optically-controlled ion-implanted gaas mesfet characteristic with opaque gate

78 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 45, NO. 1, JANUARY 1998

Optically-Controlled Ion-Implanted GaAsMESFET Characteristic with Opaque Gate

Shubha, B. B. Pal, and R. U. Khan

Abstract—An analytical modelling has been carried out foran ion-implanted GaAs MESFET having a Schottky gate opaqueto incident radiation. The radiation is absorbed in the devicethrough the spacings of source, gate, and drain unlike the othermodel where gate is transparent/semitransparent [1]. Continuityequations have been solved for the excess carriers generated inthe neutral active region, the extended gate depletion region andthe depletion region of active (n) and substrate (p) junction. Thephotovoltage across the channel and the p-layer junction and thatacross the Schottky junction due to generation in the arc regionof the gate depletion layer are the two important controllingparameters. The III–VVV characteristics and the transconductanceof the device have been evaluated and discussed.

I. INTRODUCTION

T HE optically-controlled MESFET (or OPFET) is of greatimportance because of its potential as a photo detec-

tor and pre-amplifier, r.f. switch and tunner, etc. Differentmechanisms which are responsible for the enhanced termi-nal properties of the optically-controlled MESFET are: 1)photo-induced voltage across the Schottky barrier [2], [3],2) photo generated carriers below the gate [1], [4], and3) photo conductivity effect in the source-gate and drain-gate regions and the change in the gate depletion width [5].Further, the experimental observation [6] showed a positivevoltage across the depletion region between the n-type channeland the semi-insulating substrate suggesting that the draincurrent enhancement is closely related to the channel widthmodulation of the device.

The first theoretical work on the optically-controlled ion-implanted silicon MESFET reported by Singhet al. [7] usedphotogenerated carriers below the gate due to optical ab-sorption through a transparent/semitransparent Schottky gate.Later, the work was extended for ion-implanted GaAs OPFET[1] incorporating the effect of surface traps in the activelayer of the device. An ion-implanted profile has been chosenbecause of it’s superior performance over other profiles asshown theoretically by Chattopadhyay and Pal [8]. Recently,two interesting theoretical works appeared [9], [10] in whichone [9] considered the effect of photo voltage across theSchottky junction and the channel width modulation andthe other [10] considered the photo voltaic effect acrossthe channel-substrate interface for the substantial increaseof the drain current. While the former presented a one-dimensional analytical approach for an ion-implanted GaAs

Manuscript received August 2, 1996; revised May 15, 1997. The review ofthis paper was arranged by Editor Pallab K. Bhattacharya.

The authors are with the Department of Electronics Engineering, Instituteof Technology, Banaras Hindu University, Varanasi 221 005, India.

Publisher Item Identifier S 0018-9383(98)01153-8.

Fig. 1. A schematic structure of the device and the typical ion-implanteddoping profile in the active layer.

device with medium channel, the later presented a two-dimensional numerical model for a constant doping profile.

In the present analytical model, we consider an ion-implanted GaAs MESFET which has a Schottky gate opaqueto incident radiation. The radiation is incident on the deviceand is absorbed in the active and substrate regions throughthe spacings of source, gate and drain (Fig. 1) [11]. The gatedepletion region is similar to that suggested by Takadaet al.[12] which is accurate enough even for short channel devices.The excess carriers generated in the active region move by theprocess of diffusion and recombination. Two photo voltagesare developed in this process within the device: one across theSchottky junction and the other across the n-p junction. Thesetwo voltages modulate the channel region of the active layer.In the n-p depletion region and the arc regions of the gatedepletion layer the carriers move by drift and recombination.The difference of the present structure with [10] is that weconsider an ion-implanted profile rather than a constant dopingprofile. We solve continuity equations for excess electronsand holes along with current equations which are then usedto derive the drain-source current and the transconductanceof the device. In this presentation we restrict our calculationonly for the low impedance state.

