Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Bipolar transistor
Reexamination Certificate
2001-01-31
2003-07-15
Lee, Eddie (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Bipolar transistor
C257S198000, C257S199000, C257S200000, C257S201000, C257S571000, C257S573000, C257S586000, C438S309000, C438S312000, C438S235000
Reexamination Certificate
active
06593604
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a heterojunction bipolar transistor (hereinafter, abbreviated as HBT) and manufacturing method for HTB as well as a communication device with HTB.
Conventionally, in HBTs, there has been adopted a barrier structure in which part of an emitter layer is left on an external base region, which is defined as a region where a base layer outwardly extends from an emitter region, so that holes within the base layer are inhibited from reaching a surface of the external base on the external base region. By doing so, it becomes possible to prevent decrease of reliability due to recombination of electrons diffused from the emitter layer with holes in the base layer, which combination occurs on the surface of the external base region.
The emitter layer left on the external base region needs to be of such a thickness that the emitter layer is depleted within a bias voltage range normal for transistors in order that current does not leak between an emitter and a base through the left emitter layer. Therefore, for more reliable formation of this thickness, it has been conventional practice to form a layer using V-group phosphorus as the emitter layer, thereby providing an epitaxial structure that enables selective etching.
As an example of conventional HBTs, an HBT structure of Japanese Patent Laid-Open Publication HEI 6-244195 is shown in FIG.
11
. Referring to
FIG. 11
, reference numeral
1
denotes a semi-insulating GaAs substrate,
2
denotes a GaAs collector contact layer,
3
denotes a GaAs collector layer,
4
denotes a GaAs base layer,
5
denotes an InGaP emitter layer,
6
denotes an AlGaAs emitter layer,
7
denotes an AlGaAs graded layer,
8
denotes a GaAs cap layer,
9
denotes an InGaAs contact layer,
10
denotes an emitter electrode,
11
denotes a base electrode, and
12
denotes a collector electrode
12
. In this case, the emitter layer
5
left on the external base region in the GaAs base layer
4
is very thin, 20 nm.
However, the conventional HBT disclosed in Japanese Patent Laid-Open Publication HEI 6-244195 has the following disadvantages.
The HBT having the structure shown in
FIG. 11
is fabricated as follows. First, on the semi-insulating GaAs substrate
1
, individual layers from the GaAs collector contact layer
2
to the InGaAs contact layer
9
are stacked sequentially. Next, the InGaAs contact layer
9
, the GaAs cap layer
8
, the AlGaAs graded layer
7
and the AlGaAs emitter layer
6
are etched by an etchant of phosphorus acid/hydrogen peroxide mixed aqueous solution, and the etching is ended selectively at the InGaP emitter layer
5
. Subsequently, a resist pattern
13
is formed so as to cover the external base region, and the InGaP emitter layer
5
is etched by an etchant of hydrochloric acid/phosphorus acid mixed etchant, the etching being ended the GaAs base layer
4
. Then, the GaAs base layer
4
and the GaAs collector layer
3
are etched again by an etchant of phosphorus acid/hydrogen peroxide mixed aqueous solution.
However, the HBT formed in this way, actually, has the InGaP emitter layer
5
overhanging at its ends in a protruded form by a width A, as shown in FIG.
12
. For the reason that in the process of etching the GaAs base layer
4
and the GaAs collector layer
3
with the etchant of phosphorus acid/hydrogen peroxide mixed aqueous solution, the GaAs base layer
4
and the GaAs collector layer
3
are etched even at their side faces, while the InGaP emitter layer
5
is not etched
In that case, the InGaP emitter layer
5
, which is set to such a thickness that the layer is depleted in the external base region, has quite a small thickness as thin as about 20 nm. Accordingly, in the etching process of the GaAs base layer
4
and the GaAs collector layer
3
or in later processes, the protrusion
14
may be damaged, allowing moisture or the like to penetrate through the broken places, which would cause decrease in yield and deterioration of reliability.
Therefore, in order to avoid these disadvantages, it is possible to remove the protrusion
14
of the InGaP emitter layer
5
by etching with a hydrochloric acid/phosphorus acid mixed etchant. This etching is conducted after the GaAs base layer
4
and the GaAs collector layer
3
are etched with the etchant of phosphorus acid/hydrogen peroxide mixed aqueous solution and before the resist pattern
13
is removed. In this case, however, side etching of the protrusion
14
occurs during the etching as shown in
FIG. 13
, so that the end of the InGaP emitter layer
5
are located inside the end of the GaAs base layer
4
. As a result, the base layer
4
is exposed in an outer portion of the external base region. For this reason, the base electrode ohmic material directly contacts the base layer
4
outside the external base region. Consequently, when the base electrode ohmic material is diffused via the emitter layer
5
into the base layer
4
with ohmic junction, the base electrode ohmic material is simultaneously diffused via the base layer
4
up to the collector layer
3
in the outer portion of the external base region where the base layer
4
is exposed, as shown by broken line in FIG.
13
. Accordingly, there arises a failure that a leak current flows between base and collector.
In particular, when the external base region is reduced in area for enhancement of the HBT performance, it becomes even more difficult to form the base electrode
11
while avoiding the upper surface of the base layer
4
from being exposed to the base electrode ohmic material due to the side etching of the emitter layer
5
. As a result, the base electrode
11
comes into contact with the exposed portion of the base layer
4
, giving rise to the failure that a leak current flows between base and collector.
As described above, conventional HBTs are low in reliability whether the protrusion is present in HBT or the protrusion has been removed by etching. Accordingly, the conventional HBTs have disadvantages of frequent failures and low durability when the conventional HBTs are used for power amplifiers in portable communication devices, which are used in wide-range environments and particularly which cannot sufficiently afford to provide for air-tightness because of demands for lightening of weight and reduction of size.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an HBT as well as a manufacturing method therefor which can prevent damage of a protrusion of an emitter layer present on an external base region of the HBT, and to provide a communication device using the HBT.
In order to achieve the above object, in a first aspect of the invention, there is provided a heterojunction bipolar transistor in which an emitter mesa portion is formed of emitter layers in a mesa shape on a base layer, the emitter mesa portion having a lower layer of the emitter layers to extend up to an upper surface of an external base region which is a region of the base layer outside the base layer under the emitter mesa portion, comprising: an emitter layer formed of the lower layer in a first region present on the external base region located and next to the emitter mesa portion; and emitter layers including the lower emitter layer and forming a second region located outside the first region, wherein the emitter layers in the second region is thicker than emitter layer in the first region.
Normally, thickness of the lower layer of the emitter mesa portion extending to the external base region is set to such a thickness that an emitter layer of the lower layer is depleted within a normal bias voltage range to prevent current leaks between an emitter and a base through the emitter layer. For this purpose, it is necessary to deplete the emitter layer within a range from a base electrode formed on the external base region up to the emitter mesa portion. The rest of the emitter layer present on the external base region may be made thick without causing any problem.
With this constitution, as to thickness of the emitter layer or layers o
Birch Stewart Kolasch & Birch, LLP.
Diaz Jose′ R.
Sharp Kabushiki Kaisha
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