Active solid-state devices (e.g. – transistors – solid-state diode – Specified wide band gap semiconductor material other than... – Diamond or silicon carbide
Reexamination Certificate
1998-06-09
2001-03-20
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Specified wide band gap semiconductor material other than...
Diamond or silicon carbide
C257S184000, C257S461000, C257S464000
Reexamination Certificate
active
06204522
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a device having two terminals interconnected by one or more material layers for switching between a current conducting state and a state of blocking transport of charge carriers between the terminals upon applying a voltage thereacross.
Accordingly, the invention relates to a switching device in the broadest sense of this expression and is directed to all types of devices adapted to assume a state, in which they are conducting when a voltage is applied across the terminals and a state in which a transport of charge carriers between the terminals is blocked in spite of a voltage being applied across the terminals. The device may be of the type, in which the voltage in the conducting state has an opposite direction than in the blocking state. The simplest device of this type is a rectifying diode, but the device may also be of the type capable to assume either a conducting state or a blocking state when a voltage is applied across the terminals in one and the same direction.
The definition “terminals interconnected by one or more material layers” is used for limiting the invention with respect to circuit breakers or switching devices obtaining the switching action by breaking and establishing a physical connection between the terminals of the device, i.e. connecting and disconnecting them. In contrast thereto, the physical properties of the material layers in connection with the availability of charge carriers and voltage will decide the state of the device.
A device of this type finds many applications, but the use of such a device for high power applications will hereinafter be discussed for clarity, but not in any way restricting the invention.
A device of this type may be used in equipment for handling high electric power for switching high voltages and currents for instance in circuit breakers, commutators, current valves, surge diverters, current limiters and the like. The breakdown voltage of such a device is in most of these applications considerably lower than the voltage to be held by the position in which the device is arranged in the equipment, so that it is necessary to connect a comparatively large number of such devices in series for distributing the total voltage them. The total voltage may well exceed 100 kV, whereas a single device may for instance have a breakdown voltage of 2-5 kV. A complicated and thus costly equipment is required for controlling such devices. Also, equipment for cooling them has to be rather sophisticated and expensive, especially in high frequency operation, for instance when the device is used in current valves switched according to Pulse Width Modulation (PWM) in converter stations. In fact, the major part of the costs for a converter station is caused by the controlling and cooling equipment, so that it is highly desired to reduce the number of devices required in such stations and other high power applications for saving costs.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a device of the type defined in the introduction reducing the problems of already known devices as discussed above.
This object is according to the invention obtained by providing a device with a first layer made of intrinsic diamond and a second layer arranged next to the first layer and means for switching to the conducting state by providing free charge carriers in the second layer for transport through the diamond layer through said voltage and switching back to the blocking state by terminating the provision of the free charge carriers for the transport. The diamond layer is adapted to take a major part of the voltage across the terminals in the blocking state.
The main benefit of such a device is that diamond has an extremely high breakdown field strength, which means that the number of devices to be connected in series for holding a voltage of a certain magnitude may be reduced considerably with respect to prior art devices. This results in important cost reduction even if such a device itself would be much more expensive than the prior art devices, which for the rest is not any evident fact. It has until now been very difficult to dope diamond, and intrinsic, undoped diamond has not been used in semiconductor devices in the current-conducting, active layers of the device, but it has been seen as a material primarily suited for use in insulating layers, such as as a gate insulator, in which it is possible to benefit from the excellent insulating properties thereof.
However, the present inventors have proved that a layer of intrinsic diamond may function very well in a device of this type, in which the extremely high breakdown field strength of diamond is used in the blocking state of the device. The device may nevertheless conduct a current without generating any high losses in the conducting state thanks to the provision of the free charge carriers in the second layer for allowing current conduction through the diamond layer which has a high conductivity due to the comparatively high charge carrier mobility in intrinsic diamond. “Intrinsic diamond” means that the diamond layer is either undoped or compensation doped or that the dopants are not thermally activated at temperatures of interest.
Furthermore, diamond has the highest known thermal conductivity of any solid near room temperature, which makes it well suited for high power applications, especially as a heat sink in high frequency devices, where cooling can be a limiting factor in achieving greater switching speeds. The high breakdown field strength of diamond means that a diamond layer may be made much thinner than a layer of for instance Si for the same breakdown voltage, which will considerably reduce switching losses and problems with reverse recovery, so that the switching speed may be increased. Additionally, short carrier lifetimes make possible higher switching speeds in a diamond device. Another advantage is that diamond is extremely temperature stable, in the sense that the thermal expansion thereof is very low and it remains an insulator up to very high temperatures due to the large band gap (5,4 eV) thereof, which means that it may function well under high temperature conditions, well up to 1,000 K, so that the device may be used in such applications.
According to a preferred embodiment of the invention the second layer is of a material having a substantially smaller energy gap between the valence band and the conduction band than diamond, and the means is adapted to cause switching to the conducting state and the blocking state by generating free charge carriers in the second layer for injection into the first layer and terminating the generation, respectively. An advantage of such a device is that the smaller band gap in the second layer means that the free charge carriers may be generated more readily and at a much lower cost, i.e. simpler equipment may be used therefor than should the free charge carriers instead be generated in diamond. Another advantage is that such a device may optionally assume the conducting state and the blocking state for the same direction of the voltage or an identical voltage applied across the terminals thereof by simply initiating or terminating the generation of the free charge carriers, so that a current in a determined direction may be switched on and off rapidly without any change in the direction of the voltage.
According to another preferred embodiment of the invention the means for generating free charge carriers is adapted to irradiate the second layer with photon radiation having sufficient energy to create free charge carriers in the second layer. This is one preferred potential means of generating the free charge carriers in the second layer enabling very fast switching of the device.
According to another preferred embodiment of the invention the means is adapted to generate free charge carriers by irradiating the second layer with electrons having an energy sufficiently high for creating free charge carriers in said second layer. This embodiment also resul
Bernhoff Hans
Irwin Mark
Isberg Jan
Isberg Peter
Öberg Åke
Asea Brown Boveri AB
Connolly Bove Lodge & Hutz
Crane Sara
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