II. THEORY

The schematic device structure is shown in Fig. 1. Weconsider a non self-aligned structure in which the spacingsbetween source, gate, and drain are large compared to theactive layer thickness. Since the gate is opaque and theradiation penetrates through the spacings, the excess carriersare generated in the 1) extended gate depletion region (arcregions), 2) neutral active layer, and 3) depletion region ofactive layer-substrate regions. This leads to development oftwo optical voltages: one across the Schottky junction andother across n-p (at the substrate) junction which modulates

0018–9383/98$10.00 1998 IEEE

SHUBHA et al.: OPTICALLY-CONTROLLED ION-IMPLANTED GaAs MESFET CHARACTERISTIC 79

the channel width significantly. Further, the excess electronsgenerated which flow either by diffusion and recombination(in the neutral region) or by drift and recombination (in thedepletion region) enhances the current and conductance of thedevice. Both surface and bulk recombination have been takeninto account. The dc continuity equations for excess electronsand holes generated in the neutral and depletion regions of thedevice under the steady state condition are given

(1a)

(1b)

where and are the electron and hole current densitiesand are represented by

(2a)

(2b)

In the above equations, and are the volume generationrates, and are the recombination rates, andare the diffusion coefficients, and n and p are the excesselectron and hole concentrations, suffices n and p stands forelectrons and holes, is the carrier saturated velocity alongvertical y-direction, assumed the same for both electrons andholes, , , , and are the functions of concentrationand since concentration is varying with, they also becomedependent on. However for a closed form solution we definean equivalent constant doping for the Gaussian profile such as

exp (3)

where , , and are the implanted dose, range, andstraggle parameters, respectively,, is the junction depth and

is the equivalent distance for constant doping . Theconcentration dependence of’s and ’s are defined as [13]

(4a)

(4b)

Here, is the reference concentration [13] and has thevalue /m .

A. Excess Carriers in the Neutral Channel Region

The channel being neutral, there is no field within this regionin the absence of any drain-source voltage, so andthe transport of carriers will be only due to diffusion andrecombination. Combining (1) and (2) we obtain

(5)

is the surface recombination rate andis the active layerthickness. is same as defined in [1] for electrons, whereand are replaced by and for thelow injection case. Equation (5) has the solution of the form

exp exp

(6)

Since the generation rate is assumed to decay exponentially inspace, we assume and is evaluated applying theboundary condition that at , . Thus,

where and being the electron lifetime.Substituting and in (6) we get

exp (7)

Thus, the channel charge due to the excess electrons generatedin the neutral region is

(8)

where is the extension of gate depletion region measuredfrom the surface and given by

(8a)

is the extension of the depletion of the n-p junction in thechannel also measured from the surface and given by

(8b)

is the p layer concentration, is active layer thickness,is the barrier height of the Schottky junction, is the

channel potential, is the gate-source potential, is thesubstrate to source potential, is the Fermi level below theconduction band in the neutral region, and is the built-involtage of n-p junction.

The channel current due to generation in the neutral regionof the channel can only be obtained when drain source voltage

is applied. Thus in the low impedance state the currentis given by

(9)


B. Current in the Extension Region ofthe Gate Depletion Region

In the arc region of the gate depletion the transport ofcarriers is due to drift and recommendation. So the differentialequations for electrons and holes are

(10a)

(10b)

The solutions are

exp (11)

We assume being a coefficient of exponentiallyincreasing function. Therefore

(12a)

also

exp (12b)

applying the boundary condition at ,

exp (12c)

We consider the radii of the arcs as and where

The holes generated in the arcs are given by

(12d)

These holes will produce an additional photovoltage acrossthe opaque gate.

The additional channel current due to generation in the arcregion at the source end is given by

(13)

where .The current due to right arc with radius at the drain end is

(14)

being the saturation velocity of carriers. The total contri-bution to channel current is

(15)

C. Current Due to Generation in the ActiveLayer-p-Layer Depletion Region

The differential equations governing the transport of elec-trons and holes in the active layer and p-substrate depletionregion are due to drift and recombination and are given by

(16a)

for electrons and

(16b)

for holes, respectively.The solutions are expressed as

exp (17a)

exp (17b)

In the above equations, is assumed zero due to physicalconditions and

(18)

The charge contributing to the channel current due to gen-eration in the active layer-p-layer depletion, region is givenby

(19)

where is the depletion layer width in the p-layer measuredfrom the surface. is given by

and being represented by

and

The corresponding current is

(20)

(21)


TABLE IDEVICE PARAMETERS USED FOR ANION-IMPLANTED GaAs OPFET

D. Current Due to Ion Implantation

Considering that the ion-implanted profile be representedby the Gaussian function [1] the channel charge due to ionimplantation is given by

erf erf (22)

where is the implanted dose, is the implanted rangeparameter, is the straggle parameter, and is the modifiedvalue of due to photo voltage across the p-layer-active layer junction. is the modified value of due tophoto voltage in the extended arc regions below gate.

and are written as

Thus, the current due to ion implantation is obtained as

(23)

Hence, the drain-source current of the device is sum of,, and and given as

(24)

Details of (8), (9), (13), (14), (19), (20), and (23) have beengiven in the Appendix.

