Polysilicon

By Dummy Admin
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Most silicon chips are made of single crystal silicon, which has a very low resistance to electron flow. However, silicon can exist in different forms (just as carbon can exist in the form of diamond, graphite, soot and buckminsterfullerine). As well as single crystal silicon, other forms are Poly-silicon and Amorphous silicon.

Polysilicon (p-si) is short for Polycrystalline Silicon, which is a form of silicon composed of many crystals, as opposed to Amorphous Silicon (a-si), which is an unordered form with a random internal structure. Electrons have a hard time moving quickly in amorphous silicon so the transistors in active matrix displays have to be relatively large, blocking quite a lot of light, and are relatively slow to switch. Polysilicon would be more efficient so why, then, do most active matrix LCD panels use a-si?

Making an LCD requires the silicon to be deposited on a transparent material (the substrate) and depositing p-si proved to require too high a temperature (typically 650 deg plus) to make it a practical proposition, so attention switched to a-si which could be deposited at a much lower temperature (380 deg ) allowing glass to be used as the substrate.

The speed of p-si was a strong lure, however, and eventually P-si panels were produced using quartz glass, which, though very expensive, could stand the heat of the deposition process. Small panels using quartz are used in LCD projectors and some other small applications such as camcorders and digital cameras. A major advantage of getting poly-silicon onto the glass is that the driver chips can be produced in the same process, saving cost and space and improving reliability.

Various companies have developed methods of using lasers to create p-si transistors on glass. The process is that amorphous silicon is deposited on the glass at low temperature and the silicon is then heated with a very short pulse from a laser that avoids heating the glass excessively.

As new, cheaper methods, using still lower temperatures, are developed to produce p-si displays, a-si displays will be replaced by p-si units which require less power, are brighter, more responsive, have a higher resolution and require less external circuitry to make them operate.

Most silicon chips are made of single crystal silicon, which has a very low resistance to electron flow. However, silicon can exist in different forms (just as carbon can exist in the form of diamond, graphite, soot and buckminsterfullerine). As well as single crystal silicon, other forms are Poly-silicon and Amorphous silicon.

Polysilicon (p-si) is short for Polycrystalline Silicon, which is a form of silicon composed of many crystals, as opposed to Amorphous Silicon (a-si), which is an unordered form with a random internal structure. Electrons have a hard time moving quickly in amorphous silicon so the transistors in active matrix displays have to be relatively large, blocking quite a lot of light, and are relatively slow to switch. Polysilicon would be more efficient so why, then, do most active matrix LCD panels use a-si?

Making an LCD requires the silicon to be deposited on a transparent material (the substrate) and depositing p-si proved to require too high a temperature (typically 650 deg plus) to make it a practical proposition, so attention switched to a-si which could be deposited at a much lower temperature (380 deg ) allowing glass to be used as the substrate.

The speed of p-si was a strong lure, however, and eventually P-si panels were produced using quartz glass, which, though very expensive, could stand the heat of the deposition process. Small panels using quartz are used in LCD projectors and some other small applications such as camcorders and digital cameras. A major advantage of getting poly-silicon onto the glass is that the driver chips can be produced in the same process, saving cost and space and improving reliability.

Various companies have developed methods of using lasers to create p-si transistors on glass. The process is that amorphous silicon is deposited on the glass at low temperature and the silicon is then heated with a very short pulse from a laser that avoids heating the glass excessively.

As new, cheaper methods, using still lower temperatures, are developed to produce p-si displays, a-si displays will be replaced by p-si units which require less power, are brighter, more responsive, have a higher resolution and require less external circuitry to make them operate.

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