Asian Transactions on Engineering (ATE ISSN: 2221-4267) Volume 01 Issue 02

ATE-80115029 ©Asian-Transactions 60

Abstract— The heart of the flat panel digital detector consists

of a two-dimensional array of amorphous silicon photodiodes

and thin-film transistors (TFTs), all deposited on a single

substrate. We designed 4x4 matrix TFTs arrays using current

mirror amplifiers. Advantages of current mirror amplifiers are

they need less requiring switches and the conversion time is

short. The TFT arrays 4x4 matrixes using current mirror

circuits have been fabricated and tested with success. The TFT

array directly can process signals coming from 16 pixels in the

same node. This enables us to make the summation of the light

intensities of close pixels during a reading.

Index Terms— Current mirror, pixels, TFT arrays.

I. INTRODUCTION

Flat panel imagers are based on solid-state integrated

circuits (IC) technology, similar in many ways to the imaging

chips used in visible wavelength digital photography and

video. The heart of the flat panel digital detector consists of a

two-dimensional array of amorphous silicon photodiodes and

thin-film transistors (TFTs) [1], all deposited on a single

substrate. Utilizing thin film technology similar to that used in

the fabrication of integrated circuits, layers of amorphous

silicon and various metals and insulators are deposited on a

glass substrate to form the photodiodes and TFTs matrix, as

well as the interconnections, and the contacts on the edges of

the panel technologies. At the core is an amorphous-silicon

TFT/photodiode array. The TFT/photodiode matrix [2][3] is

normally scanned progressively as shown in figure 1, one line

at a time from top to bottom.

The TFTs are essentially switches. When a large positive

voltage is applied to one of the gate lines, the switches (TFTs)

in the selected row are closed, causing them to conduct

electricity. With the TFTs energized, each pixel in the

selected row discharges the stored signal electrons onto the

data line. At the end of each dataline is a charge integrating

amplifier which converts the charge packet to a voltage. At

This work was supported in part by the Gunadarma Foundations.

N.S. Salahuddin is with the Department of Computer Engineering,

Gunadarma University, Jl. Margonda Raya No.100, Depok 16424, Indonesia

(principal corresponding author, e-mail: sultan@staff.gunadarma.ac.id ).

E.P. Wibowo, is with the Doctoral Program in Information Tehcnology,

Gunadarma University, Jl. Margonda Raya No.100, Depok 16424, Indonesia

(Also corresponding author, e-mail: eri@staff.gunadarma.ac.id ).

A.B. Mutiara is with the Faculty of Computer Science and Information

Technology, Gunadarma University, Jl. Margonda Raya No.100, Depok

16424, Indonesia (Also corresponding author, e-mail:

amutiara@staff.gunadarma.ac.id.).

M. Pandavoine is with Universite de Bourgogne, France (e-mail:

paindav@u-bourgogne.fr).

This point the electronics systems vary by manufacturer, but

generally there is a programmable gain stage and an

Analog-to-Digital Converter (ADC), which converts the

voltage to a digital number. One important aspect of the

scanning is that the FPD is continuously collecting the entire

incident signal; none is lost even during the discharge of the

pixel. The FPD is an integrating detector and the integration

time for each pixel is equal to the frame time.

Fig.1. TFT/Photodiode array schematic.

We have designed 4x4 matrix TFTs using current mirror

[4] amplifiers as shown in figure 2. Current mode was chosen

because it’s usually offers the advantages of less requiring the

switches (TFTs) and the conversion time is short. These

characteristics will be particularly beneficial at the time of the

design of the subsequent circuits on the way of data, mainly

the converters analog/digital.

Fig. 2. The 4x4 matrix TFTs with current mirror.

Design of Thin-Film-Transistor (TFT) Arrays

Using Current Mirror Circuits

N. S Salahuddin, E.P. Wibowo, A.B. Mutiara, and M. Paindavoine

Asian Transactions on Engineering (ATE ISSN: 2221-4267) Volume 01 Issue 02

ATE-80115029 ©Asian-Transactions 61

But, more important still in our case, this mode enables us

to calculate easily and instantaneously the average of the light

intensities of neighboring pixels in order to modify the

resolution of the image. Indeed, we can sum the signals

directly coming from several pixels in the same node. This

enables us to make the summation of the light intensities of

close pixels during a reading.

II. DESIGN OF THIN-FILM-TRANSISTRO (TFT) ARRAY

The circuit of active matrix composed of one line and one

row with active pixel is illustrated figure 3. It is a completely

symmetrical circuit, established in each one pixel of the

matrix. It includes a PMOS current mirror (M1-M3 are

located in pixel), two NMOS current mirror (M4-M5 and M4'

-M5 ') are common for one line and one row M6-M10 and M6'

-M10 ' are the circuits of the amplifier.

Fig. 3. The Circuit of active matrix.

III. FABRICATION AND TEST RESULTS

The circuits have been designed using the Mentor Graphics

CAD tools [5] in combination with the AMS CMOS 0.6 µm

design kit. This technology uses one layer of ploysilicium and

three layers of metal. Figure 4 shows the drawing of this

circuit, realized with the Mentor software. Figure 5 represents

Photograph of a TFTs chip.

