EP0186540A1 - Electronic circuit composed of thin-film transistors for controlling a matrix device - Google Patents

Abstract

1. Electronic control circuit with N x n outputs used for controlling N x n rows (Lj ) or columns (4) of a matrix device and more particularly a matrix display (2), characterized in that it comprises a multiplexing circuit (16), formed in N groups (G1 , ... Gn ) of n successive thin film transistors (T1 , ... Tn ), a first shift register (18) with N outputs (P1 , ... Pn ) and a second shift register (20) with n outputs (S1 , ... Sn ), the gates of the n transistors (T1 , ... Tn ) of the same group (Gi ) being connected to the same output (Pi ) of the first register, whereby to each group (Gi ) corresponds a different output (Pi ) of said first register (18), the source of each transistor being connected to a different row (Lj ) or column (4), the drain of the ith transistor (Ti ) of each group (Gi ) being connected to the ith output of the second register (20), i being an integer such that 1 =< i =< n.

Description

The present invention relates to an electronic circuit formed of thin film transistors used to control a matrix device.

More specifically, the invention relates to an electronic circuit of the shift register type used to sequentially control the rows or columns of a matrix device and in particular the rows or columns of a crystal display device liquids (flat screen with active matrix, screen for dashboard, ...), photosensitive video retinas with thin film transistors, optical sensors or rendering heads for fax systems with photodiodes lines, sensors, etc ...

In the type of active matrix screen, an electronic memory formed by memory points distributed over the entire surface of the screen stores the video signal throughout the duration of the image. An electro-optical transducer, in particular a liquid crystal transducer, is in contact with each memory point and is excited throughout the duration of the image. Each memory point is located at the intersection of a line and a connection column and it consists of a thin-film transistor produced on an insulating support and a capacitor of which the armatures are made, in the case where the transducer is a liquid crystal, by the electrodes of the liquid crystal cell itself, the insulating support constituting one of the two walls of said cell.

The electronic circuits of the shift register type envisaged for controlling such an active matrix have been studied according to. TCM technology, but their. complexities. made the realization almost impossible on a large scale.

Indeed, making a shift register with thin-film transistors is very difficult especially when it has to include a very large number of stages (300 to 500 and even more, depending on the number of rows or columns of the screen dish) ; no fault is tolerated, on pain of non-functioning of the screen.

Each stage of the shift register generally comprises two elementary inverters and a capacitor used to store the video information. In addition, each inverter is formed of at least two thin film transistors, arranged in cascade.

Since the thin film transistors of the active matrix are generally produced with amorphous silicon, these have a low transconductance and a high input capacity, which leads to obtaining a limit frequency of relatively low inverter operation and most of the time lower than the scanning frequency of the lines of a high complexity flat screen (300 to 500 lines).

Furthermore, the operation of such a shift register is very dependent on the uniformity of the characteristics of the thin film transistors, hence an estimated efficiency that is relatively low.

It has also been envisaged to produce shift registers of the charge transfer type, such as the Bucket-Brigade-Device (BBD) or the Coupled-Charge-Device (CCD) in Anglo-Saxon terminology, less greedy in number of TCM, but the density of defects in amorphous silicon has led to ineffective charge transfer.

This is why, in current active matrix flat screens, the peripheral control circuits of these screens are not integrated into said screens, the control of these screens being completely ensured by standard integrated circuits external to the screen. Unfortunately, in such a system, it is necessary to have a large number of boxes on a printed circuit connected to the flat screen, which poses problems of complex and delicate connections, or else to transfer the corresponding integrated circuits or chips onto the support in glass itself, which leads to multiple welds.

In these flat screens with external control, the cost price of these screens is high due to the cost of the control circuits and the number of connections to be made between the flat screens and the corresponding circuits, or the number of welds to be made on the glass holder. It is generally accepted that the set of peripheral integrated circuits, that is to say the set of control circuits for the lines and the control circuits for the columns of the flat screen, represents half and even more than the price of total returns from said screen.

The subject of the present invention is precisely an electronic circuit formed of thin film transistors used to control matrix devices and in particular flat screens with active matrix making it possible to remedy the various drawbacks mentioned above.

In particular, it simplifies the connection of peripheral integrated circuits with the active matrix while minimizing the cost of these circuits and therefore the overall price of flat screens by reducing the number of these peripheral circuits.

Indeed, the invention relates to an electronic circuit, used to control the rows or columns of a matrix device, using pa <t standard integrated circuits external to the device to provide the shift register function and d on the other hand, thin film transistors to provide the multiplexing function.

More specifically, the invention relates to an electronic control circuit with Nxn outputs used to control Nxn rows or columns of a matrix device, and in particular of a matrix display device, characterized in that it comprises a multiplexing circuit, formed by N groups of n successive thin-film transistors, a first shift register with N outputs and a second shift register with n outputs, the gates of the n transistors of the same group being connected to the same output of the first register, each group corresponding to a different output of said first register, the source of each transistor being connected to a different row or column, the drain of the i-th transistor of each group being connected to the i-th output of the second register, i being an integer such as 1 5 i n.

