~IEEE TRAN SACTI 0 NS ON MICROWAVE THEORY AND TECHNIQUES A PUBLICATION OF THE IEEE MICROWAVE THEORY AND TECHNIQUES SOCIETY MARCH 1994 VOLUME 42 NUMBER 3 IETMAB (ISSN 0018-9480) PAPERS Switching Characteristics of an Optically Controlled GaAs-MESFEf .................................................... . . . . . . . . . . . . . .. . . . . . . . .. . . . . .. . . . . .. . . .. . . . . . . . .. . . P. Chakrabarti, S. K. Shrestha, A. Srivastava, and D. Saxena 365 Improved Analysis Method for Multiport Microstrip Annulat-Ring Power Dividers ............ F. Tefiku and E. Yamashita 376 A Self-Diplexing Quasi-Optical Magic Slot Balanced Mixer .................................. C.-Y. Tong and R. Blundell 383 Microwave Noncontact Examination of Disband and Thickness Variation in Stratified Composite Media .............. . .. .. . .. . .. .. .. .. .. .. . .. . .. . .. .. . . .. . . .. .. .. .. .. .. .. .. S. Bakhtiari, N. Qaddoumi, S. I. Ganchev, and R. Zoughi 389 Coplanar Transmission Lines on Thin Substrates for High-Speed Low-Loss Propagation ................................ . . . .. . .. . .. . . . . . .. .. . . .. .. . . .. .. . . .. . . .. .. .. .. .. . . .. .. .. .. . .. .. M. Y. Frankel, R. H. Voelker, and J. N. Hilfiker 396 Physics-Based Expressions for the Nonlinear Capacitances of the MESFET :Equivalent Circuit ......................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S. D' Agostino and A. Betti-Berutto 403 Nonuniform Transmission Line Codirectional Couplers for Hybrid Mimic and Superconductive Applications ......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S. Uysal, C. W. Turner, and J. Watkins 407 A Multicomposite, Multilayered Cylindrical Dielectric Resonator for Application in MMIC's ... W. K. Hui and I. Wolff 415 Analysis of a Microstrip Crossover Embedded in a Multilayered Anisotropic and Lossy Media ........................ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J. Martel, R. R. Boix, and M. Horno 424 A Finite Element Cavity Resonance Method for Waveguide and Microstrip Line Discontinuity Problems .............. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J. S. Wang and R. Mittra 433 Finite Element Analysis of MMIC Structures and Electronic Packages Using Absorbing Boundary Conditions ........... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J. S. Wang and R. Mittra 441 Space Domain Approach for the Analysis of Printed Circuits ................. M. Kahriz', T. K. Sarkar, Z. A. Maricevic 450 Transient Responses of an Exponential Transmission Line and its Applications to High-Speed Backdriving in In-Circuit Test ................................................................................... C.-W. Hsue and C. D. Hechtman 458 Scattering from an Infinite Cylinder of Small Radius Embedded Into a Dielectric One .................................. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J. A. Roumeliotis and N. B. Kakogiannos 463 Analysis of the Scattering by Dielectric Bodies Using the SIE Formulation ............. A. Jostingmeier and A. S. Omar 471 Scattering of IB-Polarized EM Wave by Discontinuity in Grounded Dielectric Layer ................................... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J. Song, S. Ohnuki, D. P. Nyquist, K.-M. Chen, and E. J. Rothwell 481 Efficient Analysis of Planar Circulators by a New Boundary-Integral Technique..... G. G. Gentili and G. Macchiarella 489 (Continued on back cover) IEEE MICROWAVE THEORY AND TECHNIQUES SOCIETY The Microwave Theory and Techniques Society is an organization, within the framework of the IEEE, of members with principal professional interests in the field of microwave theory and techniques. All members of the IEEE are eligible for membership in the Society and will receive this TRANSACTIONS upon payment of the annual Society membership fee of $28.00. For information on joining, write to the IEEE at the address below. Member copies of Transactions/Journals are for personal use only. 