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Module 7: Plasma Displays
7.1. Introduction
The realization of having a large 60 inch diagonal flat panel display hanging on your
living room wall is here in the form of a [plasma display. Color plasma displays utilize
the physical phenomena of gas discharge and are sometimes referred to as glass
discharge displays - the same physical principle as fluorescent lamps. A gas
discharge generates ultraviolet light which excites a phosphor layer that fluoresces
visible light. Plasma displays are often classified as ac or dc which describes the
method of current lighting. The pioneering working gas discharge closely parallels the
early discoveries in electricity because its impressive luminous output that enticed the
early researchers to study electricity. Jean Picard is generally accredited with the first
gas discharge demonstration in 1678. However, continuous gas discharge had to
await the development of the battery by Volta in 1800. In 1838, Faraday performed
the first experiments of gas discharge between two metal electrodes placed in an
evacuated glass bulk, much like display configurations. Since the early discoveries of
plasma discharge, the technology bias progressed to its current 60 inch diagonal fullcolor configuration.
7.2. Positive Attributes of Plasma Displays
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The electrical characteristics of gas discharge (i.e. non-linearity and memory)
allow plasma display panels to be made very large (i.e. in the 20 - 60 inch
diagonal range).
Good gray scale capability. All digital gray scale allows at least 256 levels of
gray.
Plasma posses a very large non-linearity (i.e. strong threshold). For applied
voltage below threshold (i.e. firing voltage) the display emits practically no
light; therefore, a large number of lines can be addressed enabling high
resolution. 1280 x 1024 has been demonstrated. The high level of nonlinearity is a considerable advantage when compared to liquid crystal displays.
A liquid crystal material is not very nonlinear, and therefore some other
nonlinear element must be implemented on the substrate such as a thin film
transistor to increase display non-linearity.
Plasma displays possess inherent memory. Memory is a highly desirable
feature since a large number of rows (high resolution) can be addressed. This
is because a display with memory has a pixel duty cycle of one. Displays
without a memory function have a pixel duty cycle of one divided by the
number of scanned lines. For non-memory based displays get bigger and the
number of scanned lines increases, the duty cycle and therefore, the
brightness of the display decreases. Also, memory removes any flicker
sensation.
Plasma display panels have a long lifetime, comparable to CRT displays. Then
half luminance lifetimes have been reported to range between 10,000 to
30,000 hours.
Plasma displays have a very wide viewing angle since their emission is
Lambertian.
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Full video is possible since plasma can switch on the order of microseconds.
The luminous efficiency is good (450 cd/m2 at 1 L/W).
Color plasma displays have many common equipment needs as CRTs for
manufacturing, which is substantially less than a semiconductor manufacturing
line needed for active matrix LCDs.
Plasma display panels are mechanically robust because of their rugged selfsupporting structure. They are known to be high shock and vibration tolerant
and have therefore been heavily utilized in military applications.
Plasma displays have a characteristically high impedance which makes them
easy to drive. The driving electrode capacitance is very low because the
dielectric constant of the gas is essentially one. This is the lowest electrode
capacitance of any display on the order of 103 times smaller than
electroluminescent and 102 times smaller than liquid crystal displays. This
translates into current requirements and smaller drive circuit silicon for plasma
displays. They are high voltage devices (~100V), the current is very low - high
voltage drives are easier to design than high current drivers. Large area
panels can be designed with little power dissipation.
Plasma displays can be designed 100% digital - important for new future
applications such as digital video disks and high definition television.
The color gamut of full color plasma displays is adequate - slightly smaller
than that of a CRT.
Plasma displays do not scatter ambient light like their CRT counterparts.
7.3. Electro-optic Performance of Plasma Displays
The basic operation principles of gas discharge or plasma will be discussed here,
followed by a more elementary treatment of the underlying physics and physical
chemistry of gas discharge.
7.3.1. Current - Voltage Characteristics
One of the most advantageous electrical properties of plasma in the context of a
display application is its strong non-linearity at the firing voltage which enables matrix
multiplexing addressing. The current-voltage (I-V) curve of a conventional plasma
display is shown in the figure. It is important to emphasize that the I-V curve is
presented on a semi-logarithmic scale where the current transcends nine orders of
magnitude and the inherent non-linearity is 'built-in' to the material. The figure is
subdivided into several regimes, from arc to Townsed discharge and when the
discharge current has sufficient magnitude, space charge distortion sets in resulting
in a negative resistance region. A typical plasma display would operate near the
junction of the normal glow and abnormal glow regions.
