Amorphous SiGe:H TFTStructure and Operation
By,
S ARCHANA
Introduction
Objectives
Working of IR touch screen
Basic structure and operation of TFT
Amorphous Si and DOS model
Cross section of a-SiGe:H TFT
Optical characteristics
Photon incidence on tft
Results using TCAD Silvaco
Photo-stress effects
Limitation and future work
Conclusion
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Investigate application of hydrogenated amorphous SiGe thin film transistors (TFTs) as photo sensor for touch sensing displays.
Study the structure, operation and characteristics of a-SiGe:H TFT.
Study the optical characteristics, photo response and stress on device.
Study the results of photon generation of active layer using TCAD Silvaco.
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Touch-Screen displays-user-friendly interface for portable electronic devices and large-sized public displays.
Photo-type touch sensing (a-Si:H)-limitations
Difficult to distinguish between the shadow of, touching object and of a proximate one in bright ambient light.
Touch sensing impossible if on-screen image is black in backlight reflection mode.
Key to solving such problem-infrared (IR) as a touch sensing light source
a-SiGe:H photosensor-good photo sensitivity to the wavelength of IR range.
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Inverted-type OLED integrated TFT
Hydrogen passivated dangling silicon bonds, reduced the defect density
Fairly high carrier mobility
Strong photoconductivity
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PARAMETERS a-Si
Field Effect Mobility
(CM2 /VS)
0.5~1
Process Temperature (0 C) ~3500 C
TRANSPARENCY
(%)
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Density of electronic
states
Three main mechanisms of electron
transport
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Transfer characteristics
under IR and visible light
illumination.
(a)(c): Images from an a-Si visible sensor. (d)(f) images from an a-SiGe IR sensor under various
ambient conditions, the bright [470 lx, (a) and (d)], normal [170
lx, (b) and (e)], and dark [1 lx, (c) and (f)].
(g)The original images of stripe and QR code, and (h) their
scanned touch images captured from the image
grabber.
Double-gated TFT with an etch-back process
SiNx deposited on the glass substrate by
PECVD
a-SiGe:H deposited as an active layer
Mo deposited by sputtering and patterned,
forming source/drain electrodes
SiNx as a gate insulator and top gate
electrode with Mo
a-SiGe:H layer not only activates to the IR but
also to the visible light, causing the significant
photo current
To avoid the noise by visible light bottom gate
made of amorphous Si:H based material was
added
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Photo response curves in a-SiGe:H TFT stacked with
(a) normally white and
(b) normally black LCD panel in order to observe the effect of display content
dependence during the operation of LCD.
Twisted Nematic LCD
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Schematic 4
blocks operation
drawings
(a) Sensor image
when IR
LED was on
(b) sensor image
with IR LED off
(c) Ambient light
cancelled image
by subtracting the
previous two
images.
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Schematic band diagrams of a-SiGe:H double gate TFT for
(a) vertical and
(b) horizontal cross section in the active area with the bias condition of VG < VFB and Vds > 0.
(c) Corresponding region in TFT.
(d) Principle of touch sensing by photosensor embedded in display panel.
2D Simulation using SILVACO
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The photon
is incident from
(a) top-down
direction
(b) bottom-up
directions.
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Transfer characteristics
from the TFT photosensor
with source/drain
electrodes made of
(a) metal
(b) ITO for the purpose of
identifying the
dependence of the photon
incidence directions.
The external light was
given by both the bottom-
up and the top-down
directions.
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Transfer curves under IR
illumination (850 nm) and
dark state with respect to
the thickness of a-SiGe:H
active layer
Dependence of a-SiGe:H
thickness on the change of
photo sensitivity with
varying gate biases at
Vds = 5 V.
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(a) Change in the drain current as a function of stress time under a bias of Vgs=-5 V, Vds=5V with IR light soaking.
(b) The net change in the drain current in a-SiGe:H TFTs as a function of IR light soaking time. The isothermal
annealing was conducted at 50, 75, and 100 C, respectively.
(c) Logarithmic lifetime plots as a function of the reciprocal of the annealing temperature.
(d) The predicted lifetime as a function of the net changes in the drain current at 40 C
with electrical biases and IR light soaking. The captured image from the image grabber is included in the inset.
Low mobility
High dark current
Current a-SiGe:H TFT structure should be
further improved and optimized
Further study of electrical properties
Methods to reduce dark current
Modelling of defect states
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A a-SiGe:H TFT structure and various
properties was studied.
Thus a-SiGe is a strong candidate material
for an IR photosensor for future embedded
touch screen panel.
Device exhibits potential for broad use in
applications of large-sized touch screen
displays such as indoor e-boards as well as
outdoor information displays.
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