Amplifiers – With semiconductor amplifying device – Integrated circuits
Reexamination Certificate
2001-03-05
2003-06-10
Nguyen, Patricia (Department: 2817)
Amplifiers
With semiconductor amplifying device
Integrated circuits
C330S310000, C257S579000, C257S197000
Reexamination Certificate
active
06577200
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high-frequency semiconductor device, and more particularly, to an improvement in thermal stability of a multi-finger heterojunction bipolar transistor.
2. Description of the Related Art
Miniaturization is an important factor in promoting proliferation of portable terminals such as portable cellular phones. A high-output power amplifier has become a key part of a recent portable terminal.
A heterojunction bipolar transistor (hereinafter simply called “HBT”) has a high current gain &bgr;. A GaAs-based HBT having an emitter made of AlGaAs and a base made of GaAs is characterized by its high speed and is widely used for a high-output power amplifier of a portable cellular phone.
In order to attain a high output, HBTs must be arranged in shunt with each other, to there by form a multi-finger configuration. An HBT having such a multi-finger configuration will hereinafter be referred to as a “multi-finger HBT,” and individual HBTs constituting the multi-finger HBT will be called “basic HBTs.”
FIG. 8
is a plan view showing one of basic HBTs constituting a conventional high-output power amplifier.
In connection with
FIG. 8
, reference numeral
100
designates a basic HBT;
102
designates a collector layer;
104
designates a collector electrode;
106
designates a base layer;
108
designates a base electrode;
110
designates an emitter layer; and
112
designates an emitter electrode.
In a case where a multi-finger HBT is activated, the current gain &bgr; decreases with temperature. For this reason, in contrast with a homojunction bipolar transistor (for example, an Si bipolar transistor) having a current gain &bgr; increasing with temperature, the multi-finger HBT is resistant to thermal runaway.
However, it has already been reported that, because thermal nonuniformity arises among basic HBTs
100
within the multi-finger HBT, concentrated flow of current to a certain basic HBT
100
arises, thereby inducing a rapid change in an operating current. For instance, there is a report on failure arising for this reason (IEEE Transactions on Electronic Devices, Vol. 43, No. 2, February 1996, pp. 220 to 227).
Effective countermeasure against such a rapid change in an operating current is a reduction in thermal resistance of the basic HBT
100
.
Thermal resistance &THgr;TH is defined as &THgr;TH=&Dgr;Tj/&Dgr;P, where Tj is junction temperature and P is power.
FIG. 9
is a plan view showing a basic HBT constituting another conventional high-output power amplifier.
In connection with
FIG. 9
, reference numeral
120
designates a basic HBT consisting of two emitters;
110
a
and
110
b
designate split emitter layers constituting an emitter layer
110
of the basic HBT
120
; and
112
a
and
112
b
are split emitter electrodes constituting an emitter electrode of the basic HBT
120
.
FIG. 10
is a cross-sectional view taken along line X—X shown in FIG.
9
.
As shown in
FIG. 10
, reference numeral
122
designates a GaAs substrate.
The basic HBT
120
shown in
FIGS. 9 and 10
is formed from two emitters in order to reduce the thermal resistance &THgr;TH.
Two types of multi-finger HBTs are prepared: namely, a multi-finger HBT which has a single emitter
112
provided between collectors
104
, as does the basic HBT
100
; and a multi-finger HBT which has two emitters
112
provided between collectors
104
, as does the basic HBT
120
. In order to examine occurrence of thermal nonuniformity, an I-V characteristic obtained when a collector voltage Vc is changed while a base current is maintained constant is obtained as a base current parameter.
FIG. 11
is a graph showing the I-V characteristic of a multi-finger HBT using the basic HBT
100
made of a single emitter.
FIG. 12
is a graph showing the I-V characteristic of a multi-finger HBT using the basic HBT
120
made of two emitters.
As shown in
FIGS. 11 and 12
, a gradual reduction arises in an electric current within a range of small power dissipation; i.e., a range of Vc<6-volts or thereabouts, because negative feedback is applied from a power source to the HBTs
100
and
120
for reasons of temperature characteristics of current gain &bgr; of the basis HBT. In terms of such a characteristic, no difference is present between the multi-finger HBT using the basic HBT
100
made of a single emitter and the multi-finger HBT using the basic HBT
120
made of two emitters.
However, within a range of large power dissipation, i.e., a range of Vc=6V or greater, the basic HBT
100
comprising a single emitter causes breakdown of a collector current Ic at a voltage Vc lower than that at which the basic HBT
120
comprising two emitters causes breakdown of the collector current Ic. In other words, the basic HBT
100
causes breakdown of the collector current Ic in a range of power dissipation lower than that in which the basic HBT
120
causes breakdown of the collector current Ic. Broken lines A
1
shown in
FIG. 11
designate a boundary within which concentration of electric current onto the basic HBT
100
arises under the foregoing conditions. Broken lines A
2
shown in
FIG. 12
designate a boundary within which concentration of electric current the basic HBT
120
arises under the foregoing conditions. The broken lines A
2
shown in
FIG. 12
are clearly shifted toward a higher Vc range than the broken lines Al shown in FIG.
11
.
Separating an emitter to be provided between collectors into a plurality of pieces is effective for preventing occurrence of thermal nonuniformity in basic HBTs.
However, if an emitter has a plurality of parts, an area for separation must be ensured between the emitter electrodes
112
a
and
112
b
. The area of a junction between the base layer
106
and the collector layer
102
becomes large, thereby increasing the capacitance of a p-n junction. An increase in the capacitance of the p-n junction deteriorates high-frequency characteristics of a semiconductor device, thereby resulting in a decrease in gain.
Japanese Patent Application Laid-Open No. Hei. 11-102916 describes a bipolar transistor which comprises a plurality of single emitters and in which first stages of a multi-stage amplifier are connected in shunt with each other. Further, there is described proper use of a bipolar transistor of single emitter structure and a bipolar transistor of multi-emitter structure, as appropriate. However, none of the transistors correspond to a heterojunction silicon transistor.
SUMMARY OF THE INVENTION
The present invention has been conceived to overcome such a drawback and is aimed at providing a high-frequency semiconductor device having an amplifier circuit, which amplifier minimizes deterioration of a high-frequency characteristic and attains high thermal stability.
According to one aspect of the present invention, a high-frequency semiconductor device comprises a first portion of an amplifier circuit and a second portion of the amplifier circuit, which amplifies a signal output from the first portion. The first portion of an amplifier circuit is formed by means of connecting a plurality of first bipolar transistors having a hetero-junction structure in shunt with each other and is provided on a first semiconductor substrate. The second portion of the amplifier circuit is formed by means of connecting a plurality of second bipolar transistors having a hetero-junction structure in shunt with each other and being provided on a second semiconductor substrate. Further, each of the second bipolar transistors has a larger number of emitter electrodes than do the first bipolar transistor.
Accordingly, a high-frequency semiconductor device according to the present invention is advantageous in that an increase in the capacity of a p-n junction between the base layer and the collector layer of each of transistors provided in a front stage of an amplifier circuit is prevented, and that occurrence of non-uniform operations of transistors provided in a subsequent stage of an amplifier circuit having great output power can b
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