optical effect in inalas/ingaas/inp modfet

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

Optical Effect in InAlAs/InGaAs/InP MODFETH. Mitra, B. B. Pal, S. Singh, and R. U. Khan

Abstract—Analytical results have been presented for an op-tically illuminated InAlAs/InGaAs/InP MODFET with opaquegate. Partial depletion of active region is considered. The ex-cess carriers due to photo generation are obtained by solvingthe continuity equation. The energy levels are modified due togeneration of carriers. The surface recombination effect has alsobeen taken into account. The results ofIII–VVV characteristics havebeen compared under dark condition, since under illuminationexperimental results are not available. The offset voltage, sheetconcentration, III–VVV , and transconductance have been presentedand the effect of illumination discussed.

I. INTRODUCTION

A STRONG interest has been created in the study of opticaleffect in high-speed devices due to their potentiality

in fiber-optical communication and optical integration. Bothexperimental and analytical studies [1]–[6] have been carriedout by different investigators on the effect of illumination inGaAs MESFET and AlGaAs/GaAs MODFET which show thatthere is significant effect of incident light on the electricalparameters of the devices.

Recently, InAlAs/InGaAs/InP heterostructures have beenwidely studied for ultra-high-speed device application [7]. Alarge conduction band edge discontinuity between InAlAs andInGaAs provides the necessary high sheet carrier concentrationat the interface of the heterojunction with a high carrier mo-bility and saturated velocity in InGaAs region. The materialswith appropriate In-content can detect and amplify radiation ofwavelength within the range of 1.3–1.6m which is of recentinterest in fiber-optic communication systems.

In this paper, we have calculated the effect of opticalradiation on the ultra-high-speed modulation doped field effecttransistor made of InAlAs/InGaAs/InP heterostructure. Previ-ously, studies have been reported on the effect of radiationon AlGaAs/GaAs MODFET considering transparent or semitransparent Schottky gate [8]. However, the photovoltagedeveloped across the metal-semiconductor junction due toincident light reduces the depletion width below the gatein the active region. This effect has to be circumvented byapplying a negative gate voltage equal to the photovoltage.This degrades the overall device performance [8]. To makethe photoeffect more meaningful in a MODFET Pal andMitra [9] have suggested that the radiation be absorbed in

Manuscript received May 30, 1996; revised May 7, 1997. The review ofthis paper was arranged by Editor P. K. Bhattacharya.

H. Mitra is with D. L. W., Varanasi 221 004, India.B. B. Pal and R. U. Khan are with the Department of Electronics

Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India.

S. Singh is with the Department of Physics, Udai Pratap College, Varanasi221 002, India.

Publisher Item Identifier S 0018-9383(98)01152-6.

the semiconductor through the spacings of source, gate anddrain, and the Schottky gate may behave as an opaque medium.This allows the radiation to create electron-hole pairs in theheterojunction regions. The excess electrons move toward theheterojunction interface and the holes move either toward thesurface or substrate. A photovoltage is developed across theheterojunction which drags the electrons into the interfaceenhancing the sheet concentration of the two-dimensionalelectron gas (2-DEG). Both one-dimensional (1-D) and two-dimensional (2-D) analyses have already been presented fora AlGaAs/GaAs MODFET considering total depletion of theactive region [10] under the condition of optical illumination.

In this paper, we assume the radiation to fall on the spacingsof source, gate and drain of a InAlAs/InGaAs/InP MODFETand the active layer is considered to be partially depleted sothat the analysis is valid even at low temperature. The excesscarriers are solved using the continuity equations for electronsand holes. The Poisson’s equation is used for electric fieldand voltage. The effect of radiation on– characteristicsand transconductance have been presented.– results havebeen compared under dark with the published data [7]. Thetheory is presented below.

II. THEORY

Fig. 1 shows the In Al As/In Ga As/InP modu-lation doped field effect transistor with radiation falling withinthe gaps of source, gate and drain, the Schottky gate beingopaque to the radiation. The active layer is of n-InAlAs 200A thick, followed by an undoped layer of the same materialof 20 A thickness. The heterojunction is made with undopedInGaAs on InP substrate. The low field mobility as high as10 000 cm/V/s has been reported for the 2-DEG system at300 K [7]. Our interest is to first calculate the excess carriersgenerated in the active region and undoped region by solvingthe continuity equations for electrons and holes. These are thensubstituted in the Poisson’s equation to obtain the expressionsfor electric field and voltage by applying appropriate boundaryconditions.