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IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 42, NO. 3, MARCH 1994 365 Switching Characteristics of an Optically Controlled GaAs-MESFET P. Chakrabarti, S. K. Shrestha, A. Srivastava, and D. Saxena Abstract-The switching characteristics of an optically con ion-implanted GaAs-MESFET under illumination was first trolled Metal Semiconductor Field Effect Transistor (MESFET), proposed in [7]. Unfortunately, the model fails to account popularly known as Optical Field Effect Transistor (OPFET), for a number of important physical phenomena whi~h shape have been derived analytically. The limitations of the existing the device characteristics in the illuminated condition. An model have been overcome in the present model. Calculations improved de model of the device is very recently proposed are being carried out to examine the effect of illumination on the current-voltage characteristics, drain-to-source capacitance [8]. (Cdc), internal gate-to-source capacitance (C8.), drain-to-source The switching characteristics of a GaAs MESFET was first resistance (Rds), the transconductance (gm), the input RC time reported in [9]. The model was later extended in [10], for an constant and the cutoff frequency (h) of a GaAs-MESFET. optically controlled GaAs-MESFET . The model tlOJ suffers The variations of these parameters with gate length L and the doping concentration Nd have also been studied in 8d ark and from a number of major limitations. In their work the authors illuminated conditions. The results of numerical calculations show appear to have ignored the photovoltage developed across that there is an overall decrease in the input RC time constant of the Schottky gate of the MESFET under illumination. It is the device in the illuminated condition arising from the internal understood that in presence of a high gate bias resistance gate-to-source capacitance and the transconductance. The results the photovoltage developed across the Schottky gate is nearly obtained on the basis of the model show a close agreement with the reported experimental findings. The simple model presented equal to the open circuit voltage of an illuminated Schottky here is fairly accurate and can be used as a basic tool for circuit contact and the voltage strongly influences the characteristics simulation purposes. of the MESFET in the illuminated condition [8]. Furthermore, in calculating the intemal gate-to-source capacitance in the I. INTRODUCTION illuminated condition they have considered the excess photo generated charge in the channel region [ 10) in place of the I N recent years, considerable interest has been shown in excess charge in the depletion region below the gate. In their studying and modelling Optically Controlled Field Effect work [10), the authors have also ignored the effect of illumina Transistors (OPFET's) fabricated with Schottky gate config tion on the minority carrier lifetime and various losses of the uration.These OPFET's are expected to emerge as promis incident radiation due to reflections at various interfaces. The ing detectors for use in integrated optoelectronic circuits. present paper makes an attempt to overcome the limitations of A number of theoretical and experimental investigations on the previous models and provide a simple but fairly accurate the effect of illumination on MESFET structures have been model suitable for use in integrated optoelectronic circuit recently reported [1)-[8]. Preliminary investigations reveal simulation purposes. The outline of the proposed model has that photo response of illuminated MESFET is due to the been described in the next section. optically generated carriers which increase the conductivity of the channel and the photovoltage developed across the Schottky barrier which affects the applied reverse voltage II. FORMULATION OF THE MODEL on the gate [5], [8]. Simple models have been developed The GaAs-MESFET structure under consideration is similar by several workers [2)-[4] in order to explain the results of to one in [10) and is reproduced in Fig. l(a). In the present experimental investigations on the effect of illumination on analysis the channel has been considered to be uniformly commercially available GaAs MESFET's. The large signal doped. The radiation has been assumed to be incident on characteristics of an illuminated GaAs MESFET has been the semitransparent metal