No sooner than the TFT LCD became
technologically competitive with the incumbent Cathode Ray Tube
and gained broader acceptance in the marketplace, challengers for its
position have begun to appear. The second wave in flat panel display
technology is rapidly approaching production, and promises to exceed
the impressive capabilities of the TFT LCD.
The two most mature candidates for the crown of leading
flat panel display technology are the Field Emission Display (FED) and
the Thin Film Transistor Light Emitting Polymer (TFT LEP). Both of
technologies are emissive, rather transmissive as is the TFT LCD,
therefore both offer very wide viewing angles and high contrast ratios
in brightly lit environments.
With the impending proliferation of High Definition
Television (HDTV) into the marketplace, the first live broadcasts
taken place in the US during Sen John Glenn's well publicised Shuttle
flight, flat panel display technology will be will be of key
importance. Large and wide screens are not the forte of the CRT
which suffers geometrical distortions with increasing screen size, and
therefore will never be a serious candidate for 0.75 x 1.5 metre
domestic cinema environment.
What is no less important is that consumer commodity
volume production of large and wide flat panel displays will make them
rapidly affordable for computer applications, and since they are
designed for a digital drive mechanism, interfacing will not be an
issue. Unlike analogue TV technology, HDTV is wholly digital and
a frame buffering scheme not unlike that in every computer graphics
adaptor. Therefore adaptation of large flat panel HDTV displays to
computing will happen rapidly, as the technology enters the
Both the TFT LEP and the FED promise major improvements
over the CRT and therefore there is much merit in exploring these
technologies more closely.
Light Emitting Polymer
The Light Emitting Polymer is a very recent scientific
discovery. Most conventional organic polymer materials are excellent
insulators, indeed most modern insulation materials are polymer based.
The reason for this behaviour is simple, since such materials typically
posses few if any free electrical charge carriers (electrons or holes),
in comparison with semiconductors such as Silicon, or metals. A typical
polymer comprises interlocking chains of large organic molecules.
carriers in conventional polymers can be liberated only by the
application of extremely high electric fields which typically have the
effect of damaging the material, literally ripping it apart at the
polymer chain level.
Within the last three decades researchers have
determined that some types of polymer materials can exhibit properties
more traditionally associated with semiconductors or conductors. An
example of such are Conjugated Polymers, in which the structure of the
polymer chains creates conditions, under which carriers can be freed
readily, and achieve useful mobilities along the polymer chain. The
number of carriers and their mobility in a material determine its
The maturity of modern solid state physics at this time
is such, that techniques have been developed to model the behaviour of
such materials well enough to produce repeatable and predictable
results. Classical semiconductor properties such as bandgap energies,
have been observed in conjugated polymers. Many well established
component structures in Silicon or other conventional semiconductors
be duplicated in polymers, although with significantly larger
and much higher defect densities. We will not be seeing a plastic
microprocessor in the near future.
The result of this research has been a concerted effort
to produce commercially useful semiconducting and conducting polymers.
The first materials to mature to useful level have been conducting
polymers, based on polyaniline (known for at least a century now), and
polypyrrole. Suitably doped, such materials can exhibit conductivities
similar to that of copper, a mainstay of our industry. One of the
critical issues for such materials is stability, since they must last
least five to ten years even for consumer products. It would appear
this goal is close to being met now, and some manufacturers, such as
Matsushita, are now producing polymer based capacitor electrodes,
battery electrodes (incidently a key technology for electrically
cars since weight is a major issue) and connections on speaker
membranes. A long term goal is to replace the traditional etched copper
printed circuit board with a polymer based technology.
By the early nineties researchers found another
application for semiconducting polymers - the display. By using device
engineering techniques developed for Light Emitting Diode (LED)
technology, they were able to replicate the recombination mechanism
which allows the LED to work - recombining carriers releasing energy as
visible band photons of light. The heterojunction structure of
dissimilar materials used in GaAs/GaAsP LEDs has its analog in polymer
technology, with a similar ability to covert electrical energy into
Initially conversion efficiencies were abysmal, but
refinement and exploitation of LED engineering methods have now allowed
the design of polymer materials and structures which achieve genuinely
useful efficiencies, comparable to LEDs, in excess of 5%. Wavelengths
have now been demonstrated from the infrared band up to blue. In one
half a decade from initial demonstration, the LEP has achieved both the
conversion efficiency performance and colour range which LED designers
took more than two decades to achieve. Importantly LEPs are
fast, at less than a microsecond their switching speeds are competitive
with the LED, which is no mean feat for an essentially amorphous thick
film material. Drive voltages for LEP emitters are low, of the order of
3V or less, similar to those of semiconductors, which makes them
suitable for battery powered applications. Since the LEP is emissive,
wide viewing angles can be achieved. In the simplest of terms, an LEP
a light emitting ink, which can be readily printed on to a solid or
flexible substrate using inkjet printer technology.
