Miscellaneous active electrical nonlinear devices – circuits – and – Gating – Utilizing three or more electrode solid-state device
Reexamination Certificate
2001-03-26
2002-04-16
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Gating
Utilizing three or more electrode solid-state device
C327S430000, C327S434000
Reexamination Certificate
active
06373318
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to an electronic switching device. The electronic switching device includes a first semiconductor component. The first semiconductor component has a first cathode connection, a first anode connection, and a first grid connection. The electronic switching device further includes a second semiconductor component. The second semiconductor component has a second cathode connection, a second anode connection and a second grid connection. The first anode connection and the second cathode connection are electrically short-circuited.
Such an electronic switching device is known from WO 97/34322 A1 corresponding to commonly-owned U.S. Pat. No. 6,157,049, and from U.S. Pat. No. 5,396,085. The respectively disclosed electronic switching device also includes an electrically conductive connection between the first cathode connection and the second grid connection. This interconnection of two semiconductor components is also referred to as a cascode circuit. The electronic switching device is used for switching high electric current, and is also configured for a high reverse voltage. The first semiconductor component is made of silicon (Si) and, owing to the high charge carrier mobility in the silicon, ensures a high switching speed. The second semiconductor component is composed of a semiconductor material having a breakdown field strength of more than 10
6
V/cm, in particular of silicon carbide (SiC), and ensures a high reverse voltage.
In contrast, an electronic switching device that is produced using only silicon, for example a voltage-controlled Si-MOSFET (Metal Oxide Semiconductor Field Effect Transistor), has steady-state losses in the switched-on state which rise with the reverse voltage which has to be coped with by the Si-MOSFET in the switched-off state. In silicon, the steady-state power loss of a power MOSFET configured for a reverse voltage of more than 600 V is excessive for a forward current of more than 5 A. For this reason, Si-MOSFETs are no longer used, despite the high switching speed, for applications with a reverse voltage and a forward current in the said order of magnitude.
According to WO 97/34322 A1, the first semiconductor component is composed of Si and, overall, for a given polarity of the operating voltage, the electronic switching device also can be switched between a switched-on state and a switched-off state using a control voltage that is present at the first grid connection. When the electronic switching device is switched off, a depletion zone (zone with a reduced number of charge carriers and thus a high electrical resistance; space-charge zone) of at least one p-n junction constricts at least one channel region of the semiconductor component, which is composed of SiC. The majority of the operating voltage which is to be switched off and is applied between the first cathode connection and the second anode connection is dropped across this depletion zone. Owing to the high breakdown field strength of the silicon carbide that is used, the p-n junction, in particular its depletion zone, can withstand a considerably greater reverse voltage than a p-n junction formed from silicon and with the same charge carrier concentrations and dimensions. Because the majority of the reverse voltage is dropped within the second semiconductor component, the first semiconductor component thus need be configured only for the remaining part of the reverse voltage. This results in considerably reduced power losses in the first semiconductor component, which is composed of silicon, when switched on.
When switched on, the depletion zone of the p-n junction in the second semiconductor component is flooded with charge carriers, and the channel region is opened. An electric current now can flow through the channel region. The total power loss in the electronic switching device then includes the losses in the first and second semiconductor component. These total losses are now considerably less than those with a pure silicon semiconductor component configured for the same reverse voltage.
Integration of the two semiconductor components to form a hybrid semiconductor structure is also known from WO 97/34322 A1. The metallization, which is applied to the entire area of the surface of the second semiconductor component composed of SiC, for the second cathode connection is in this case at the same time used as the metallization for the first anode connection of the first semiconductor component, which is composed of Si.
A similar electronic switching device have a cascode circuit formed by a first semiconductor component composed of Si and a second semiconductor component composed of SiC is known from U.S. Pat. No. 5,396,085. One difference, however, is the use of a composite substrate, which contains not only an area composed of silicon but also an area composed of silicon carbide. The two semiconductor components are each produced in one of these areas of the composite substrate.
Furthermore, a cascode circuit formed from a normally-off MOSFET composed of silicon and an SIT (Stated Induction Transistor) composed of a composite semiconductor, for example of gallium arsenide (GaAs) or indium phosphide (InP) is also known from JP 61-161015 A1. This electronic device is in this case primarily used for extremely fast switching for a radio-frequency application.
In general, in the described cascode circuit, the forward resistance of the first semiconductor component has a negative-feedback effect to the second grid connection of the second semiconductor component. As the current through the electronic switching device increases, the negative bias voltage on the second grid connection also rises in comparison to the second cathode connection. The depletion zone of the p-n junction, which is located between the two connections, is thus further enlarged into the channel region intended for the current flow. Thus, as the current through the electronic switching device rises, the forward resistance of the second semiconductor component is increased, however.
In order to at least partially overcome this effect, the electrical switching device disclosed in German patent DE 34 07 975 C2, corresponding to U.S. Pat. No. 4,523,111, provides for a p-n junction in the second semiconductor component to be appropriately biased. This p-n junction is located between the second grid connection and the second cathode connection within the second semiconductor component, which is in the form of a junction field-effect transistor (JFET). The bias voltage is in this case dimensioned such that the p-n junction, and thus also the JFET overall, are in a bipolar conduction state. The bias voltage is thus greater than the diffusion voltage of this p-n junction. For silicon, the diffusion voltage is in the order of magnitude of 0.6 to 0.7 V. For bipolar operation of the p-n junction, the second semiconductor component is now no longer driven without any power consumption. A current flows via the p-n junction. Owing to this current flow, the second grid connection needs to be configured to be more stable and, in particular, also larger, as a result of which space is lost for the actual active area of the second semiconductor component. This reduces the current switching capacity of the electrical switching device. The current flow at the second grid connection furthermore leads to a capacitance of the p-n junction first of all having to be charged up or having its charge reversed when a switching process is initiated. The achievable switching speed thus also falls.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide an electronic switching device having at least two semiconductor components that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that has a low forward resistance and, at the same time, a good current switching capacity and a high switching speed.
With the foregoing and other objects in view, there is provided, in accordance with the invention, an
Dohnke Karl-Otto
Mitlehner Heinz
Stephani Dietrich
Weis Benno
Greenberg Laurence A.
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
Tran Toan
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