gate in the vertical direction while reported in [6]. Unfortunately, the models proposed so far the drain-to-sorce current flows in the horizontal fdirection. are either too complicated or not adequate to be used for The incident radiation enters the underlying semiconductor circuit simulation purposes. A simple yet fairly accurate model after encountering a loss at the gate metallization as well which takes into account all the important physical phenomena as at the metal-semiconductor interface. This loss can be involved in an illuminated MESFET need to be considered reduced substantially by suitably designing the structure [8]. for this purpose. A closed form de analytical model of an Absorption of the radiation in the semiconductor results in the generation of excess electron-hole pairs. These excess carriers Manuscript received May 14, 1992; revised April 22, 1993. P. Chakrabarti is presently with the Department of Electronics Engineering, change the charge distribution below the gate and the channel Institute of Technology, Bamaras Hindu University, Varamasi-221005, India. due to photoconductive effect. The excess photo generated The authors are with the Department of Electronics and Communication carriers also reduce the minority carrier life time. Further, the Engineering, Birla Institute of Technology, Ranchi-835 215, India. IEEE Log Nµmber 9215171. excess carriers also result in the development of a photovoltage 0018-9480/94$04.00 © 1994 IEEE 366 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 42, NO. 3, MARCH 1994 V0p across the Schottky gate due to photovoltaic effect. In presence of a high gate bias resistance the photovoltage is nearly equal to the open circuit voltage of an illuminated .-----.v• Schottky junction and it strongly influences the characteristics of the device under illuminated conditions. In the analysis, it has been assumed that in the illuminated condition also the 1 internal space charge region below the gate can be divided into sections I, II, and III (in Fig. l(a)) similar to the one a cosidered in [10] with charge distributions Q10P, Q20P and l Q3op• respectively. In the dark condition, these charge amounts correspond to Q1, Q2 and Q3, respectively [9]. The MESFET channel can thus be considered to be divided into three sub channels. The centre subchannel below Section I is associated with the gate metallization while the other two are due to r----------------- --- - - ----, inter electrode spaces [10]. Absorption of incident radiation in · Intrinsic mod ti the active region of the channel through the surface features Gate Drain of MESFET gate ·affects all the three subchannel regions and causes a change in the I-V characteristics of the device in the illuminated condition. The spacings between the gate and source and that between the gate and drain will offer series resistances. Since the gate of MESFET has been assumed to I be illuminated only, the two series resistances will remain I ------- ~-- unaffected in the presence of illumination. In the present analysis, the incident radiation has been characterized in terms of incident optical power density 2 Popt(W/m ) which undergoes reflection at the metal surface Source Source as well as at the metal semiconductor interface. Assuming the optical power density to decrease exponentially with the Fig. I. (a) Internal space charge distribution in the photo-MESFET before pinch off region. (b) Equivalent circuit of the MESFET. depth of penetration in the semiconductor, the excess carriers generated per unit volume within the semiconductor can be written as [8] From (1) and (2), TL can be obtained as 112 uA n = Go p · TL = (1 - Rm)(al h-v Rs)PoptTL ( 1 - exp ( - aa )) TL = ~{ 1~ +~ 4(~l2-~(Ra1,n-.