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Gas-Discharge Displays: the Stateof the Art

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Gas-Discharge Displays: the Stateof the Art IEEE TRANSACTIONS ON ELECTRONDEVICES, VOL. ED-24, NO. 7, JULY, 1977 835 Gas-Discharge Displays: The Stateof the Art ALAN SOBEL, SENIOR MEMBER, IEEE Abstract-The physics of the cold-cathode gas discharge is re- viewed, with emphasis onsuch display-relatedaspects as the visible light output and the use of priming as a method of moving the light-e:mittingsite through the display. Displays usedfor numerics, alphanumeric characters, and computer graphics are described, with attention being given both to commerciqllyavailable units and to those which are still in the laboratory. Attempts to produce gas-discharge TV displays are reviewed. Such related problems as drive circuitry apd overall system cost are discussed, with emphasis on considering the entire display system, including all the required 11 I I power supplies and drive-cireuit compoqents. I I. INTRODUCTION N SURVEYINGthe state of the gas-discharge- I display art,I shall describe currently availabledevices and summarize researchwhich may lead to new devices. This kindof survey requires attention to the interactions I posilive space among the display devices themselves, their driving cir- 4 chorge dens@ cuitry, and the enduses for the equipment. Because of the 1 I intimate relations among these three elements, is it nec- I negottve space essary to consider them together rather than separately; 4 charge -3 in general, we must be concerned with display systems rathe.r than justdisplay devices. Most display research and development is market ori- ented; managements see little point in funding work on devicles for whichthere is not likely to be ademand. With this in mind, thesurvey mixes an accountof the technology I’ I with a limited assessment of the competitive situation, not Fig. 1. The morphology of the discharge. In most display applications, only among typesof gas-discharge displays but also among the cathode dark space is very small and thevarious cathode layers the other typesof displays that vie with gas discharges in indistinguishable. It is possible to have a self-sustaining discharge without a positive column; in fact, the anodemay even be located in the mlarketplace. the Faraday darkspace, although thisgives rise to some unusual effects We begin with a review of the physics of the,gas dis- [I]. charge, and then go on to describe displays used for nu- merals, alphanumeric characters in general, and graphic about V. They thusacquire enough energy to ionize gas and ilmage displays. There is some discussion of research 150 molecules. This ionization produces the cathodeglow-a on TV displays,which are the most difficult display region in the immediate vicinity of the cathode where light problem because of the greattechnical difficultiesand the severe cost constraints. and more ions are produced. The cathode glow is the visible output for anumber of devices, including the well-known “Nixie” and mostof the 7-segment numeric displayslike 11. REVIEWOF THE PHYSICS OF THE DISCHARGE the Burroughs “Panaplex” and the Beckman “Screened In all of the devices discussed here, the electrodes are Image Displays.” cold (except for the possible effects of electron or ion At the endof the cathodeglow there is a relatively dark bombardment); thermionicemission plays nopart. Elec- volume, the Faraday darkspace, which is succeeded by the trons are released from the cathode(see Fig. 1) by posi- positive column. The column may be almost indefinitely tive-i’onand TJV-photon bombardment. Once the discharge long, and the additional voltage required per additional has built up, there is a sheath of positive ions near the column length is small. However, the positive column may cathode. Electrons are acceleratedacross the gapbetween be vestigial or nonexistent if the anode in located in or near this sheath and the cathodeby a potentialthat is typically the Faraday darkspace; 7-segment numeric displays are usually built thisway. The positive column is an essentially Manuscript received October 29,1976; revised February 1,1977. neutral, lightlyionized plasma. Since electrons are much Theauthor is withthe Zenith Radio Corporation, Chicago, IL 60639. more mobile than ions, they areresponslible for most of the 836 IEEE TRANSACTIONSDEVICES, ON ELECTRON JULY 1977 current. Thereis ionization and recombination throughout repeated pulses with the aid of the charge stored in the the region, with electrons andions being lost to the wtds series capacitance at each firing.The local firing is extin- and replaced by more charged particles producedby Lon- guished when the stored charge is reducedby supplying ization. another, smaller “erase” pulsewhich causes firingat too The positive column can be usedto produce light e.X- low an amplitude to recharge the series capacitances ciently, as in the fluorescent lamp. The most efficimnt [13]-[18]. gas-discharge displays yet described have used the positive AC panels can also be operated ina nonmemory mode column in mercury to produce UV which excites pl.13~- in which an element emits light