Simulation of GaAs MESFET and HEMT Devices for RF Applications

olume, Issue, January February 03 ISSN 78-6856 Simulation of GaAs MESFET and HEMT Devices for RF Applications Dr.E.N.GANESH Prof, ECE DEPT. Rajalakshmi Institute of Technology ABSTRACT: Field effect transistor is a monopolar device that displays superior high frequency and low noise performances. GaAs MESFETs are commonly found in RF amplifier, mixers and oscillators. The electron mobility of GaAs is five times higher than silicon. N channel MESFET are used in RF and Microwave applications. MODFET another FET type exhibits higher frequency response than MESFET due to dissimilar semiconductor material doped together. It has difference in band gap energy between dissimilar semiconductor materials and surpass frequency limit of MESFET while maintaining low noise performance and high power rating. This paper discusses about the simulation of MESFET and MODFET in its output and transfer characteristics. GaAs MESFET device with S parameter design is investigated here for the stability of the device at high frequency and discussed maximum gain, Figure of merit and stability from the proposed design. This simulation is useful to study high frequency effects of FET structure and finds extensive usage in high frequency devices. It is found that gain of lesser than desired value and lesser Figure of merit gives stability at higher frequency of operation. Keywords: Metal Semiconductor FET, High electron Mobility Transistors, Figure of merit, Transconductance. Its principle is based on a heterojunction which consists of atleast two different semiconducting materials brought in to intimate contact.. Metal Semiconductor FET MESFET Figure a MESFET, Operation in linear region []. INTRODUCTION Field effect transistors are classified as Metal semiconductor field effect transistor (MESFET) and High electron mobility transistor (HEMT or MODFET). Due to presence of large capacitance formed by gate electrode and the insulator in MISFET, it finds use in relatively low and medium frequency applications. MESFET and HEMT find applications up to 60 to 70 GHz and HEMT even up to 00 GHz [3]. Since our interest only on RF applications our focus is on the above FET s. Figure a and b shows the MESFET operated in linear and saturation region. The MESFET differs from the common insulated gate that there is no insulator under the gate over the active switching region. This implies that the MESFET gate should, in transistor mode, be biased such that one does not have a forward conducting metal semiconductor diode instead of a reversed biased depletion zone controlling the underlying channel. HEMT is s a field effect transistor incorporating a junction between two materials with different band gaps. Figure b MESFET, Operation in saturation region [] Here the transistor is operated in depletion mode; analysis is based on this mode only. The schottky contact between the semiconductor and gate builds up a channel space charge region that affects the current flow from source to drain. The space charge region is controlled by gate source voltage gs. Let d be the barrier voltage, ds - olume, Issue January - February 03 Page 3

olume, Issue, January February 03 ISSN 78-6856 charge of carriers L length of channel W width of channel, Nd concentration of electron in channel. Equation and gives space charge region and resistance between source and drain. is possible to fabricate semi-insulating (SI) GaAs substrates, which eliminates the problem of absorbing microwave power in the substrate due to free carrier absorption. High Electron Mobility Transistors HEMT ds R q( d gs) Nd L ( d ds) W ---- () --------- () nn D Substituting Equation into Equation yields the draincurrent equation 3 I D ds R qd ( d gs) D o[ ] ds ND I G G 0 Conductance, W width, L length ---(3) The drain source voltage increases, the space charge region near drain contact also increases as well, resulting non-uniform distribution of depletion region along the channel. The drain current is obtained by integrating drain current from 0 to L and voltage up to drain source voltage which yield drain current as in equation 4. I G 3/ 3/ D o[ DS [{ } { } ]] DS d Gs d GS 3 qn Dd ------------- (4) When we try to increase voltage, space charge region occupies entire channel, resulting current and voltage are saturation voltage sat and Isat. The drain saturation current is given by P / IDss Go[ ( d Gs ) { P} ]-------------- (5) 3 3 Therefore Drain current at linear and saturation regions are found by increase in gate voltage channel region at one side which is bulky. This investigation of difference in drain current with and without channel modulation gives idea about operation at high frequencies. GaAs rather than silicon MESFETs provides two more significant advantages: first, the electron mobility at room temperature is more than 5 times larger, while the peak electron velocity is about twice that of silicon. Second, it Figure Generic hetero structure of depletion mode HEMT[]. The basic structure of GaAlAs n doped semiconductor is followed by undoped GaAlAs spacer layer of the same material, an undoped GaAs layer and high resistive semiinsulating GaAs substrate.hemt exploits the difference in band gap energy between dissimilar semiconductor materials such as GaAlAs and GaAs in an effort to substantially surpass the upper frequency limit of MESFET while maintaining low noise performance and high power rating. The high frequency is due to separation of the carrier electrons from their donor sites at the interface between the doped GaAlAs and undoped GaAs layer where they confined to very narrow layer in which motion is possible only parallel to interface. So mobility of order of 9000 cm /vs. is major improvement over GaAs MESFET. The threshold voltage for HEMT is given by to b Wc ----------- (6) ( Q ) p is pinch off voltage P qndd P ---------(7) H From the above we find electron drain current d I q N W -----------(8) dy D P D D q Charge of electron n Mobility of electron N D - Concentration of donor atoms W width of channel olume, Issue January - February 03 Page 4

