30

Int. J. Elec&Electr.Eng&Telecoms. 2014 Namrata Soni and Kanchan Cecil, 2014

COMPARATIVE STUDY OF GaN AND GaAs MESFET

Namrata Soni1* and Kanchan Cecil1

*Corresponding Author: Namrata Soni,namratasoni21may@gmail.com

In the information, science and technology such as computer science, telecommunication,processing of the signals or images transmission the field effect transistor plays a major role. Inthis paper, we have determined the comparative study of various qualities of GaN and GaAsMESFET here we have presented the result of calculating the influence of gate length on inputand output impedance of GaAs and GaN MESFET, these two physical models are based on theanalysis of iterative method in the active region under the gate. The theoretical results based onanalytical expression that we have established are discussed and compared with those of result.

Keywords: Image transmission, Iterative method, Influence of gate, Telecommunication,Gate lengths

INTRODUCTIONThe properties of Ill-V nitride heterostructuresare attracting increasing attention for a widerange of device applications, including blue,green, and ultraviolet LED’s, short wavelengthlasers, and high power, high temperature, andhigh frequency electronic devices. There arethree important binary nitride materials: AIN,GaN, and InN. Among these, GaN shows greatpromise for microwave applications. The wideenergy band gap of GaN (3.43 eV, ascompared to 1.4 eV for GaAs) leads to lowintrinsic carrier concentration over a widerange of temperatures. This in turn allows GaNbased devices to be operable at hightemperatures. Also this wide band gap allows

ISSN 2319 – 2518 www.ijeetc.comVol. 3, No. 4, October 2014

© 2014 IJEETC. All Rights Reserved

Int. J. Elec&Electr.Eng&Telecoms. 2014

1 Department of Electronics and Communication Engineering, JEC, Jabalpur, India.

very high electric breakdown fields (1.5 x 107V/m as compared to 2.5 x 105 V/m for GaAs).As a result GaN based devices can be biasedat very high drain voltages (breakdown voltage= 50-500 V depending on the application), andbecause of the large thermal conductivity ofGaN (1.7 W/cm. K as compared to 0.46 W/cm. K for GaAs), the channel temperature canreach 300 °C. Even though the low-fieldmobility is not high (the best value reported sofar for GaN is about 2000 cm2/V.s ascompared to a value about 8500 cm2/V.s forGaAs), it is not very sensitive to ionizedimpurity concentration. Furthermore, a largerpeak velocity can be reached (2.7 x 107 cm/sat room temperature as compared to 1.5 x 107

Research Paper

31

Int. J. Elec&Electr.Eng&Telecoms. 2014 Namrata Soni and Kanchan Cecil, 2014

cm/s for GaAs), which permits high currentsand high operating frequencies. The energyband gap difference between AIN and GaN isalso quite significant (e.g., 2.57 eV, ascompared to 0.73 eV for GaAs and AlAs),which permits high concentrations of freecarriers to be confined at the AlGaN/GaNhetero interface paving the way for the highperformance AlGaN/GaN high electronmobility transistor.

GaN-based microwave power MESFETshave defined the state-of-the art for outputpower density and have the potential toreplace GaAs-based transistors for a numberof high power applications. The GaN-basedmaterial system, consisting of GaN, AlN, InNand their alloys, has become the basis of anadvanced, microwave-power-devicetechnology for a number of reasons. GaN hasa breakdown field that is estimated to be 3MV/cm, which is ten times larger than that ofGaAs, and a high peak electron velocity of 2.7x 107 cm/s.

As a result of these properties, excellenthigh-frequency, high-power performance hasbeen achieved with GaN based MESFETs.Although significant progress has been madein the past few years, additional developmentalwork is required for GaN MESFETs tobecome a viable technology. This paperexamines the microwave frequencyapplications of GaN MESFET. Starting from I-V characteristics, I analyzed properties usingphysics based model for GaN. Variouspossible impacts of the parasitic resistancesand capacitances are considered, withemphasis to discuss the potential applicationsof GaN MESFET and a better realizabledesign

THEORETICAL ANALYSISGaN FETs are known to be a wide band gapsemiconductor material with a high breakdownelectric field, high saturation drift velocity andbetter thermal conductivity in comparison withthe more commonly used GaAs and SiCMESFETs.

