Mesfet -Review Report

8/6/2019 Mesfet -Review Report

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PROJECT REPORT

TITLE OF THE PROJECT

STUDY OF MESFET AND HEMT

Supervisor Name -Mrs. Maitreyi Roy Kanjilal



SHORT DESCRIPTION



MESFET:

MESFET is Metal semiconductor field effect transistor. In thisdevice, the gate is created as a schottky diode i,e. metalcontact and here the semi-insulating SiC(Silicon Carbide) orGaAs(Gallium Arsenide) substrate defines thickness of N-typeelectron channel.

Fig. MESFET STRUCTURE

MESFET in SiC are made of n-type material because of thehigher electron mobility and is fabricated using epitaxial layerson semi-insulating substrates to minimize parasiticcapacitances. A MESFET has three metal-semiconductorcontacts. The ohmic contacts are labelled source and drain, andthe Schottky barrier is labelled gate. A MESFET is oftendescribed in terms of the gate dimensions.



The effective thickness of channel can be altered by reversebias of schottky diode which is created by deposition of appropriate metal on the N-type GaAs layer.

4H-SiC PLANAR MESFET:

SiC (Silicon Carbide) is very good material due to its superiorelectrical, chemical and thermal properties and its high electricbreakdown field, high saturated electron drift velocity and highthermal conductivity makes it suitable for high powermicrowave devices.

4H-SiC means four bi-layer stacking periodicity and hexagonal symmetry of Silicon Carbide, and it is one type of Polytype (i, e.variations of the same chemical compound that are identical intwo dimensions and differ in the third) . 4H-SiC MESFET has thecapability of high voltage, high output impedance, easymatching and wide bandwidth through X-band (8.2-12.4 GHz).

This is fabricated using ion-implantation without recess gateetching to eliminate potential damage to gate region and tolower contact resistance.

http://en.wikipedia.org/wiki/Hexagonal

http://en.wikipedia.org/wiki/Chemical_compound

http://en.wikipedia.org/wiki/Hexagonal

http://en.wikipedia.org/wiki/Chemical_compound



Fig. small-signal current for 4H-SiC

Fig. power gain for 4H-SiC MESFET

This device produced 15dB at 1 GHz. These results clearlydemonstrate the advantages of 4H-SiC for high-powermicrowave application where its high-thermal conductivity,high-voltage and high-power density capability are veryattractive.

For applications in microwave power amplifiers, MESFETs musthave large gate width because it causes decreased impedancesthat result in increased power gain. And with decreased gatelength, the maximum oscillation frequency and cut-off frequency are increased.



4H-SiC MESFET WITH 42 GHz (MAXIMUM) FREQUENCY:

MESFET structures are fabricated on high resistivity SiCsubstrates of 4H-polytype. For 4H-SiC MESFET, due to its highelectron mobility and lower ionization energy for donors, boththe frequency and power performance can be obtained. These

devices with 0.5 µm gate length gives maximum frequency of 42 GHz and RF gain of 5.1 dB at 20 GHz while biased at a highoperating voltage of 40 volts. These devices exhibit high gate-drain breakdown voltage of about 100 volts (at 0.5 mA/mm).

High resistivity 4 H-SiC substrate provides good RF isolationand introduces negligible parasitic effects.

The 4H-SiC MESFETs are suitable for high power microwave

applications where its high-thermal conductivity, high voltageand high power density capabilities are desired.



INVESTIGATION OF SCALABILITY OF 4H-SiC MESFET FOR HIGHFREQUENCY APPLICATIONS:

A 4H-SiC MESFET with maximum operating frequency i,e. 42GHz indicates that it is capable of producing excellent RFperformance to X-band and potentially up to K-band.

The high frequency capability of this device can also be

determined by lateral scaling of the device i,e. gate lengthreduction.

The scalability of SiC MESFET can be determined by using athin highly doped p-buffer, block gate and Г-gate. For bothblock gate and Г-gate, the drain current increases withreducing gate length of SiC MESFET.

For MESFETs with block gates, the channel-on resistance and

pinch-off voltage decreases with decreasing gate length andtransconductance is almost constant. Hence, short channel-effect is moderate here.