Now, let's revisit one of the most attractive features of plasma display - the extreme
non-linearity of the the gas discharge phenomena. The figure depicts the firing
voltage at approximately 250 V. Note the extreme sensitivity of the current scale in
this region; for small small charges in voltage the current can change by as much as
three orders of magnitude. The light output of the plasma display is approximately
proportional to the current; therefore, plasma is ideal for multiplexing schemes.
The gas discharge process requires external priming as identified in the low current
(nA region) region of the graph. The gas discharge process requires some energetic
particles begin the avalanche process (discussed next) for at least one electron to
initiate the growth discharge process. Without priming, the discharge process will
never commence at any voltage. Priming electrons can come from a number of
different active particles created by a prior discharge or even by a neighboring
discharging pixel. The active particles can include many different species - free
electrons, free ions, metastable atoms and ultraviolet photons.
The I-V characteristics of a plasma gas discharge display show that current levels
can reach very large values, and to avoid a catastrophic arc, some current limiting
mechanisms must be introduced. The figure below illustrates two ways of
accomplishing this which have been utilized successfully in commercial displays.
Plasma displays based on dc use a resistor, a semiconductor current source, or a
short voltage pulse to limit current and have the electrode in intimate contact with the
gas. For the ac mode, on the other hand, the current is limited with an internal glass
dielectric that couples the electrode's capacity to the gas discharge.
For systems used commercially, dc displays have the resistors or current sources
connected to the display electrode external to the panel which allows one discharge
to be ignited along that electrode at any one time. For scanned displays, this
approach works fine. Multiple discharges and dc memory require placing the resistor
internal to the panel in series with each pixel, or a resistor per sub-pixel.
The ac displays achieve memory and current limiting with a dielectric layer. The
dielectric layer acts as a capacitor in series with each pixel. The discharge deposits a
charge on the wall that reduces the voltages across the gap when a voltage pulse is
applied to an ac panel. After a short time period, the discharge will extinguish and the
light output will end until the applied voltage reverses polarity and a new discharge
pulse occurs. This results in an inherent memory function which greatly increases the
size of the display while maintaining brightness.
7.3.2. Wavelength Output Distribution
For display applications, the optical output in the visible region is of most importance.
The figure below shows the wavelength distribution.
This is a typical spectrum of gas discharge phenomena of neon gas. The major
reason for using neon in plasma displays is its high luminous efficiency among the
noble gases. The above spectrum gives a common neon orange color emitted by
most plasma displays. Other colors are achieved by using other gases or by
introducing phosphors into the display.
7.3.3. Regions of the I-V Curve
There are several regimes of the I-V curve that need to be treated. These are the
following:
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

Low current regime
Townsed discharge regime
Subnormal glow regime
Normal glow regime
Abnormal glow regime
Arc regime
Low Current Regime: The lowest currents are in the 10-8 - 10-9 regime that are
strongly dictated by the priming process. These low currents are below the firing
process.
Townsed Discharge Regime: As firing voltage is exceeded, current grows rapidly, as
discussed earlier. In this regime, current is constant in a very wide range. This regime
is often referred to as the self sustaining regime because the gas discharge
phenomenon will continue after priming current is removed.
Subnormal Glow Regime: These can eventually result in a negative resistance region
because the density of charged particle in the gas will reach a point where it
influences the applied electric field.
Normal Glow Regime: Once, the space charge distribution is established, the
discharge will configure itself such that the current density on the cathode surface is
constant. The variation in current in the normal glow region occurs by varying the
area of the negative glow that fills the cathode so constant density is maintained.
Abnormal Glow Regime: At some point upon increasing the current, the cathode is
fully covered with negative glow - constant current density can no longer be
sustained. The increase in current density requires a less efficient electric field profile
that requires higher voltage across gas. Therefore this region has a positive
resistance.
Arc Regime: At high currents, the power dissipation in the discharge is so large and
temperature is so enormous, that the cathode material is vaporized. The cathode
vapor creates a very high pressure gas that can support the high currents which
generates a high degree of field distribution. At this point, since the temperature and
fields are so high, both the atomic and field emissions are the dominant cathode
emission mechanism. High currents can be generated in this regime for voltages of
<10 Volts. The arc is therefore characterized by a sharp negative resistance curve.