~,R.;)h(,.v~,l. )-(R~1,-)R~ Ps.o) ~ ppt ~T P ~((1l ~-_ ~eexxp~p((-~-aaa~a))~))} ~ ~--1 (1) an;hv opt (3) where G0p is the optical generation rate per unit volume, a is the optical absorption coefficient, TL is the minority carrier B. Photo-Induced Voltage lifetime in the illuminated condition, Rm and R s are the reflection coefficients of the metal and semiconductor surfaces Due to direct illumination of the gate, there is a development respectively at the operating wavelength >. [11], a is the width of photovoltage V0P at the Schottky gate contact which effec of the active region, h is the Planck's constant, and v is the tively reduces the applied reverse bias on the gate. However, operating frequency. the photo-induced voltage is greatly influenced by the surface recombination, the lifetime of the minority carriers in the illuminated condition and the external resistance present in A. Effect of Illumination on the Minority Carrier Life Time the gate bias circuit. The previous models [2], [4], [6] fail to The excess carriers generated in the GaAs region below the account for the effect of surface recombination and the change gate affects the minority carrier lifetime. The lifetime TL of the in minority carrier life time on the photo-induced voltage. The minority carriers in the illuminated condition can be obtained present model calculates the photo voltage by considering the from surface recombination and the lifetime of minority carriers in the illuminated condition (TL) which has been found to be significantly different from the equilibrium minority carrier TL n; Tp = n; + t1n (2) lifetime (Tp). The electron hole pairs generated in the depletion region are seperated out by the existing field in the region. The electrons move towards the channel while the holes move where n; is the intrinsic carrier concentration of the semicon towards the surface where they recombine via the surface traps. ductor, Tp is the minority carrier lifetime in equilibrium and The surface recombination results in a leakage current that t1n is the excess photo generated carriers in the semiconduc affects the photo-voltage which, in tum, gives a feed back tor. effect either through the source or substrate [7]. The excess CHAKRABARTI et al.: SWITCHING CHARACTERISTICS OF AN OPTICALLY CONTROLLED GaAs-MESFET 367 TABLE I PARAMETER VALUES REFERENCES Tn 1 x 10-6 s [7] 1611 X13 Tp 1 x 10-s s [7] kn 3.1 x 10-15m3s [7] 12 k p 3.1x10-17m3s [7] Nr 4 x 1017 m2s [7] II vb; 0.8V [II] µn 0.85m2 /Vs [II] µp 0.04m2/Vs [II] 10 Rm 0.1 [8] a 106 Im [ll] 9 n; 1.79 x 1012/m3 [ll] >. 0.83µm [5] f e photo generated carriers also affect the minority carrier lifetime -;:; 7 .... in the illuminated condition under steady state. Both the effects ."....', tend to reduce the magnitude of the photo-voltage. .J !-' 6 It has been observed that for a very high external gate bias resistance only, the photovoltaic effect is dominant [2]. In 5 our analysis, we have assumed a very high resistance to be present in the gate circuit. This approximates the photovoltage 4 developed to be equal to the open circuit photo voltage (Vp) that affects the applied gate bias voltage in presence 0 of illumination. The open circuit photo voltage generated at 3 the Schottky gate in presence of surface recombination can be obtained as 2 I 0 2 3 4 5 6 7 8 OPTICAL POWER DENSITY, Popi (W/m2)-+ Fig. 2. Variation of minority carrier lifetime, TL with the optical power where T is the absolute temperature, k is the Boltzmann density, Popt· constant, q is the electronic charge, 'T/ is the constant depending on the semiconductor [11], µP is the hole mobility of GaAs and the channel. This photovoltage tends to forward bias the ls is the saturation current density of the metal-semiconductor Schottky contact and reduces the width of the depletion region. Schottky contact. For a high mobility semiconductor like This causes an increase in the effective channel thickness. In GaAs, the current density ls can be approximated by the other words, the photovoltage modulates the effective channel thermionic emission theory and can be expressed as [11]: opening resulting in a change in the channel conductance. The ls = A*T2 exp ( - q:;n) effective channel thickness at the drain end, H in presence of optical illumination can be written as qan being the barrier height of the Schottky contact at thermal equlibrium and A* is the effective Richardson constant for thermionic emission. The surface recombination rate R can be written as [7] R = NrKnKp(nsns - ntPt) where vbi is the built-in potential at the metal semiconductor Kn(ns + nt) + Kp(Ps +Pt) junction, Vd is the drain voltage, V is the gate voltage, 9 Nd