For MESFETs with Г-gates, the decrease in transconductanceand pinch-off voltages with respect to decreasing gate lengthindicates larger influence and short channel effect.



HIGH EFFICIENCY CLASS-E POWER AMPLIFIER USING SiC MESFET :

The efficiency improvement of a power amplifier decreases thepower consumption and heat-sink requirement but it increasesthe output power because power amplifiers consume a majorityof powers.

Hence, switched mode amplifiers can be used to increase theefficiency of power amplifiers and class-E power amplifier issuitable for this purpose because of its simple topology andhigh efficiency capability in high frequency range.

In general, the switched mode device in class-E amplifier usesSi-LD MOS (Silicon Laterally Diffused Metal OxideSemiconductor) at frequency of 1 GHz. Hence, wide band-gap

devices like SiC MESFET are used to supply high power andhigh efficiency to power amplifiers at high frequency regionover 2 GHz because it provides high breakdown voltage, highelectron saturation and high temperature robustness.



fig . Full Schematic of the class-E SiC MESFET poweramplifier

EVALUATION OF COMMERCIALLY AVAILABLE SiC MESFETs FORPHASED ARRAY RADAR APPLICATIONS:

Wide band-gap materials like Silicon-Carbide (SiC) and Gallium-Nitride (GaN) are used for high power microwave applications.

Now SiC MESFETs are commercially available whose majorapplication is in Phased Array Radar (PAR) because it requiresthe transmitter to develop high output power with little or nopulse drop.

PAR applications include high power density which results inhigh voltage operation and leads to low current. Because of itshigh output impedance, the matching is very simple.

Table. Material parameters of SiC and GaN compared toGaAs and Si



Few measurements of these devices are done with respect tocurrent hysteresis, CW conditions, varying pulse lengths,varying duty cycles and varying Base-plate temperatures todetermine whether these (SiC and GaN) are much better thanprevious generation devices or not. The results of thosemeasurements are as follows:

The current hysteresis of these device exhibits less instabilityas compared to previous devices. Current hysteresis is thetendency for the

Transistor to draw different drain current for the same gatevoltage when the voltage is being increased against when it isbeing decreased.

When these devices are measured under CW conditions, theyexhibit better output power compared to other devices.



SiC MESFET devices are measured for pulse lengths between100µs and 5ms with constant duty cycle of 30% which resultedin good amplitude and phase drop than others.

Fig. Power droop for a SiC MESFET device for a 5mspulse length

Fig. Phase droop for a SiC MESFET device for a 5mspulse length



When these devices are measured for duty cycles between 5%and 50% with constant pulse length of 500µs, the increasingduty cycle reduces the bulk performance of the device. But themagnitude of amplitude and phase drop remains constantcompared to others.

This device is mounted on a temperature controlled hot plate. The output power and gain are measured for base-platetemperatures between room temperature and 100°C, which arealmost constant and same as others.

Fig. Power vs. baseplate temperature for SiCMESFET device



MOSFET ANALYTICAL CHARGE MODEL WITH QUANTUM EFFECTSUSING A TRIANGULAR POTENTIAL APPROXIMATION:

When the MOS device is in depletion or weak inversion, thenthe triangular potential approximation is taken to determinethe charge of that device.

The investigation of quantum mechanical effects in MOSstructure is done by using self consistent numerical andanalytical techniques, which are used to solve Schrödinger-Poisson equations.

The triangular potential well approximation is used to getanalytical solutions from Schrödinger equation for application inSPICE circuit simulations. But it is only valid for weak inversionregion of MOS structure. The central assumption of triangularpotential approximation is that the charge in electrostaticpotential in direction perpendicular to transistor channel isconstant.



DIRECT DETERMINATION OF BIAS-DEPENDANT SERIES PARASITICELEMENTS IN SiC MESFETs:

A simple series extrinsic element extraction scheme isemployed in which the parasitic elements are determined by

equating the measured Z-parameters with equivalent analyticexpressions which are derived for a SiC MESFET equivalentcircuit model incorporating the charging resistance.

By this method, the bias dependant (both on gate and drainvoltage) series drain and source resistance can be measuredfrom active-bias S-parameters. The correlation betweenmeasured and modelled S-parameters is 20 GHz.