olume, Issue, January February 03 ISSN 78-6856 Since the current flow on the surface is very thin, the drain current at saturation region can be found by integrating over the entire surface length from 0 to L. HEMT is operated as an enhancement or depletion mode. For the depletion we require t0 < 0, for enhancement t0 >0. Here only enhancement type taken in to consideration. There is no much channel variation in HEMT.. CHARACTERISTICS OF MESFET & MODFET. I - characteristics of MESFET The I- characteristics of MESFET with gs = -,-.5,-,-.5 and plotting drain current Id as a function of drainsource voltage ds. Drain source voltage is in the range of 0 to 5 volts.id vs ds curves are plotted using matlab program.. Figure 3 shows output characteristics and Figure 4 shows transfer characteristics of MESFET. Figure 4 Transfer Charecteristics Drain current and gate source voltage of MESFET. Table Output Parameters from MESFET Transfer Characteristics S.No Name of the parameter alue Pinch off voltage 4.4 Threshold voltage -. 3 Transconductance Gm 8.8 4 Idss 6.889 ma Figure 3 Drain characteristics of MESFET. channel length modulation is zero, when channel length modulation is taken into account the characteristics gives thick slolid curves. Therefore channel length modulation parameter is dependent on applied voltage. So as ds is changed channel length is also changed. Additional increase in drain current beyond saturation voltage results in minor increases of drain current due to shortening of channel. At saturation the Id equation is multiplied by ( channel pinch off occurs. Figure 4 shows plot between drain current and gate source voltage. From the above characteristics it is possible to find the parameters given in table. Large Pinch off voltage gives negative threshold voltage and called depletion type or on MESFET because negative voltage must be applied to the gate to deplete the channel fully to turn off drain current. The transconductance of the device is one of the most important indicators of the device quality for microwave and millimeter wave applications. When all other characteristics are equal, a device with high transconductance will provide greater gains and superior high frequency performance. The value of Gm greater than gives usage of MESFET at higher frequencies for depletion mode operations.. Frequency response of MESFET The high frequency response of MESFET performance is determined by transit time of charge carriers traveling between source and drain and RC time constant of device. L Sat ----------- (0) Assuming saturation velocity 0 7 cm / sec and gate length f t ------------() The frequency is found to be 5 GHz hence can be used as high frequency (micro and milli region) devices. olume, Issue January - February 03 Page 5

olume, Issue, January February 03 ISSN 78-6856.3 I- characteristics of High Electron Mobility Transistors. The I- characteristics of HEMT for constant gs = -, - 0.75,-.5,-0.5 and 0 olts with Id as function of drain source voltage. Assuming N D = 0 8 cm -3, b = 0.8 v, W = -0 W.s. We found pinch off voltage and threshold voltage using 6 and 7. Threshold and pinch off voltage can be calculated as in MESFET. p =.8, to = -.v.figure 5 shows the output characteristics of HEMT. The above calculations are performed by square law modeling, from which the output characteristics are drawn. The Knee voltage, breakdown and swing voltage are calculated from the characteristics apart from maximum output power and efficiency. Table Output parameters from Drain characteristics S.No Parameters alues Knee voltage knee.0 volt breakdown 5 olt 3 Imax 0.3 A /mm 4 Pout 0.5 watts /mm as the corresponding output power increases, which can be modeled considering device physics. This technique can be used to optimize device performance by determining which part of the device to modify for greatest impact.4 Frequency response of HEMT The high frequency response of HEMT is determined by the transit time similar to the MESFET. However the transit time is expressed best through the electron mobility and electric field. Therefore transit time is given by L L ----() sat n ds f t -----------(3) n = 8000 cm / v.s, which HEMT can be operated is between 5 GHZ to 90 GHz. 3. COMPARISON OF HEMT AND MESFET The HEMT device is identical with MESFET, but exploits the differences in bandgap energies between heterogeneous semiconductors. Here the current flow is restricted to a very narrow, quantum well layer where the charge mobility can attain twice the value of MESFET. Because of carrier separation from the donor sites, extremely high operational frequencies have been reported. The frequency of HEMT is almost 00 times fast than MESFET. Figure 5 Output characteristics of HEMT. The difference between figure 3 and 5 is channel length modulation which cannot be noted down in HEMT. For the calculated maximum output power, the output gain is around 5 db i.e output power is 30 times that of input power. Given DC power of 0 Watts, the power efficiency is around 7% which is high enough to operate at high frequencies. The power and gain characteristics of HEMT were investigated by using these simulations that were self consistently solved with the I- characteristics, The results showed the expected drop in gain with frequency, 4. DESIGN OF GAAS MESFET AMPLIFIER USING S PARAMETERS. (UNILATERAL DESIGN ONLY) MESFET amplifier is designed using s-parameters for different frequency[5]. For exact matching condition of RF MESFET transistors F007_pf transistor with two port condition s parameters are found from microcap software. Figure 6,7 and 8 gives N-port circuit for s parameters and ac analysis of s-parameters. 4. DESIGN PROCESS STEPS: Device choices are chosen based on the requirements of the power device and its output stage. The following steps are used to find the maximum output power as given in block diagram of figure 6.b.. Choose a matching topology based on frequency, bandwidth, cost goals Choose a bias circuit based on class of operation and power supply requirements. MESFETS require negative gate voltage, which must be provided with the biasing circuit. olume, Issue January - February 03 Page 6