The dc continuity equations used are

(1a)

for electrons, and

(1b)

for holes.

0018–9383/98$10.00 1998 IEEE

MITRA et al.: OPTICAL EFFECT IN InAlAs/InGaAs/InP MODFET 69

(a)

(b)

Fig. 1. Schematic structure of MODFET and the typical energy band diagram.

The current density equations are

(2a)

(2b)

where is the volume generation rate and is the surfacerecombination rate. Other symbols have their usual meaning asin [9]. We assume partial depletion of the active region so thatthe analysis is valid even at low temperature. The transport ofexcess carriers in the neutral region is due to the process ofdiffusion and recombination and that in the depletion region is


due to drift and recombination. The recombination takes placeat both bulk and the surface and is due to positive or negativetraps present in the bulk and at or close to the surface. Thedifferential equation at the neutral region is given by

(3)

where , being the absorption coefficient, andthe radiation flux density.In the depletion region, the equation governing the flow of

carriers is

(4)

Here, is the carrier velocity along the vertical direction andis equal to the saturated velocity of carrier.

The solutions for (3) and (4) are given by

(5)

and

(6)

respectively, where

The velocity-field relation for electrons in InAlAs is assumedin the form [12]

exp (7)

where is the low field mobility, is the saturated velocity,and is the field. The velocity tends to saturate at high electricfield, is taken from [9]. The boundary conditions usedfor the evaluation of the constants are at the interface of theheterojunction, at , and at the surface, at

, . The value of the constants are evaluatedusing these boundary conditions.

The total excess carrier is the sum of (5) and (6). Similarequations are valid for holes also. They are given in theAppendix.

III. POISSON’SEQUATION AND THE TOTAL CHARGE

Considering the partial depletion of the active region, thePoisson’s equation is represented as

(8)

where is the free electron concentration in the activeregion. The effect of free hole is neglected. Since the doping

concentration is high in a MODFET the degenerate statisticsis applied for the concentrations [11].

Thus

exp(9a)

and

exp

exp(9b)

where is the doped layer concentration and is thedensity of state function at the conduction band. Equation (9b)is an approximation to Fermi–Dirac integral [11]. , ,and are the Fermi, minimum of conduction band, anddonor energy levels, respectively.Let us putand , where zero refers to the case whenthere is no heterojunction. When gate is in contact withthe semiconductor the Fermi energy is written as

where is taken as the reference energy.Thus, (8) may be rewritten as

exp

exp

exp(10)

Using the transformation

The field at the surface is given by

lnexp

exp

lnexp exp

exp exp(11)

where the boundary conditions are

is the offset voltage, is the metal semiconductorwork function difference, is the conduction band edgediscontinuity between InAlAs and InGaAs materials, and


Fig. 2. Offset voltage versus radiation flux density.

is the sheet concentration of 2-DEG at the heterojunction. Thetotal charge in 2-D gas is thus obtained using (11) as

(12)

The total charge includes charge due to surface, bulk, and thecharge due to photo generation.

The offset voltage is calculated as follows.From Fig. 1,

(13)

Furthermore, [9] is also given by

(14)

Using the expressions for excess carrier concentration fromthe Appendix, we write

(15)

being the photogenerated term as given in the Appendix.The sheet concentration is given by

(16)

where

(17)

is the photovoltage across the heterojunction and isthe voltage generated due to the excess carrier.


Fig. 3. Sheet concentration versus gate voltage at different radiation flux density.

IV. CURRENT–VOLTAGE RELATION

The current in the 2-DEG electron gas at the interface ingiven by

(18)

where . Equation (18) covers both low field andhigh field regions. Integrating from to , beingthe gate length

ln

and , and beingthe source and drain parasitic resistances. Thus

ln (19)

Equation (19) represents the current–voltage ( ) relationfor the MODFET under optically illuminated condition. Thetransconductance is obtained by differentiating (19) with re-spect to keeping constant.

V. RESULTS AND DISCUSSIONS

Numerical calculations have been carried out for aIn Al As/In Ga As/InP MODFET considering the


Fig. 4. Drain source current versus drain voltage under optical illumination and dark for a gate voltageVG = 0:4 V.

optical effect. The dimensions and other basic parameters usedin the calculations are given in Table I.

