Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-01-19
2001-02-27
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S214100, C257S077000
Reexamination Certificate
active
06194699
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION AND PRIOR ART
The present invention relates to a photoconductive switch comprising at least a first layer of a first material and two contact layers arranged on opposite sides of the first layer and connectable to different potentials for applying a voltage across the first layer, said first layer being adapted to be conducting upon applying a voltage across said contact layers when irradiated by light of an energy high enough for lifting charger carriers from the valence band to the conduction band of said first material, a first of said contact layers being provided with apertures for allowing light applied on the switch from the side of said first contact layer to reach said first layer for making the switch conducting upon applying said voltage thereacross.
A switch of this type finds many applications, and it may for instance be used in equipment for handling high electric power for switching high voltages (which may be 2-400 kV) and currents, for examples in surge diverters, current limiters and the like. An advantage of a photoconductive switch is that light control provides for a very fast switching, which is of particular importance in high power applications for protection of equipment when faults occur.
A common problem of such photoconductive switches being irradiated from the same side as one of the contact layers, so-called vertical switches, is the difficulty to obtain a good contact with a low contact resistance on the side of the irradiation and at the same time an efficiently high generation of free charge carriers in the first layer by the irradiation. The regions of the first layer covered by said contact will be in the shadow, i.e. no light will reach these regions and generate free charge carrier there, which means that the conductivity will be low in these regions. It is no practical solution to make the apertures very large and accordingly the contact layer portions separated by said apertures very small, since this will reduce the active area, i.e. the area which may be reach by charge carriers in the conducting state of the switch, of the switch to much and make a substantial contact resistance contribution to the total resistance of the switch.
This problem is there irrespectively of the material of said first layer, but it is more accentuated for some materials, such as diamond, being hard to dope and having a large band gap (energy gap between the valence and the conduction band thereof). a possible way of obtaining a partial solution to this problem for some other materials is namely to dope said first layer very highly in the regions next to the first contact layer, which means that free charge carriers created in the first layer by said irradiation will easier reach the first contact also through the regions in “shadow”. Another possible way to such a partial solution of the problem is to deposit a layer of conducting transparent In-doped SnO
2
on the first layer as contact layer, but this solution is only available for materials of the first layer having a band gap being less than 4.2 eV (the band gap of SnO
2
).
Diamond, which has a band gap of approximately 5.4 eV and being hard to dope will hereinafter for illuminating but not in any way limiting the invention be discussed, since this is a material for which none of the partial solutions mentioned above may be used. Thus, it is not possible to reduce the contact resistance by highly doping the diamond layer next to the first contact layer, and a use of SnO
2
in the way mentioned above is not possible, since SnO
2
would absorb most of the high-energy light required for making the diamond conductive, so it will never reach the diamond layer.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a photoconductive switch of the type defined in the introduction having a lower contact resistance on the side of said first contact layer than known photoconductive switches of this type.
This object is according to the invention obtained by arranging a thin second layer on the side of the first contact layer, at least covering the surfaces of the first layer exposed through said apertures and forming an interface to said first layer in said apertures, that the second layer is made of a material being able to form a well ordered interface to said first material and having the same or a larger energy gap between the valence band and the conduction band thereof as or than said first material for allowing said light to pass therethrough without being to a substantial degree absorbed by said second layer.
The active contact area of the first contact layer will for a given geometric area of the first contact layer be increased, since the charger carriers may reach the first contact layer through said second layer in said apertures without travelling though the regions “in shadow”. It is for this sake important that the interface between the first layer and the second one is well ordered, so that the charger carriers and the light will not at said interface be caught by electrical and optical traps, respectively, increasing the resistance of the switch. It will in this way be possible to reduce the geometric area of the first contact layer, i.e. make the apertures larger, and still have a sufficiently large active contact area for not limiting the current handling capability of the switch to much. This results in a smaller total area of the regions in “shadow” reducing the resistance of the switch when irradiated.
By making said second layer thin and of a material having a band gap being at least as high as the band gap of said first material there will be no risk that any major part of the light is absorbed by this second layer.
According to a preferred embodiment of the invention said second layer covers also the first contact layer, which is very preferable, since this will increase the active area of the first contact layer even more, since it is now possible for charge carriers to reach the contact layer through said apertures also at the surfaces of the first contact layer directed away from the first layer. This means that the geometric area of the first contact layer may be reduced further resulting in an increasing of the advantages of the application of said second layer mentioned above.
According to another preferred embodiment of the invention the material of the second layer has substantially the same gap between the valence band and the conduction band as said first material. This means that some of the light will be absorbed by said second layer, which may lower the resistance of the switch further.
According to another preferred embodiment of the invention the material of the second layer is at least next to the interface to the first layer the same as said first material, which is advantageous, since this means that it will normally be easy to form a well ordered interface between the first and the second layer, since the lattice-match will be exact, and it is then preferred that the material of the second layer is the same as the material of the first layer. Using the same material also means that the requirement concerning the energy band gap will automatically be fulfilled.
According to another preferred embodiment of the invention the energy gap between the valence band and the conduction band of the first material exceeds 4,5 eV, and the invention is particularly applicable to switches being built of such material, since there is no longer any possibility to use a conducting transparent SnO
2
-layer as mentioned above.
According to another very preferred embodiment of the invention said first material is diamond. Diamond is, as mentioned, very hard to dope and it has a considerably larger band gap than SnO
2
, namely 5.4 eV compared to 4.2 eV, but it would be very well suited as a material in a photoconductive switch thanks to some excellent physical properties of diamond. Diamond has namely an extremely high breakdown field strength, which means that a photoconductive switch of diamond may hold very high voltages in the blocking
Bernhoff Hans
Isberg Jan
Asea Brown Boveri AB
Le Que T.
Pollock Vande Sande & Amernick
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