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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: [email protected] ).
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: [email protected] ).
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:
M. Pandavoine is with Universite de Bourgogne, France (e-mail:
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
ATE-80115029 ©Asian-Transactions 62
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.
Asian Transactions on Engineering (ATE ISSN: 2221-4267) Volume 01 Issue 02
ATE-80115029 ©Asian-Transactions 63
Fig.2. The 4x4 matrix TFTs with current mirror.
Asian Transactions on Engineering (ATE ISSN: 2221-4267) Volume 01 Issue 02
ATE-80115029 ©Asian-Transactions 64
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
Asian Transactions on Engineering (ATE ISSN: 2221-4267) Volume 01 Issue 02
ATE-80115029 ©Asian-Transactions 67
Fig. 9. Response in dynamic response of active matrix with several pixels