Miscellaneous active electrical nonlinear devices – circuits – and – Gating – Compensation for variations in external physical values
Reexamination Certificate
2001-07-26
2003-07-22
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Gating
Compensation for variations in external physical values
C327S123000, C327S257000, C327S258000, C327S375000, C333S018000, C333S203000
Reexamination Certificate
active
06597231
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor switching circuits for interrupting and switching high frequency signals in the VHF and UHF bands, and it also relates to semiconductor devices using the semiconductor switching circuits.
2. Description of the Related Art
Various kinds of wireless devices operating in the ultrahigh frequency range incorporate semiconductor switching circuits to interrupt and switch transmitted high frequency signals. In order to reduce power consumption, such switching circuits use metal semiconductor field effect transistors (MESFET) formed of a GaAs compound semiconductor or the like.
FIG. 9
shows an example of a semiconductor switching circuit of the above-mentioned type. As shown in the figure, in the semiconductor switching circuit, between an input terminal
71
for inputting a high frequency signal and an output terminal
72
outputting the high frequency signal, there is arranged a first field effect transistor (hereinafter referred to as a first FET)
77
for opening and closing the input/output terminals. The drain of the first FET
77
is connected to the input terminal
71
and the source thereof is connected to the output terminal
72
. The gate of the first FET
77
is connected to a switching terminal
73
via a resistor
81
. The switching terminal
73
receives a switching signal for controlling the first FET
77
.
Between the input terminal
71
and a ground potential terminal
76
there is arranged a second field effect transistor (hereinafter referred to as a second FET)
78
for obtaining isolation characteristics. The drain of the second FET
78
is connected to the input terminal
71
and the source thereof is connected to the ground potential terminal
76
. The gate of the second FET
78
is connected to a switching terminal
74
via a resistor
82
. The switching terminal
74
receives a switching signal for controlling the second FET
78
. The first FET
77
and the second FET
78
are MESFETs.
In the above arrangement, when the first FET
77
and the second FET
78
are depletion-type N-channel FETs, the FETs
77
and
78
are driven by applying a positive voltage. The source of the second FET
78
is connected to a terminal (external bias terminal)
75
via a resistor
83
. A positive bias voltage is applied to the terminal
75
. As a result, in the semiconductor switching circuit shown in
FIG. 9
, by applying either a positive switching voltage higher than a predetermined threshold voltage or a ground potential to the FETs
77
and
78
from the switching terminals
73
and
74
which receive the switching signals, the circuit between the input/output terminals
71
and
72
can be opened and closed.
For example, with constant bias voltage on the terminal
75
, when the same voltage is applied to the terminal
74
, the FET
78
becomes ON; and when ground potential is applied to the terminal
74
, the FET
78
becomes OFF.
Thus, by applying appropriate switching voltages to the terminals
73
and
74
, it can be arranged that when the first FET
77
is conducting, the second FET
78
is not conducting; and when the first FET
77
is not conducting, the second FET
78
is conducting. By operating the second FET
78
in this way, sufficient isolation characteristics between the input terminal
71
and the output terminal
72
can be maintained, particularly when the first FET
77
is not conducting.
Between the source of the second FET
78
and the ground potential terminal
76
, a parasitic inductance component
85
is generated by a bonding wire and a lead frame, when the semiconductor switching circuit is formed into an IC chip to be used as a semiconductor device. In this case, in terms of the parasitic inductance
85
, the higher the frequency, the higher the impedance. Thus, since the impedance between the second FET
78
and the ground potential terminal
76
becomes higher in a high frequency region, the impedance of the input terminal cannot be sufficiently lowered. As a result, when the first FET
77
is not conducting and the second FET
78
is conducting, satisfactory isolation characteristics between the input terminal
71
and the output terminal
72
cannot be maintained.
Therefore, in this semiconductor switching circuit, in order to obtain sufficient isolation characteristics between the input terminal
71
and the output terminal
72
, a capacitance element
84
is connected in series with the parasitic inductance component
85
. In other words, the capacitance element
84
has a value set to permit serial resonance with the parasitic inductance
85
at a specified frequency. In this case, a resonance frequency necessary to improve the isolation characteristics between the input terminal
71
and the output terminal
72
is represented by the symbol f, the value of the inductance component
85
is represented by the symbol L, and the value of the capacitance element
84
is represented by the symbol C. A condition for producing the serial resonance is represented by C=1/(4&pgr;
2
×f
2
×L). When the value C of the capacitance element
84
is determined and thereby a serial resonance is produced at a specified frequency, the impedance between the input terminal
71
and the ground potential terminal
76
can be minimized. Accordingly, when the first FET
77
is not conducting and the second FET
78
is conducting, good isolation characteristics between the input terminal
71
and the output terminal
72
can be maintained. In addition, besides the above function, the capacitance element
84
has a DC blocking function that isolates the power supply voltage applied to the external bias terminal
75
from the ground potential terminal
76
.
The capacitance element
84
is generally formed as a metal-insulation capacitor on the semiconductor chip. After the capacitance clement
84
and the FET have been integrated into a chip to form a monolithic microwave integrated circuit (hereinafter referred to as MMIC), the value of the parasitic inductance
85
generated by the bonding wire and the lead frame can no longer be adjusted. Thus, it requires a lot of time and experimentation to set the value of the capacitance element
84
most appropriately.
In addition, the capacitance element
84
is formed not by a pure capacitance component but by a capacitance component including a parasitic inductance component generated by metal electrodes, wires and the like. Consequently, since an inductance component required for the serial-resonance condition is equivalent to a sum of the inductance components of the parasitic inductance
85
and the capacitance element
84
, the configuration of the metal wire used needs to be considered when setting the value of the capacitance element
84
.
The entire inductance component, which is equivalent to the sum of the inductance components of the parasitic inductance
85
and the capacitance element
84
generated by the bonding wire and the lead frame, usually has a small value of a few nH or lower. Therefore, in order to produce a serial resonance in a low frequency region, a large capacitance component relative to this small inductance is required. When there is provided a large capacitance component, changes in the impedance near a frequency at which the impedance of the serial resonance circuit is zero become smaller. Thus, a frequency band in which the impedance of the serial resonance circuit is small is broadened with respect to the resonance frequency, and therefore, sufficient isolation can be provided over a wide frequency range. In contrast, in order to produce a serial resonance at high frequencies, a small capacitance component relative to the inductance is required. In this situation, near the frequency at which the impedance of a serial resonance circuit is zero, the impedance changes increase. As a result, the frequency band in which the impedance of the serial resonance circuit is small is narrowed, which greatly narrows the frequency band where sufficient isolation is obtainable.
Specifica
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
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