06_Part_02_Dipsplay And Photosensing Systems

(PART 2)
Field Effect Transistors(FET)
Field effect transistors (FET) consist of a piece of semiconductor made into four regions
that are referred to as the source, drain, gate, and body (Figure 1). The source and drain
components are doped with charge carriers that will provide excess holes (p-doped) or
electrons (n-doped) in the semiconductor. Between the source and drain is the body,
which is
semiconductor that is either non-doped or oppositely doped from the source and drain.
Figure 1: Diagram of a planar Field Effect Transitor (FET) with labeled parts.
FET configurations
The gate is placed near the body region, but separated by an electrical insulator,
(Often oxides) and may be either a conducting metal or semiconductor oppositely
doped from the source and drain. In an n-channel, or npn, FET, applying positive
voltage to (a negative voltage in the case of a hole-doped p-channel, or pnp,
transistor) to the gate, the gate provides an electric field that causes electrons to
be mobile in the body of the transistor, which becomes conductive and even
current amplifying.
Effect of varying the gate voltage
 gate volt controls the thickness of the channel
 consider an n-channel device
 making the gate more positive attracts electrons to the gate and makes the
gate region thicker – reducing the resistance of the channel. The channel is
said to be enhanced
 making the gate more negative repels electrons from the gate and makes the
gate region thinner – increasing the resistance of the channel. The channel is
said to be depleted
Thin Film Transistors(TFT)
The Thin Film Transistor (TFT) is a special kind of field-effect transistors that are made by the
deposition of thin films of the different materials that make up the transistor. In an a-Si TFT, the
conducting carriers are electrons. Electrons drift from the source electrode to the drain, which
results in current flow from drain to source. The switching structure is based on the metal–
insulator–semiconductor (MIS) structure, which is also used widely in the IC industry with
crystalline silicon. In such a structure, the metal can attract and repel carriers at the
semiconductor side, which changes the carrier concentration of the semiconductor. It effectively
engineers the conductance and hence can be used to switch on and off with high and low
conductance, respectively. Although operation modes for crystalline silicon, a-Si and poly-Si are
different, the basic physics is similar.
Schematic representation of and notation for a TFT.
Thin-film transistor characteristics
Typical TFTs can be categorized as normal or inverted structures, depending on whether their
gate is on the top or bottom, respectively.
Another classification depends on whether the drain/source and gate are
on the same or opposite sides; these are called coplanar or staggered structures,
respectively. Hence, there are basically four different kinds of TFT structures. Different
structures may be the result of different fabrication procedures and may result in different
device performance.
Typically, a-Si TFTs use the inverted staggered structure configuration because it makes them
easy to fabricate and gives them better electrical characteristics. The a-Si channel is sensitive
to photons of the external ambient that mainly illuminates from the bottom side in a display
system: an inverted structure can shield the device from this light by virtue of the gate metal.
Non-silicon-based thin-film transistors
this section, will introduce two kinds of TFTs where the active layer consists of organic
molecules and ZnO-based materials. Organic TFT (OTFT) can be fabricated with a low
temperature deposition and a solution process, which is less complex compared with
conventional Si-based technologies. In addition, due to the mechanical flexibility of organic
material, OTFT can be used as the backbone of the flexible display, and this has the benefit
of being rugged, thin and lightweight. Also, it is potentially of low cost due to the possibility
of using the roll-to-roll process. When fabricating a flexible display on a plastic rather than a
glass substrate, one important issue arises concerning the durability of the substrate.
chemical structure of some common organic semiconductors are showed below. Intrinsically, organic
materials are ambipolar which means that they can conduct both holes and electrons. Extrinsically, however,
electron conduction is often several orders of magnitude smaller: first of all, injection of electrons from a
contact into organic materials is more problematic than hole injection. Second, electron-rich molecules and
organic anions are very sensitive to oxidation by water or oxygen and the resulting oxidized molecules have
a strong tendency to trap electrons which effectively lowers electron mobility. In practice, it is very difficult to
exclude water and oxygen and therefore organic molecules are sometimes substituted with electronwithdrawing groups like fluor to make them more oxidation-resistant which often yields higher extrinsic
electron mobilities. Pentacene is one of the most popular and promising ‘small molecule’-semiconductors
with reported p-type room-temperature mobilities typically around 100 cm2/Vs.
P-type organic semiconductor materials
n-type organic semiconductor materials
For practical applications, organic semiconductors are typically used as thin films ranging from
a few nanometers to a few micrometers thick. To deposit such thin films, several methods can
be used depending on the organic material’s properties, cost, tolerances, substrate type etc.
For polymers, deposition typically occurs from solution phase which of course requires the
polymer to be soluble. The polymer solution in a volatile solvent is dispensed on a substrate
surface. When the solvent evaporates, it leaves behind a coating of the polymer. The resulting
polymer film is amorphous or semicrystalline depending on the processing conditions. The
simplest methods, are:
· solution casting (where the solution is simply poured on a substrate, A),
· inkjet printing (where small drops of solution are ejected in a patterned way on a substrate,
· aerosol spray (where a ‘mist’ of droplets is homogenuously sprayed on a substrate, C),
· ‘doctor blading’ (where a blade scrapes the solution over a substrate, D),
· screen printing (where the solution is pressed through a screen onto a substrate, E),
· dip coating (where a substrate is dipped in the polymer solution, F),
· spin coating (where the solution is ‘spun’ on a rotating substrate, G).