By matrix device can be understood a device comprising only one row or one column of components.

These electronic control circuits only comprise two external integrated circuits, which are the two shift registers, making it possible to supply, via the multiplexing circuit, Nxn lines or columns of a matrix device and in particular of a screen. active matrix dish. This therefore makes it possible to reduce the number of connections to be made between the external circuits and the flat screen as well as to reduce the cost price of these flat screens.

When the electronic circuit of the invention is used to control the Nxn lines or the Nxn columns of a matrix device comprising thin-film transistors produced on the same insulating support, the transistors in thin layers of the multiplexing circuit of the control circuit of the invention can advantageously be produced on said support and simultaneously with the transistors of the matrix device.

Other characteristics and advantages of the invention will emerge more clearly from the description which follows, given by way of illustration and in no way limiting. For clarity, this description refers to the control of the lines of a flat screen with active matrix, but of course, the invention is of much more general application, as indicated above.

• - la figure 1 représente schématiquement un circuit électronique, conformément à l'invention, servant à commander les lignes d'un écran plat à matrice active, et
• - la figure 2 représente les différents signaux d'entrée et de sortie du circuit de commande de la figure 1.

The description refers to the appended figures, in which:

• FIG. 1 schematically represents an electronic circuit, in accordance with the invention, used to control the lines of a flat screen with active matrix, and
• FIG. 2 represents the various input and output signals of the control circuit of FIG. 1.

In FIG. 1, an electronic control circuit has been shown in accordance with the invention making it possible to control Nxn lines of a flat screen with active matrix, bearing the general reference 2. This active matrix 2 is formed, in a conventional manner, of several conductive columns 4 and Nxn conductive lines denoted L _j , j being an integer ranging from ₁ to Nxn. At each crossing of a column 4 and a line L _j is located a memory point 8 of the active matrix 2, formed of a thin film transistor 10 and a capacitor 12, connected in series.

This active matrix 2 further comprises Nxn capacitors 14, one of the plates of each capacitor being connected to ground, the other to one of the conductive lines L _j of the matrix 2.

According to the invention, the control circuit of NxN conductive lines L _j of the matrix 2 is formed, on the one hand to a multiplexing circuit, of general reference ₁ 6, consisting of N groups G ,, ..., G _N thin-film transistors, each group G _i comprising n successive transistors T ₁ , ... T _n , and on the other hand a first shift register 18, comprising N outputs, and a second shift register 20 with n outputs.

The multiplexing circuit 16 comprises a thin film transistor per conductive line L _j , ie in total Nxn TCM; for example for a flat screen of 320 lines, we could take N = ₄₀ and n = 8.

Such a control circuit makes it possible to considerably reduce the number of connections to be made between the control circuit and the active matrix, these connections being reduced to N + n connections instead of Nxn connections for the matrix devices of the prior art.

According to the invention, each conductive line L _j of the active matrix 2 is supplied by the source of a single thin film transistor; for example the source of the first transistor T, of group G, is connected to line L ₁ , the source of the second transistor T ₂ of group G, is connected to line L, and so on and the source of the last transistor T _n of group G _N at line L _{N. n} .

The gates of the N consecutive transistors T ₁ , ... T _n of the same group G; are connected to the same output P; from the first shift register 18, to each group corresponding to a different output P _{i from} said first register. In other words, the gates of the transistors T ₁ , ... T _n of the group G, of transistors are all connected to the output P, of the first register 18, and the gates of the transistors T ₁ , ... T _n of the group G _N of transistors are all connected to the output P _{N of} said first register 18.

For the drains of the Nxn thin film transistors, they are supplied so that the drain of the i th transistor T _i of each group Ci is connected to the i th output Si of the second register 20, i being an integer such that 1 ≦ i ≦ not. In other words, the first transistors denoted T, of each group of transistors G ₁ , ... G _N are all connected to the first output S, of the second shift register 20, the second transistors T, of groups of transistors G ₁ , ... G _N are all connected to the output S2 of the second register 20, ... and the last transistors T of the groups G ₁ , ... G _N of transistors are all connected to the last output S _n of the second register 20 .

In FIG. 2, the various input and output signals of the control circuit of FIG. 1 are shown. The signal R (n) corresponds to the output signal of the shift register 20, the signal R (N) corresponds to the output signal of the shift register 18 and the signal L, corresponds to the input signal of the first conductive line of the matrix 2. Each pulse of duration r, delivered by the register 20, corresponds to the access time to a conductive line of the matrix, r being close to 64 µs and the pulse r ', delivered by the register 18, is equal to nx τ.