The rapid rate of evolution in this technology owes
to the fact that there is no need for an expensive Silicon Valley fab
produce prototypes, therefore the time to fabricate test a new
structure or material variant can be as short as a week.
Some key engineering issues remain before the
hits the market en masse. The most important at this time is material
lifetime, since the polymers will degrade if exposed to the atmospheric
oxygen or moisture. Much of the current research effort lies in
exploring the longevity of the materials, suitable sealants, and
production techniques to avoid contamination. The target is an
life of more than 20,000 hrs (about 2.3 years nonstop operation).
Another area of major effort is the design of materials
which emit suitable red, green and blue wavelengths for practical RGB
displays. If the wavelengths are displaced too far from conventional
CRT phosphors, a conventional video signal will need to have its
vector" rotated and scaled so that images have the correct colour.
Otherwise, people viewed on such a display may look either seasick, or
The architecture of displays based upon LEP technology
is another major issue. Test displays have been fabricated using
drive techniques analogous to the passive LCD display (Systems March
98, pp 28). While passive LEP displays do not exhibit the nasty
crosstalk properties of passive LEDs, they suffer similar drive
capacitance speed limitations.
UK based Cambridge Display Technology (http://www.cdtltd.co.uk),
key developers at this time, have
licenced their patents to Seiko Epson, Philips, Hoechst, and Uniax. In
joint effort with Seiko Epson, CDT are now concentrating on the
adaptation of the LEP technology to the Thin Film Transistor (TFT)
display substrate technology used with the established LCD. Using
essentially similar drive and addressing techniques to the LCD, each
acts as a drive transistor switch for an LEP pixel or subpixel.
The glass substrate is produced as for an LCD, the TFT
and addressing structures are fabricated on the substrate surface, and
the LEP then applied. Using inkjet techniques, two layers are printed
to the substrate, geometrically registered so that the TFT connects to
the anode of the LEP heterojunction structure. Between four and six
printing passes will be required for an RGB display, depending on the
LEP materials required. Since there is no need to laminate with LCD
materials and polarising filters, the TFT LEP process is potentially
much cheaper than the TFT LCD process, producing a much thinner (ie
millimetres) and lighter display with no backlight. While the LEP is a
power glutton compared to an LCD, this will be offset by the absence of
the backlight tube and its power supply.
The TFT LEP flat panel display is an attractive
for established manufacturers of TFT LCDs since they can reuse their
existing investment in fabrication plant. The down side is that TFT LEP
panels will be limited in size and penalised in cost by the
TFT/substrate fabrication defect rates, the limitation of TFT LCD
at this time.
It is reasonable to predict that if no major obstacles
are encountered, the TFT LEP display will supplant the TFT LCD display
in most applications by the end of the coming decade. My advice is not
to invest in chemical manufacturers who specialise in LCD materials!
The Field Emission Display is a cousin to the incumbent
Cathode Ray Tube, and is based upon the same idea of exciting an
emissive phosphor material with accelerated electrons. That is where
similarity ends alas.
Whereas a CRT employs three electron guns for the whole
screen, and uses magnetic or electric fields to scan the beam across
face of the tube, the FED uses one or more miniature "electron guns" or
cathodes for each pixel, or subpixel. Since the electrons need only
travel a short distance in an FED, the display can be as thin as a
The key to the FED is the little known technology of
vacuum microelectronics, pioneered by Stanford Research Institute in
early sixties. The central idea in vacuum microelectronics is the
fabrication of microscopic vacuum tube structures using the
photolithographic and thin film techniques which are today common in
Most current FED designs employ one or more
conically shaped cathode per each pixel. The cathode sits below a
conductive gate structure, and the electron beam from each cathode
passes through a hole in the gate, to the face of the FED which forms
the anode. A typical design uses the sixties developed Spindt process,
using amorphous silicon, and metal, to form the structures. The cathode
itself may be silicon, or molybdenum, palladium oxide, carbon or
The physics of the anode-gate-cathode operation are in
principle no different from those of the classical triode tube. A high
voltage is applied between the cathode and the anode, which causes an
electron beam to be emitted from the cathode and accelerated to the
anode. Application of a negative voltage to the cathode controls the
current in the beam, or shuts it off altogether. Thus the pixel or
subpixel phosphor can be turned on or off, or modulated in brightness.
Unlike a CRT, the FED is a "cold cathode" device, in
which the electrons are literally ripped out of the cathode by a very
stong electric field. Whereas a typical thermionic tube uses a flat
cathode, coated with a material which easily lets go of its electrons
when heated (those ancients out there may recall the quaint practice of
rejuvenating exhausted cathodes by burning off the surface layer with
excess current), an FED uses most frequently an unheated conical
- the electric field at the tip of the cone is so strong that electrons
leap out of the cathode material at room temperature. The best
comparison in field strengths can be made by considering that a large
CRT uses 35-45 kV across a two foot anode-cathode gap, whereas an FED
uses hundreds of Volts up to 10 kV across a gap of millimetres.