These large currents will result in catastrophic failure of a display, so the significance
of understanding the arc phenomenon is not to let it happen.
Within the normal glow discharge regime there are three regions commonly
observed:
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
Negative glow
Faraday dark space
Positive column
Negative Glow Region: There is a high E field region near the cathode where most of
the voltage drop occurs. In this region, strong avalanches develop which generate
light output associated with the negative glow.
Faraday Dark Space: After the field is near the cathode is terminated, the electron
current is still high. However electrons do not have very much energy because E is
low and thus light - therefore light generated is low resulting in a dark space.
Positive Column: The next region toward the anode is the positive column. and
electrons are contained in this region. In this region, electron and ion density are
comparable, the E- field in this region is of low constant value, and mobility of ions
and electrons is unimportant since field is low.
7.4. The Underlying Physics of Gas Discharge
Here we will review the underlying physics of gas discharge in a relatively simple
form. There are several reactions that will be covered here. We will work with the
example of the Penning Mixture - a neon gas doped with a small percentage of argon
(< 0.1% Ar).
We will categorize the reactions as gas volume reactions and cathode surface
reactions. The gas volume reactions are ionizations (labeled I), excitations (E),
metastable generations (MG) and Penning Ionization (PI). The cathode surface
reactions are concerned with the ejection of electrons from the cathode by ions,
metastable atoms or photons.
Electron
Ne
Ne+
Ar
Ar+
Ne*
Nem
n
- Drift Right
Neutral Neon Atom - Diffuse
Positive Neon Atom - Diffuse
Neutral Argon Atom - Diffuse
Positive Argon Atom - Drift Left
Excited Neon Atom - Diffuse
Metastable
Neon
- Diffuse
Atom
Photon
- Propagate
Even in the simplest explanations, the gas discharge is complex. It has many species
that are transported by different mechanisms. The type of species found in this
example are the following:
1.
2.
3.
4.
5.
6.
7.
electrons
neutral neon atom
positive neon ion
positive argon ion
excited neon atom
metastable neon atom
photon
The charges species in the gas discharge reaction include electrons, positive neon
ions, and positive argon ions and are transported by the electric field i.e. field induced
drift. Diffusion transport is considered secondary to the field induced drift mechanism.
The neutral species include the neutral neon atoms, the neutral argon atoms, the
excited neon atom, and the metastable neon atom. Because of their neutrality, their
transport is governed by diffusion. Photons are governed by the laws of
electromagnetic radiation.
7.5. Gas Discharge Reactions
7.5.1. Energy Level Diagram for Neon
The figure below shows the allowed transition for an outer electron of the neon atom.
optical output
due
to
transition
between 2p
and
1s
energy levels
The optical output of the gas discharge process is strongly correlated to the above
energy level diagram. The optical output spectrum shown above is a consequence of
photon emitted when an excited neon atom makes a transition between the 2p and
1s energy levels.
Ionization is the most vital reaction of a gas discharge used in plasma displays. When
an electric field is applied across the gas, the electrons within the gas are accelerated
and collide with neutral gas atoms. The figure above demonstrates that the new atom
has NO allowable energy levels between 0 and 16.6 eV j; therefore most collisions will
be elastic in nature. Elastic collisions means that election does not gain or lose
energy. As shown in the above figure, 21.6 eV is the ionization limit of the neon atom
and the electric field will undoubtedly have accelerated elections to this level.
These collisions can cause an electron to be ejected from the neon atom theory
creating a positive neon ion and a new free electron.
The ionization reaction is given by:
Ne + e- --> Ne+ + 2eBoth the ion (+) and electron (-) drift in the field in opposite directions. Since the neon
ion is massive compared to the electron, it drifts 102 times slower.
Excitation will occur. Some of the electrons will gain sufficient energy to excite the
neutral neon atom during a collision. This reaction occurs between 16.6 - 21.6 eV.
For energies less than this only elastic collisions result and above this, ionization is
favored.
The time scale of the excited atom is 10 ns, which is short, before it radiates a photon
and returns to the ground state. The reaction is described by:
Ne + e- --> Ne* + eNe*--> Ne (or Ne*, or Nem) +v
The spectrum observed for a plasma display panel are layers, a result of the
transitions between 2p and 1s levels. This leaves the atom in the excited 1s state that
will undoubtedly be deexcited by some later reaction.
