JPH06283707A - Variable-capacitance circuit - Google Patents

Abstract

PURPOSE:To provide a variable-capacitance circuit in which the series resistance of an element can be made extremely small in the variable-capacitance circuit wherein a gated Schottky diode for a Schottky gate field-effect transistor is used as a variable capacitance. CONSTITUTION:In a variable-capacitance circuit, a gated Schottky diode 3 which is provided with a drain 15 and with a source 14 is used as a variable capacitance. The variable-capacitance circuit is provided with a drain bias electrode 20 which applies a DC bias to the drain 15, with an MIM capacitor 5 which shott-circuits the drain 15 with the source 14 in terms of a high frequency, with a via-hole 16 and with a back grounding electrode 17.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable capacitance circuit used for a voltage controlled oscillator or the like in a microwave band or a millimeter wave band.

[0002]

2. Description of the Related Art Conventionally, a voltage control oscillator (VCO) is used in a communication device that uses a microwave band or a millimeter wave band to stabilize the frequency modulation or oscillation frequency or a feedback loop. ing. Normally, in this circuit, a variable capacitance element such as a varactor is used in the resonance circuit of an oscillator using an active element such as a transistor or a Gunn diode to change the oscillation frequency.
As an active element for oscillation used here, it is advantageous to use a heterojunction field effect transistor (FET) such as a high electron mobility transistor (HEMT) having a high cutoff frequency and a high gain particularly in millimeter waves. . Therefore, in order to make such a VCO into a monolithic integrated circuit, it is convenient to use a gate Schottky diode of a heterojunction FET having a structure similar to that of the transistor as the variable capacitance circuit. In such a Schottky diode, it is necessary to minimize the series resistance in order to increase the capacitance Q, and normally, the gate and the ohmic electrode are interleaved as shown in FIG. 2 showing a part 2 of the millimeter wave monolithic integrated circuit. Digital type, in other words,
The source and drain of the FET are connected and short-circuited to form the source / drain electrode 34. In FIG. 2, 31 indicates a semi-insulating GaAs (gallium arsenide) substrate, and 32 indicates an active region. This active area is
For example, undoped GaAs, undoped I on the substrate 31
nGaAs (indium gallium arsenide) channel,
This is a region in which N-AlGaAs (aluminum, gallium, and arsenic) is stacked. 33 is a gate electrode. 34
Is a source / drain electrode, and is connected to the back surface ground electrode 37 through the Via-Hole 36. A control voltage is applied from the electrode 38 to the gate Schottky diode, and is connected to the entire oscillation circuit by a transmission line 39.

[0003]

However, in the variable capacitance circuit using the conventional gate Schottky diode as shown in FIG. 2, even if the source and the drain are short-circuited as described above, the result of the experiment is as follows. It has been found that its series resistance is large and it is not suitable for an oscillation circuit. The present invention has been made to solve the above problems, and an object of the present invention is to provide a variable capacitance circuit that uses a gate Schottky diode as a variable capacitance and that can reduce the series resistance of elements to an extremely small value. To do.

[0004]

In order to solve the above problems, a variable capacitance circuit according to the present invention is a variable capacitance circuit using a gate Schottky diode of a Schottky gate field effect transistor having a drain and a source as a variable capacitance. Therefore, it is configured to include a DC bias applying unit that applies a DC bias to the drain, and a short-circuiting unit that short-circuits the drain with the source in high frequency.

[0005]

In the above conventional variable capacitance circuit, the drain and the source thereof are short-circuited in terms of both DC and high frequency and are not biased with DC, and in the heterojunction FET, the ohmic electrode (source) and the channel (source) are connected. The AlGaAs layer existing between (InGaAs or GaAs) serves as a kind of barrier, and the current across the barrier is small unless an electric field is applied, and therefore the resistance is considered to be large. According to the capacitor circuit, since a DC bias can be applied to the drain to accelerate the electrons, the barrier can be easily crossed and the resistance can be reduced. Further, in the variable capacitance circuit of the present invention, since the drain is short-circuited to the source at high frequency, the resistance value can be further reduced.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a part of the configuration of a millimeter wave monolithic integrated circuit (MMIC) which is an embodiment of the present invention. Figure 1
As shown in, the MMIC 1 is formed on the semi-insulating GaAs substrate 11 having the back surface ground electrode 17 on the back surface. The figure shows a part of the gate Schottky diode 3 in the MMIC.

The gate Schottky diode 3 has source electrodes 14a and 14b, a drain electrode 15, and a gate electrode 13. Source electrodes 14a and 14b
Is grounded to the ground electrode 17 on the rear surface of the substrate through Via-Hole 16a, 16b. In addition, the drain electrode 15 is configured to include a capacitor upper electrode 21, an insulating film 23, and a capacitor lower electrode 22, and an MIM (metal-metal).
Insulating film-metal) capacitor 5 and Via-Hole 16
It is grounded at a high frequency by means of c and a direct current (DC) voltage is applied by the drain bias electrode 20.
Therefore, the drain electrode 15 of the gate Schottky diode 3 is connected to the MIM capacitor 5 and Via-Hol.
e16c, back surface ground electrode 17, Via-Hole16
a and 16b, the source electrodes 14a and 14
It is short-circuited with b. Here, MIM capacitor 5, V
ia-Hole 16c, backside ground electrode 17, and Via
-Hole 16a, 16b constitutes a short-circuit means.
The drain bias electrode 20 corresponds to a DC bias applying means.