is the doping concentration in the channel, and E is the where Nr is the area density of the traps, Kn and Kp are the permittivity of the semiconductor. capture factors. The surface carrier concentraions ns and Ps under illuminated condition are given by: C. Internal Gate-to-Source Capacitance ns = aPopt(l - Rm)(l - R )Tn/hv 8 Assuming the n channel to be doped uniformly with donor Ps = aPopt(l - Rm)(l - Rs)TL/hv concentration Nd(m-3) and considering a forward voltage which take the values nt and Pt. respectively, when the Fermi (V p) to be developed across the Schottky gate under illu 0 level lies in the traps. mination, the total charge below the gate in the space charge The photovoltaic effect discussed previously arises from region can be written as the collection of excess photo generated carriers in the high electric field in the depletion region which is transverse to 368 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 42, NO. 3, MARCH 1994 The internal gate-to-source capacitance Cgs in the illuminated - - - ILWMINATED CONDITION (WITHOUT RECOMBINATION) condition can be obtained as: - · - ILLUMINATED CONDITION (WITH RECOMBINATION) 290 C gs = ( 8Qavsso p)TvT g d = constant. (5) 2BO The total space charge below the gate in the illuminated condition can be obtained as 270 ..,.,-· ,, .,,...-· / / 260 / -·- / - t / / 250 / / / / ~ I / / ::a. 240 I / / I . ·-r-·-· .... I; · I ./ C~l 230 / ./ ......- ...... Nrz4XI017/m2 >f0? 2 20 I,' /. !/ /. / NT =3 X I01Jm 2 :0r //// NT •2X 1017/ m2 Q. 210 fl/ Nr=-0 fl. where W is the width of the gate, L is the gate length, v;, 200 is the source voltage, and V,, is the p9i nch off voltage and is I given by: 190 V. _ qa2Nd P - 2c The total gate to source capacitance in the illuminated condi OPTICAL POWER DENSITY,Popt CW/m~ - tion has been obtained as Fig. 3. Variation of photovoltage, V0p with incident optical power density, Popt (with and without recombination). channel region under illumination. The excess electron hole pair generated in the channel modulates the conductivity. The photovoltage developed across the Schottky barrier reduces the width of the depletion region. Thus, both the channel conductivity as well as the channel conductance are affected in the illuminated condition. This fact results in a large change in a drain current under illumination. However, in GaAs (7) MESFET surface recombination tends to reduce this effect to some extent. Thus, the internal gate-to-source capacitance in the illuminated The channel charge due to doping and optically generated condition is an increasing function of the gate-to-source volt carriers taking surface recombination effect into account can age and depends on the value of V0P, which is determined by be obtained as the incident optical power density. From (7), it is also clear that the gate-to-source capacitance shows a very high value qWL Nd for a particular level of illumination when Vgs (= V9 - Vs) is Qcop(Vx) = aqW L9Nd - 2 9 in the neighborhood of (Vbi - V p). [( 0 q~d) 1/2 { (Vbi - Vg + V,, - Vop)1/2 D. The Current-Voltage Characteristics l The drain-to-source resistance of the MESFET under illumi nation (Rds) can be obtained from the current voltage relation + (Vbi - Vg + Vx - Vop)112} of the device under illumination. In order to find the current voltage relation of the device, both photovoltaic effect and + ~~t (1 - Rm)(l - Rs) photoconductive effect should be taken into consideration. In the previous model [10), the photovoltaic effect was not con (1- exp(-aa))TLWqL9 - qRWL9TL (8) sidered. The current voltage characteristics of the device can be obtained by integrating the total charge in the undepleted The carrier concentration per unit area in the channel in CHAKRABARTI et al.: SWITCHING CHARACfERISTJCS OF AN OPTICALLY CONTROLLED GaAs-MESFET 369 - DARK CONDITION - DARK CONDITION - - ILLUMINATED COM>lllON (WITHOUT llECOMelNATION) 8 - - ILLUMINATED CONDITION (WITHOUT RECOMBINATION) - · -LLUMINATED COl\OITION (WITH RECOM!ltlAOON) 6 -·-ILLUMINATED CONDITION (WITH RECOMBINATION) Vd = O·I V ././ vd = o.1v Vg =-0·2V ,, ,, ./ Vg =·0·2V NPTo pt=• 4S XWI0/m17l/ m1 ,,"',,,/" -/ NPoTp t == 4SXW 1/0m17/lm l // 6·5 -:-/·;-· t t 4 5 6 .. .... .... ~ ~.6 .j. .