Fig. 4. Layout of the TFTs with 4x4 matrixes in 0.6 µm CMOS process

Fig. 5. Photograph of the TFTs with 4x4 matrixes in 0.6 µm CMOS process

In order to test the circuits of active matrix of flat panel, we

used several photodiodes where each of one is connected to

one input of the circuit. Output of the circuit is connected to a

matrix of LEDs in order to validate this circuit. Figure 6

illustrates the validation method.

Fig. 6 Experimental of active matrix

The principle of the circuit is that if value of the pixel is

higher than the value of reference of tension (Vr), the exit of

the comparator is in a high state (5V). On the other hand, if

value of the pixel is less than the value of the tension Vr, the

exit of the comparator is in a low state (0V). After the

comparison, the exits of the comparator are entered to the

logical gating circuit AND. In the logical gating circuit AND,

if signal comparator from line and row are 5V and

corresponding LED is active. After the test of the response of

our active matrix, we have analysed the dynamic response of

active matrix. We used the luminescent blue diode (source)

driven with an impulse generator as shown in figure 7.

Fig. 7. The luminescent blue diode (source) driven with a impulse generator

Figure.8, shows the results of measurement of the dynamic

responses of active matrix with a blue LED calibrated for an

illumination of 0.4 lux

Asian Transactions on Engineering (ATE ISSN: 2221-4267) Volume 01 Issue 02

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Fig. .8 Response in dynamic mode of active matrix

We measured the signals directly coming from several

pixels in the same node. Figure 9, shows the results of

measurement of the dynamic responses of active matrix.

Fig. .9 Response in dynamic response of active matrix with several pixels

IV. CONCLUSION

We have designed a 4x4 matrix TFTs using current mirror

amplifiers for image sensor. We can sum directly the signals

coming from several pixels in the same node. This enables us

to make the summation of the light intensities of close pixels

during a reading. These results allow us to consider using of

this technology in new the flat panel digital detector.

ACKNOWLEDGMENT

The author would like to thank the Gunadarma

Foundations for financial support.

REFERENCES

[1] GEs Digital Flat Panel Technology,” Digital Flat

Panel”.(http://www.eradiography.net/radtech/f/flat_panel.htm)

[2] VARIAN Medical System, ”Flat Panel X-Ray

Imaging”.http://www.varian.com/xray/pdf/Flat_20Panel_20Xray_20I

maging_2011-11-04.pdf )

[3] Wei Zhao and J.A. Rowlands , ” Digital radiology using active matrix

readout of amorphous selenium: Theoretical analysis of detective

quantum efficiency,” Medical Physics, Vol.24, No.12, December 1997

[4] B. M. Wilamowski, E. S. Ferre-Pikal, O. Kaynak,” Low power, current

mode CMOS circuits for synthesis of arbitrary nonlinear fuctions,” 9th

NASA Symposium on VLSI Design 2000.

[5] Mentor Graphics Corporation. (http://www.mentor.com)

[6] Nur Sultan Salahuddin, Michel Paindavoine, Dominique Ginhac,

Michel Parmentier, Najia Tamda,“A CMOS Image Sensor Array

dedicated to Medical Gamma Camera Application,” In 26th

International Congress on High-Speed Photography and Photonics,

September 2004,.5580-144

N.S. Salahudin was born in Jakarta, Indonesia, in 1968. He obtained a

degree (MT) from the University of Indonesia in the field of optoelectronics

and laser applications, Indonesia, in 2000 and a PhD from the Universite de

Bourgogne in the field of informatics, image and electronics, Dijon, France,

in 2007. Currently he is the head of the center of multimedia studies and

robotics (Mechatronics) at Gunadarma University, Indonesia. His main

interests are control systems and their applications.

Eri Prasetyo Wibowo was born in Kendal, Indonesia, in 1966. He received

the B.S degree in Electronics and Instrumentation from Gadjah Mada

University, Indonesia, in 1992, the M.S. degree in Information System from

Gunadarma University, Indonesia, in 1994 and the Ph.D. degree in

Electronics Informatics from the Burgundy University, France, in 2005. He

was as Secretary of Doctoral Program in Information Technology at

Gunadarma University. He was member of EACOVIROE Project to

promoted Master VIBOT which founded by Erasmus Mundus. His current

interests are in Real time Image processing applications and System On

CHIPs Design.

A.B. Mutiara was born in Jakarta, Indonesia, in 1967. He obtained a

PhD from the Universitaet Goettingen, Germany. Currently he is the Dean of

Faculty of Computer Science and Information Technology at Gunadarma

University, Indonesia.

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Fig.2. The 4x4 matrix TFTs with current mirror.

Asian Transactions on Engineering (ATE ISSN: 2221-4267) Volume 01 Issue 02

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Fig. 3. The Circuit of active matrix.

Asian Transactions on Engineering (ATE ISSN: 2221-4267) Volume 01 Issue 02

ATE-80115029 ©Asian-Transactions 65

Fig.4. Layout of the TFTs with 4x4 matrixes in 0.6 µm CMOS process

Asian Transactions on Engineering (ATE ISSN: 2221-4267) Volume 01 Issue 02

ATE-80115029 ©Asian-Transactions 66

Fig. 8. Response in dynamic mode of active matrix

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Fig. 9. Response in dynamic response of active matrix with several pixels