Active solid-state devices (e.g. – transistors – solid-state diode – Transmission line lead
Reexamination Certificate
1999-07-12
2004-10-05
Lee, Eddie (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Transmission line lead
C257S758000
Reexamination Certificate
active
06800929
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to a microwave integrated circuit, and more particularly relates to a microstrip line structure for a semiconductor device operable at radio frequencies, which contributes to downsizing and performance enhancement of mobile communications unit terminals.
In recent years, the applications of mobile communications terminals of various types, including cellular phones and portable communications terminals, have been spanning a wider and wider range over the world. In Japan, cellular phones, operating on 900 MHz and 1.5 GHz bands, and personal handy phone systems (PHS), operating on 1.9 GHz band, have been popularized. Globally speaking, European GSM and DECT phones and American PCs phones are very popular.
Among these numerous types of mobile communications unit terminals, portable communications terminals, in particular, are required to be as small in size and as light in weight as possible. Thus, first of all, components for a portable communications terminal should have its size reduced and its performance enhanced. For example, to downsize a power amplifier for use in a radio-frequency transmitter for a portable communications terminal (hereinafter, simply referred to as an “RF power amplifier”), it is strongly needed to implement the RF power amplifier as a monolithic microwave IC (MMIC) of GaAs. In an MMIC, active components, matching circuit and bias supply are all integrated on a single chip. Thus, an MMIC can more effectively contribute to downsizing than a hybrid IC (HIC), in which matching circuit and bias supply are implemented as discrete chip components.
From a viewpoint of performance enhancement, however, an MMIC is said to be inferior to an HIC. This is because if an RF power amplifier is implemented as an MMIC, for instance, then parasitic resistive components, such as interconnection resistance, which are involved with semiconductor device processing for fabricating the MMIC, adversely increase, thus causing a considerable loss of the power to be transmitted. For that reason, a power amplifier implemented as an MMIC often results in lower power gain, lower power amplification and deteriorated distortion characteristic compared to a power amplifier implemented as an HIC. Thus, according to the currently available techniques, it is determined based on a necessary trade-off between downsizing and performance enhancement which part of a power amplifier should be implemented as an MMIC.
Hereinafter, an exemplary MMIC implementation of output matching circuit and drain-biasing circuit will be described with reference to
FIGS. 10 through 15
. An exemplary microstrip line structure will also be described with reference to
FIG. 16. A
microstrip line structure is a basic structure of a spiral inductor used as a passive component in the out-put matching circuit and drain-biasing circuit.
FIG. 10
illustrates a planar pattern for a final-stage MESFET and an output matching circuit thereof used for a high-output power amplifier transmitting a power of about 1 W. Following is respective parameters of the final-stage MESFET.
Unit finger length: 300 &mgr;m
Total gate width: 24 mm
Frequency: 900 MHz
Power supply voltage: 3.5 V
Saturated output power with idle current of 400 mA supplied: about 1.5 W
Operating current: about 550 mA
Gain: about 12 dB
The gate of the MESFET
410
is connected to a gate-biasing pad
412
via a gate electrode extended line
411
. The source of the MESFET
410
is connected to an MIM capacitor
409
via a source pad
413
. The drain of the MESFET
410
is connected to a drain-biasing pad
415
via a drain extended line
414
. One terminal of a spiral inductor
408
is connected to a part of the drain extended line
414
, while the other terminal thereof is connected to an output pad
416
via the MIM capacitor
409
.
The line of the spiral inductor
408
is made of gold plated to be about 3 &mgr;m thick. The extended lines thereof are formed by evaporating and depositing titanium and gold thereon. The upper-layer conductor of the MIM capacitor
409
is made of gold plated, while the lower-layer conductor thereof is formed by evaporating and depositing titanium and gold thereon. The interlayer dielectric film of the capacitor
409
is formed by depositing silicon nitride (SiN
x
) with a dielectric constant of about 7 by a CVD process.
In the MMIC including the output matching circuit, the final-stage MESFET operates at a frequency of 900 MHz with a current of about 560 mA supplied. The MESFET provides a saturated output power of about 1.0 W with a power supply voltage of 3.5 V applied, and shows a gain of about 10 dB.
FIG. 11
illustrates an equivalent circuit of the MESFET and output matching circuit thereof shown in
FIG. 10. A
MESFET shown in
FIG. 11
, including gate
302
, source
303
and drain terminals
304
, corresponds to the MESFET
410
shown in FIG.
10
. Equivalent series inductor
305
, equivalent series resistor
306
and equivalent parallel capacitor
307
are connected to the drain terminal
304
of the MESFET. The equivalent series inductor
305
and resistor
306
form an equivalent circuit of the spiral inductor
408
. In this example, the inductance value of the equivalent series inductor
305
is about 2.5 nH, the resistance value of the equivalent series resistor
306
is about 4 &OHgr; and the capacitance value of the equivalent parallel capacitor
307
is about 12 pF. The equivalent parallel capacitor
307
corresponds to the MIM capacitor
409
shown in FIG.
10
.
FIG. 12
illustrates a location of load impedance Z
L
301
of the MESFET on a Smith chart showing impedance matching from a 50&OHgr; line. The load impedance Z
L
301
can be impedance-matched with the center of the Smith chart at 50&OHgr; by tracing paths formed by the parallel capacitive components and series inductive components. In this case, the value of the load impedance Z
L
301
is 7&OHgr;+j4 &OHgr;.
Next, a drain-biasing circuit and a MESFET, in which a drain choking inductor is implemented as a part of an MMIC, will be described with reference to FIG.
13
. The choking inductor is a device used for preventing radio frequency power from leaking to the drain power supply.
Following is respective parameters of the MESFET.
Unit finger length: 100 &mgr;m
Total gate width: 1 mm
Frequency: 900 MHz
Power supply voltage: 3.5 V
Saturated output power with idle current of 20 mA supplied: about 120 mW
Operating current: about 23 mA
Gain: about 13 dB
The gate of the MESFET
505
is connected to a gate-biasing pad
507
via a gate electrode extended line
506
. The source of the MESFET
505
is connected to a source pad
508
. The drain of the MESFET
505
is connected to a drain extended line
509
. Part of the drain extended line
509
is connected to a drain-biasing pad
510
via a spiral inductor
504
.
In the MMIC including the drain choking inductor, the MESFET operates at a frequency of 900 MHz and with a power supply voltage of 3.5 V applied and a current of about 19 mA supplied. The MESFET provides a saturated output power of about 90 mW with an idle current of 20 mA supplied, and shows a gain of about 11 dB.
The line of the spiral inductor
504
is made of gold plated to be about 3 &mgr;m thick. The extended lines thereof are formed by evaporating and depositing titanium and gold thereon.
FIG. 14
illustrates an equivalent circuit of the MESFET
505
and the drain-biasing circuit shown in FIG.
13
. An equivalent series inductor
502
and an equivalent series resistor
503
constitute an equivalent circuit of the spiral inductor
504
shown in FIG.
13
. In this example, the inductance value of the equivalent series inductor L
502
is 21 nH and the resistance value of the equivalent series resistor R
503
is 7.5 &OHgr;.
FIG. 15
illustrates the location of choke impedance Z
c
501
on a Smith chart. The choke impedance is located at a drain terminal of the MESFET, which is short-circuited at an end through which a drain voltage is applied. Usually, the choke impedance Z
c
501
i
Lee Eddie
Nixon & Peabody LLP
Owens Douglas W.
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