only as long as itis supplied phors. However, these displays have important dis,d- with drive pulses [19], [ZO]. This kindof operation is tol- vantages, includinghigh voltage and high operating tern- erant of a muchwider range of applied voltages and of such peratures, and so far there are no commercial positi,Je- panel parameters asgas fill and interelectrode spacing. column displays. In either memoryor nonmemory modes, the series ca- Because the electric field in the positive column is ml.Ich pacitance provides the isolation andcurrent limiting lower than it is in the cathodeglow, different statescan be needed for parallel operation of a number of cells. This excited and thevisible colors produced in thetwo regicm capacitance equalizes the currentdensity in differentparts can be quite different. Bothregions are sources of both ’IJV of large-area cells, thus keeping the luminance uniform and visible light. over the entire area. Several review articles anda book covering the physics of the discharge are listed in thebibliography [1]-[6]. B. Firing, Priming, and Scanning The firing voltage is afunction of the gas, its pressure, A. General Types of Displays and the geometry of the device. Uniformity of electrode In so-called dc gas discharges, the electrodes are ~iin- size and spacing is necessary to secure uniform firing mersed in the gas and are generally supplied with unilii- voltage among allthe elementsof a display; this can gen- rectional pulses. Once a cell or display element is fired, tl at erally be achieved with reasonable mechanical tolerances. is, a self-sustaining discharge is produced, the impeda:nce Most display elements thatuse the visible glow from the between cathode and anodebecomes low oreven negatjiIrt2. discharge (as distinguished from those that use the UV It is therefore necessary to limit the current, usually 3y output or the electrons) are filled with a Penning mixture means of a series resistor for each cathode or anode. :[:E to reduce the firing voltage. Typically, this consists of neon matrix arrays can be operated in amemory mode, inwhich plus about0.5 percent argon. a dc voltage lower than thefiring voltage is applied to 311 The voltage across a self-sustaining dischargeis usually elements and individual elements are either firedor EX- significantly lower than thevoltage required to initiate the tinguished by suitable pulses superimposed on this [ 71. discharge, and thisvoltage difference or breakback is one This typeof operation requires a limitingresistor for ea ::h of the ingredientsof memory in ac panels.It can also be element, and these areexpensive and difficult tofabric2 t,e used, in conjunction with appropriate circuit arrange- [8],[9]. An alternative is to make the cathodesof graphi te; ments, toprovide memory in dc displays. While the firing this changes thecell operation so that itsON resistance is voltage for a typical cathode-glow display may be 170 V, positive and external limiting resistance is not needed [:I 01. a swing of only 60 V or less is needed to turn thedischarge Still another alternativeis a pulsedoperation inwhich .t ne on and off, so that relatively low-voltage devices can be applied pulses are too short for a self-sustaining discharge used to control thedisplay. to become stable 1111, [12]. Neither of these alternatives The discharge does not begin immediately when the has yet reached the stageof commercial application. firing voltage is applied. There must be at least oneelec- In ac discharges, the electrodes are usually insu1at:d tron or ion in thegap, to be accelerated and toproduce the from the gas by thin dielectric films and the electroje first of the ionizing or electron-emitting events necessary configuration is symmetrical, whereas in dc displayst 7e for breakdown. This requirement for initiating particles cathodes and anodes may be quite different. Charge is leads to a statistical time lag in breakdown. In addition, stored in thecapacitances formed by these dielectric filrns there is a formative time lag,usually a few microseconds, and the resultingvoltage aids in thereignition of the dl: s- during which the breakdownprocess proceeds to the charge on eachhalf cycle of the appliedvoltage. The dl.s,- self-sustaining state. Thesedelays can be reduced by in- charge current and theresulting light occur in shortpulses creasing the applied voltage. Their existence imposes a for the gas mixtures used in ac panelswith memory. A.1, a minimum pulse-width, below which the statistical timelag typical operating frequencyof 50 kHz, the cumulativePI’- and theresulting fluctuation in light output areintolerably fect of many pulses can be an average luminance of t:le large fractions of the overall light-pulse length and the light order of 30 &. A complet,e discharge forms in each htdf output, respectively. In ac discharges, this initial timelag cycle. The stored charge also gives the display memoiryr: is important in the initial firing, butonce an element is The entire matrix arrayis supplied with a repetitivepu!lse fired, the sustain-pulse discharges recur with very low voltage (the sustaining voltage) belowthat required to st2 rt jitter.
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