Display device

Abstract: By arranging a Schottky diode which is insensitive to light in parallel with a photodiode which is sensitive to grazing light, a switching element is obtained which, when used in, for example an LCD, has a current-voltage characteristic which is comparable with that of a zener diode. A picture display device provided with these switching elements can then be driven in a reset mode.

Description: The invention relates to a display device comprising an electro-optical display medium between two supporting plates, a system of pixels arranged in rows and columns, with each pixel being formed by picture electrode provided on the facing surfaces of the supporting plates, and a system of row and column electrodes for presenting via switching elements selection and data signals by means of which a range of voltages dependent on the electro-optical display medium can be presented across the pixels for the purpose of picture display, said device comprising means for applying, prior to selection via said switching elements an auxiliary voltage across the pixels, which auxiliary voltage is beyond or on the edge of the voltage range to be used for picture display, the switching elements comprising semiconductor elements each being provided with a layer of semiconductor material which forms a Schottky contact with a connection conductor over a part of a surface of the semiconductor material, the Schottky contact determining the current-voltage characteristic of the switching element if a forward bias voltage is applied to the connection conductor. A display device of this type is suitable for displaying alphanumerical information and video information. A display device of the type described in the opening paragraph is described in EP-A-0 299 546. In a display device described in this Application asymmetrical non-linear switching elements which may be Schottky-diodes are used for applying voltages across the pixel. The use of Schottky-diodes in display devices is known e.g. from US-A-4.709.992, in which diode rings are implemented with Schottky-diodes. In one of the embodiments shown in EP-A-0 299 546 both the voltages for picture display and said auxiliary voltage are presented by means of a zener diode which is operated in the breakdown mode for presenting the auxiliary voltage. The technologies which are used for realising active matrices of switching elements for liquid crystals (amorphous silicon, polycrystalline silicon technology) are, however, generally less suitable for realising zener diodes with a reproducible current-voltage characteristic in the forward direction. Moreover, the light sensitivity, paticularly of amorphous silicon is found to be detrimental. A display device according to the invention is characterized in that the connection conductor outside said part is separated from the semiconductor material by a layer of insulating material, a back bias voltage, if applied to the connection conductor, allowing a conduction channel to be produced by grazing incident light for collecting charge carriers for supplying enough current for charging or discharging the associated pixel to the auxiliary voltage. The invention is based inter alia on the recognition that the light sensitivity of the semiconductor material, which is considered to be detrimental, can be utilized to obtain a switching element having a current-voltage characteristic which is comparable with that of a zener diode, at least for the range used. The Schottky contact guarantees a satisfactorily reproducible current-voltage characteristic in the forward direction so that the voltage across a pixel can be adjusted accurately. Simultaneously, sufficient charge carriers are generated by (grazing) incident light for supplying enough current at a comparatively low back-bias voltage for discharging or charging to a voltage value beyond the range to be used for display. The switching element thus obtained may be considered, as it were, as an anti-parallel circuit of a Schottky diode and a photodiode. For an optimum operation of the photodiode the connection conductor is preferably made of an opaque material such as, for example tungsten, molybdenum, platinum or nickel. A display device according to the invention can be driven with similar control signals as those described in EP-A-0299 546. Further aspects of the invention will be described and explained in greater detail with reference to the drawings and an embodiment. Fig. 1 shows diagrammatically a display device according to the invention. Fig. 2 shows diagrammatically a switching element for a display device according to the invention and Fig. 3 shows a current-voltage characteristic of a switching element according to Fig. 2. Fig. 1 shows diagrammatically the electrical equivalent of a display device 1 according to the invention. Pixels 2 arranged in a matrix are located at the area of crossings of row electrodes 3 and column electrodes 4, while the pixels 2 can be supplied with a voltage via switching elements 5 which in this embodiment connect the column electrodes 4 to the pixels 2. The switching elements 5 are shown as Schottky diodes but in practice they form an anti-parallel circuit of a Schottky diode and a photodiode. Fig. 2 shows in a diagrammatic cross-section a part of a picture display device according to the invention. The switching element 5 is contacted at its lower side by a column conductor 4 which extends across a support or substrate 6 of, for example glass or quartz. The actual semiconductor body 7 of, for example amorphous silicon may extend on both sides of the conductor 4 across the substrate. The conductors 4 have 250 nm thick chromium tracks in this embodiment, which tracks are obtained by means of vapour deposition techniques or sputtering techniques. A layer of amorphous silicon 8 (approximately 500 nm) is provided on these tracks in the conventional manner, which layer is substantially intrinsic, except for a thin layer 9 (approximately 60 nm) at the lower side which is of the highly doped n type for the purpose of satisfactory contacting. The separate semiconductor areas 7 are obtained by subsequently patterning the layer thus obtained in a photolithographic etching step. Subsequently, a passivating layer 10 is provided, for example a layer of SiO₂ having a thickness of 0.4 µm by means of PECVD (Plasma Enhanced Chemical Vapour Deposition), or a layer of polyimide. Contact holes 11 are formed in this layer 10 by means of photolithographic techniques at the area of a Schottky contact to be formed, whereafter a layer 12 of molybdenum, tungsten or another suitable, preferably opaque metal is provided and patterned. If such a switching element 5 is driven in the forward direction, the current-voltage characteristic (Fig. 3) is exclusively determined by the Schottky contact which is formed by the semiconductor body 7 and the (Schottky) metal layer 12. This Schottky contact and hence the forward characteristic are hardly influenced by grazing incident light (denoted by arrows 13 in Fig. 2) because the thickness d of the passivating layer 8 is much smaller than the total length l by which the opaque metal layer 10 covers the semiconductor body 7 (d << l). Although charge carriers are generated by the incident light in the peripheral area 14 of the semiconductor body, they do not reach the central area 15 where the actual current flows. When a sufficiently high back-bias is applied to the metal layer 12, it is possible that holes accumulate in the amorphous silicon along the interface of the passivating layer 10 and the metal layer 12. As a result, a conducting channel 16 is introduced by which an effective collection of the charge carriers generated by the light is possible. This becomes manifest in a photocurrent and a current-voltage characteristic as shown in Fig. 3. The current-voltage characteristic thus obtained is comparable with that of a zener diode so that a display device comprising a switching element according to Fig. 2 can be described in a similar manner as that described in EP-A-0299546, with reference to such a display device comprising a zener diode as a switching element.

