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Technology Stocks : THREE FIVE SYSTEM (TFS) - up from here? -- Ignore unavailable to you. Want to Upgrade?

To: michael c. dodge who wrote (1274)	4/8/1998 5:12:00 PM
From: benjaminf	Read Replies (3) \| Respond to of 3247

Michael, After the recent exposure to the non-technical side of this thread, it's refreshing to see a return to an honest discussion of the relative merits of competing technology. I would like to address some of the problems and concerns I have with low-temperature (and high temperature) polysilicon as competition to silicon based microdisplays. I will also discuss where I think polysilicon-based displays have a great future; it's just not in high-resolution microdisplay applications. Single crystal silicon-based microdisplays will always have a huge advantage over polysilicon microdisplays. The reason for this is simple. The charge mobility, or the rate with which current flows through a transistor, is much lower for polysilicon than it is for single crystal silicon devices. (Note: If this was not the case, then most semiconductor manufacturers would use polysilicon over single crystal silicon). Due to its reduced charge mobility, a polysilicon transistor has to be fabricated on a much larger scale just to be able to approach the performance of single crystal silicon. Due to its size, a polysilicon transistor will never attain the performance level of a single crystal silicon transistor. Thus, many, many more single crystal silicon transistors can be placed on a substrate than polysilicon transistors. The key to performance of a microdisplay is its physical size and its pixel resolution. This makes transistor density the crucial factor for microdisplay fabrication and resolution. Because single crystal silicon transistors can be fabricated on the smallest scale known, they are the obvious choice for microdisplay applications. To bring home my point, polysilicon based microdisplay devices can never be compared to a single crystal silicon microdisplay on a technical basis. Silicon-based microdisplay technology will be the technology of choice for high-resolution projection and virtual viewing applications. With that said, let's explore the area where polysilicon transistors can be used for display applications. Analogous to the discussion above, a polysilicon transistor is better than an amorphous silicon transistor. Amorphous silicon transistors are used almost exclusively for direct view, laptop-type displays. It is logical to conclude that, as the techniques of large area deposition of low temperature polysilicon are perfected, polysilicon will find its place direct-view display applications. In a typical laptop display panel, the size of the transistor is very large, especially when compared to the size of a transistor in a microdisplay. Since the size of the transistor, in the direct view application, is now not a defining issue, the polysilicon transistor is in a position to supplant the amorphous silicon transistor. Some of the large amorphous silicon TFT panel manufacturers, such as Sharp and Toshiba, have started to produce large, direct-view panels (e.g., greater than 7"), using polysilicon transistors. My guess is that it is only a matter of time before they perfect the polysilicon process for 14-inch applications and above. In my estimation, the producers of amorphous silicon-based displays are the ones that have the most to fear from polysilicon devices. Comparing the use of polysilicon versus single crystal silicon in microdisplay fabrication is like comparing apples to radios. Ben F.