There are a number of metastable levels in the energy level diagram that is shown
above. There are two of four 1s levels of neon that cannot emit a photon, i.e.
metastable. The reaction can be written as:
Ne + e- --> Nem + eThese so called metastable states can also be created as a by-product by an
excitation reaction. Many of the radiating transitions shown in the energy level
diagram end up in one of the two metastable levels. A two step process can therefore
cause metastable generation - e.g. > 18.5 eV excites a neutral neon atom to the 2p
state and that atom radiates a photon, forcing the neon atom in the 1s metastable
state. This two step process is:
Ne + e- --> Ne* + eNe*--> Nem + v
The transport diffusion process of metastable atoms is slow since the neon atoms do
not radiate a photon and are therefore neutral. Positive neon ions are virtually the
same mass, but move much faster since they are charged. Metastable atoms survive
for long times - on the order of 1-10 µs - significantly longer than the 10 ns lifetime of
excited atoms. Metastable states do not naturally decay but are typically dexcited by
collisions with the chamber walls or the Penning ionization process.
Metastable atoms have ~16.6 eV of energy in the energy level diagram. This
additional ionization phenomenon enables plasma display panels to switch at lower
voltages. The argon atom concentration strongly dictates the additional ionization.
Penning described the concentration, about 0.1% in Neon, to provide the largest
amount of ionization per applied volt. This is known as the Penning mixture. Other
Penning mixtures include neon, xenon, krypton, and noninert gases.
There are a number of important reactions at the cathode surface. Electrons can be
ejected from the cathode surface, which can be simulated by cathode collision from
positive ions, metastable ions, and photon. The most important cathode surface
reaction for plasma displays is the electron ejection due to positive ions. The energy
of neon and argon is:
Energy
Argon Ion
Neon Ion
15.8 eV
21.6 eV
Upon collision with the cathode, the ions capture an electron from the surface and
become neutral - thereby saving up their energy to the surface. The energy is
sufficient to allow the electron to escape the work function energy of the cathode
surface, which is in the range 3-10 eV. The significant portion of electron energy
results in a high probability that an ion colliding with the surface will result in electron
ejection. The ejection reaction is dominant in plasma displays because all ions in field
are transported in the field, and additionally drift toward the cathode.
The photo emission phenomenon is also an important mechanism for electron
ejection. Because of the nature of the work function of the cathode, ~3 eV, only
ultraviolet photons have significant photoemission. These ultraviolet photons have
transitions between the 1s and ground state energy levels. These ultraviolet photons
are emitted isotropically and even though there are a lot of them, only a few are
directed toward cathode.
Metastable neon atoms have an energy of 18.8 eV that can be given to the surface to
eject an electron and dexcite the metastable atoms. The metastable atoms are not as
nearly important as the ions since they tend to diffuse in random directions at very
slow rates as compared to the transport of ions towards the cathode.
There is also a phenomenon in plasma display called an Avalanche. Because the
ionization reaction discussed above results in two free electron and a positive ion, an
avalanche can occur. The two electrons can then go on to create two more
ionizations, and continue to do this over and over again. As the avalanche grows
and progresses to the anode, the number of ionizations increases exponentially. For
plasma displays, 10-300 ionizations can occur in an avalanche.
7.5.2. Modeling Gas Discharge
Simple models are useful in understanding plasma display phenomenon - firing
voltage, current growth rate, and priming.
The avalanche process can be modeled as a simple amplifier with gain circuit shown
above. The gain is given by
Gain µ = e  v - 1
Where  is the number of ion pairs per volt and V is the voltage across the anode
and cathode.
 strongly depends on the gas mixture, gas pressure P, and the
The  for neon doped with 1% argon is much larger than for pure
The magnitude of
electric field E.
neon because of the Penning reaction.
Avalanches create ions, metastables, and photons which can find their way back to
the cathode and eject secondary electrons. The avalanche gain increases with
increasing voltage. Since secondary electrons can now start new avalanches, these
process can be modeled as a feedback path in the circuit. The delay depicts transient
time for the ions, metastables, or photons between point of creation and cathode.
There is a finite probability,

that the particle will cause an ejection of an electron
when it reaches the cathode. The
term is a multiplicative to the feedback term.