The gate electrode 13 is connected to an oscillation circuit (not shown) by a transmission line 19, and a control voltage Vc is applied by a voltage control electrode 18. Figure 1
The active region 12 in, for example, is undoped GaAs, undoped In0.15G on the semi-insulating GaAs substrate 11.
a0.85As (indium gallium arsenide) channel layer and N-Al0.15Ga0.85As (aluminum
A gallium / arsenic) electron supply layer is used which has been laminated and grown. The source electrodes 14a, 14 of this active region 12
Below the b and drain electrodes 15, an n ⁺ -GaAs contact layer (not shown) is further provided on the above-mentioned electron supply layer.

As a result, the gate length is 0.15 μm and the gate width is 50.
When the equivalent circuit is estimated from the measurement of the S parameter at 0 to 60 GHz for the case of 100 μm in total of μm × 2, in the conventional example shown in FIG. 2, when the control voltage Vc = -2V, the capacitance C = 0.076 pF, series resistance R = 12.2 Ω, and capacitance C = 0.163 p when control voltage Vc = 0V
In contrast to F and the series resistance R = 19.8 Ω, which is large, in the embodiment of the present invention shown in FIG. 1, when the voltage applied to the drain bias electrode 20 is 0.3 V, the control voltage Vc = -2V, capacitance C = 0.068 pF, series resistance R = 6.7Ω, and control voltage Vc = 0V, capacitance C = 0.158 pF, series resistance R = 5.8Ω. And, sufficient capacity change was realized.

The reason why the series resistance value is reduced is that in the conventional variable capacitance circuit such as the gate Schottky diode 4 shown in FIG. 2, the drain electrode and the source electrode are short-circuited both in terms of direct current and high frequency. In a heterojunction FET that is not biased, the AlGaAs layer existing between the ohmic electrode (source) and the channel (InGaAs or GaAs) serves as a kind of barrier, and the current across the barrier is small unless an electric field is applied. For this reason, it is considered that the resistance is large, whereas in the variable capacitance circuit such as the gate Schottky diode 3 according to the present invention having the above-described configuration, a DC bias is applied to the drain electrode 15 to accelerate electrons. Therefore, it is considered that the above barrier can be easily crossed and the resistance can be reduced. . Further, in the variable capacitance circuit such as the gate Schottky diode 3 according to the present invention, since the drain electrode 15 is short-circuited to the source electrodes 14a and 14b at high frequency, it is considered that the series resistance value can be further reduced. To be

In the above description, the heterojunction FET type was explained as the gate Schottky diode. However, even in the case of the GaAs MES type FET type, this is a tunnel in which the characteristics of the source and drain ohmic electrodes slightly exceed the barrier. It is considered to be governed by the current, and it is considered that the resistance becomes smaller when a voltage is applied to the drain, so that the present invention can be applied to this case.

The present invention is not limited to the above embodiment. The above-mentioned embodiment is an exemplification, has substantially the same configuration as the technical idea described in the scope of the claims of the present invention, and has any similar effect to the present invention. It is included in the technical scope of the invention.

[0013]

As described above, in the conventional variable capacitance circuit described above, the drain and the source thereof are short-circuited both in terms of direct current and high frequency, and no direct current bias is applied. The AlGaAs layer existing between the conductive electrode (source) and the channel (InGaAs or GaAs) serves as a kind of barrier, and if the electric field is not applied, the current across the barrier is small, and therefore the resistance is considered to be large. According to the variable capacitance circuit of the present invention having the configuration, since a DC bias can be applied to the drain to accelerate electrons, the barrier can be easily crossed and resistance can be reduced.
Further, in the variable capacitance circuit of the present invention, since the drain is short-circuited to the source at high frequency, there is an advantage that the resistance value can be further reduced. From the above, there is also an advantage that a low resistance variable capacitance circuit suitable for an MMIC can be realized, and a high-performance voltage controlled oscillator, and further, a high-performance small millimeter wave communication device can be realized.

[Brief description of drawings]

FIG. 1 is a perspective view showing a partial configuration of a millimeter wave monolithic integrated circuit according to an embodiment of the present invention.

FIG. 2 is a perspective view showing a partial configuration of a millimeter wave monolithic integrated circuit as a conventional example.

[Explanation of symbols]

 1, 2 Millimeter-wave monolithic integrated circuit 3, 4 Gate Schottky diode 5 MIM capacitor 11 Semi-insulating GaAs substrate 12 Active region 13 Gate electrode 14a, 14b Source electrode 15 Drain electrode 16a-16c Via-Hole 17 Backside ground electrode 18 Voltage control Electrode 19 Transmission line 20 Drain bias electrode 21 Capacitor upper electrode 22 Capacitor lower electrode 23 Insulating film 31 Semi-insulating GaAs substrate 32 Active region 33 Gate electrode 34 Source drain electrode 36 Via-Hole 37 Backside ground electrode 38 Voltage control electrode 39 Transmission line

Claims (1)

[Claims] 1. A variable capacitance circuit that uses a gate Schottky diode of a Schottky gate field effect transistor having a drain and a source as a variable capacitance, the direct bias applying means for applying a direct current bias to the drain, and the drain having a high frequency. And a short-circuit means for electrically short-circuiting the source.