Vidicon target consisting of silicon dioxide layer on silicon

Abstract

The present invention relates to targets for VIDICON tubes comprising monocrystalline semiconductors. The target comprises a uniform monocrystalline semiconductor layer with N-type doping, (17), having a narrow forbidden band upon which a transparent metal layer (18) is deposited which is exposed to the incident light radiation. A dielectric layer (19) is deposited upon the layer (17) in order to receive the impact of the electron beam.

Parent Case Text

This is a continuation of application Ser. No. 130,625, filed Apr. 2, 1971, now abandoned.

Description

The present invention relates to a target comprising a monocrystalline semiconductor, designed for electronic tubes of the vidicon type, for the television camera applications.

Known targets of this type are in the form of mosaics of photo-diodes, each with a pn-junction.

These targets are complex and, moreover, it is difficult to manufacture a pn-junction with a semiconductor having a narrow forbidden band. However, semiconductors of this type are the only ones which give a vidicon tube a good spectral response within the long wave range, that is to say the infra-red.

According to the invention, threre is provided a target for cathode-ray tubes intended for producing a video signal, comprising an n-doped semiconductor layer having a face exposed to the incident light radiation, and an other face and deposited on said other face, a dielectric layer upon said semiconductor layer in order to take the impace of the electron beam;

THE SAME CONDUCTOR AND THE DIELECTRIC LAYERS HAVING AN INTERFACE, HAVING FREE ENERGY LEVELS IN THE FORBIDDEN BAND OF THE SEMICONDUCTOR, FOR TRAPPING POSITIVE CHARGE CARRIERS AND MEANS FOR APPLYING TO SAID FACE OF SAID SEMICONDUCTOR LAYER A PREDETERMINED D.C. POTENTIEL.

The invention will be better understood from a consideration of the ensuing description and by reference to the attached drawings in which:

FIG. 1 schematically illustrates a VIDICON tube.

FIG. 2 illustrates a schematic transverse section through the target in accordance with the invention.

FIG. 3 illustrates the electrical charges in the target of FIG. 2.

FIG. 4 is the target energy diagram, in accordance with the transverse section of FIG. 2.

FIG. 1 illustrates the essential elements of a conventional VIDICON tube. The electron gun 1 comprises a source of electrons which successively scan the target 3 through the meshes of the grid 4, by deflection of the electron beam 2. The symbols 5, 6 and 7 respectively designate the collimating coil, the focusing coil and the deflection unit. The objection or scene being reproduced, 8, transmits the light rays 9 onto the lens 10, the latter forming an image of said object or scene upon the target, through the glass wall 11 of the tube.

The operation of the tube is well known.

The video signals in particular, are picked up at the output 12.

These signals are transmitted by the resistance-capacitance arrangement (13-14) to the video output 15.

The output 12 is at a positive potential V.sub.T in relation to that of the cathode. The corresponding voltage source 16 is connected to the output 12 across the resistor 13.

An embodiment of the target according to the invention is shown in FIG. 2. It comprises an N-type semiconductor layer 17, for example silicon, upon which a transparent conductive layer 18 has been produced, for example in the form of a deposit of tin oxide having a thickness of some few hundreds of A. The layer 18 is exposed to the incident light radiation. A dielectric layer 19 is deposited upon the layer 17 in order to receive the impact of the electron beam from the cathode. It is constituted for example by a silicon oxide layer formed by anodic oxidation on the silicon semiconductor layer which is preferably doped at a level of around 10.sup.14 to 10.sup.15 atms/cm.sup.3.

The operation of the target is as follows:

The electron beam coming from the electron gun produces upon the exposed surface of the layer 19, negative charges 20 represented by the - sign in FIG. 3. The charges 20 induce in the semiconductor 17 a positive space charge 21 constituted by ionised impurity atoms. These ions 22 are fixed. In a general way, the charges 20, under the influence of the electric field, created by the difference of potentiel between source 16 and cathode 1 pass with a greater or lesser degree of difficulty through the layer 19. In the following paragraph the conditions governing this transfer will be explained. In the dark condition, the charges 20 are arrested by the space charge and cannot reach the layer 18. Under illuminated conditions, the photons of higher energy than the forbidden band 23 shown in FIG. 4, will be absorbed and will create "electron-hole" pairs. In the energy diagram of FIG. 4, the abscissae OX plot the distances of points on the target from a plane parallel to the front face of the dielectric layer 19 and located upstream thereof in relation to the electron beam originating from the cathode. The ordinates OE, plot the energies or the potentials of changed sign. The charge carriers thus created, shown in FIG. 4 by the signs - and +, will be subjected to the electric field due to the difference of potential between V.sub.T and the potential of the cathode. The electrons will diffuse towards the layer 18, remaining in the conduction band 24. The holes will diffuse towards the dielectric, remaining in the valence band 25. When they arrive at the "interface" between semiconductor and dielectric, they may either be neutralised by electrons which have passed through the dielectric, or may accumulate there. In the latter case, they will occupy free energy levels in the forbidden band and will be trapped there at 26. According to the invention, the semiconductor and the dielectric are chosen in such a manner that these energy levels exist at the interface.

It is because of this phenomenon that the lateral conductivity is low at the interface. The lateral conductivity is low at the surface of the dielectric layer submitted to the electron bombardment. Thus, the resolving power is improved.

We will now see how the electrons can pass through the dielectric which has been assumed to be relatively thick for example in order of 500 A.

The free charge carrier movement in the semiconductor layer allows a decreasing of the space charge. The resistivity of the semiconductor decreases, and the potential difference between the interface and the face of the layer exposed to the light decreases. The electric field in the dielectric increases, and also the leakage current in the dielectric. Thus, the electric charges deposited by the electron beam at each scanning tune, flow across the structure. The electric current due to this movement is a direct function of the light beam intensity.

A second possible mode of conduction which arises in the case of a dielectric layer having a thickness in the order of 50 A, in fact silicon oxide at the surface of the doped silicon, will now be described. The dielectric is traversed by a tunnel effect mechanism. The conductivity then depends upon the number of cases available for electrons at the surface of the semiconductor, that is to say upon the number of free or trapped holes at the interface and consequently upon the illumination.

Amongst the advantages of the invention it will be observed that the target is constituted by a single type (N) of semiconductor, there being no necessity to produce a pn-junction within its thickness.

In addition, the semiconductor layer is uniform, that is to say it is not necessary to produce a mosaic of elements.

Finally, by using a semiconductor material with a narrow forbidden band, this being made possible a good spectral response to visible light and infra-red radiation, is obtained.

The invention can be applied to any cathode ray tube in which it is desired to reproduce an image formed on the target.

Claims

What I claim is:

1. In a cathode ray tube intended for producing a video signal, a target consisting of:

an N-type silicon semiconductor layer having a face exposed to the incident light radiation and another face directed towards said cathode and deposited on said other face, a silicon dioxide dielectric layer upon said semiconductor layer having a uniform thickness comprised between 50 and 500 A in order to take the impact of the electron beam, the semiconductor and the dielectric layer having an interface, having free energy levels in the forbidden band of the semiconductor for trapping positive charge carriers, and means for applying to said face of said semiconductor layer a predetermined d.c. potential.

2. A target as claimed in claim 1, characterised in that said dielectric layer has a thickness in the order of 50 A.

3. A target as claimed in claim 1, wherein the dielectric layer thickness is of the order of 500 angstroms.