Amorphous Silicon Electronics
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e first commercial amorphous silicon devices were photovoltaic solar cells, which in 1980 were introduced by Sanyo into consumer products such as handheld calculators. As the efficiency of devices increases, and manufacturing costs decrease, the goal of employing photovoltaic power on a large scale becomes more achievable. Today, amorphous silicon is being used as a photoreceptor in copiers, utilizing the ability to deposit the material
Figure 1. Model of the atomic structure of a-Si:H illustrating the bonding disorder and the presence of hydrogen. Large disks represent silicon atoms and small disks are hydrogen.
over large areas. The most exciting and rapidly growing technological developments in this area, however—and the focus of this paper —are the matrixaddressed arrays. These are multiple pixel devices used for imaging and display. Amorphous silicon optical scanners were first introduced into fax machines in the mid-1980s and now comprise about half the market. Mass production of liquid crystal displays driven by amorphous silicon transistors began in 1987, and their development is particularly rapid now. The prospects for large-area detector devices, including medical x-ray imaging, appear attractive. The application of amorphous semiconductors to large-area electronics is not new. Selenium photoreceptors enabled the first Xerox copier machines 35 years ago. What is new is the extra functionality provided by the matrix-addressed silicon arrays, which permits electronic reprographics and display in a way not possible with the simple photoreceptor. Amorphous Silicon Amorphous silicon is deposited from silane gas (SiH4) by plasma-enhanced chemical vapor deposition. The plasma provides the energy to dissociate the silane which initiates film growth, although the complex chemical process of deposition is still not completely understood. Films are usually grown at 200-300°C, and virtually any material that can stand this temperature is a suitable substrate, although glass is the most common. The reactor can be scaled to large size, and deposition systems with substrates measuring many square feet presently produce solar cells. Amorphous silicon has standard semiconducting properties. It has an optical bandgap of about 1.7 eV, exhibits n-type and p-type doping by the addition of phosphorus- or boron-containing gases to the deposition reactor, and is highly photoconductive.3 The material owes most of its useful properties (but also some problems) to the presence of about 10 at.% of hydrogen, which originates from the SiH4 deposition gas. The hydrogen reduces the
Table I: Some Typical Room-Temperature Electrical Properties of Hydrogenated Amorphous Silicon, as Compared to Crystalline Silicon. a-Si:H Electron mobility Hole mobility n-type conductivity p-type conductivity
1 cm 2 /Vsec 3X1CT3 cmVVsec
io-2n-'cm-1 10 " a 1cm_1
Crystalline Si 1300 cm2/Vsec 500 cm2/Vsec 10 2 ir'cm -1
MRS BULLETIN/NOVEMBER 1992
Amorphous Silicon Electronics
Dark Forward Bias
illumination
Light Reverse Bias
ITO a-Si:H n
metal
Dar
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