Fig. 2 represents the plot of offset voltage of the deviceagainst the radiation flux density. Offset voltage physicallymeans that one has to apply a higher gate voltage for theformation of 2-DEG at the interface of the heterojunction.It is clearly indicated that we need higher gate voltage forthe creation of 2-DEG because of the application of radiationin MODFET as increases with flux density. This hasbeen observed in earlier results also [10]. Fig. 3 is the plot ofsheet concentration versus gate voltage at different radiationflux density including the dark case . At a fixedgate voltage, we have more sheet concentration as we goon increasing the radiation intensity. maintains a linearrelation with .

Fig. 4 represents the – characteristics for InAlAs/InGaAs/InP MODFET both under optical illumination and

dark for a gate voltage V. We have significantincrease in the current with a higher pinch off as fluxdensity increases. This indicates the sensitivity of thedevice to radiation and also the amplifying and switchingcapability of the device with respect to incident opticalpower density. Fig. 5 shows the transconductance of thedevice as a function of gate voltage at dark and at differentilluminating conditions indicating a significant enhancementin the maximum cut off frequency of the device due toillumination.

Lastly, Fig. 6 represents a comparison of– charac-teristics of our results with the published data under darkcondition. Since the experimental results are not available forInAlAs/InGaAs/InP MODFET under illumination, we havecompared our results under dark for gate terminal open cir-cuited. A good agreement is observed between our calculatedresults with those given in [7].


TABLE IPARAMETER VALUES USED FORCALCULATION

VI. CONCLUSION

In Al As/In Ga As/InP MODFET with partialdepletion condition of the active layer has been analyzed underthe condition of optical illumination. The offset voltage, sheetconcentration of 2-DEG,– and transconductance have beenplotted and discussed. Results of– is compared under darkwith already published data [7] showing a good agreement.No experimental data is available for InAlAs/InGaAs/InPMODFET under optical illumination. The device will workas a high-speed photodetector and amplifier in MMIC andcommunication systems.

APPENDIX

Calculation of excess charge due to illumination and relatedparameters.

From (5) and (6), the solution for total excess electronconcentration due to optical, absorption is given by

(A1)

where

(A2)

(A3)

(A4)

Similarly for holes

(A5)

where

(A6)

and are similar to those of electron except thatandare replaced by and where


Fig. 5. Transconductance versus gate voltage at different radiation flux density.

2-DEG Charge Control Regime

The voltage developed across the InAlAs/InGaAs interfaceis given by

(A7)or

where

(A8)


Fig. 6. Comparison of the drain source current versus drain source voltage under dark condition at zero gate voltage —— represents results and� � �

correspond to Chenet al. [7].

(A9)

is the voltage contributed by the excess carriers generateddue to photoabsorption.

REFERENCES

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

[2] 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.

[3] 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.

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

[5] R. N. Simons, “Microwave performance of an optically controlledAlGaAs/GaAs high electron mobility transistor and GaAs MESFET,”IEEE Trans. Electron Devices, vol. 34, p. 1444–1445, 1987.

[6] A. A. Desallers and M. A. Romero, “Al0:3Ga0:7As/GaAs HEMT’sunder optical illumination,”IEEE Trans. Microwave Theory Tech., vol.39, p. 2010–2017, Dec. 1991.

[7] J. W. Chen, M. Thurairaj, and M. B. Das, “Optimization of gate-to-drain separation in submicron. Gate-length modulation doped FET’s for


maximum power gain performance,”IEEE Trans. Electron Devices, vol.41, pp. 465–475, Apr. 1994.

[8] H. Mitra, D. P. Singh, and B. B. Pal, “Effect of signal-modulated opticalradiation on the characteristics of a MODFET,”Appl. Phys. A, vol. 56,pp. 335–341, 1993.

[9] B. B. Pal and H. Mitra, “Enhanced optical effect in a high electronmobility transistor device,”Opt. Eng., vol. 32, no. 4, pp. 687–691, 1993.

[10] B. B. Pal, H. Mitra, and D. P. Singh, “Enhanced optical effect in a high-electron mobility photo-transistor device: Two-dimensional modelingconsidering a realistic velocity-field related,”Opt. Eng., vol. 33, no.4, pp. 1250–1254, 1994.

[11] G. George and J. R. Hauser, “An analytic model for MODFET capac-itance voltage characteristics,”IEEE Trans. Electron Devices, vol. 37,pp. 1193–1198, May 1990.

