IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 47, NO. 7, JULY 2000 1469

Light Dependence of SOI MOSFET with NonuniformDoping Profile

George K. Abraham, B. B. Pal, and R. U. Khan

Abstract—The light dependence of a fully depleted short channel sil-icon-on-insulator (SOI) MOSFET is investigated. As the photon flux den-sity increases, there is a lowering in the surface potential barrier called thephoton induced barrier lowering (PIBL). The threshold voltage shows alogarithmic reduction with the increase in the incident flux density. Thedrain source current and the transconductance significantly increase underthe incident optical flux density. The device will be useful for high speed ap-plication in optical systems.

Index Terms—OPFET, SOI MOSFET.

I. INTRODUCTION

Silicon-on-insulator (SOI) devices are very attractive candidates forhigher performance of VLSI. SOI devices offer some interesting fea-tures, such as reduction of junction capacitance, increase of packingdensity (compared to bulk devices), suppression of latch up, reductionof short channel effects, and radiation hardness [1].

The photosensitivity of metal insulator semiconductor (MIS) struc-ture has opened up the possibility of their use as infrared photode-tectors, optically variable reactors, and optical CCD’s for solid stateimaging [2]–[4]. This basic MIS structure can also be used to realizeoptically gated MISFET photodetectors.

The possibility of SOI MOSFET as an optically sensitive device waspresented by Boschet al. [5]. Werneret al. [6] realized a comparatorcircuit making use of the threshold voltage shift due to optical illumi-nation on SOI MOSFET. The realized circuit allows the integration ofthe optically sensitive device without any additional process steps. Inthis context, the realization of simple low cost optical switches and de-tectors are of great importance. The possibility of SOI MOSFET as aphotodetector is a most important field of study.

In this brief, we have considered the optical dependence of anonuniformly doped SOI MOSFET and solved the two-dimensional(2-D) Poisson equation for the surface potential and threshold voltage.The variation of surface potential with channel position, and thresholdvoltage with channel length with different flux densities have beenstudied. Studies have also been made on the drain current andtransconductance of the SOI MOSFET under dark and illumination.

II. THEORY

The schematic structure of an illuminated SOI MOSFET is shown inFig. 1 along with the coordinates. The illumination is incident on the de-vice at a perpendicular direction to the surface of the transparent/semi-transparent metal gate. The intensity of illumination is uniform alongthe direction parallel to the surface. Thus, the generation rate variesonly in the verticalx direction and decreased exponentially. The ex-cess carries generated also vary alongx direction and is uniform inydirection. The illumination causes production of excess electron holepairs (E–H pairs) in the silicon film. The holes move toward the sourceand electrons move toward the drain under the influence of drain source

Manuscript received May 28, 1999; revised December 27, 1999. The reviewof this brief was arranged by Editor J. M. Vasi.

The authors are with the Department of Electronics Engineering, Institute ofTechnology, Banaras Hindu University, Varanasi 221 005, India (e-mail: bb-paul@banaras.ernet.in).

Publisher Item Identifier S 0018-9383(00)04269-6.

Fig. 1. Four terminal n-channel SOI MOSFET structure.

voltage. A photo voltage (Vop) across the source film junction is devel-oped increasing the effective gate voltage. The doping concentration ofthe film is considered to be non uniform and assumed a Guassian dis-tribution type [7]. In subthreshold and linear regimes with small drainto source voltage, the drain current is small and the Poisson’s equationalone is sufficient [8] to describe the device properties. At low drainsource voltages the potential�(x; y) can be approximated by a simpleparabolic function [9]. Solving the Poisson’s equation under illumina-tion by applying the appropriate boundary conditions given in [9], thesurface potential distribution in the silicon film is obtained as

�f(y) = A:0 exp �ypaf + A1 exp y

paf � �Df + �If (1)

where we have (2.a)–(2e), shown at the bottom of the next page.NA(0)is the value of the impurity distributionNA(x) at the surface, atx = 0.

