationstudy for ZnO thin film transistors.

The paper is ordered as follows: SectionII presents proposed device structure of bottom gate ZnO TFT.Section III deals with results and discussion section which further includesthe I-V characteristics curves at different technology nodes. Section IVpresents study and comparison of simulated results with the theoretical resultsusing Matlab and finally conclusion is drawn in section V.  LITERATURESURVEY This section represents the papers presentedand published by various people related to Attendance monitoring systems.   (A) GaAs MESFET Large Signal Oscillator Design-K.M.

Johnson Techniques forlarge signal GaAs MESFET oscillator design are described which do not requirerepeated large signal measurement. In the first technique, small signalS-parameter measurements are used with a computer program to compute thepackaged and mounted device equivalent circuit. Large signal measurements aremade to determine a mathematical relationship between only those parameterswhich vary under large signal conditions. These relationships are included in thecomputer program. Then, once the equivalent circuit has been computed from thesmall signal S-parameter measurements, those parameters varying under largesignals are incrementally altered until large signal S parameters are obtainedwhich correspond to maximum oscillator output power. These values are used tocalculate embedding element values for six oscillator topologies. A coaxialcavity FET oscillator was built and tested using the large signal designtheory, and it substantially verified the design technique.

The second designtechnique is based on the fact that S/sub 21/ varied more than other S parameters under large signals.By making design calculations based on S/sub 21/ reduced to the pointcorresponding to maximum oscillator power, it was possible to get usable designinformation for an FET oscillator.  (B)Fabrication ofGaAs MESFET Ring Oscillator on MOCVD Grown GaAs/Si(100) Substrate -ToshioNonaka, Masahiro Akiyama, Yoshihiro Kawarada and Katsuzo Kaminishi JapaneseJournal of Applied Physics, Volume 23, Part 2, Number 12 GaAs MESFET crystal oscillators were successfullyfabricated on silicon substrate. GaAs epitaxial layers were grown directly byMOCVD on Si(100) substrate. A typical transconductance of 200 mS/mm wasobserved for the FET of 1.0 µm × 10 µm gate.

A minimum propagation delay timeof 51 ps/gate at a power dissipation of 1.1 mW/gate was observed for an E/Dgate ring oscillator with gate length of 1.0 µm.GaAs MESFET ring oscillatorswere successfully fabricated on silicon substrate. GaAs epitaxial layers weregrown directly by MOCVD on Si(100) substrate. A typical transconductance of 200mS/mm was observed for the FET of 1.0 µm × 10 µm gate.

A minimum propagationdelay time of 51 ps/gate at a power dissipation of 1.1 mW/gate was observed foran E/D gate ring oscillator with gate length of 1.0 µm1.    (C)Low frequencynoise physical analysis for the improvement of the spectral purity of GaAs FETsoscillators-J.Graffeuil.

Tantrarongroj,.J.F.

Sautereau Low frequency (L.F.) noise in GaAs FETs was investigatedboth theoretically and experimentally.

The main contribution to the overallnoise at frequencies over 103 Hz was found to be flicker noise generated in thegradual region of the channel.A new simple relationship is proposed to derivethe noise voltage intensity referred back to the input at normal operatingconditions: it is reported that this noise spectral intensity does not dependon bias voltages for micrometer or submicrometer devices. This relationshipprovides a fast and easy way for assessing devices for their L.

F. noise: animprovement in the spectral purity of GaAs FETs oscillators designed with lowL.F. noise FETs is reported2. (D)Design of GaAsMESFET Oscillator Using Large-Signal S-Parameters -Y. Mitsui ; M.

Nakatani; S. Mitsui A design method ofGaAs MESFET oscillator using large-signal S-parameters has been discussed.Together with the measurement results of  the dependence of Iarge-signall S- parameterson power levels and bias conditions, computer analysis of the equivalentcircuit for MESFET’S has qualitatively clarified the large signal properties ofMESFET’S. On the basis of these findings, S-parameters have been designed forthe MESFET oscillator over the frequency range of 6-10 GHz, which has resultedin powerthe dependence of Iarge-signall S- parameters on powerlevels and bias conditions, computer analysis of the equivalent circuit forMESFET’S has qualitatively clarified the large signal properties of MESFET’S.On the basis of these findings, S-parameters have been designed for the MESFEToscillator over the frequency range of 6-10 GHz, which has resulted in poweroutput of 45 mW at 10 GHz with 19-percent efficiency, and 350 mW at 6.

5 GHz with26-percent efficiency, respectively. Good agreements between predicted andobtained performances of MIC positive feedback oscillator have beenascertained, verifying the validity of the design method using large-signalS-parameters3. (E)TemperatureStabilization of GaAs MESFET Oscillators Using Dielectric Resonators C. Tsironis ; V.