E. Transconductance and Channel Conductance

The transconductance and the channel conductance of thedevice are calculated using the following relations:transconductance

constant

constant(25)

and channel conductance

constant

constant(26)

The current is independent of and hence .In these calculations, the photo voltage plays an important

role and is calculated using the relation

ln ln (27)

Where is the hole current density crossing the junctionat and

ln ln (28)

where is the excess hole current density crossingthe junction at and is the corresponding holeconcentration. The solution for is given by 9(b) wherethe constant is evaluated at at which inequilibrium.

III. RESULTS AND DISCUSSIONS

Fig. 2(a) and (b) show the plots of– characteristicsfor different flux densities and gate voltages, respectively.Fig. 2(a) is for both dark and illumination at a zero gate sourcevoltage and Fig. 2(b) is for a constant flux density. Under darkand low illumination intensity it is observed that current tendsto saturates at a higher value of beyond 1.0 V which isdue to the nonlinearity of the impurity profile. If we comparethe results with those of Mishraet al. [1] and Lo and Lee [10],we observe that our results are more sensitive to illumination.Mishraet al.have considered only the generation of carriers inthe gate depletion region along with surface recombination andLo and Lee considered photovoltaic effect across the interfaceof the active and buffer layers in addition to generation andrecombination due to traps present in the buffer layer. Inour calculation we assume the low injection case and largercarrier life time s compared to Loo and Lee

s , so the effect of illumination is more in ourcase. Further comparing our results with those (experimental)


(a)

(b)

Fig. 2. (a) Drain-source current versus drain-source voltage under dark andilluminated condition for different flux densities at zero gate-source voltage.(b) I–V characteristics for different gate voltages at a fixed radiation fluxdensity.

of Simons [14] we find that the– characteristics are quiteclose to each other.

Fig. 3(a) and (b) represents the plots of channel conductanceagainst drain-source voltage for different flux densities

(a)

(b)

Fig. 3. (a) Channel conductance against drain-source voltage at zerogate-source potential and dark and illuminated conditions. (b) Channelconductance versus drain-source voltage for different gate-source potentialsand at fixed flux density.

and gate source voltages, respectively. Channel conductancedecreases with and reaches a saturation value beyond1.0 V where it also becomes almost independent of .So becomes bias independent beyond pinch off. Thereis significant increase in due to illumination from itsdark value as expected. Fig. 4(a) and (b) shows the plotsof transconductance against gate-source voltage for differentflux densities and drain-source potentials respectively. Thetransconductance shows a gradual increase as the device


(a)

(b)

Fig. 4. (a)Transconductance versus gate-source voltage both at dark and illu-mination for different flux densities. (b) Transconductance against gate-sourcevoltage at different drain-source potentials under illuminated condition.

moves from enhancement mode to depletion mode. It alsoincrease significantly with the increase in radiation flux den-sity. changes the transconductance very little. The resultsare similar as in [1]. The results are also in good agreementwith the experimental results of Simons [14].

IV. CONCLUSIONS

The photo effects in an ion-implanted GaAs MESFET havebeen considered for theoretical analysis. The Schottky gateis considered non transparent and that the radiation entersinto device through the spacings between source, gate anddrain. Thus the depletion region just below the gate is notaffected due to radiation whereas the channel and the activelayer and p junction depletion region have excess carriersdue to absorption of photons. This induces a photo voltageacross the junction leading to channel width and hence channelconductance modulation resulting in significant increase in de-vice current, channel conductance, transconductance comparedto previous models [9], [10]. Also the results are in goodagreement with the experimental results of Simons [14].