We have assumed that the silicon film is thin enough to be fullydepleted and the front channel turns ON before the back channel. Thethreshold voltage is taken to be that value of gate voltage for which thedifference between the extrinsic Fermi level and the intrinsic fermi level'f is equal to the surface potential minimum. The threshold voltage iscalculated as

Vth = [Vfbf + �NA(0)]�A2 + A3 � 'f

1� A4

� [�VI + �If ] (3)

where

A2 =�bi � (�bi + Vds) exp �Lpaf

1� exp �2Lpaf� exp �ymin

paf (4a)

A3 =(�bi + Vds)� �bi exp �Lpaf

1� exp �2Lpaf� exp ymin

paf exp �L

paf (4b)

and

A4 =1� exp �Lpaf1� exp �2Lpaf

exp �ymin

paf

+ exp ymin

paf � L

paf (4c)

� =tsitfq

"oxand �VI = �� � + Vop � �If : (4d)

ymin is the channel position where the front channel potential is min-imum. The last term in the equation (3) represents the reduction in thethreshold voltage due to illumination.

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1470 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 47, NO. 7, JULY 2000

For an n channel SOI MOSFET the current in the front channel isgiven by

ID =W�e�(y)jQI(y)j d�f(y)dy

(5)

whereW is the width of the channel and�e� (y) is the effective mo-bility of the electrons in the channel.QI(y) is the charge in the inver-sion layer and is given by

QI(y) = �Cf [Vgs + Vop � Vfbf � �f(y) + Vsub]�QBI(y): (6)

QBI(y) is the depletion layer charge density considering the effect ofillumination, and can be calculated as

QBI = Cf'f + Cb(Vsub � Vfbf) +A2 +A3 � 'f

1�A4

� Cf � qNA(0)tsi +�QBI (7)

where�QBI is the change in the depletion charge density due to illu-mination and is given by

�QBI =q� �tsiCf

+ Vop Cf :

Integrating (5) from source (y = 0) to drain (y = L), the total draincurrent is obtained as in (8), shown at the bottom of the page, where

� =WEc"si�no

LCf

and K = 1� (aVdS) (9a)

a is an empirical constant [10]

�D1 =Cf

Ec"si(�bi � Vgs + Vfbf + Vds) (9b)

�D2 =Cf

Ec"si(�bi � Vgs + Vfbf)

and

�I =Cf

Ec"siVop: (9c)

The transconductance of the device is obtained as

gm =@IDI

@Vgs V =constant: (10)

The values of the parameters used in the calculations are given as fol-lows.L = 0:3 �mNA = 7:939 � 1022=m3

tsi = 500 A0

tf = 200 A0

tb = 4000 A0

Vsub = 0:0 VQ = 2:0 � 1016 ions/m2

Rp = 5:3 � 10�2 �m� = 3:8 � 10�2 �m"si = 1:04 � 10�10 F/m"ox = 0:34 � 10�10 F/mT = 300Ka = 0:633Ec = 6:01 � T 1:5 � 102Vm�1

� = 1:0 � 104 m�1

�n = 2:5 � 10�3 sW = 5:0 �m

III. RESULT AND DISCUSSIONS

For a channel length of 0.3�m the variation of the surface potentialalong the channel in both dark and illuminated conditions are studiedand presented in Fig. 2. The difference between the minimum poten-tial in the channel and the source potential decides the potential barrierwhich is present in the channel against the flow of electrons. With theincrease in the photon flux density there is a reduction in the potentialdifferences between the channel potential minimum and the source po-tential. This may be called photon induced barrier lowering (PIBL).

Calculations have been carried out for threshold voltage of thedevice under both dark and illumination. The plot ofVth versus thechannel length is shown in Fig. 3. Illumination causes a reduction inthe threshold voltage. In other words, the channel will be conductivefor lesser gate voltage under illumination. From the graph, it is clearthat as the flux density increases, the threshold voltage decreases, butnot in same proportion. There is an approximate logarithmic variationsimilar to those reported earlier [6].

