Thin Film Transistor Technologies
Flat-panel displays
(FPDs) are becoming increasingly commonplace in today's commercial electronic
devices. FPDs are finding widespread use in many new products, such as
cellular phones, personal digital assistants (PDAs), camcorders, and laptop
personal computers (PCs). This generation of handheld electronics places
stringent demands on their displays. FPDs in these devices are expected
to be lightweight, portable, rugged, low-power and high-resolution. Displays
having all these attributes will enable a wide variety of commercial applications
in the future.
Active-matrix liquid-crystal displays (AMLCDs)
are the leading flat-panel display technology. These displays are ubiquitous
in laptops, often dubbed "active-matrix TFT," (an abbreviation for "active-matrix
thin-film transistor"). What exactly does this name mean?
A display is composed of a grid (or matrix) of picture elements
("pixels"). Thousands or millions of these pixels together create an image
on the display. Thin-film transistors (TFTs) act as switches
to individually turn each pixel "on" (light) or "off" (dark). The TFTs
are the active elements, arranged in a matrix, on the display. Thus, the
name "active-matrix TFT."
Most commercially available AMLCDs use glass as the starting material
in the display fabrication process. Glass has excellent optical clarity
and is compatible with chemicals used in standard semiconductor processing.
However, glass has the undesirable characteristic of being extremely fragile.
As a result, displays must be handled carefully to avoid breakage. However,
if plastic is employed as the starting material for display fabrication,
we can achieve a display that is not only lightweight and rugged but also
flexible. The realization of such a technology will have a significant
impact on the display industry. It is, however, not a trivial task to fabricate
displays on plastic. Many significant challenges arise when plastic substrates
are used in place of glass. Research in the TFT group is aimed at addressing
and overcoming those challenges.
The development of a thin-film transistor (TFT) technology
for use with plastic substrates is still in its infancy. There is significant
room for improvement in ultra-low temperature fabricated poly-Si TFTs.
High
mobilities, low leakage currents and threshold
voltages are desirable for high-performance active-matrix LCD applications,
particularly for the
integration of driver circuitry,
but low processing temperatures (<150ºC) must be maintained for
compatibility with low-cost plastic substrate materials. In general, superior
poly-Si TFT performance is achieved with higher-temperature fabrication
processes, because the quality of the critical gate-dielectric interface
is highly sensitive to process temperature.
An ultra-low-temperature (100ºC) fabrication process, which
would be compatible with flexible plastic substrates, is being developed
by the TFT technology group at UCB. The goal is to achieve polycrystalline-silicon
(poly-Si) thin-film transistors (TFTs) with current-driving capability
far exceeding that of conventional amorphous-silicon TFTs
typically employed in high-performance active-matrix liquid-crystal displays
today. Techniques for formation of the poly-Si and gate-dielectric
materials are being investigated in order to determine the optimum
processes for high-performance TFTs. Various device and process architectures
for attaining low leakage current are being studied. The degradation of
TFT performance under high-voltage bias stressing will
also be characterized. To enable device modeling and circuit design, a
physically
based model for ultra-low-temperature-fabricated TFTs will be developed.
Contributed by Yeh-Jiun
Tung
Click here
for a paper on Silicon Transistor Fabrication on Plastic Substrates.
This work is supported by a grant (ECS-9733247) from the National
Science Foundation, under the Career Advancement Program.
Go back
to Tsu-Jae's home page
Definitions of Terms
-
Liquid-Crystal Displays (LCD's):
LCD's are currently the leading flat-panel display technology. Liquid
crystals change orientation under an applied electric field and can thereby
block or pass light. For more information, click here.
Back
to top
-
Active-Matrix Liquid-Crystal Displays (AMLCD's):
LCD technology that incorporates an active-matrix, as opposed to a
passive-matrix or "dual-scan" technology. Back to top
-
Thin-Film Transistor (TFT):
A transistor whose active, current-carrying layer is a thin film (usually
a film of silicon), in contrast to MOSFETs, which are made on Si wafers
and use the bulk-silicon as the active layer. In a flat-panel display,
light must be able to pass through the substrate material to reach the
viewer. Opaque silicon wafers obviously will not be suitable for these
transmissive displays. Glass is the most commonly used starting substrate
because it is highly transparent and is compatible with conventional semiconductor
processing steps. Since glass is not a semiconductor like silicon, a thin
film of silicon is deposited on top and the transistors are fabricated
using this thin layer. Hence, the name "thin-film transistor." Click here
for a diagram of a TFT. Back to top
-
High Mobilities:
Mobility is the proportionality constant that relates the drift velocity
to the electric field strength in a semiconductor. Mobility essentially
gauges how easily current carriers (i.e. electrons, holes) can move through
a piece of silicon. Electrons move most easily through single-crystalline
silicon because of the uniform arrangement of the atoms. Unfortunately,
single-crystalline films are difficult to deposit due to the low melting
point of glass. In poly-crystalline silicon, individual grains of crystalline
Si are randomly oriented to one another. In this state, electrons can move
easily through each crystalline grain, but are likely to scatter upon reaching
a boundary between grains. Electrons have the lowest mobility in amorphous
silicon, which has neither short- nor long-range atomic order. Back
-
Low Leakage Currents:
Leakage current refers to the small amount of current that flows (or
"leaks") through a transistor when it is "turned off." In an ideal transistor,
leakage current would be zero, but in practice, leakage current always
has a finite value. Leakage current causes the voltage in the pixel capacitor
to drop between each frame refresh, and thus changes the pixel brightness.
Leakage current most significantly affects the fineness of the display's
grayscale. With a low leakage current, finer levels of grayscale can be
achieved. Back
-
Threshold Voltages:
The voltage necessary to turn on a transistor. Threshold voltages should
be low so that it takes lower voltages to charge and discharge the display's
pixels (thereby turning them on and off). Back
-
Integration of Driver Circuitry:
A display needs row and column drivers to properly read the image data
into the pixels. Most displays are "dumb" and have external IC drivers
that require bonded connections to the rows and columns. Poly-Si TFTs have
enough current-driving capability to be used in driver circuitry, enabling
the driver circuitry to be built directly on the display periphery. This
is called "integration of driver circuitry." Back
-
Amorphous Silicon TFT's:
TFTs that are made using a thin layer of amorphous silicon. Atoms in
amorphous silicon have no short- or long-range order. When a film of silicon
is deposited at low-temperature on glass or plastic, the atoms are normally
arranged in this amorphous state. High temperatures are required if films
are to crystallize into poly-Si. Back
-
Gate-Dielectric Materials:
In a transistor, we want the current to flow from the source to the
drain, and not into the gate. Thus, we must put an insulating material
between the gate and the channel of the transistor. The most common gate-dielectric
material is silicon dioxide. Back
-
High-Voltage Bias Stressing:
This refers to a testing procedure in which newly fabricated electronics
are tested for reliability by applying high voltages to them. Back
-
Physically Based Model:
A model of transistor operation based on measured data, as opposed
to a theory-based model. Back
Go back to
Tsu-Jae's home page
References:
[Carey97] P. G. Carey, P. M. Smith, P. Wickboldt, M. O. Thompson and T.
W. Sigmon, Conference Record of the 1997 International Display Research
Conference (Toronto, Canada), pp. M36-M39, 1997
[Young96] N.D. Young, R.M. Bunn, R.W. Wilks, D.J. McCullough,
G. Harkin, S.C. Deane, M.J. Edwards, and A.D. Pearson, EURO Display '96,
pp. 555-558, 1996
Go
back to Tsu-Jae's home page