Surface-conduction Electron-emitter Display - Description

Description

A conventional cathode ray tube (CRT) is powered by an electron gun, essentially an open-ended vacuum tube. At one end of the gun electrons are produced by "boiling" them off a metal filament, which requires relatively high currents and consumes a large proportion of the CRT's power. The electrons are then accelerated and focused into a fast-moving beam, flowing forward towards the screen. Electromagnets surrounding the gun end of the tube are used to steer the beam as it travels forward, allowing the beam to be scanned across the screen to produce a 2D display. When the fast-moving electrons strike phosphor on the back of the screen, light is produced. Color images are produced by painting the screen with spots or stripes of three colored phosphors, one each for red, green and blue (RGB). When viewed from a distance, the spots, known as "sub-pixels", blend together in the eye to produce a single picture element known as a pixel.

The SED replaces the single gun of a conventional CRT with a grid of nanoscopic emitters, one for each sub-pixel of the display. The surface conduction electron emitter apparatus consists of a thin slit across which electrons jump when powered with high-voltage gradients. Due to the nanoscopic size of the slits, the required field can correspond to a potential on the order of tens of volts. A few of the electrons, on the order of 3%, impact with slit material on the far side and are scattered out of the emitter surface. A second field, applied externally, accelerates these scattered electrons towards the screen. Production of this field requires kilovolt potentials, but is a constant field requiring no switching, so the electronics that produce it are quite simple.

Each emitter is aligned behind a colored phosphor dot, and the accelerated electrons strike the dot and cause it to give off light in a fashion identical to a conventional CRT. Since each dot on the screen is lit by a single emitter, there is no need to steer or direct the beam as there is in an CRT. The quantum tunneling effect that emits electrons across the slits is highly non-linear, and the process tends to be fully on or off for any given voltage. This allows the selection of particular emitters by powering a single horizontal row on the screen and then powering all of the needed vertical columns at the same time, thereby powering the selected emitters. Any power leaked from one column to surrounding emitters will cause too small a field to produce a visible output; if that emitter was not turned on the leaked power will be too low to switch it, if it was already on the additional power will have no visible effect. This allows SED displays to work without an active matrix of thin-film transistors that LCDs and similar displays require, and further reduces the complexity of the emitter array. However, this also means that changes in voltage cannot be used to control the brightness of the resulting pixels. Instead, the emitters are rapidly turned on and off using pulse width modulation, so that the total brightness of a spot in any given time can be controlled.

SED screens consist of two glass sheets separated by a few millimeters, the rear layer supporting the emitters and the front the phosphors. The front is easily prepared using methods similar to existing CRT systems; the phosphors are painted onto the screen using a variety of silkscreen or similar technologies, and then covered with a thin layer of aluminum to make the screen visibly opaque and provide an electrical return path for the electrons once they strike the screen. In the SED, this layer also serves as the front electrode that accelerates the electrons toward the screen, held at a constant high voltage relative to the switching grid. As is the case with modern CRT's, a dark mask is applied to the glass before the phosphor is painted on, to give the screen a dark charcoal grey color and improve contrast ratio.

Creating the rear layer with the emitters is a multi-step process. First, a matrix of silver wires is printed on the screen to form the rows or columns, an insulator is added, and then the columns or rows are deposited on top of that. Electrodes are added into this array, typically using platinum, leaving a gap of about 60 micrometres between the columns. Next, square pads of palladium oxide (PdO) only 20 nm thick are deposited into the gaps between the electrodes, connecting to them to supply power. A small slit is cut into the pad in the middle by repeatedly pulsing high currents though them, the resulting erosion causing a gap to form. The gap in the pad forms the emitter. The width of the gap has to be tightly controlled in order to work properly, and this proved difficult to control in practice.

Modern SEDs add another step that greatly eases production. The pads are deposited with a much larger gap between them, as much as 50 nm, which allows them to be added directly using technology adapted from inkjet printers. The entire screen is then placed in an organic gas and pulses of electricity are sent through the pads. Carbon in the gas is pulled onto the edges of the slit in the PdO squares, forming thin films that extend vertically off the tops of the gaps and grow toward each other at a slight angle. This process is self-limiting; if the gap gets too small the pulses erode the carbon, so the gap width can be controlled to produce a fairly constant 5 nm slit between them.

Since the screen needs to be held in a vacuum in order to work, there is a large inward force on the glass surfaces due to the surrounding atmospheric pressure. Because the emitters are laid out in vertical columns, there is a space between each column where there is no phosphor, normally above the column power lines. SEDs use this space by placing thin sheets or rods on top of the conductors that keep the two glass surfaces apart. A series of these is used to reinforce the screen over its entire surface, which greatly reduces the needed strength of the glass itself. A CRT has no place for similar reinforcements, so the glass at the front screen has to be thick enough to support all the pressure. SEDs are thus much thinner and lighter than CRTs.