The secondary electrons that are ejected from the cathode must be added to the
electrons due to the priming current I0. The priming current comes about from
external means such as photoemission from photons (generated by an independent
nearby gas discharge). The sum of the priming and secondary current represents the
flow of electrons that can start new avalanches, and can feedback into gain amplifier
- i.e. positive feedback.
The loop gain model is easy to understand. If the loop gain is greater than 1, it will
become unstable. The loop gain is defined as
µ =  (e  v - 1)
where µ = 1 is a point of instability. This condition defines the firing voltage of the gas
discharge.
The transient behavior of the current depends output current for V above firing is
shown in (b). the current grows exponentially with a positive time constant
is correlated to µ and the feedback delay time:
, which
where A and B are constants, and I0 is the priming current. The output current I is
proportional to the priming current I0. Once initiated, the output current will
experience growth until some other phenomenon limits it. Catastrophic failure results
if the current continues to grow until an arc appears - therefore some current limiting
mechanism must take over.
The current growth for V below firing is also shown in the figure. In this situation
so the output current asymptotically approaches a steady state value.
<1,
As with the situation above firing, the coefficients C and D are independent of time,
and I I0.
Since I I0, the gas discharge phenomenon will not occur without some external
priming current. Priming can be as simple as a single electron to initiate the
avalanche. Without a single electron, the gas discharge process cannot occur, no
matter how much voltage is applied. The three priming procedures are pilot cell
priming; self-priming; and radioactive priming.
Pilot cell priming is the most common on plasma display technology. The pilot cell will
generate electrons, ions, photons, and metastables derived from the discharge. The
various reactions occurring in gas discharges, thus will create electrons in the cell to
be initiated.
The importance of these particles is the priming process largely depends on the
space (distance) between the pilot cell and prime cell. The electron and ions will
usually follow the electric filed lines and will therefore be influenced if the primed cell
and pilot cell are close. The metastable ions are only important in the priming process
when the prime and pilot cell are in close proximity. The metastables contribute to
priming by the Penning effect or by colliding with the cathode and ejecting an
electron by the cathode secondary process.
A sufficient number of photons will also be created in the pilot cell, which can in turn
create electrons in the primed cell by photo-emission. Typically, photoemission only
occurs for deep ultraviolet photon. This is typically not an effective prime because the
ultraviolet is strongly attenuated through the glass. If the photons are unobstructed,
most photons will travel freely without much absorption by gas. For this case,
photons have a large range priming influence, and distance between the pilot cell
and prime cell can be large.
Self priming is another option in the case when the pilot cell and primed cell are the
same. This is utilized when the cell is periodically pulsed. If the period since the last
discharge pulse is relatively short, there still may be priming particles left around
(electrons, ions, metastables, and photons). These can be utilized in the next
discharge. However, for self priming, the display needs some priming mechanisms
when it is first turned on. Care must therefore be taken to ensure that the critical
starting particle decay time between discharges is not exceeded by normal operation,
otherwise cell will not execute a discharge.
A type of priming rarely used is radioactive where small amounts of radioactive gas
is doped into the gas mixture (Kr 85, Ni 63). The radioactive particles emitted will have
sufficient energy to ionize a large number of atoms. This is an attractive solution to
priming because no additional prime cell is needed but radioactive particles do
present a health risk during manufacture. Once enclosed in the glass envelope, beta
particles do not penetrate the glass for the Kr 85 and Ni 63 elements.
One of the most important issues to plasma display engineering is the dependence of
firing voltage on the gas discharge cell design. This is characterized by the Paschen
curve shown below.
The curve provides the breakdown voltage as a function of pd, the gas pressure and
the separation between the cathode and anode. It is important to note that the firing
voltage will remain unchanged for differing cathode - anode distances as long as the
pressure is changed so that the product pd is a constant. This is commonly utilized
by display engineers to minimize cost. In order to reduce drive circuit costs, it is
desirable to have a low firing voltages. The value of d is usually dictated by resolution
so the display engineer has the freedom to choose p for a given d to minimize the
firing voltage.
7.3.3. Regions of the I-V Curve
There are several regimes of the I-V curve that need to be treated. These are the
following:






Low current regime
Townsed discharge regime
Subnormal glow regime
Normal glow regime
Abnormal glow regime
Arc regime
Low Current Regime: The lowest currents are in the 10-8 - 10-9 regime that are
strongly dictated by the priming process. These low currents are below the firing
process.