[12] R. A. Giblin, E. F. Scherer, and R. L. Wierich, “Computer simulationof instability and noise in high-power avalanche devices,”IEEE Trans.Electron Devices, vol. ED-20, pp. 404–418, 1973.

H. Mitra received the B.Sc. and M.Sc. degreesin physics in 1982 and 1985, respectively, fromBanaras Hindu University, Varanasi, India, and thePh.D. degree in 1995 from Purbanchal University,Jaunpur, India.

He joined Diesel Locomotive Works (Indian Rail-ways), Varanasi, India, in 1987, where he worksas an Account Assistant. His areas of researchinterest are optoelectronic and high-speed devicesusing III–V compounds. He has published morethan 15 papers in various journals and conferenceproceedings.

B. B. Pal was a gold medalist at the First Examination of the Board ofSecondary Education, State Assam, India, in 1962. He received the B.Sc.Honors degree in physics from Presidency College, Calcutta, India, in 1965,and the B.Tech., M.Tech., and Ph.D. degrees from the Institute of RadioPhysics and Electronics, Calcutta, in 1967, 1968, and 1975, respectively. HisPh.D. thesis on Avalanche Transit Time Diodes included the Double AvalancheRegion (DAR) IMPATT diode which was a new proposition.

He was a Lecturer at the Institute of Radio Physics and Electronicsfrom 1971 to 1979. He became a Reader at the Department of ElectronicsEngineering, Institute of Technology, Banaras Hindu University in 1979 wherehe was promoted to Professor in 1986. He worked as the Chairman of thedepartment from 1988 to 1990. During his chairmanship, the departmentwas recognized as the Centre for Advanced Study in Microelectronics andMicrowave Engineering. He worked extensively on heterojunction IMPATT’s,DAR IMPATT’s, superlattice APD’s, MESFET, HEMT, optical effect inMESFET (OPFET), and HEMT. His present areas of interest are high-speeddevices, optical effects in devices, and computer-aided simulation. He haspublished more than 150 papers in journals and conference proceedings.

Dr. Pal was the guest editor of three special issues of theJournal ofthe Institution of Electronics and Telecommunication Engineers. He was aFoundation Life Member of Semiconductor Society (India) in 1984 and wasits Vice President for four years starting in 1989. He was a Fellow of OpticalSociety of India. He also founded Varanasi Chapter of Semiconductor Society(India) in 1984 and was its Chairman for several years. He was a foundationmember of Bangiya Samaj, Varanasi, India. His name is included in Asia’sWho Who, World’s Who Who, ABI’s Directory, and ABI’s Research Boardof Advisors.

S. Singh received the B.Sc. and M.Sc. degrees inphysics from Agra University in 1967 and 1969,respectively, and the Ph.D. degree in physics fromBanaras Hindu University, Varanasi, India,, in 1980.

He joined the Department of Physics, Udai PratapCollege, Varanasi, as a Lecturer in 1970. From 1981to 1983, he served as a Lecturer at the Department ofPhysics, University of Nairobi, Kenya, East Africa.He has been to I.C.T.P., Trieste, Italy, twice; fromJuly 11 to Sept. 9, 1983, and from April 26 to Aug.15, 1984. Presently, he is a Senior Member of the

teaching staff at the Department of Physics, Udai Pratap College (AutonomousInstitution). He is also working as a part-time Academic Counselor inphysics at Varanasi Study Centre of Indira Gandhi National Open University,New Delhi, India. His research interests lie in phonon transport in dopedsemiconductors and semiconductor devices physics. He has published morethan 30 papers in journals and conference proceedings.

Dr. Singh is a Life Member of the Semiconductor Society India, IndianPhysics Teachers Association, and Indian Physics Association.

R. U. Khan received the B.E., M.E., and Ph.D.degrees in electronics engineering from the Instituteof Technology, Banaras Hindu University, Varanasi,India, in 1971, 1973, and 1987, respectively.

He joined the Department of Electronics Engi-neering, Institute of Technology, Banaras HinduUniversity as a Lecturer in 1980. His present areasof interest are millimeter wave solid-state devices,photodetectors, MESFET’s MOSFET’s, and III–Vsemiconductor compounds. He has published morethan 25 papers in journals and conference proceed-

ings.Dr. Khan is a Foundation Life Member of the Semiconductor Society, India.

optical effect in inalas/ingaas/inp modfet

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