Fig. 4 shows the variation of drain current versus channel voltagewith photon flux density as a parameter. As we increase the photonflux density the current increases. The pinch off voltage beyond which

A0 =(�bi + �Df � �If)� (�bi + �Df � �If + Vds) exp �Lpaf

1� exp �2Lpaf(2a)

A1 =(�bi + �Df � �If + Vds)� (�bi + �Df � �If) exp �Lpaf

1� exp �2Lpafexp �Lpaf (2b)

�Df =bDf

af�If =

bIfaf

(2c)

bD =q

"siNA(0)� af (Vgs � Vfbf) (2d)

bIf =q� �

"si+ afVop and af=

"ox"sitsitf

(2e)

IDI = �KEc"sif(�2D � �1D)� [ln(1� �D1 + �I)� ln(1� �D2 + �I)]g�f(CfVsub +QBI)fln(1� �D1 + �1)� ln(1� �D2 + �I)gg (8)

IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 47, NO. 7, JULY 2000 1471

Fig. 2. Surface potentialV channel position under dark and differentilluminating conditions.

Fig. 3. Threshold voltageV channel length under dark and illumination.

Fig. 4. Drain currentV drain voltage at dark and for two different photon fluxdensities.

the current is saturated increases with photon flux density. The satura-tion current can be controlled by varying the flux density. The variationof transconductance under dark and illuminated conditions are givenin Fig. 5. The transconductance curve shows an increase in nonlinearfashion with gate voltage. AsVgs increases, more and more electronsare attracted toward the surface, resulting in an increase in the conduc-

Fig. 5. Transconductance as a function of gate source potential under dark andilluminating conditions.

tion. Illumination causes higher value of transconductance comparedto dark condition.

IV. CONCLUSION

Theoretical investigation on the effect of illumination on SOIMOSFET with Guassian distribution of impurity profile has beencarried out. Illumination causes reduction in the potential barrier dueto PIBL.Threshold voltage variation shows a logarithmic reductionagainst photon flux density. The drain current and the transconduc-tance are increased by illumination and the saturation of the draincurrent can be controlled by photon flux density. These attractivefeatures of SOI MOSFET makes it potential candidate as a photosensitive device. Furthermore, the advantages of SOI-based opticalsensors lies in the higher integration density. The device will thereforebe useful in optical integration and optical computation.

REFERENCES

[1] J. P. Colinge, “Some properties of thin film SOI MOSFET’s,”IEEE Cir-cuits Devices Mag., vol. 3, pp. 16–20, 1987.

[2] J. Grosvalet and C. Jund, “Influence of illumination on MIS capaci-tance in the strong inversion region,”IEEE Trans. Electron Devices, vol.ED-14, pp. 777–780, 1967.

[3] A. Sheret al., “Si and GaAs photocapacitive MIS infrared detectors,”J.Appl. Phys., vol. 51, pp. 2137–2148, 1980.

[4] B. B. Pal and S. S. Nelson, “Optoelectronc charge coupled devices(OECCD’s) using InP capacitor arrays,”IEEE Trans. Electron Devices,vol. 40, pp. 1878–1880, 1993.

[5] M. A. Bosch, D. Herbst, and S. K. Tewsbury, “The influence of light onthe properties of NMOS transistor in Laser�-zoned crystallized siliconlayer,” IEEE Electron Device Lett., vol. EDL-5, pp. 204–206, June 1984.

[6] R. Werner, C. Zimmermann, and A. Kalz, “Light dependence of partiallydepleted SOI MOSFET’s using SIMOX substrates,”IEEE Electron De-vices, vol. 42, pp. 1653–1656, Sept. 1995.

[7] S. M. Sze,Physics of Semiconductor Devices, 2 ed. New York: Wiley,1981, p. 469.

[8] J. A. Greenfield and R. W. Dutton, “Non planar VLSI devices analysisusing the solution of Poisson equation,”IEEE Trans. Electron Devices,vol. ED-27, p. 1520, 1980.

[9] K. K. Young, “Short channel effect in fully depleted SOI MOSFET,”IEEE Trans. Electron Devices, vol. 36, p. 394, Feb. 1989.

[10] V. Aggrawal and R. S. Gupta, “A two dimensional short channel modelfor the drain current voltage characteristics of fully depleted SOIMOSFET,” Int. J. Electron., vol. 79, pp. 293–301, 1995.