Townsed Discharge Regime: As firing voltage is exceeded, current grows rapidly, as
discussed earlier. In this regime, current is constant in a very wide range. This regime
is often referred to as the self sustaining regime because the gas discharge
phenomenon will continue after priming current is removed.
Subnormal Glow Regime: These can eventually result in a negative resistance region
because the density of charged particle in the gas will reach a point where it
influences the applied electric field.
Normal Glow Regime: Once, the space charge distribution is established, the
discharge will configure itself such that the current density on the cathode surface is
constant. The variation in current in the normal glow region occurs by varying the
area of the negative glow that fills the cathode so constant density is maintained.
Abnormal Glow Regime: At some point upon increasing the current, the cathode is
fully covered with negative glow - constant current density can no longer be
sustained. The increase in current density requires a less efficient electric field profile
that requires higher voltage across gas. Therefore this region has a positive
resistance.
Arc Regime: At high currents, the power dissipation in the discharge is so large and
temperature is so enormous, that the cathode material is vaporized. The cathode
vapor creates a very high pressure gas that can support the high currents which
generates a high degree of field distribution. At this point, since the temperature and
fields are so high, both the atomic and field emissions are the dominant cathode
emission mechanism. High currents can be generated in this regime for voltages of
<10 Volts. The arc is therefore characterized by a sharp negative resistance curve.
These large currents will result in catastrophic failure of a display, so the significance
of understanding the arc phenomenon is not to let it happen.
Within the normal glow discharge regime there are three regions commonly
observed:



Negative glow
Faraday dark space
Positive column
Negative Glow Region: There is a high E field region near the cathode where most of
the voltage drop occurs. In this region, strong avalanches develop which generate
light output associated with the negative glow.
Faraday Dark Space: After the field is near the cathode is terminated, the electron
current is still high. However electrons do not have very much energy because E is
low and thus light - therefore light generated is low resulting in a dark space.
Positive Column: The next region toward the anode is the positive column. and
electrons are contained in this region. In this region, electron and ion density are
comparable, the E- field in this region is of low constant value, and mobility of ions
and electrons is unimportant since field is low.
7.6. Time Varying Characteristics
Pulse voltages are applied to plasma panels so the transient behavior must be
understood. The engineer needs to know the time for the discharge to grow to a
steady state value so the duration of the pulse, pulsewidth, can be incorporated to
ensure that the discharge will emit light for a given duration. The after glow, or delay
of gas discharge, is important because it determines the subsequent initial densities
of priming particles for subsequent discharge pulses. The afterglow characteristics
influence the repetition rate.
The two important times during initial growth of discharge are statistical decay and
formative decay. The decay is the sum of the two, in the range of 100 ns - 100 µs.
Statistical Decay: A decay time scale is required for one electron in the gas volume to
initiate the avalanche. This electron is derived from the priming current I 0.
Formative Decay: A time associated with waiting for the discharge to reach its
desired current level once the growth discharge has emerged,
Afterglow Decay: A duration where glow persists after the applied voltage is
removed. This visible light decays very rapidly, but many other particles in the
discharge decay much more slowly and influence the priming of subsequent
discharges.
7.7. Plasma Display Technology Fundamentals
There are two types of plasma panels, ac and dc. In the figure below one can
compare and contrast them. The main difference between ac and dc is the dielectric
film in the ac mode that serves to limit the discharge current and stores charge to
increase memory.
7.7. AC Plasma Displays
Color plasma displays are persuaded and dominated by ac technology, so we will
focus on ac here. First, starting with the monochrome plasma display to build up, the
fundamentals understanding to be applied to full color.
The above ac monochrome structure is produced by depositing thin film electrodes
on the front and back glass substrates, and by depositing a thin dielectric layer over
the electrodes. The dielectric coatings serve as a capacitator to limit discharge
current. The dielectric also stores charge, providing a bistable memory function., The
substrates are sealed around the perimeter and filled with neon gas. The simple
structure enables pitches to be isolated simply by the action of electric fields.