Pauker A simple model ofthe temperature stabilization of dielectric resonator FET oscillators (DRO’s)is presented. Deduced from the oscillation condition, the model furnishesrelations for oscillation power and frequency stability with temperature. Astack resonator with an appropriate linear resonance frequency/temperaturecharacteristic                                   PROPOSED SYSTEM -Device Structure and its Operation: Fig.1: Circuit diagram of Mesfet OscillatorRg- gate series resistance, Ri- channel resistance between source and gate,R- source series resistance, L- common source lead inductance, c -g drain gate capacitance, Cg-gate source capacitance, g- transconductance. The figure below showsa diagram of gallium arsenide (GaAs) MESFET (metal-semiconductor field-effecttransistor).

MESFET is nothing but a JFET fabricated in GaAs which employs ametal-semiconductor gate region (a Schottky diode). The device operates inessentially the same way as does a junction-gate FET, except that instead of agate-channel on junction there is a gate-channel Schottky barrier. Thedepletion region associated with this barrier will control the effective heightof the conducting channel and can thereby control the drain-to-source currentof the device.Fig2:MESFET StructureThe width of this depletion region will increase withincreasing gate voltage so that we see again that the gate will be the controlelectrode and as long as the Schottky barrier is reverse biased, the gatecurrent will be very small.Electron mobility inGaAs (8500 cm2/v-s) is much higher than that of silicon andallows MESFET operation at frequencies higher than can be achieved with silicondevices. The MESFET also possesses very short channel length. This results invery short channel transit times for electrons. As a result, MESFET can operatewell into the range of 1 to 10 gigahertz (GHz).

Thus, applications of MESFETswere initially in microwave circuits for high frequency performance.However, since 1984,high-speed logic circuits employing MESFETs have been produced commercially.These logic circuitsare made compatible with the high-speed bipolar logic family calledemitter-coupled logic (EGL).Working principle:Fig3: Data Flow modelof MESFETFIG4:Cross sectionalview of mesfet model- Perspectiveof a MESFET The Metal-Semiconductor-Field-Effect-Transistor (MESFET)consists of a conducting channel positioned between a source and drain contactregion as shown in the figure.

The figure above flow from source to drain iscontrolled by a Schottky metal gate. The control of the channel is obtained byvarying the depletion layer width underneath the metal contact which modulatesthe thickness of the conducting channel and thereby the current between sourceand drain.Large signal measurements are made todetermine a mathematical relationship between only t hose parameters which varyunder large signal conditions. These relationships are included in the computerprogram.

Then, once the equivalent circuit has been computed from the smallsignal Sparameter measurements, those parameters varying under large signalsare incrementally altered until large signal S parameters are obtained whichcorrespond to maximum oscillator output power. These values are used tocalculate embedding element values for six oscillator topologies. A coaxialcavity FET oscillator was built and tested using the large signal designtheory, and it substantially verified the design technique. The second designtechnique is based on the fact that S21 varied more thanother S parameters under large signals. By making design calculations based onS21 reduced to the point corresponding to maximum oscillator power, it was possible to getusable design information for an FET oscillator. Themetal-semiconductor field-effect transistor (MESFET) is a unipolar device,because its conduction process involves predominantly only one kind of carrier.The MESFET offers many attractive features for applications in both analog anddigital circuits.

It is particularly useful for microwave amplifications andhigh-speed integrated circuits, since it can be made from semiconductors withhigh electron mobilities (e.g., galliumarsenide, whose mobility is five times that of silicon). Because the MESFET isa unipolar device, it does not suffer from minority-carrier effects and so hashigher switching speeds and higher operating frequencies than do bipolartransistors.

Aperspective view of a MESFET is given in It consists of a conductive channelwith two ohmic contacts, one acting as the source and the other asthe drain. The conductive channel is formed in a thin n-type layer supported by a high-resistivitysemi-insulating (nonconducting) substrate. When a positive voltage is applied to the drain withrespect to the source, electrons flow from the source to the drain. Hence, thesource serves as the origin of the carriers, and the drain serves as the sink.The third electrode, the gate, forms a rectifying metal-semiconductorcontact with the channel. The shaded area underneath the gate electrode isthe depletionregion of the metal-semiconductorcontact. An increase or decrease of the gate voltage with respect to the sourcecauses the depletion region to expand or shrink; this in turn changes thecross-sectional area available for current flow from source to drain.

TheMESFET thus can be considered a voltage-controlled resistor.FIG5:Cross sectionalview of mesfet model- current-voltagecharacteristics of a MESFETAtypical current-voltage characteristic of a MESFET, where the draincurrent ID is plotted against the drain voltage VD for various gate voltages. For a given gatevoltage (e.g., VG = 0), thedrain current initially increases linearly with drain voltage, indicating thatthe conductive channel acts as a constant resistor.

As the drain voltageincreases, however, the cross-sectional area of the conductive channel isreduced, causing an increase in the channel resistance. As a result, thecurrent increases at a slower rate and eventually saturates. At a given drainvoltage the current can be varied by varying the gate voltage. For example,for VD = 5 V, one can increase the current from 0.6 to0.

9 mA by forward-biasing the gate to 0.5 V or one can reduce the current from0.6 to 0.2 mA by reverse-biasing the gate to ?1.0 V.

