High-isolation semiconductor device

Details High-isolation semiconductor device High-isolation semiconductor device

2003-01-14

2004-09-07

Flynn, Nathan J. (Department: 2826)

Active solid-state devices (e.g., transistors, solid-state diode

Regenerative type switching device

C257S701000, C257S150000, C257S178000

Reexamination Certificate

active

06787815

ABSTRACT:

BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a high-isolation semiconductor device and, more particularly, to a high-isolation semiconductor device suitable for use in a satellite television receiver.
(b) Description of the Related Art
In recent years, satellite-broadcasting systems are widely spread among a large number of households, and thus the number of satellites used for the satellite-broadcasting systems is increasing. In a satellite broadcasting receiver system for receiving the multiple-channel programs, a switching device is often used for switching a plurality of high-frequency signals, or radio-frequency signals (RF signals), supplied through respective channels to deliver a selected one of the RF signals to each of a plurality of receivers. The switching device for the RF signals is generally implemented by a matrix switch having a plurality of inputs and a plurality of outputs, wherein a selected one of the inputs can be coupled to each of the outputs.
FIG. 7
shows a typical circuit configuration of a switching device
500
for receiving two input RF signals, signal A and signal B, to deliver a selected one of the input signals to each of two receivers
80
and
81
. The switching device
500
is implemented by a 2×2 matrix switch having two inputs and two outputs, or two SPDT (single-pole-double-throw) switches. Each of the receivers
80
and
81
is connected to a corresponding output terminal of the switching device
500
to receive one of the RF signals, signal A and signal B, independently of each other.
The matrix switches incorporated in a communication SWIG or communication LSI are generally implemented by transistors, such as a junction PET, or a pin-junction photo-diode The communication SWICs, for example, include a SPDT SWIC having a plurality of separate SPDTs which are the minimum switch units, and a twin-SWIC having a plurality of SPDT pairs, each pair being connected in parallel. The SWIC is mounted on a mounting board having a dielectric substrate and patterned interconnections formed on top and bottom surfaces of the dielectric substrate. In an alternative, the twin-SWIC, for example, may include interconnections therein and may be received in a package together with the interconnections.
The performance of a matrix switch with respect to the isolation characteristic is estimated by an isolation D/U (desired-to-undesired) ratio of each signal line receiving a desired output signal, which is calculated by the power ratio of the desired signal component to the undesired signal component, both the components being included in the output signal. It is desired that transmitters and receivers used in the up-to-date satellite broadcasting system have an isolation D/U ratio as high as 40 dB or above.
The isolation D/U ratio as high as 40 dB or above can be achieved in a SWIC itself as a separate unit by suitably selecting the constituent elements, circuit configuration, circuit constants and layout pattern of the SWIC. However, since a plurality of SWICs are connected together by signal lines for achieving a matrix switch having a plurality of inputs and a plurality of outputs, there are crossing points between these signal lines which transmit the RF signals. This makes it difficult to achieve a high isolation D/U ratio with respect to the signal lines, whereby the switching device cannot achieve a higher isolation characteristic as a whole. In the example shown in
FIG. 7
, the signal line for transmitting the signal A and the signal line for transmitting the signal B cross each other at the crossing point C
1
, wherein the signal leakage arises therebetween to degrade the isolation D/U ratio of the signal lines.
As described above, a 2×2 matrix switch, for example, is generally implemented by mounting a plurality of SPDT SWICs or twin-SWICs on a mounting board having patterned interconnections or mounting a single 2×2 matrix SWIC on such a mounting board. In the former case, a crossing point arises between RF signal lines on the mounting board, whereas in the latter case, a crossing point arises between RF signal lines within the 2×2 matrix SWIC. In either case, the signal leakage should be suppressed by enlarging the distance between the RF signal lines and/or reducing the projected area of the opposing RF signal lines at the crossing point.
FIG. 8A
is the top plan view of a switching device implemented by a single SWIC including a 2×2 matrix switch, whereas
FIG. 8B
is the sectional view of the crossing point shown in FIG.
8
A. The switching device includes RF signal lines which include a plated signal line
46
plated with gold and overlying a dielectric film (oxide film)
42
, and a metallic signal line
47
underlying the dielectric film
42
, wherein the signal lines
46
and
47
oppose each other at the crossing point C
1
with an intervention of the dielectric film
42
.
The dielectric film
42
has a small thickness on the order of micrometers, whereby the distance between the RF signal lines
46
and
47
is small and a large cross-talk may arise therebetween. Assuming that the projected area of the opposing RF signal line is around a several-micrometer square, the level of the cross-talk between the RF signal lines is around 30 dB, which is not negligible for the switching device.
FIGS. 9A and 9B
show, similarly to
FIGS. 8A and 8B
, another conventional switching device
600
implemented by two SWICs
10
and
11
each including SPDT switch and mounted on a common mounting board
40
. In this example, metallic signal lines
47
oppose each other with an intervention of the mounting board
40
having a thickness of around 1 mm or below.
In the example shown in
FIGS. 9A and 9B
, since the RF signal lines
47
on the mounting board
40
have a characteristic impedance of 50 &OHgr;, the RF signal lines
47
have a width of around 1 mm, assuming that the mounting board
40
has a dielectric constant &egr; of 3.38 and a thickness of 0.51 mm, for example. This may cause a significant cross-talk level between the RF signal lines
47
, e.g., an isolation D/U ratio around 30 dB with respect to the RF signal lines
47
. In addition, if the mounting board
40
has a micro-strip line structure, there arises a mismatching of the characteristic impedance due to the fact that a ground plane cannot be provided at the crossing point C
1
on the surface of the mounting board
40
opposite to the surface on which the subject RF signal line
47
is formed.
FIGS. 10A and 10B
show, similarly to
FIGS. 8A and 8B
, another conventional switching device
700
implemented by two SWICs
10
and
11
each including SPDT switch and mounted on a common mounting board, which includes a pair of dielectric substrates
40
and a ground plane
80
sandwiched therebetween. In this example, RF signal lines
47
oppose each other with an intervention of the mounting board including the ground plane
80
and the dielectric substrates
40
. This structure allows significant reduction of the cross-talk level between the RF signal lines
47
and suppresses the mismatching of the characteristic impedance. However, the mounting board has a larger thickness and the fabrication cost thereof is higher.
FIG. 11
is a top plan view of another conventional switching device
800
implemented by a twin-SWIC
20
. In this example, a semi-rigid cable
50
is used as a part of one of the RF signal lines
47
and raised from the mounting board
40
at the crossing point C
1
. This structure reduces the cross-talk between the RF signal lines
47
; however, the junction for connecting the signal line
47
on the mounting board
40
and the semi-rigid cable
50
causes miss-matching of the characteristic impedance and thus degrades the signal characteristics, such as a return loss. In addition, arrangement of the semi-rigid cable
50
above the mounting board
40
raises the costs for the switching device
800
.
As described above, the conventional techniques for the switching device do not achieve a high isolation

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