Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Bipolar transistor
Reexamination Certificate
1999-06-15
2001-11-06
Munson, Gene M. (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Bipolar transistor
C257S077000, C257S197000, C257S586000, C257S592000
Reexamination Certificate
active
06313488
ABSTRACT:
FIELD OF THE INVENTION AND PRIOR ART
The present invention relates to a bipolar transistor having at least a low doped drift layer of crystalline SiC.
SiC has some excellent physical properties, such as a high thermal stability, a high thermal conductivity and especially a high breakdown field being approximately ten times higher than for Si, which makes SiC well suited as a material for high power devices operating under conditions were high voltages may occur in the blocking state of the device. The high breakdown field makes it possible to make transistors of SiC comparatively thin and still able to hold high voltages in the blocking state thereof reducing on-state losses.
The present invention is particularly occupied with bipolar transistors for high power applications, specifically in the fields of distribution and transmission of electric energy, for example in converters of different types as in HVDC converter stations, and will hereinafter be explained with respect thereto, but the invention should not be considered to be restricted to transistors for handling high powers and/or voltages. Other possible applications are for example as current breakers and current limiters.
“Drift layer” as defined above is to be interpreted broadly and may be a part of the collector as well as of the base in some designs of such transistors.
This type of transistor already known is normally called Bipolar Junction Transistor (BJT) and a major drawback of such transistors already known for high power applications is the high control current that must be supplied to the contact for the base of the transistor. To achieve a low control current the base width must be very small, but a thin base layer suffers from punch-through breakdown unless the doping is high. A high base doping lowers the emitter injection efficiency and this increases the control current. This means in practice that a high control current must be supplied to the base contact in the on-state of the transistor resulting in unnecessarily and mostly unacceptably high power losses.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a bipolar transistor of the type defined in the introduction, which operates more efficiently, i.e. with low on state losses and low base current, than such transistors already known and in which the drawbacks discussed above are reduced.
This object is according to the invention obtained by providing such a transistor with at least one first layer of a semiconductor material having a wider energy gap between the conduction band and the valence band than an adjacent layer of SiC. Accordingly, such a transistor may be called a Heterojunction Bi-polar Transistor (HBT), since it will have semiconductor layers of at least two different materials. However, it is emphasized that this definition also comprises the case of forming said semiconductor material with a wider bandgap by a polytype of SiC having a wider bandgap than the polytype of SiC used for an adjacent layer.
It has been found that the introduction of such a first layer of such a wider bandgap material will make it possible to reduce the on-state losses of the transistor by making it possible to obtain the on-state of the transistor by using a lower base current than would be necessary for a bipolar transistor with all the layers made of the same semiconductor material of crystalline siC.
According to a first preferred embodiment of the invention the emitter of the transistor is one said first layer. This results in a heterojunction between the emitter and the base, and this heterojunction will maintain a high carrier injection from the emitter through a low barrier therefore and suppresses the carrier injection from the base thanks to an increased valence band barrier against such injection, even with high base doping, solving the problem of a high base doping in an ordinary bipolar transistor resulting in a low emitter injection efficiency and an increased base control current.
According to another preferred embodiment of the invention the transistor comprises a continues base layer doped according to a first conductivity type and physically separating the emitter and the collector of the transistor, which are doped according to an opposite, second conductivity type, and means are arranged next to an electrode making contact to the base layer for introducing an energy barrier for minority charge carriers injected into the base layer from the emitter for reducing recombination of such minority charge carriers at the base contact electrode. It has been found that it is not possible to achieve full benefit of the HBT-structure with an heterojunction between the emitter and the base without solving the problem of recombination of minority charge carriers at the base contact. If this is not done the benefits of the HBT-structure may be lost, since a great part of the minority charge carriers injected into the base layer from the emitter will be “absorbed” by the base contact through re-combination with opposite charge carriers. This results in a higher collector-emitter voltage at a given base current, i.e. a higher base current is needed for a certain collector-emitter voltage. However, this problem is solved by the introduction of said means forming an energy barrier for the minority charge carriers resulting in a diversion of such charge carriers coming close to the base contact in the direction of the collector instead.
According to another preferred embodiment of the invention said means are formed by a sub-layer of said base layer located next to said contact electrode and having a higher doping concentration of said first conductivity type than the rest of the base layer, and a further preferred embodiment of the invention has said means formed by the arrangement of one said first layer next to the base layer for forming the base layer contact electrode and which is doped according to the first conductivity type, so that the energy barrier for the minority charge carriers will in the first case be formed by said higher doping concentration and in the second case by the introduction of a heterojunction at the base contact, and both measures will efficiently decrease electron re-combination at the base contact.
According to another preferred embodiment of the invention the base of the transistor is formed by a grid doped according to a first conductivity type and buried in the drift layer doped according to an opposite, second conductivity type while leaving drift layer regions between adjacent grid bars, the emitter and the collector of the transistor being also doped according to said second conductivity type. A bipolar transistor with this totally new design will also have the on-state losses efficiently reduced. This is mainly achieved by the fact that the charge carriers injected into the drift layer from the emitter has not to be transported in any layer in which they are minority charge carriers, which means a longer life time of the charge carriers. It will also be possible to obtain the plasma in the drift layer at a lower current of charge carriers of the first conductivity type, so that the base control current needed for obtaining a low collector-emitter voltage will be reduced. This means in the case of an emitter of n-type and accordingly a base layer of p-type that the electrons injected from the emitter have not to be transported through a p-doped region and possible limitations of very low life-times in the highly doped p-base of standard HBT due to re-combination central in e.g. B- and Al-doped SiC, are avoided, and the plasma may be obtained at a lower hole current supplied through the grid.
According to another preferred embodiment of the invention being a further development of the embodiment last mentioned the transistor comprises two base grids, one arranged close to the emitter and the other close to the collector, so that the transistor is bi-directional with respect to current conduction and turn-off capabilities and the base grid closest to the emitter in the respective type of operati
Bakowski Mietek
Breitholtz Bo
Gustafsson Ulf
Zdansky Lennart
ABB Research Limited
Connolly Bove Lodge & Hutz
Munson Gene M.
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