Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
2002-05-10
2004-05-25
Tokar, Michael (Department: 2819)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C330S084000
Reexamination Certificate
active
06741144
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a field-effect transistor (hereinafter, referred to as an “FET”) used in a microwave band. In particular, the present invention relates to a GaAs power FET element having an internal matching circuit.
2. Description of the Related Art
There is an increasing demand for a GaAs FET as a device for mobile communication equipment such as a mobile phone, due to its excellent high-frequency characteristics. Particularly, a GaAs power FET is applied, as an amplifier of power for transmission, to a base station as well as a terminal of a mobile phone, and such an FET contributes to the miniaturization and low power consumption of the base station due to its high-output and high-efficiency characteristics. In the present specification, various high-frequency devices such as a power FET, a low-noise FET, and a mixer will be referred to collectively as a high-frequency semiconductor device.
Hereinafter, a conventional high-frequency semiconductor device will be described.
FIGS. 12A and 12B
are schematic views of a conventional GaAs power FET element having an internal matching circuit.
FIG. 12A
is a plan view showing the inside of the FET element, and
FIG. 12B
is across-sectional view taken along a line E—E′ in
FIG. 12A
In
FIGS. 12A and 12B
, a package
17
has a configuration in which a frame
16
made of ceramic is welded to a bottom portion
13
mainly made of copper. The bottom portion
13
is plated with gold. FET chips
1
a
and
1
b
are mounted substantially at a central portion of the package
17
. An incoming dielectric substrate
91
made of ceramic is mounted on an input side of the FET chips
1
a
and
1
b
. An incoming distributed constant line
93
is formed on the surface of the incoming dielectric substrate
91
. An outgoing dielectric substrate
92
is mounted on an output side of the FET chips
1
a
and
1
b
. An outgoing distributed constant line
94
is formed on the surface of the outgoing dielectric substrate
92
. An input terminal
10
and the incoming distributed constant line
93
are connected electrically to each other via bonding wires
19
. Similarly, the incoming distributed constant line
93
and the FET chips
1
a
,
1
b
; the FET chips
1
a
,
1
b
and the outgoing distributed constant line
94
; and the outgoing distributed constant line
94
and an output terminal
12
respectively are connected to each other via the bonding wires
19
.
In order to obtain a high-frequency power from the power FET, it is required to form an input impedance matching circuit and an output impedance matching circuit outside of the power FET so as to reduce the reflection of a high-frequency power.
Since the total gate width of the FET chips
1
a
and
1
b
is very large, the input and output impedances thereof are very low (i.e., 1 &OHgr; or less). Thus, when it is attempted to obtain an impedance matching circuit directly in such a low impedance state, optimum matching conditions are not obtained, and a power loss becomes very large. In order to obtain power from the FET efficiently, it is important that the impedance of the FET is once converted to a high level (about 10 &OHgr;). In general, the incoming distributed constant line
93
and the outgoing distributed constant line
94
also are called internal matching circuits and designed so as to realize such impedance conversion.
An abnormal oscillation, which can be a serious problem in using the power FET element, will be described below.
In the FET elements in
FIGS. 12A and 12B
, the case where there is a variation in a threshold value (V
th
) and a mutual conductance (gm) between regions M and N of the FET chip
1
a
will be considered. For example, in the case where a high-frequency power output from the region M of the FET chip
1
a
is larger than that output from the region N, a roundabout power
96
is generated on the outgoing distributed constant line
94
. The roundabout power
96
becomes a reflection power to the region N, whereby the impedance on the output side seen from the region N is changed. More specifically, a difference in impedance on the output side is caused between the regions M and N. As a result, a power imbalance further is increased, resulting in an abnormal oscillation. According the actual measurement, when there is a difference of 0.2 V in a threshold voltage between the regions M and N, an abnormal oscillation was caused in the vicinity of the maximum output.
Next, the case where there is a variation in V
th
and gm between the FET chips
1
a
and
1
b
will be considered. For example, in the case where a high-frequency power output from the FET chip
1
a
is larger than that output from the FET chip
1
b
, a roundabout power
97
is generated on the outgoing distributed constant line
94
. When the roundabout power
97
is generated, a reflection power to the FET chip
1
b
is increased, resulting in a change in impedance on the output side seen from the FET chip
1
b
. More specifically, a difference in impedance on the output side is caused between the FET chips
1
a
and
1
b
, and the difference in high-frequency power to be output further is increased. A power imbalance is increased, resulting in an abnormal oscillation.
The abnormal oscillation is caused not only when the FET chips
1
a
and
1
b
are varied, but also when an imbalance is likely to be caused in an operation of the FET chips
1
a
and
1
b
(e.g., under a transient condition (during power-on) or when an unnecessary signal is input instantaneously). When an abnormal oscillation is caused, an interference wave not only has an adverse effect on radio communication, but also damages the FET element, which is a serious problem in terms of reliability.
SUMMARY OF THE INVENTION
Therefore, with the foregoing in mind, it is an object of the present invention to provide a high-frequency GaAs power FET element with high performance, capable of suppressing an abnormal oscillation.
In order to achieve the above-mentioned object, a first high-frequency semiconductor device of the present invention includes: an amplifier; a dielectric substrate provided on an input side or an output side of the amplifier; a plurality of transmission lines formed on a surface of the dielectric substrate and connected electrically to the amplifier; and a resistor formed on the surface of the dielectric substrate and connected electrically between the plurality of transmission lines.
In the first high-frequency semiconductor device, it is preferable that the plurality of transmission lines have an electrical length of substantially &lgr;/4 with respect to an operation frequency, and the resistor has the same length as that of the plurality of transmission lines in a traveling direction of a high-frequency power.
In order to achieve the above-mentioned object, a second high-frequency semiconductor device of the present invention includes: first and second amplifiers; a dielectric substrate provided on an input side or an output side of the first and second amplifiers; a first transmission line formed on a surface of the dielectric substrate and connected electrically to the first amplifier; a second transmission line formed on a surface of the dielectric substrate and connected electrically to the second amplifier; and a resistor formed on a surface of the dielectric substrate and connected electrically between the first and second transmission lines.
In the second high-frequency semiconductor device, it is preferable that the first and second transmission lines have an electrical length of substantially &lgr;/4 with respect to an operation frequency, and the resistor has the same length as those of the first and second transmission lines in a traveling direction of a high-frequency power.
In order to achieve the above-mentioned object, a third high-frequency semiconductor device includes: first and second amplifiers; a dielectric substrate provided on an input side or an output side of the first and second amplifiers; a first transmission line form
Maeda Masahiro
Morimoto Shigeru
Matsushita Electric Industrial Co. Ltd
Merchant & Gould P.C.
Nguyen Linh V
Tokar Michael
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