Equivalent circuit parameter extraction procedure:-

Here, 500µm x 0.6µm SiC MESFETs are used for the extractionprocedure. The gate resistance is extracted by a cold-FETmethod, where the parasitic elements are determined from S-parameter measurements at zero drain bias and hence it iskept constant for whole extraction process.

The S-parameters of SiC MESFET biased at V d=30V and V gs =-4.5V are measured and then converted into corresponding Y-parameters. These Y-parameters are then converted into Z-parameters and then by using analytic formulations and linearregression techniques, the gate, source & drain inductors aredetermined prior to bias- dependent source and drainresistance.



ELECTRICAL OPERATION OF 6H-SiC MESFET AT 500°C FOR 500HOURS IN AIR AMBIENT:

The reliable operating temperature envelop of integratedsemiconductor devices has been expanded from 125°C toabove 200°C, but for further benefits in aerospace combustionengine applications, the temperature may approach 600°Calso. Some wide band-gap semiconductors like SiC, diamondand GaN demonstrated short term durability when operatedunder high temperatures. Hence, to achieve stable and longterm operation under high temperatures and that too withoutsignificant changes in electrical operating parameters, a hightemperature n-channel 6H-SiC MESFET is fabricated.

In fabrication process, multiple levels of high temperaturemetal and dielectric passivation is used to prevent

contamination i, e. to prevent the oxygen to reach theelectrically sensitive interfaces.



Fig. Schematic drawing of the structure of theMESFET.When 6H-SiC MESFET was operated at 500°C for over 500hours in an air ambient, then there is a 10% change inoperational transistor parameters. The long term testing under500°C was possible due to a thick film metallization basedceramic package with conductive die attach material and Au-wires, which electrically interconnect the device tometallization traces on Alumina substrate using thermo-sonicbonding.

After the whole operation, it is observed that most of therelevant aspects of transistor structure (i,e. conductive channelcharge, ohmic contacts, dielectric surface passivation, hightemperature packaging etc.) are stable in these harshconditions. Only degradation is that the leakage of gate-channel diode is increased with the increased anneal time.Hence, 6H-SiC MESFET is suitable for harsh combustion engineenvironments.



POTENTIAL PERFORMANCE OF SiC & GaN BASED METAL

SEMICONDUCTOR FIELD EFFECT TRANSISITOR:SiC & GaN are wide band gap semiconductors and thereforehave a high breakdown field and low thermal generation rate.Also their good thermal conductivity and stability make themsuitable material for high power, high temperature and harshenvironment electronic devices.

A Monte Carlo simulation is used to model steady state electron

transport in SiC & GaN field effect transistors. This simulationsprovides a peak electron velocity of 3x10^5 m/s & 2.3x10^5m/s for SiC & GaN materials respectively. Hence, thisparameter makes these suitable for high frequency operationdevices.

MESFET is mostly used in construction of large scale integratedcircuits because of its simplicity, comparative lack of dopantdiffusion problems and high packaging densities. But SiC & GaNoffers high electron mobility and hence prospect of high



frequency operating rates. Their direct band gap allows easierintegration with optical devices.

TRANSITION OF SiC MESFET TECHNOLOGY FROM DISCRETETRANSISTORS TO HIGH PERFORMANCE MMIC TECHNOLOGY:

SiC MESFETS offer significant advantages for the nextgeneration commercial & military systems. Its increased powerdensity & higher operating voltage enable higher performance,lighter weight and wide bandwidth systems.

A Monolithic Microwave Integrated Circuit or MMIC is a type of

integrated circuit (IC) device that operates at microwave frequencies (300 MHz to 300 GHz). These devices typicallyperform functions such as microwave mixing , poweramplification, low noise amplification , and high frequencyswitching.

