Development of an MMIC (monolithic microwave integrated circuit) using CMOS processes has been pursued in a field of wireless communication using millimeter wave frequency bands, and reducing chip's area by providing a matching circuit with a lumped parameter has also been pursued.
In general, providing a matching circuit with a lumped parameter poses a problem of a band being narrowed by a self-resonant frequency of inductance or capacitance.
In a matching circuit using a lumped parameter element, the matching circuit is generally made variable in order to enhance a narrow band characteristic. A conceivable way to make internal capacitance of the matching circuit variable is to employ a varactor or make the capacitance parallel and control the capacitance by turning a switch on or off. However, using the varactor or the switch at millimeter wave frequency bands causes a problem of an increase in loss of the matching circuit.
A configuration for controlling an inductance value of an inductor; for instance, has been proposed as a method for solving the problems (see; for instance, Patent Document 1 and Patent Document 2).
FIG. 13 is a schematic diagram of a variable inductor described in Patent Document 1. In FIG. 13, an inductance variable element 101 has a swirl-like spiral electrode 102 and switches 105 and 106 that are intended to short-circuit an electric current flowing between input-output electrodes 103 and 104 by short-circuiting turns of the spiral electrode 102.
The switch 105 is formed from a gate electrode 107 and diffusion regions 109 and 110 which are made in such a way that the gate electrode 107 is sandwiched therebetween, and the switch 106 is formed from a gate electrode 108 and diffusion regions 110 and 111 which are made in such a way that the gate electrode 108 is sandwiched therebetween. The inductance variable element 101 and the switches 105 and 106 are formed on a silicon substrate 113 by way of a dielectric layer 112.
The diffusion regions 109, 110, and 111 are made over a surface of the n-type silicon substrate 113 by thermal diffusion or ion implantation of p-type impurities.
An inductance value of the variable inductor is determined from a length of the spiral electrode 102 between the input-output electrodes 103 and 104. In the variable inductor, the length of the spiral electrode 102 between the input-output electrodes 103 and 104 can be shortened by turning on the switch 105 and shortened further by additionally turning on the switch 106.
FIG. 14 shows a schematic diagram of a variable inductor described in connection with Patent Document 2. In FIG. 14, the variable inductor includes a spiral coil 201, a first layer 202 of the spiral coil, a second layer 203 of the spiral coil, a control circuit 204, a switch 205, a silicon substrate 206, circumferential wiring 207, a first layer 208 of the circumferential wiring, and a second layer 209 of the circumferential wiring.
The variable inductor of Patent Document 2 is configured as follows. The circumferential wiring 207 is laid around the spiral coil 201. The switch 205 is put at one end of the circumferential wiring 207. The switch is turned on or off by a control signal sent from the control circuit 204, thereby opening or short-circuiting the one end of the circumferential wiring 207.
In a state in which the one end of the circumferential wiring 207 is short-circuited when compared with a state in which the one end of the circumferential wiring 207 is opened, a magnetic flux developing in the spiral coil 201 penetrates through the circumferential wiring 207, whereby an induction current flows through the circumferential wiring 207, to thus cancel the magnetic flux of the spiral coil 201 and reduce an inductance value of the spiral coil 201. Namely, the one end of the circumferential wiring 207 is opened or short-circuited by turning on or off the switch 205, thereby making the inductance value variable.