Schematical depiction of the various methods for obtaining thin films by solution
Organic thin-film transistor array fabricated
entirely by printing process on a flexible plastic
Optical microscopic image of the 100 ppi
organic thin-film transistor array fabricated
entirely by printing process (pixel size: 254
The availability of organic semiconductors allows the construction of organic electronic devices
analogous to classic devices such as transistors, solar cells and (light-emitting) diodes and gave
rise to the field of organic or ‘plastic’ electronics. Due to their relatively low performance, they are
no competition for classic, inorganic semiconductors in high performance applications but they
can maybe bring electronics to areas where silicon can not. Thanks to their relative ease of
fabrication and deposition and their compatibility with printing techniques and low-cost, flexible
substrates, they could be used for flexible, large area and potentially low-cost applications
Electronic papers
memory devices including radio frequency
identification cards (RFIDs)
Sony Corporation (‘Sony’) announced that it developed a super-flexible 80 μm-thick 4.1-in 121
ppi OTFT*1-driven full color OLED display which can be wrapped around a thin cylinder.
To create the display, Sony developed OTFTs with an original organic semiconductor material (a
PXX derivative) with eight times*2 the current modulation of conventional OTFTs.
Sony will continue to improve the performance and reliability of its flexible
organic displays because the application of these developments are expected
to yield thin, light-weight, durable and mobile devices with enhanced formfactor.
Technology Features
1. High performance OTFT with originally developed high-mobility and highlystable organic semiconductor materials, PXX derivatives.
Sony has developed organic semiconductor material which is stable under
exposure to oxygen, moisture, light and heat and improved current
modulation of eight-times*2 that of conventional OTFT with organic
semiconductor of pentacene. Improvement of this OTFT achieved the
world’s highest-resolution OTFT-driven OLED display with resolution of 121
ppi and 432 x 240 x RGB (FWQVGA) pixels*4.
2. Integration of a flexible gate-driver circuit with OTFTs
This is the world’s first demonstration*3 of an OLED display with an integrated
gate-driver circuit with OTFTs.
3. Enhancement of flexibility with all organic insulators in the OTFT and OLED
integration circuit In order to enhance flexibility of the display, Sony has
developed organic insulators for all the insulators in the OTFT and OLED
integration circuit.
4. Achieved display capable of reproducing moving images while rolled-up around cylinder with
4mm radius.
Even after 1000 cycles of repeatedly rolling-up and stretching the display, there was no clear
degradation in the display’s ability to reproduce moving images.
Specification of the OTFT
organic semiconductor: peri-Xanthenoxanthene(PXX) derivative
hole mobility: 0.4 cm2/Vs
current on/off ratio: 106
channel length: 5μm
threshold voltage: -5V
Specification of the rollable OTFT-driven OLED display
size of a panel: 4.1 inch wide
number of pixels: 432 x 240 x RGB pixels
size of a pixel: 210μm x 210μm
resolution: 121 ppi (pixels per inch)
number of colors: 16,777,216
peak luminance: >100 cd/m2
contrast ratio: >1000:1
minimum bending radius: 4 mm
driving scheme: 2T-1C voltage programming with OTFTs
thickness of a panel: 80μm
Device configurations
There is two common device configurations. Both are of the inverted structure type, since the insulator
must be formed before the organic thin film. For a bottom contact device, the organic layer is deposited at
the top of the device. Drain, source, gate and insulators can be defined by conventional photolithography
which provides a high resolution (less than 1 m). For a top contact device, the organic material is formed
first, followed by drain and source electrode evaporation through a shadow mask. The resolution is
limited to several tens of micrometers in this configuration. As regards device performance, OTFTs with a
top contact typically exhibit a superior performance compared to bottom contact types due to the larger
contact area and lower contact resistance. Typically, a molecular thin film exhibits a much lower mobility
value than semiconductor materials with covalent bonds, which means the electric current provided by
OTFTs is typically smaller than that of a-Si and poly-Si based TFTs.
Benefits of an OTFT:
Does not require glass substrate as amorphous Si does. It could be made
on a piece of plastic.
Manufactured at lower temperatures
Deposition techniques could reduce costs dramatically.
Challenges involved:
Work arounds for complications with photo resists.
To find organic semiconductors with high enough mobilities and switching times.
OTFT technology’s application is diverse.
Organic thin-film transistor (OTFT) technology
involves the use of organic semiconducting
compounds in electronic components,
notably computer displays. Such displays are
bright, the colors are vivid, they provide fast
response times (which need to be developed
in OTFT), and they are easy to read in most
ambient lighting environments.
Organic substrates allow for displays to be fabricated on flexible surfaces, rather than on
rigid materials as is necessary in traditional TFT displays. A piece of flexible plastic might be
coated with OTFT material and made into a display that can be handled like a paper
document. Sets of such displays might be bundled, producing magazines or newspapers
whose page contents can be varied periodically, or even animated. This has far-reaching
ramifications. For example, comic book characters might move around the pages and
speak audible words. More likely, such displays will find use in portable computers and
communications systems.
1. Jiun-Haw Lee , David N. Liu, Shin-TsonWu, “ Introduction to Flat Panel
Displays”, chapter3: thin film transistors, Wiley-SID Series in Display
Antonio Facchetti, Tobin J. marks, “Transparent Electronics”.
3. Alberto Salleo, “Fabrication and characterization of highperformance
polymer thin-film transistors”, Palo Alto research center.

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