Active solid-state devices (e.g. – transistors – solid-state diode – Bipolar transistor structure
Reexamination Certificate
2000-08-18
2003-10-14
Fahmy, Wael (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Bipolar transistor structure
C257S587000
Reexamination Certificate
active
06633075
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to heterojunction bipolar transistors including those connected in parallel and methods for fabricating the same. The invention also relates to a high-frequency transmitter or receiver having a heterojunction bipolar transistor as an amplifier.
As a high-power device for microwave band, there has been developed GaAs-based heterojunction bipolar transistors (hereinafter, referred to as HBTs). Generally, HBTs, which are high in thermal resistance, have a problem that when used as a high-power device, HBTs would involve high junction temperature. On this account, as shown in
FIG. 22
, a structure for improving heat radiation property has been proposed in Japanese Patent Laid-Open Publication HEI 8-279562.
FIG. 22A
shows a planar pattern of HBTs connected in parallel for high-power operation, and
FIG. 22B
shows a cross section taken along a line B—B of FIG.
22
A. In this structure, a plurality of HBTs
90
each having a collector electrode
106
, a base electrode
105
and an emitter electrode
104
are included on the surface side of a GaAs substrate
113
, where via holes
110
are provided between adjacent HBTs
90
so as to be cut through the substrate from its top to rear surface side. Heat generated at a junction
127
on the top surface side of each HBT
90
is conducted from the emitter electrode
104
of the transistor to a metal body
99
within its adjacent via holes
110
via an air bridge
111
, and further conducted from the metal body
99
to a plated heat sink (hereinafter, referred to as PHS) layer
112
provided on the substrate rear surface, thus being radiated.
However, this conventional structure has a first drawback that for implementation of even higher power output, electric resistance of the air bridge
111
is not negligible, with heat radiation effect insufficient, so that the junction temperature inside the transistor cannot be reduced sufficiently. In this conventional structure, there is a second drawback that because of limitations in reducing the emitter inductance, there may arise variations in high frequency characteristics or the gain in high frequency operation may decline.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a heterojunction bipolar transistor (including those connected in parallel) which is capable of improving the heat radiation and reducing the emitter inductance.
Another object of the invention is to provide a fabricating method capable of fabricating such a heterojunction bipolar transistor.
A further object of the invention is to provide a high-frequency transmitter or receiver having such a heterojunction bipolar transistor as an amplifier.
In order to achieve the above-mentioned object, the present invention provides a heterojunction bipolar transistor comprising: an emitter layer, a base layer and a collector layer laminated on a top surface of a semiconductor substrate; and a heat sink layer made of a metal and provided on a rear surface of the substrate, wherein a via hole is cut through the emitter layer, the base layer, the collector layer and the substrate, and a surface electrode of the emitter layer and the heat sink layer are connected to each other by a metal wiring line running through within the via hole.
In the heterojunction bipolar transistor of this invention, heat generated during operation at junctions (mainly, an interface between the base layer and the collector layer) on the top surface side of the semiconductor substrate is dissipated through two paths. One of the paths is a path along which the heat conducts from the junction via the surface electrode of the emitter layer to the metal wiring line on the substrate top surface side, and further conducts from the metal wiring line within the via hole to the heat sink layer on the substrate rear surface side. The other path is a path along which heat conducts from the junction via an interior of the substrate to the metal wiring line within the via hole, and further conducts from there to the heat sink layer on the substrate rear surface side. Since the heat generated at the junction is dissipated through two paths as described above, heat radiation property of the heterojunction bipolar transistor is improved. Also, since the via hole extends through the emitter layer, the base layer, the collector layer and the substrate, the surface electrode of the emitter and the top surface of the via hole are very close to each other. Therefore, the metal wiring line is led from the surface electrode of the emitter layer into the via hole at a very short distance. As a result, emitter inductance is reduced and high-frequency characteristics are improved, as compared with the case where an air bridge is used.
In an embodiment of the invention, the via hole has a cross section formed into a polygonal shape in which apex angles are obtuse angles, or a circular shape.
When the cross-sectional shape of the via hole has acute angles, there is a possibility that electric field concentration may occur at the acute-angle portions during operation, causing the device reliability to lower. Thus, in the heterojunction bipolar transistor of this embodiment, the via hole has a cross section formed into a polygonal shape in which apex angles are obtuse angles, or a circular shape. As a result, electric field concentration around the via hole is suppressed. Therefore, the device reliability is improved.
In an embodiment of the invention, an interior of the via hole is buried with a same material as that of the metal wiring line.
In the heterojunction bipolar transistor of this embodiment, since the interior of the via hole is buried with the same material as that of the metal wiring line, the heat radiation effect through the via hole is enhanced, so that the heat radiation property is further improved. As a result, stabler device characteristics as well as higher device reliability can be obtained.
In an embodiment of the invention, a peripheral edge portion of the emitter layer is formed so as to be thinner in thickness than residual portion of the emitter layer.
In the heterojunction bipolar transistor of this embodiment, the thickness of the peripheral edge portion of the emitter layer is thinner than the thickness of the residual portion of the emitter layer, that is, what we called an edge-thinning structure is formed. Therefore, re-combination of holes and electrons generated between the peripheral edge portions of the emitter layer and the base layer during operation is prevented. As a result, the device reliability can be enhanced.
In an embodiment of the invention, a plurality of heterojunction bipolar transistors are arrayed on a common semiconductor substrate and electrically connected to one another so as to be enabled to operate in parallel.
In this parallel-connected heterojunction bipolar transistors, since any of the heterojunction bipolar transistors are electrically connected to one another so as to be enabled to operate in parallel, a high-power output operation is enabled. Also, heat generated at the junction of each transistor is dissipated to the heat sink layer on the substrate rear surface. Therefore, heat concentration due to performance variations among the transistors is suppressed so that the reliability is improved.
In an embodiment of the invention, a groove extending from the top surface of the substrate to the rear surface of the substrate is provided in the common semiconductor substrate so as to partition adjacent heterojunction bipolar transistors from one another.
Generally, in parallel-connected heterojunction bipolar transistors, adjacent transistors would thermally affect one another during operation. When one transistor is unequally heated with the result of nonuniform heat generation, a transistor adjacent to the transistor is affected with the result of heat generation, which in some extreme cases leads to breakage. Also, when no transistor is present in adjacency to one transistor, there is a possibility that the thermal balance collaps
Birch & Stewart Kolasch & Birch, LLP
Fahmy Wael
Farahani Dana
Sharp Kabushiki Kaisha
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