The ac plasma panels require an additional coating of magnesium oxide (MgO),
which enables lower drive voltages and extends the life of the panel. The ac displays
require an ac signal, often called the sustaining voltage, to be applied during
operation. The frequency of the applied voltage is usually 50 KHz. The figure below
shows a driving waveform for a pixel in both the on and off states. When a pixel is
discharging, charge accumulates on the dielectric glass walls and therefore
influences the voltages across the gas. the component of voltage due to this change
is simply known as the wall voltages. In the on state, the voltages change for each
polarity reversal of the sustained voltage. The change in wall voltage coincides with a
pulse of light due to the gas discharge. When the pixel is off, there are no light
pulses, and the wall voltage remains at zero level. Pixel addressing is achieved
through a partial discharge by introducing an addressing pulse that has time between
sustain pulses. A write pulse causes the wall voltage to transmit from zero to the final
wall voltage level. An erase pulse causes the wall voltage to return to zero.
7.8. Color Plasma
Full color plasma displays are achieved using phosphor. The phosphors are excited
by ultraviolet light resulting from the gas discharge process. This is like a fluorescent
light. Xenon, doped with neon and helium buffer gas, generates ultra violet light.
The basic structure of two ac configurations are shown above. The double substrate
structure is very simple. The single substrate structure separates the discharge
cathode areas from the phosphor by applying the sustained voltage only to the lower
electrodes while the phosphor is on top. The single substrate technique results in
longer phosphor life because it is not directly sputtered by the energetic ions that are
directed toward the cathodes.
The structure for a single ac plasma display is presented above, showing that front
and back substrates have a single one dimensional feature. Since the two substrate
structures are positioned orthogonally, alignment is not critical between substrates
because pixel will automatically arise wherever electrodes intersect, i.e. easy to
manufacture.
The phosphors are pumped with the ultraviolet light. In both cases the sustaining
electrodes are on the top surface plate. The ac voltage is applied in the conventional
way, and the fringing field profile provides the optimal pump.
The structure above has glass barrier ribbed spacers between each subpixel, in order
to reduce cross-talk between pixels. Reducing cross talk is important in order to
retain good color, putting the attenuation of the ultraviolet photons within the ribs
does not transmit the 147 nm to 173 nm wavelengths radiated by the xenon gas. The
phosphors are placed on the subpixel channel except for the front plate which has
the phosphor damaging sputtering activity at the cathode.
Also notice in the above figure that the electrodes are buried beneath the phosphors
of the neon substrate. These are the column electrodes that are selectively pulsed
depending on the input data. The address activity could potentially cause sputter
damage of the phosphor, the address pulse frequency is well above the sustain
frequency (orders of magnitude) the amount of address damage is minimal.
The sustain electrodes are made of a conductive transparent material such as tin
oxide - however the resistance is orders of magnitude too high. Narrow bus
electrodes of high conductivity material, such as silver chrome - copper - chrome, are
placed on the tin oxide to reduce electric resistance to values ~100.
The double ac device has a few differences. The main differences are the
introduction of glass banner ribs to maintain color purity and the introduction of
phosphors. The phosphors are placed on the substrate and are painted to avoid the
location directly over the electrode, since the sputtering action and the electrode will
cause phosphor degradation.
The figure above shows another color plasma structure. In this configuration there is
a placement of resistors in series with each subpixel to limit discharge current for the
dc case. Another difference is that the dc device needs to have barrier ribs for all four
edges of each subpixel. This is needed to prevent dc discharge for spreading to
neighboring pixels. The color arrays are patterned along each row and each column
In a two dimensional structure, requiring careful alignment.
Inherent memory can be utilized for the dc plasma structure with the pulse memory
waveforms shown below.
The basic principle is a metastable priming from the preceding discharge pulse, then
the discharge will build up in a sufficiently short time to fully mature during the very
short electrode voltage pulse. The electrode voltage pulse is adjusted to be short to
inhibit a sequence of discharges if there has not been an initiating higher amplitude
address pulse.
Memory cannot be used to create gray levels since the pixels must be on or off
(binary). Instead memory displays achieve gray levels by the percentage of time that
the pixel is on in a given frame - therefore pixels must be addressed multiple times
per frame.
DRIVING SEQUENCE FOR 256 GRAY SCALE
In the figure above for 256 levels of gray, each frame is divided into 8 subfields and
each subfield consist of an address period and sustain period. During a given
address period, address pulses are applied to all pixels in he panel according to the
image date. Each of the eight subfields has a sustain period with a different number
of sustain cycles which emits an amount of light proportional to the number of sustain
cycles. Each bit of a given pixel intensity is allowed to control one of the subfield,
then the total number of sustain cycles (and light) per frame is proportional to 8 bit
intensity value. This is digital gray scale.