Millimeter-band semiconductor switching circuit

Details Millimeter-band semiconductor switching circuit Millimeter-band semiconductor switching circuit

1999-10-12

2001-11-20

Pascal, Robert (Department: 2817)

Wave transmission lines and networks

Plural channel systems

Having branched circuits

C333S262000

Reexamination Certificate

active

06320476

ABSTRACT:

BACKGROUND OF THE INVENTION
1. (Field of the Invention)
The present invention relates to a semiconductor switching circuit used in the millimeter-wave band.
2. (Description of Related Art)
Field effect transistors (FET) are typically used as a switching element for switching between transmitting and receiving signals in a communication, receiving, or transmission module used in microwave and millimeter-wave communications and radar systems.
FIG. 17A
is a front view of a FET
600
used as a single-pole single-throw (SPST) switch in a typical semiconductor switch, and
FIG. 17B
is a sectional view taken along the line XVIIB-XVIIB′ in FIG.
17
A. Drain interconnect
601
and drain electrode
602
are connected together by means of a conductive air bridge
617
bridging source electrode
605
and gate electrode
612
. Drain electrode
602
and drain electrode
603
are connected together by a conductive air bridge
618
bridging source electrode
606
and gate electrodes
613
and
614
. Drain electrode
603
and drain interconnect
604
are connected together by a conductive air bridge
619
bridging source electrode
607
and gate electrodes
615
. Source electrodes
605
,
606
, and
607
are connected to via hole
609
by way of a generally comb-shaped source interconnect
608
. Gate electrodes
612
,
613
,
614
, and
615
are interleaved with a gate current supply interconnect
616
between the above-noted source and drain electrodes. The drain interconnect
601
is connected to a transmission line
610
forming a part of an MMIC (Microwave and Millimeter-wave Integrated Circuit). Drain electrode path
604
is similarly connected to a transmission line
611
also forming another part of the MMIC.
FIG. 18
shows an equivalent circuit of the FET
600
. Inductances
623
and
624
disposed in front and rear stages of the FET
600
, respectively, have an inductance component L peculiar to the FET
600
as shown in
FIG. 17A
, and inductance
625
is an inductance component Ls of the via hole
609
shown on the left side of source electrodes
605
,
606
, and
607
in FIG.
17
A.
Switching is accomplished by controlling the voltage (which is hereinafter referred to as “gate voltage Vg”) applied to the gate electrodes, that is, to gate current supply interconnect
616
, of FET
600
. More specifically, FET
600
is on when gate voltage Vg is set to a level lower than or equal to a specific threshold value, such as when the gate voltage Vg is set to approximately 0 V, to thereby connect transmission line
610
to ground conductor
622
. As a result, there is no signal flow to transmission line
611
. When the gate voltage Vg exceeds the above-noted threshold value, FET
600
is off, signal flow from transmission line
610
to ground conductor
622
is interrupted, and signals thus flow from transmission line
610
to transmission line
611
.
FIG. 19
is an equivalent circuit of FET
600
in the ON state. Resistor
626
is an ON resistance R
on
. Impedance Z
on
of the FET observed at node B is expressed by the following equation:
Z
on
=R
on
+j
2
&pgr;f
(2
L+Ls
).
As will be known from this equation, impedance Z
on
increases as the frequency f of the RF signal input increases. When impedance Z
on
reaches a particular high level, resistance division allows part of the signal that should flow from transmission line
610
to ground conductor
622
to leak to transmission line
611
, and switching characteristics deteriorate, that is, signal loss increases and isolation deteriorates.
FIG. 20
is an equivalent circuit of FET
600
in the OFF state. Capacitance
627
is an OFF capacitance C
off
. Impedance Z
off
of the FET observed at node B is expressed by the following equation:
Z
off
=−j
/2
&pgr;fC
off
+j
2
&pgr;f
(2
L+Ls
)=−
j
[1-4 &pgr;
2
f
2
C
off
/(2
L+Ls
)]/(2
&pgr;fC
off).
As will be known from this equation, impedance Z
off
decreases as the frequency f of the RF signal increases. When impedance Z
off
reaches a particular low level, resistance division allows part of the signal that should flow from transmission line
610
to transmission line
611
to leak to ground conductor
622
, and switching characteristics again deteriorate, that is, signal loss increases and isolation deteriorates.
FIG. 21
is a Smith chart showing impedance Z
on
and impedance Z
off
, indicated by the black dots in the figure, at node B in FIG.
19
and
FIG. 20
when an RF signal of frequency f=75 GHz is passed. As noted above, impedance Z
on
when in the ON state and impedance Z
off
in the OFF state are proportional to the frequency f of the RF signal. To improve switching characteristics with high frequency RF signals, particularly in the millimeter-wave band, inductances
623
,
624
, and
625
, or more specifically the inductance L of the FET design and the inductance Ls of the via hole, must be suppressed to low levels.
SUMMARY OF THE INVENTION
An object of the present invention is therefore to provide an field effect transistor capable of exhibiting excellent switching characteristics such as a low loss and high isolation with respect to high frequency, particularly millimeter-wave RF signal, by suppressing the inductance component peculiar to the shape of the FET to a low level.
To achieve the above object, a millimeter-band semiconductor switching circuit according to the present invention comprises a field effect transistor (FET) as a switching element for the millimeter-band transmission line disposed between the millimeter-band transmission line and ground. This semiconductor switching circuit comprises a generally comb-shaped gate electrode having a plurality of gate electrode prongs and connected to a current supply path; a first electrode and a second electrode interleaved in alternating sequence with the plurality of gate electrode prongs with a specific interval therebetween; a first electrode interconnect interconnecting the plurality of first electrodes at each lengthwise end of the first electrodes; a second electrode interconnect for connecting adjacent second electrodes by means of an air bridge; and a ground line for connecting to ground the first electrode interconnect, or two second electrodes located at both ends in the connection direction and connected by way of the second electrode interconnect. A transmission line is connected to the first electrode interconnect, or the second electrodes located at both ends in the connection direction and connected by way of the second electrode interconnect, that is not connected to the ground line.
Accordingly, it is possible to reduce the inductance component between an electrode and the ground layer to thereby improve the switching characteristic, as compared with the device in which a first electrode interconnect disposed at both ends of a first electrode, or one of two second electrodes that are connected by means of a second electrode interconnect and are disposed at both ends in the connection direction, is connected to a ground layer of a semiconductor substrate. In addition, the transmission line can be connected in the same wiring pattern, thereby increasing the freedom of design incorporating the semiconductor switching circuit.
The first and second electrodes can be the drain and source electrodes, or the source and drain electrodes, respectively.
It is to be noted that the ground line can connect to ground by way of a via hole, the first electrode interconnect or two second electrodes located at both ends in the connection direction and interconnected by a second electrode interconnect. Alternatively, the ground line can directly connect to a ground plate the first electrode interconnect or two second electrodes located at both ends in the connection direction and interconnected by a second electrode interconnect.
The first electrode interconnect and second electrode interconnect can be further mutually connected by means of a resonance circuit having a specific reactance.
A further aspect of the present invention relates to a millim

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