Adevice related to the MESFET is the  junctionfield-effect transistor (JFET).The JFET, however, has a p-n junctioninstead of a metal-semiconductor contact for the gate electrode. The operationof a JFET is identical to that of a MESFET.Thereare basically four different types of MESFET (or JFET), depending on the typeof conductive channel. If, at zero gate bias, a conductive n channel exists and a negative voltage has to beapplied to the gate to reduce the channel conductance, as shown in then thedevice is an n-channel “normally on” MESFET. Ifthe channel conductance is very low at zero gate bias and a positive voltagemust be applied to the gate to form an n channel, thenthe device is an n-channel “normally off” MESFET.

Similarly, p-channel normally on and p-channel normally off MESFETs are available.Toimprove the performance of the MESFET, various heterojunction field-effecttransistors (FETs) have beendeveloped. A heterojunction is a junction formed between two dissimilarsemiconductors, such as the binary compound GaAs and the ternary compound AlxGa1 ? xAs. Such junctions have many unique features that arenot readily available in the conventional p-n junctionsdiscussed previously.

Figure 3 shows across section of a heterojunction FET. The heterojunction is formed between ahigh-bandgap semiconductor (e.g.

, Al0.4Ga0.6As, with abandgap of 1.9 eV) and one of a lower bandgap (e.

g., GaAs, witha bandgap of 1.42 eV). By proper control of the bandgaps and the impurityconcentrations of these two materials, a conductive channel can be formed atthe interface of the two semiconductors.

Because of the high conductivity inthe conductive channel, a large current can flow through it from source todrain. When a gate voltage is applied, the conductivity of the channel will bechanged by the gate bias, which results in a change of drain current. Thecurrent-voltage characteristics are similar to those of the MESFET shownin  If the lower-bandgap semiconductor is a high-purity material, themobility in the conductive channel will be high. This in turn can give rise tohigher operating speed.Fig6:Crosssection of a heterojunction FET having a conductive channel at theheterojunction interface.  -MOTION OF ELECTRONS IN ENERGY BANDBlock parameter k:-For free particles k = wave number =  expected value of momentumFor a particle bound to a periodic potential crystal momentum.is not the actual momentumbut the momentum related to the constant of motion which incorporates thecrystal interaction.

This crystal momentum parameter k is also periodic with aperiod of . The E-K diagram is thereforethe Energy versus crystal momentum characteristics of an electron in thecrystal.EnergyBand SolutionEnergy band solution indicates only the allowed energyand momentum states but not about time evolution of electron’s positionetc.given E, k gives possible values of position of finding the electron with acertain probability i.e, position is uncertain. If E is known exactly,uncertainty in t is infinity, we can not find anything about the electron’sposition.

Therefore for a particle motion we need wave packets constantE wave function grouped about a peak energy.Probability of finding the represented particle in agiven region of space = 1 for some specified time. Center of mass of a particle moving with a velocity – classical idea. wave packet also a mass – QM idea Packet of travelingwave with center frequency and center wave number k thendescribes the particle motion represented by this group. group velocity,  E, k gives the center values of energy and crystalmomentum. FLOWCHART:                                                                                                                                                                                                               METHODOLOGY   We have done simulation in license CogendaVisual TCAD software,We are designing and simulating MESFET. We have useddifferent body material and compared with each other to evaluate bestperformance among the designing of the other models we are also going toimplement this device in the oscillator circuit.We have so far designed the P-Njunction on visual Tcad Software for getting the proper Idea about our project& seen the results of the same.

We have seen the I-V characteristics,channel length, changing material used of both the design model. After designwe will going to compare it with other devices such as Mosfet, Hemt which areused in many such applications where Mesfet can be used.   APPLICATION •      Communication technology satellite and fibreoptic •       Cellphone                                                                                        •       Usedfor higher breakdown voltage 100kV                                           •      Used for higher thermal conductivity                                    •       Simplertechnology than MOSFETs or HEMTs.

                   •       Militaryapplication •       Usedin Microwave frequencies more than 10^15ghz                REFERENCES 1)       GaAs MESFET Large Signal OscillatorDesign- K.M. Johnson 2)      Fabricationof GaAs MESFET Ring Oscillator on MOCVD Grown GaAs/Si(100) Substrate- ToshioNonaka, Masahiro Akiyama, Yoshihiro Kawarada and Katsuzo Kaminishi JapaneseJournal of Applied Physics, Volume 23, Part 2, Number 12  3)       Low frequency noise physical analysisfor the improvement of the spectral purity of GaAs FETs oscillators- J.Graffeuil.K.Tantrarongroj,.

J.F.Sautereau  4)      Designof GaAs MESFET Oscillator Using Large-Signal S-Parameters- Y.Mitsui ; M. Nakatani ; S. Mitsui 5)      TemperatureStabilization of GaAs MESFET Oscillators Using Dielectric Resonators- C.

Tsironis ; V. Pauker                                                                                                                                                                                                                                                                                                                                                     

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