· '.u-.~. '. 5.Su~ u"~" u(uz! u~z (u"z"! uz < u< ~ .<... 4 Q~. . ":'. :"0"u.oa.>. :''. 4 45.".0::0.>... '.,' :> .0"0.. '. "!"<" ""tc" 0;aCI017/ m2 I I I . I . 3·5 \ I I 3-0 f \ /I // 530~ b \ I I I / . I 2·5 t ~ 1\ f/ { / ~."...,',- \ i / I I . / ~ ~~ I\,\ : I . ' '' . 1 ·5 \ \ \/ \/\ ' . >\.~/ < /.. _x_ / ../1 /, . // / / . / 11··50 ."~.u5""..',,' ' :o~~z III'~ 1 !~i1 - ji 433800 g1<0z" ;-' ~ 1·0 / ' ." .0. . 2-l I,. , '. \ / / / / . ..... '--~·---·::::-.:-- 0·5z: I•I. I '\ ' \ . 0·5 ........ 0 '~~·-·-·-·-·- 330 - - ·- '\:r--- -------- -0·7 -0·6 -0·5 -0'4 -0·3 -0·2 -0·1 0 l~L--....L..----L. __. ___.......__---L._ __,..__-!:---~--'280 GATE TO SOURCE VOLTAGE ,Vgs (VJ 0 I 2 3 4 5 6 8 Fig. 8. Variation of drain current, Ids and drain-to-source resistance, Rds OPTICAL POWER DENSITY,Popt CWfff the optical generation . rate and causes V p to saturate. The has been found to occur at Vgs = -0. 71 V for. the assumed 0 previous models [2],.[4], [5] cannot predict the ultimate limit values of recombination.p arameters. The numerical values of various paraineters used iii the calcuiation are given in Table L of V0P (i.e., the built-in potential of the Schottky contact) as they fail to account for the change in lifetime of the minority The excess photo generated carriers in the active region carriers under illumination. Fig. 3 also shows the variation below the gate cause a change in the minority earner. lifetime under illumination. The variation of the minority carrier life of the photovoltage, V0P, with incident optical power density in presence of surface recombination for various values of time under illumination (TL) With the incident Optical power density has been shown in Fig. 2. It is seen that TL decreases ttora pa plionwg edr evnasliutye iNn Tth. eI tp creasne nbcee eoafs isluyr fsaeceen rtehcaotm Vb0ipn astaiotunr aatneds with increase in the incident optical power density. This· the value ts less for higher values of NT. decrease in TL in presence of illumination is an important Fig. 4 shows the variations of gate-to-source capacitance consideration in the present model. Results of our calculation Cgs. as well as drain-to-source capacitance Cdc· below pinch show the equilibrium mean lifetime of the minority. carriers off with the gate-to-source voltage in the dark and illuminated decreases from 10-S seconds to 3.93 X 10-ll seconds for an condition for Vds = 0.1 V. The incident optical power iii.ciderit optical power density of 5 W/m2. This decrease in the density has been assumed to be Popt = 5 W/m2. It is seen minority carrier lifetime, in turn, limits the value of the photo that, in both dark arid illuminated conditions, the Cgs and voltage Vop· This means that V p cannot increase. indefinitely Ccic decrease with increasing reverse gate-to-source voltage. 0 with the increase in the optical generation rate caused by an However, for a fixed applied gaie-to-source reverse voltage, increase in the incident optical power density. This is clear the capacitance increases significantly in the presence of from Fig. 3, which shows the variation of the open circuit illumination in both the cases. This is due to the fact that, in photo-voltage devefoped across the Schottky barrier with the presence of illumination, the photovoltage V0P developed the incident optical power density (with and without surface across the Schottky contact reduces the gate-to-source reverse recombination). From the graph, it is seen that V p increases voltage which in turn enhances the capacitances. This result 0 with the increase iri the incident optical power density in th~ is in accordence with the experimentally measured variations beginning and finally tends to saturate for a higher value of reported in [5]. A comparison of the results of our theoretical Popt even in the absence of surface recombination. This opt is calculations with those of experimentally measured values of because, at higher values of incident optical power, the lifetime [5] reveal that the percentage change in the capacitance values 372 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 42, NO. 3, MARCH 1994 ·7·5X10-4 - DARK CONDITION 1·25 - - - ILLUMINATED CONDITION CWITHOUT