Polycrystalline Wide Band-gap Semiconductors

Although visible-blind UV photodiodes with good performances can be made from crystalline SiC and GaN, their introduction to the market is hampered by their high production costs. In many market segments, a photodiode made with 

Polycrystalline GaN UV detectors have been realized with 15% quantum efficiency [4], This is about 1 /4 of the quantum efficiency obtained by crystalline devices. Available at a fixed price, however, their increased detection range may well compensate their lack in sensitivity. Furthermore, new semiconductor materials with a matching band gap appear as promising candidates for UV detection if the presumption of the crystallinity is given up. Titanium dioxide, zinc sulfide and zinc oxide have to be mentioned. The opto-electronic properties and also low-cost production processes for these compound semiconductors have already been investigated to some extent for solar cell applications [5]. 

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In the near future, UV photodiodes made from polycrystalline wide band-gap semiconductors may fill the gap in the market. Although they have a lower sensitivity (photocurrent per area) they promise to have a better merit-rating in terms of photocurrent per sensor costs. The other major drawback of polycrystalline photodiodes, the risetime of micro- to milliseconds, is not relevant for household applications. Fuji Xerox Laboratories in Japan are developing visible-blind UV photodiodes made from polycrystalline GaN [12], while twlux AG in Berlin, Germany is developing visible-blind UV photodiodes made from polycrystalline titanium dioxide [13]. A prototype is shown in Fig. 5.45. 

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