MMICs were originally fabricated using gallium arsenide (GaAs),a III-V compound semiconductor . It has two fundamentaladvantages over Silicon (Si), the traditional material for ICrealisation: device ( transistor ) speed and a semi-insulating

http://en.wikipedia.org/wiki/Integrated_circuit

http://en.wikipedia.org/wiki/Microwave

http://en.wikipedia.org/wiki/Radio_frequency

http://en.wikipedia.org/wiki/Frequency_mixer

http://en.wikipedia.org/wiki/Low-noise_amplifier

http://en.wikipedia.org/wiki/Gallium(III)_arsenide

http://en.wikipedia.org/wiki/Extrinsic_semiconductor#Semiconductor_doping

http://en.wikipedia.org/wiki/Silicon

http://en.wikipedia.org/wiki/Transistor

http://en.wikipedia.org/wiki/Integrated_circuit

http://en.wikipedia.org/wiki/Microwave

http://en.wikipedia.org/wiki/Radio_frequency

http://en.wikipedia.org/wiki/Frequency_mixer

http://en.wikipedia.org/wiki/Low-noise_amplifier

http://en.wikipedia.org/wiki/Gallium(III)_arsenide

http://en.wikipedia.org/wiki/Extrinsic_semiconductor#Semiconductor_doping

http://en.wikipedia.org/wiki/Silicon

http://en.wikipedia.org/wiki/Transistor



substrate . Both factors help with the design of high frequencycircuit functions. The speed of Si-based technologies hasgradually increased as transistor feature sizes have reducedand MMICs can now also be fabricated in Si -technology.

The ability to fabricate complex circuitry, high power SiC MMICsallows another degree of freedom of systems engineers indevelopment of next generation RADAR and communicationsystems.

High Electron Mobility Transistor (HEMT) :

http://en.wikipedia.org/wiki/Wafer_(electronics)

http://en.wikipedia.org/wiki/Wafer_(electronics)



It is a form of FET (Field Effect Transistor) which is used toprovide very high level of performance at Microwavefrequencies (30-300 GHz). It offers low noise and high gain.

A special PN junction is used in HEMT which is known as hetero-

junction, which consists of a junction that uses differentmaterials in either side of it. The common material used areAluminium Gallium Arsenide (AlGaAs) and Gallium Arsenide(GaAs).

GaAs is used because it provides high level of basic electronmobility which is crucial to operation of this device.

Since Si (Silicon) has much lower level of electron mobility,hence it is never used in HEMT.

Two main structures are generally used for HEMT:-

1. Self-Aligned Ion Implanted Structure



2. Recess-Gate Structure

Self-Aligned Ion Implanted Structure:

Here the gate, drain & source are generally metallic contacts,although source and drain are sometimes made up of Germanium (Ge).

The gate is generally made up of Titanium (Ti) and it forms aminute reverse biased junction similar to GaAs FET.

Recess-Gate Structure:

Here another layer of n-type GaAs is set down to enable thedrain and source contacts. The thickness of the gate is alsovery critical since the threshold voltage of FET is determined bythis.

The size of gate and hence the channel is very small. The gate

is only 0.25 Microns or less, enabling the device to have highfrequency performance.

OPERATION:



To allow conduction, semiconductors are doped with impuritieswhich donate mobile electrons (or holes), which are slowed

down because of collisions with impurities (dopant).HEMTs avoid this by using high mobility electrons generatedusing the hetero-junction of a highly doped wide-band gap n-type donor-supply layer (i,e. AlGaAs) and a non-doped narrowband gap channel layer with no dopant impurities (GaAs).

The electrons generated in thin n-type AlGaAs layer dropcompletely into GaAs layer to form a depleted -AlGaAs layer,

which is known as Two-Dimensional Electron Gas (2DEG) layer.

Within this Two-Dimensional Electron Gas (2DEG) layer, theelectrons are able to move freely because there are no otherdonor electrons or items with which the electrons will collide(because the GaAs layer is un-doped, and from which theycannot escape). Hence, these create a very thin layer of highlymobile conducting electrons with high concentration and thus

giving the channel very low resistivity. This provides highelectron mobility which is the main purpose of HEMT.

A bias applied to the gate formed a Schottky barrier diode,which is used to modulate the number of electrons in thechannel formed from 2DEG layer and these controls theconductivity of the device.

APPLICATION:



Because of their low noise performance, HEMTs are used in lownoise-small signal amplifiers, power amplifiers, oscillators andmixers operating at high frequencies.

It is also used in a wide range of RF design applicationsincluding cellular telecommunication, RADAR, radio astronomyand any application which requires combination of low noiseand high frequency performance.

ETHICAL AND LEGAL CONSIDERATION



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Mesfet -Review Report