APPENDIX

Calculation of Channel Charge and the Drain-Source Current

The channel charge due to the excess carriers generated inthe neutral region given by (8) is

exp exp

exp exp

(A1)

The corresponding current is calculated by integrating equation(A1) in terms of channel voltage and is given by

exp exp

exp

exp

exp

exp

exp exp (A2)

where


The channel current due to generation in the external arc regionat the source end is given by

exp

(A3)

The current due to right arc with radius at the drain end is

exp

(A4)

The charge contributing to the channel current due to gen-eration in the active layer-p-layer depletion region is givenby

exp exp (A5)

The corresponding current is

(A6)

where

The current contributed by impurities having a Gaussianprofile is represented as

erf

exp erf

exp

erf erf

exp exp

erf

exp erf

exp

erf erf

exp exp (A7)

where

REFERENCES

[1] S. Mishra, V. K. Singh, and B. B. Pal, “Effect of radiation and surfacerecombination on the characteristics of an implanted GaAs MESFET,”IEEE Trans. Electron Devices, vol. 37, pp. 2–10, Jan. 1990.

[2] J. Graffeuil, P. Rossel, and H. Martinot, “Light induced effects in GaAsFET’s,” Electron. Lett., vol. 15, no. 14, pp. 439–441, July 1979.

[3] R. N. Simon and K. B. Bhasin, “Analysis of optically-controlledmicrowave/millimeter wave device structures,”IEEE Trans. MicrowaveTheory Tech., vol. MTT-34, pp. 1349–1355, Dec. 1986.

[4] S. N. Mohammad, M. S. Unlu, and H. Morkoc, “Optically-controlledcurrent–voltage characteristics of ion-implanted MESFET’s,”Solid-State Electron., vol. 33, no. 12, pp. 1499–1509, 1990.

[5] J. Pan, “GaAs MESFET for high speed optical detection,” inSPIE Int.Tech. Symp., San Diego, CA, 1978.

[6] W. D. Edwards, “Two and three terminal gallium arsenide FET opticaldetectors,”IEEE Electron Device Lett., vol. EDL-1, pp. 149–150, Aug.1980.

[7] V. K. Singh, S. N. Chattopadhyay, and B. B. Pal, “Optically-controlledcharacteristics of an ion-implanted Si MESFET,”Solid-State Electron.,vol. 29, pp. 707–711, 1986.

[8] S. N. Chattopadhyay and B. B. Pal, “A unified model for MESFETanalysis,”Semicond. Sci. Technol., vol. 3, pp. 185–189, 1988.

[9] B. B. Pal and S. N. Chattopadhyay, “GaAs OPFET characteristicsconsidering the effect of gate depletion width modulation due to incidentradiation,” IEEE Trans. Electron Devices, vol. 39, pp. 1022–1027, May1992.

[10] S. H. Lo and C. P. Lee, “Numerical analysis of the photo effects in GaAsMESFET’s,” IEEE Trans. Electron Devices, vol. 39, pp. 1564–1570,July 1992.

[11] B. B. Pal and H. Mitra, “Enhanced optical effect in a HEMT device,”Opt. Eng. (SPIEJ), vol. 32, no. 4, pp. 687–691, April 1993.

[12] T. Takada, K. Yokoyama, M. Ida, and T. Sudo, “A MESFET variablecapacitance model for GaAs integrated circuit simulation,”IEEE Trans.Microwave Theory Tech., vol. MTT-30, p. 719, 1982.

[13] S. Selberherr,Analysis and Simulation and Semiconductor Devices.New York: Springer-Verlag, 1984, p. 106.

[14] R. N. Simons, “Microwave performance of an optically-controlledAlGaAs/GaAs high electron mobility transistor and GaAs MESFET,”IEEE Trans. Microwave Theory Tech., vol. MTT-15, pp. 1444–1455,Dec. 1987.

Shubha received the B.Sc. degree from St. John’sCollege, Agra, India, in 1989, and the M.Sc. de-gree from Dayalbagh Deemed University, Agra, in1991. She is currently pursuing the Ph.D. degreein the Department of Electronics Engineering, theInstitute of Technology, Banaras Hindu University,Varanasi, India. Her research interest involves thecharacterization and modeling of GaAs OPFET.

B. B. Pal, for a photograph and biography, see this issue, p. 77.

R. U. Khan, for a photograph and biography, see this issue, p. 77.

optically-controlled ion-implanted gaas mesfet characteristic with opaque gate

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