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Solid-Bate Electronics Vol. 38, No. 1, pp. 255-257, 1995 Copyright 0 1995 Elsevier Science Ltd

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NOTE

AN OPTICALLY CONTROLLED InP MIS CAPACITOR

(Received 26 October 1993; in revised form 16 May 1994)

INTRODUCTION

Optical effects in MIS device have been an area of increasing interest and a number of works have been reported for semiconductor materials with direct bandgap as well as with iso-electronic trap centres in the indirect bandgap ma- terials[l4]. Among the III-V semiconductors InP MIS configurations utilizing A&O, layer as the insulator have drawn special attention in recent years. However, at the interface between InP crystal and insulator film, there exist a considerable number of surface states and their energy distribution widely spreads throughout the crystal forbidden gap. It has been reported that a large amount of surface state density prevails below mid-gap energy, while the density is rather low near the conduction band edge. This makes n-channel inversion type InP MIS configuration more attractive than a p-channel one[5]. In the present paper we have examined theoretically the effect of illumination on the characteristics of an InP/AI,O, MIS structure. The structure is similar to a conventional MIS configuration except for the fact that the former uses a semi-transparent metal gate. This type of gate can be obtained by making the gate metalisation very thin (u 200A). The theoretical result presented in this paper is based on the semi-numerical model[l] outlined in the next section.

THEORY

The structure under consideration has been shown as an insertion in Fig. 1. Application of a large positive voltage on the metal gate causes strong inversion to occur in the semiconductor at the interface. When an optical radiation of suitable wavelength shorter than the absorption edge of InP, is incident on the semi-transparent metal gate, a portion of the incident power is transmitted through the Al,O, layer into the semiconductor. Absorption of this radiation results in the generation of excess electron-hole pairs in the de- pletion region. These excess carriers change the minority carrier lifetime significantly enabling the device high fre- quency capacitance to increase in presence of illumination. The excess carriers also reduce the surface potential in the illuminated condition which in turn reduces the width of the depletion region and enhances the capacitance of the device.

For a quantitative study, we begin solving one-dimen- sional Poisson’s equation in the illuminated condition (under steady-state condition) given by:

d2+, G=_pLo (1) c,

where, pt.(x) = q]No - NA + (ppo + Ann) exp(-BU - ($I + An) exp(/?tiL)], is the volume density of the charge in the depletion region under illuminated condition. Here, q is the electronic charge, B = q/H’, k is the Boltzmann’s constant, T is the absolute temperature, L, is the permittivity of the semiconductor, No and NA are donor and acceptor concen- tration respectively, tjL is the potential at any point in the illuminated condition, np and p,,,, are the equilibrium elec- tron and hole concentration at the edge of the depletion

region in the substrate, respectively. An is the excess elec- tron-hole pairs generated per unit volume due to absorption of incident optical power and is given by[l]:

An = tLJ’op,U - &)(I - R,)(r - Rm) hvW,,,

[I -ev-aW,)l (2)

where Popt is the incident optical power density, a is the optical absorption coefficient of the semiconductor, R,, R, and R, are the reflection co-efficients at the metal-gate, metal-insulator and insulator-semiconductor interface, re- spectively, h is the Plan&s constant, Y is the frequency of the incident radiation, W,,, is the maximum width of the depletion region[l] and tL is the minority carrier lifetime in the illuminated condition[l].

The surface charge per unit area in the illuminated condition under steady-state can be obtained by solving eqn (1) following[l]. as:

exp( - BiL ) + PtirL - 1

Metal

Insulator

Ohmic contact

I .2

0.9

z 0.6

‘;;i ._ 2 2 0.3 a

8 z ; -0.0 WY

-0.3

-0.6 -2.C

I I I I I I I I 1 -1.0 0.0 I.0 2.0 3.0 4.0 5.0 6.0

Applied voltage (V)

Fig. 1. Variation of surface potential with applied voltage for dark and various illuminated conditions along with a

schematic of the structure shown as an insertion.

255

256 Note

with negative sign for (IrL > 0 and positive sign for $IL < 0, where, $IL is the surface potential in the illuminated con- dition, L,, is the extrinsic Debye length in the illuminated condition[ I].

The applied voltage in the illuminated condition can be written as:

~=~(Q~+Q~~-Q~+~~. (4)

where t, is the permittivity of the insulator (A&O,), d is the thickness of the insulator, Q, is the fixed charge per unit area located at the Al,O,/InP interface, Q,, is the charge due to additional midgap states in the insulator.