The current voltage characteristic ofMESFET shows the comparison among thetwo devices which can be seen from thetheoretical analysis that GaN based devicesare better for high power devices where theother GaAs devices where fails to withstandhigh temperature and power.

The current-voltage relationships of aMESFET are illustrated in Figure 1. Thechannel current is plotted as a function ofapplied drain-source potential for differentgate-source voltage levels. Three regions ofoperation can be identified from the figure.They are the linear region, the saturationregion and the breakdown region. In the linearregion, current flow is approximately linear withdrain voltage. As drain potential increases, thedepletion region at the drain end of the gatebecomes larger than at the source end. Sincethe device is taking constant current throughthe channel region, the electrical field increasesas the channel region narrows, and thereforea related increase in electron velocity occur.

However, if the gate reverse bias isincreased while the drain bias is held constant,the depletion region widens and the conductivechannel becomes narrower, reducing thecurrent. When Vgs = Vp, the pinch-off voltage,the channel is fully depleted and the draincurrent is zero, regardless of the value of Vds.Thus, both Vgs and Vds can be used to control

32

Int. J. Elec&Electr.Eng&Telecoms. 2014 Namrata Soni and Kanchan Cecil, 2014

the drain current. When the MESFET isoperated under such bias voltages, where bothVgs and Vds have a strong effect on the draincurrent, it is said to be in its linear or voltagecontrolled resistor region channel near thedrain becomes narrower, the electrons mustmove faster.

However, the electron velocity cannot,increase indefinitely; the average velocity ofthe electrons in GaAs cannot exceed a velocitycalled the saturated drift velocity,approximately 1.3 x 107 cm/s. If Vds isincreased beyond the value that causesvelocity saturation (usually only a few tenths ofa volt), the electron concentration rather thanvelocity must increase in order to maintaincurrent continuity. Accordingly, a region ofelectron accumulation forms near the end ofthe gate. Conversely, after the electrons transitthe channel and move at saturated velocity intothe wide area between the gate and drain, anelectron depletion region is formed.

The fundamental physical limitations of Sioperation at higher temperature and powersare for these applications. For phase arrayradars, wireless communication market andother traditional military applications requiredemanding performance of microwavetransistors. In several applications, as well asin radar and military systems, the developmentof circuits and sub-systems with broadbandcapabilities is required. From transmitter pointof view the bottleneck, and the critical key factor,is the development of high performance PA.

Next generation cell phones require widerbandwidth and improved efficiency. Thedevelopment of satellite communications andTV broadcasting requires amplifiers operatingboth at higher frequencies and higher powerto reduce the antenna size of terminal users.The same requirement holds for broadbandwireless internet connections as well. This highpower and high frequency applications requiretransistor with high breakdown voltage, highelectron velocity and high thermal conductivity.The wide band gap materials, like GaN andSiC are preferable.

The most important factor that characterizethe properties of GaN MESFET can be giveby its current voltage characteristic.

This work has been conducted by thephysics based analytical model. So far, many

Figure 1: Current-Voltage Characteristicsof MESFET

Material Properties GaAs GaN

Band gap (eV) 1.43 3.4

Electron Mobility (cm2/V-sec) 8500 2000

Electron Saturation Velocity(106 cm/sec) 12 25

Thermal Conductivity (Watts/cm. K) 0.5 1.3

High/Low Power Device Low High

Table 1: Properties of GaN and GaAs

33

Int. J. Elec&Electr.Eng&Telecoms. 2014 Namrata Soni and Kanchan Cecil, 2014

analytical model has been developed toevaluate the I-V characteristics, where themodel for I-V characteristics cannot combinethe non-saturation and saturation region(linearity and non-linearity regime).