Claims: A display device (1) comprising an electro-optical display medium between two supporting plates, a system of pixels (2) arranged in rows and columns, with each pixel being formed by picture electrodes provided on the facing surfaces of the supporting plates, and a system of row (3) and column (4) electrodes for presenting via switching elements (5) selection and data signals by means of which a range of voltages dependent on the electro-optical display medium can be presented across the pixels for the purpose of picture display, said device comprising means for applying, prior to selection via said switching elements (5) an auxiliary voltage across the pixels, which auxiliary voltage is beyond or on the edge of the voltage range to be used for picture display, the switching elements (5) comprising semiconductor elements (7) each being provided with a layer (18) of semiconductor material which forms a Schottky contact with a connection conductor (12) over a part of a surface of the semiconductor material, the Schottky contact determining the current-voltage characteristic of the switching element (5) if a forward bias voltage is applied to the connection conductor, characterized in that the connection conductor outside said part is separated from the semiconductor material by a layer of insulating material, a back bias voltage, if applied to the connection conductor (12), allowing a conduction channel (16) to be produced by grazing incident light (13) for collecting charge carriers for supplying enough current for charging or discharging the associated pixel (2) to the auxiliary voltage. A display device as claimed in Claim 1, characterized in that the connection conductor (12) comprises an opaque metal. A display device as claimed in Claim 1 or 2, characterized in that the part of the connection conductor (12) not associated with the Schottky contact extends substantially as far as the edge of the semiconductor material. A display device as claimed in Claim 1, 2 or 3, characterized in that the semiconductor material is amorphous silicon. A display device as claimed in Claim 1, 2, 3 or 4, characterized in that the electro-optical medium comprises liquid crystalline material.

Inventors: HUISMAN FREDERIKUS ROELOF JOHA, HUISMAN, FREDERIKUS ROELOF JOHANNES
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1991
Main classification: G09G3
IPC codes: G02F1, H01L29, G09G3, H01L27, H01L31