Polysilicon LCD

Displays n e w s Pa en s Polysilicon LCD Large area liquid crystal displays have been discovered by workers at Standard Telecommunication Laboratories to be addressable by a silicon layer adjacent to the LC cell (see Displays 3 (1982) 46). A team of researchers at the laboratories has filed three UK patent applications covering a display on silicon and its matrix addressing. By retaining the sharp thresholds for display activation within the access circuitry by using field effect transistors the problems of direct matrix addressing are avoided. The FETs ensure that each display element feels either the full drive voltage or no voltage at all. A dyed phase change

liquid crystal mixture is used in the display in preference to the dynamic scattering mode used in earlier research attempts because of its lower current requirements. The number of display elements is only limited by the semiconductor matrix, ignoring leakage effects. Page write time is determined by the liquid crystal's response time to the full drive voltage. The LCDs keep the viewing angle, temperature and contrast properties associated with other dyed phase change mixtures. Some viewing restrictions are introduced however when a reflector is placed behind the LC layer. A conventional specular reflector is necessary in the

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units since the display cannot be used in transmission because of the opaque silicon layer. Standard polysilicon technology is used to form in the silicon layer the array of FETs, one for each display element. In operation the FETe are in the off state except while they are being addressed. A whole row is activated in the addressing process, with appropriate voltages then applied to the column conductors.

UK Patent Applications 2 078 421A, 2 078 422A, 2 079 022A Standard Telephones and Cables Ltd, fgO Strand, London WC2R 1DV, UK

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