Thus the same can be explained for theGaAs mesfet by getting the I-V characteristicwe can compare the performance of twomaterials.

I-V CHARACTERISTIC OFGaN MESFETThe most important factor that characterize theproperties of GaN MESFET can be give byits current voltage characteristic.

This work has been conducted by thephysics based analytical model. So far, manyanalytical model has been developed toevaluate the I-V characteristics, where themodel for I-V characteristics cannot combinethe non-saturation and saturation region(linearity and non-linearity regime).

I-V CHARACTERISTIC OFGaAs MESFETThus the same can be explained for the GaAsMESFET by getting the I-V characteristic wecan compare the performance of twomaterials.

The current voltage characteristic has beenevaluated by new imperial equation.

Ids = (Vgs – Vto)2 * (1 + Vds) * tanh(Vds)

Thus the result will be given in Figure 3.

Thus the current voltage characteristic oftwo material has been shown in the twoanalytical model.

RESULTS AND DISCUSSIONThe analytical model of GaAs MESFET hasbeen developed to evaluate the I-Vcharacteristics, and extrinsic parameters:gate-source capacitance and gate-draincapacitance. This model incorporates a newempirical equation for simulating the biasdependence junction capacitance of GaAsMESFET. The results of gate-sourcecapacitance and gate-drain capacitance showan improvement of device performance byincorporating the new empirical equation. Butin comparison with that of GaN MESFET itdoes not shows the non-saturation andsaturation region (linearity and non-linearityregime).

In GaN MESFET the present work showsan tremendous impact of numerical iterativeto find the I-V characteristics of GaN MESFETin linear and non-linear regions where parasiticresistance has been considered which ishelpful for the high power and high temperaturemicrowave devices.

Figure 2: Ids-Vds Characteristicsof GaN MESFET

0 2 4 6 8 10 12 14 16 18 200

5

10

15

Dra

in-s

ourc

e cu

rrent

(mA)

Drain-source voltage(V)

Ids-Vds characterist ics for GaN MESFET

34

Int. J. Elec&Electr.Eng&Telecoms. 2014 Namrata Soni and Kanchan Cecil, 2014

CONCLUSIONIn this work, we have developed the transportmodel to analyze the performance of twomaterial device with there analytical modelsthus the result conclude GaN to be the bestsuitable material for high frequency microwaveapplications.

REFERENCES1. Arabshahi H, Benam M R and Salahi B

(2007), Modern Physics Letters B,Vol. 21, p. 1715.

2. Shigematsu H, Inoue Y, Akasegawa A,Yamada M, Masuda S, Kamada Y,Yamada A, Kanamura M, Ohki T,Makiyama K, Okamoto N, Imanishi K,Kikkawa T, Joshin K and Hara N (2009),“C-Band 340-W and X-Band 100-W GaNPower Amplifiers with Over 50% PAE”,IEEE MTT-S Int. Microwave Symp. Dig.,pp. 1265-1268.

3. Takagi K, Matsushita K, Kashiwabara Y,Masuda K, Nakanishi S, Sakurai H,Onodera K, Kawasaki H, Takada Y andTsuda K (2010), “Developing GaNHEMTs for Ka-Band with 20W”, IEEECSIC Symp. Dig.

4. Takagi K, Takatsuki S, Kashiwabara Y,Termite S, Matsushita K, Sakurai H,Onodera K, Kawasaki H, Takada Y andTsuda K (2009), “Ku-Band AlGaN/GaN-HEMT with Over 30% of PAE”, IEEEMTT-S Int. Microwave Symp. Dig.,pp. 457-460.

5. Tsuda (2010), “Developing GaN HEMTsfor Ka-Band with 20W”, IEEE CSICSymp. Dig.

Figur 3: Plot of Current VoltageCharacteristics of GaAs MESFET for

Different Gate-Source Biasing

0 0.5 1 1.5 2 2.5 30

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5x 10-3

Vds(v)

Id (m

A)