The differential depletion layer capacitance per unit area at the low frequency can be found as:

(5)

The high frequency capacitance can be obtained by using Lindner’s formula[l] given by:

where C,, is the flat-band capacitance and $,,, is the match point as discussed in[l].

The total device capacitance per unit area under illumina- tion can be obtained by considering the effect of insulator capacitance, C, in series with Co,. The value of the low and high frequency capacitance of the device in the dark con- dition can be obtained in a similar way by assuming P,,, = 0.

RESULTS OF NUMERICAL CALCULATION AND DISCUSSION

Computation starts with the numerical solution of eqn (4) by using Newton-Raphson technique. Equation (4) is solved iteratively to find an accurate value of $sL corre- sponding to each value of the applied voltage. The process is repeated for dark condition assuming P,,, = 0. Calcu- lations have been carried out for an InP/Al?O, MIS capaci- tor at 300 K. The doping concentration in p-InP has been assumed to be 10”/m3 and the thickness of the insulator has been assumed to be 6OOi(. The value of the surface-state density at the Al,O,-InP interface has been taken to bc 10”/m2/eV near the conduction band[5].

1350

i? 2 750 3 2 600 z? ‘2 450 P u 300

Low frequency

’ ’ ’ ’ ’ ’ ’ ’ ’ -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Applied voltage (V)

Fig. 2. Variation of the low and high frequency capacitance (per unit area) with applied voltage for dark and various

illuminated conditions.

800 High frequency

Applied voltage = 0.8V

01 0.03 30 3 x 103 3 x 105

Optical power density (W/m’)

Fig. 3. Variation of the high frequency capacitance (per unit area) with optical power density at a constant applied

voltage.

Figure I shows the variation of the surface potential with the applied voltage for dark and various illuminated con- ditions. It can be easily seen that for negative applied voltage (in the accumulation mode), the surface potential is not affected by illumination. The reason is that in the accumu- lation mode the optically generated carrier are negligible in comparison with the majority charge carriers. On the other hand, the surface potential decreases significantly in the illuminated condition for positive applied voltage. It is seen from Fig. 1 that for a constant value of the positive applied voltage, the decrease in surface potential strongly depends on the incident optical power density. The decrease in surface potential is due to the accumulation of excess photogenerated electrons in the inversion layer under illumi- nation.

Figure 2 shows the variation of both the high and low frequency capacitance of the device with voltage in dark and illuminated condition. It is found that the high frequency capacitance of the device is significantly affected by illumi- nation in the strong inversion region and the value of the capacitance tends to the value Ci with the increase in the incident optical power density. This may be accounted for by the fact that the excess photogenerated carriers reduce the lifetime of the minority carriers in the illuminated condition and makes the generation-recombination process much faster in the illuminated condition than in the dark condition. This enables the device to follow high frequency measuring a.c. signal in the illuminated condition. The dependance of the high frequency capacitance of the device in the strong inversion region on the incident optical power density is depicted in Fig. 3. For an applied voltage of 0.8 V, the high frequency capacitance of the device can be varied from 145 pF/m2 to 333 pF/m* by changing the incident optical power density from 30 mW/m* to 3000 W/m2. Thus the device can be used as an optically controlled capacitor to perform a variety of functions[l].

CONCLUSION

The paper presented here shows that InP/AI,O, MIS structure can be used as an optically controlled capacitor by making the gate semi-transparent. The faster generation- recombination process in the illuminated condition has been

Note 257

found to be responsible for the increase in the high frequency capacitance of the device in the strong inversion region.

Department of Electronics Engineering B. K. MlSHRAt

Institute of Technology P. CHAKRABARTI

Banaras Hindu University Varanasi-221005 India

tThe author is with the Department of Electronics Engin- eering, Bhilai Institute of Technology, Bhilai House, Durg, Pin-490001, India.

REFERENCES

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2. J. Grosvalet and C. Jund, IEEE Trans. Electron Devices ED-14, 777 (1967).

3. A. Sher, Y. H. Tsuc and John Moriarty, J. appl. Phys. 51, 2137 (1980).

4. P. Chakrabarti and B. K. Mishra, 1992 Asia-Pact&- Microwave Conf. Proc. 1, 383 (1992).

5. E. Yamaguchi and T. Kobayashi, Jap. J. appl. Phys. 21, 104 (1982).