olume, Issue, January February 03 ISSN 78-6856 Optimize the input circuit for gain and input match.find S parameters from smith chart. Extract the package parasitic elements, they will be part of the overall output matching circuit. Optimize the output circuit for best match to RL (this condition will maximize RF power output). If necessary, add circuit elements to insure wideband unconditional stability. The above points are used to find power and its optimization. coefficients, circuit is unconditionally stable or not, load reflection coefficient for desired design [4]. Figure 7 smith chart of s and s of AC analysis. Figure 6.a two port network for finding angle and magnitude of s parameters. Figure 8 smith chart of s and s of ac analysis [5]. Figure 6.b. Block diagram of finding maximum power output Figure 7 and 8 shows magnitude and phase response of s, s, s, s and operating up to 6 GHz. It is considered here s parameters s,s,s,s from the table 3 as S = 0.5 60 0, S = 0.0 0 0, S = 6.5 5 0, S = 0.6-35 0 at a frequency of 6 GHz. We are going to find maximum power gain, optimal choice of reflection Stability factor K S k ( ( S ) ( S ) ( S ) ------- (4) K Stability factor. K =.7 When K > transistor is always stable. olume, Issue January - February 03 Page 7

olume, Issue, January February 03 ISSN 78-6856 Maximum gain Gmax. G smax = ( ( S) ) =.33 0r.5 db --------- (5). G Lmax = --------(6) ( ( S ) ) =.56 0r.94 db G smax and G Lmax are gain at input and output matching network. Go is insertion gain of transistor. 3. Go = (S) = 4.5=6.6 db Desire gain by adjusting load reflection coefficient Since the design is for 8 db MESFET amplifier overall gain is 7.5 db, we have to choose G L = 0.49 db. Figure of merit U = U = 0.08 S S S S --------------(7) ( ( S ) )( ( ) ) S Figure of merit tells about how much gain deviates from maximum gain. Nearly 8 % gain deviates. The MESFET considered here as channel with Gaussian profile and channel length of 0. µm. The ds is set at p as the device is very sensitive at the pinch off and gs=-0.5. The Schottky barrier depletion region under the gate extends into the active region. It controls the cross section of the conduction channel under the gate and it modulates the channel conductivity. Figure 7 and 8 gives you s parameters of the MESFET device from which stability and deviation in gain can be observed. It is found that K > stability can be achieved and hence device transconductance increases gain increases with increase in conduction in channel region of the transistor. Hence it was fond that operating point in the stable region may be at a gain less than desired value so that stability can be achieved for the given frequency of operation. Figure of merit gives idea of deviation of gain for given S parameters at higher frequencies. It can be concluded that the simulated transistors with higher stability and maximum gain used for microwave and other higher frequencies. s parameters values are taken from [6] [7]and its corresponding magnitude and phase response are plotted designed and gain, stability and figure of merit are calculated. It was found that maximum stability can be achieved with gain lesser than desired gain value and lesser figure of merit gives more stability and suitable at higher frequency operation. In future we further extend our work for MODFET design and calculation of s- parameters. REFERENCES [] Reins fold Ludwig RF CIRCUIT DESIGN Theory and applications. Pearson education II edition [] D..MORGAN and Parkman PHYSICS AND TECHNIOLOGY OF HETEROJUNCTION John Wiley New York 977. [3] A.S.Grove PHYSICS AND SEMICONDUCTOR TECHNOLOGY wiley 967. [4]S.Y.ZIAO MICROWAE DESIGN AND AMPLIFIER DESING prentice hall 987. [5]G.D.endilin Design of amplifiers using S parameters Jhon wiley New york 98. [6] Infineon Tech Discrete & RF Semiconductors Technologiesmanual.www.infineon.com [7] www.datasheetarchive.com AUTHOR Dr.E.N.Ganesh received M.tech degree in Electrical Engineering from IIT Madras, P.hD from JNTU Hyderbad. He has over 6 years of academic experience and published more than 0 papers in international journals and around 30 papers in conferences. in 997 and 999, respectively. At present working as Professor in Rajalakshmi group of institutions. 5. CONCLUSION High frequency devices like MESFET and MODFET are simulated and simple MESFET designed using S parameters. In conclusion HEMT gives higher frequency response than MESFET besides same output characteristics. A simple single stage MESFET is olume, Issue January - February 03 Page 8