A voltage pulse r, delivered by the shift register 18 which operates at the scanning frequency of the lines of the flat screen, will only be transmitted to a line of said screen in the group G _i made passing by the pulse τ ' , provided by the shift register 18. When the thin film transistors are blocked, the corresponding lines of the flat screen are kept low thanks to the line capacitor 14 which remains charged throughout the duration of an image on the 'screen.

According to the invention, when the transistors 10 of the active matrix 2 are thin film transistors produced on an insulating support such as glass, constituting in particular one of the two walls of the flat screen between which is disposed the liquid crystal, the thin film transistors of the multiplexing circuit 16 may advantageously be produced on said support and simultaneously with the manufacture of the transistors 10 of the active matrix 2 of the screen. On the other hand, the two shift registers 18 and 20 will be standard integrated circuits, produced independently of the active matrix 2.

One of the methods usable for manufacturing the thin film transistors of the multiplexing circuit 16 at the same time as those of the active matrix 2 has in particular been described in a French patent application No. 8215499 filed on September 14, 1982 in the name of the applicant and entitled "Method of manufacturing electronic circuits based on thin film transistors and capacitors". As the title of this application indicates, this process also allows the simultaneous production of capacitors such as 12 and 14 of the matrix.

In a simplified way, this manufacturing process consists in depositing, on an insulating support such as glass, constituting one of the walls of the flat screen, a transparent conductive layer in particular of tin and indium oxide and then a layer of n ⁺ doped amorphous silicon. These layers are then photo-etched using a first mask, so as to produce: the sources and drains of the thin-film transistors of the multiplexing circuit and of the matrix, one of the armatures of the capacitors 12 and 14, the conductive columns 4 of the matrix as well as the drain bus of the multiplexing circuit 16.

A layer of hydrogenated amorphous silicon is then successively deposited, an insulating layer, in particular of silicon oxide, and a conductive layer, for example of aluminum, then a photogravure of this stack of layers is carried out, using a second mask, so as to define the gate of the thin film transistors of the multiplexing circuit 16 and of the matrix 2 as well as the conductive lines L _j of said matrix.

The assembly is then passive using a deposition of a layer of silicon oxide for example, then openings are made in this passivation layer (by photoengraving using a third mask) , at the ends of the conductive lines of the matrix, on the grids of the thin film transistors of the multiplexing circuit 16 and on the drain bus of said multiplexing circuit.

A metallic deposit, for example made of aluminum, is then produced, then the photo-engraving is carried out using a fourth mask, in order to make the connections between the multiplexing circuit 16 and the active matrix 2, between the drain bus and the drains of the thin film transistors of the multiplexing circuit and between the gates of the transistors T ₁ , ... T _n of the same group G _i of transistors of the multiplexing circuit 16.

For further details concerning the manufacture of such electronic circuits based on thin film transistors and capacitors, reference may be made to the patent application cited above.

The application of the electronic circuit according to the invention to the control of the lines of a flat screen with active matrix is of course given only by way of example. In particular, the circuit of the invention can advantageously be used to control the lines of a photosensitive video retina with thin film transistors; such a retina has in particular been described in French patent application No. 82 04003 filed on March 10, ₁ 982 in the name of the applicant and entitled "Photoconductive element in hydrogenated amorphous silicon carbide and video retina cell using such an element". Furthermore, the circuit of the invention can be used to control a photod ⁱ odes array used in facsimile, these photodiodes and the thin film transistors of the multiplexing circuit being produced simultaneously on the same support.

In general, the circuit of the invention can be used to control any line of electronic components of the integrated transistor or diode type.

Claims (2)

1. Electronic control circuit with Nxn outputs used to control Nxn rows (L _j ) or columns (4) of a matrix device, and in particular those of a matrix display device (2), characterized in that comprises a multiplexing circuit (16), formed of N groups (G ₁ , ... G _N ) of n successive thin-film transistors (T ₁ , ... T _n ), a first shift register (18) to N outputs (P ₁ , ... P _N ) and a second shift register (20) with n outputs (S ₁ , ... S _n ), the gates of the n transistors (T ₁ , ... T _n ) of the same group (G _I ) being connected to the same output (P _i ) of the first register, to each group (G;) corresponding to a different output (P _i ) of said first register (18), the source of each transistor being connected to a different line (L _j ) or to a column ( ₄ ), the drain of the i-th transistor (T _I ) of each group (G;) being connected to the i-th output of the second register (20), i being a integer such as 1 ≦ i ≦ n. 2. Electronic circuit according to claim 1, used to control the Nxn rows or columns of a matrix device (2) comprising thin film transistors produced on the same insulating support, characterized in that the thin film transistors (T ₁ , ... T _n ) of the multiplexing circuit (16) are produced simultaneously with said transistors (10) of the matrix device on said support.

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