RECOMBINATION> - ILLUMINATED CONDITION (WITH RECOMBINATION) - DARK CONDITION - -- ILLUMINATED CONDITION (WITHOUT RECOMBf.IATIONl I - ·-ILLUMINATED CONDITION lWITH RECOMBINATION) vd =0·1 v HPor p t== 54 Wit/om127 /m2 VVdg :~CoH·2VV -- 1-0 Popi-= 5W/m2 Nr = :o 101'lm2 6·5 i > 6 0·75 ..5 .l w' ~ uz ~ 5'5 ~z !u) t3 80 ;:) 0·5 0z "z' 5 0~ ,a<_:t z ,a<_:t 0·25 4.5 -0·5 -0-4 -0·3 -0·2 -0·1 GATE TO SOURCE VOLTAGc:, Vg, (Vl 4 5 6 B 9 10 11 12x1022 Fig. I0 . Variation of transconductance, 9m with reverse gate-to-source DOPING CONCENTRATION;-Nd (m-1)- voltage in dark and illuminated condition (with and without recombination). Fig. 11. Variation of transconductance, 9m with the doping concentration, Nd in dark and illuminated condition (with and without recombination). is more in our case. This is because, in our analysis, we have assumed the gate to be semitransparent, which is not true for ing concentration, both capacitances increase in the presence the commercially available MESFET used by Simons. On the of illuminatfon. However, surface recombination reduces the other hand, a quantitative comparison of our results with those capacitance values in the illuminated condition as shown in the obtained by [4 ] on the basis of their theoretical model indicates figure~ The variations of Cgs and Cdc at constant gate-to-source that the percentage change in the corresponding capacitance voltage have been plotted in Fig. 6 under dark and illuminated values under illumination is less in our case. This is perhaps conditions (with and without recombination) by taking Lg as a because their model ignores the reflection of incident optical parameter. In all the cases, both capacitances increase with an power at the entrance as well as at the metal semiconductor increase in Lg. This is due to the fact that, at constant applied interface. Their model also neglects the decrease of minority voltages, an increase in Lg causes an increase in the depletion carrier lifetime in the presence of illumination, which ~ends to region charge below the gate, which results in an increase in reduce the V0P and hence the capacitance. The effect of surface the capacitance value. The surface recombination has a similar recombination on the variations of Cgs and Cdc with the effect on the capacitace values as already discussed. reverse gate-to-source voltage has also been depicted in Fig. Fig. 7 shows the current voltage characteristics of the device 4. It is seen that due to surface recombination the capacitance in the dark and illuminated condition up to pinch off. It is values slightly decrease in presence of illmT\ination. seen that, at pinch off, there is a significant change in the The gate-to-source capacitance Cgs and the drain-to-source drain current Ids in the illuminated condition as compared capacitance Cdc variations with the doping concentration have with that in the dark condition. The huge change in the drain been shown in Fig. 5 for dark and illuminated conditions current in the illuminated condition is due to both photovoltaic (with and without recombination) at a constant gate-to-source and photoconductive effect. These characteristics are similar to voJtage. It is clear that both the capacitances increase with the those of the experimental results reported in [5]. However, the increasing doping concentration in dark as well as illuminated illumination effect ·on the drain current somewhat decreases conditions. This may be accounted for by the fact that, at when the surface recombination effect is taken into account. a constant applied gate-to-source and drain-to-source voltage, A quantitative comparison of our results with those of [4], the width of the depletion region below the gate decreases with [5] reveals that the percentage change in the dark current at an increase in Nd. This decrease in the width of the depletion saturation under illumination in our case is more than that region, in tum, results in a corresponding increase in the of [5] and less than that of [4]. This discrepancy is due capacitance values. It is further seen that, for a particular dop- to the reason already discussed. The variation of Ids and

IEEE MTT-V042-I03 (1994-03)

Similar IEEE MTT-V042-I03 (1994-03)