The FED is not without its warts. A classical problem
all such devices is contamination of the vacuum by outgassing of
materials with the tube. Molecules of species such as a oxygen will be
trapped in the crystalline structures of most materials used in the
production of the device, and these eventually work their way out and
into the vacuum. Once loose in the vacuum, the high electric field
ionises them, and the positively charged ions do what physics dictates
and collide with the cathode, lodging in its surface and binding with
the cathode material. The result is that the cathode can become coated
with insulating compounds, such as oxides, and its ability to emit
electrons is degraded. The phosphors may also be degraded by loose ions
in the vacuum. While classical vacuum tube fabrication methods, such as
the use of getters (basically a reactive metal is released after the
tube is pumped out and sealed, to trap loose ions), have brought the
problem down to a manageable level, it is still considered to be a
limitation to the usable life of the FED. Devices have been fabricated
with 10,000 hr operating lives.
Other issues with FED revolve about the phosphors to be
used, which must be different from those used in CRTs. Whereas the
electron beam in a CRT is highly energetic (ie electrons accelerated
over a decent distance), in an FED the distance is short and thus the
electrons have only modest energy. To achieve similar brightness to a
CRT, either a more sensitive phosphor must be used, or many more
electrons pumped into the phosphor. The problem with hitting the
phosphor with more electrons is that this tends to be at the expense of
operating life. These are engineering issues which are still being
worked on at the time of writing.
Fabrication of FEDs is also non-trivial, since these
devices combine classical vacuum tube fabrication techniques with
semiconductor fabrication techniques.
The addressing architecture for the FED is, as we would
expect, based upon a row and column addressing scheme. The two most
popular schemes are switched or unswitched anode addressing.
In the former, the phosphors for red, green and blue
laid down as stripes, and overlayed with three sets of anode
metallisation, one for each colour. The three subpixels forming each
pixel share a single cathode emitter, and gate. Each addressing frame
(in time) is divided into a red, green and blue "field". For each
field/row, the anodes are all connected to a common high voltage, upon
which the gates across the entire column of pixels are driven to their
respective control voltages. Like this all of the pixels in a row are
excited concurrently (essentially the same scheme commonly used in TFT
LCDs), and the drive logic cycles its way through the frame, row by
row. The drawback of this approach is that high voltages must be
switched fast, and dielectric breakdown between anodes can be a
problem, limiting the technique to anode voltages of hundreds of Volts.
The alternative scheme is common anode addressing,
each subpixel requires its own cathode emitter and controlling gate
shared across a column. In this arrangement, all anodes in a row are
switched on at once, and the time is then divided into three slots
during which the red, green and blue subpixel gates of each column are
driven with their unique control voltages. This approach is frequently
used with high voltage FEDs, but triples the amount of gate drive
As with passive LCD addressing schemes, the FED is
limited by the electrical capacitance of the gate electrodes, typically
metal over silicon dioxide.
The current state of the art in FEDs are 14 cm diagonal
panels, with prototypes as large as 27 cm demonstrated. Resolutions are
still modest, with VGA level performance, or lesser, the norm at this
stage. Anode drive voltages vary between 300V and 15 kV. Colour
are typically in the 16-bit range, although Futaba claim a 24-bit
display. Nearly all current designs employ the Spindt process conical
cathode technique. Luminescence performance ranges between 50 and 1000
cd/m^2 for RGB displays. Typical contrast ratios are 100:1.
Unlike the TFT LEP, which is very new technology, the
FED has been in the development phase for some time now. Motorola,
Futaba, Micron and Pixtech are offering production displays, and Canon,
Candescent, FED Corp, Fujitsu, Raytheon and Samsung all claim to have
working prototypes (readers interested in more detail are referred to
the April 98 issue of IEEE Spectrum).
While the FED is still well behind the CRT in the key
areas of resolution performance, cost and durability, it would appear
that the major technical issues have been overcome and the device is
ready to enter volume production.
Clearly the days of the CRT display are numbered. With
the three competing technologies of the TFT LCD, TFT LEP and FED, the
venerable three gun tube is facing formidable opposition with
technological growth potential which the CRT has no more.
Because considerable market pressure for consumer HDTV
electronics will develop over the next decade, the computer industry is
likely to significant long term benefits in available technology,
especially for large desktop screens.
In the shorter term, we will see these maturing display
technologies first in the palmtop and laptop marketplace, where modest
volume high cost pilot production can be amortised.
What is certain at this time is that flat panel
have a very bright future.