JPH03108377A - Variable capacity diode device - Google Patents

Abstract

PURPOSE:To reduce high frequency resistance by forming, by diffusion, a diffusion layer, which nearly determines capacity varying ratio so that it exhibits a hollow columnar shape. CONSTITUTION:An N conductivity type high resistivity epitaxial layer 2 is formed on a high impurity concentration N-type conductivity semiconductor substrate 1. An N conductivity type high impurity concentration circular hollow columnar diffusion layer 3 is formed to reach the substrate 1 from the main surface of the semiconductor base substance of this layer 2. Next, a P conductivity type high impurity concentration diffusions layer 4, which covers the layer 3, is made to form PN junction J1 with the layer 3. Next, the element center of an SiO2 film 5 being formed on the main surface of the semiconductor base substance in diffusion process is removed, and there a conductor film 6 is overlaid to form an electrode. To this film 6, a gold wire 7 is wire-bonded. According to this constitution, it is possible to easily enlarge the area of a peripheral wall, whereby high frequency resistance can be reduced further, and also performance index Q can be improved.

Description

[Detailed Description of the Invention] [Industrial Application Field of the Invention] The present invention relates to a variable capacitance diode device suitable for high frequencies and used as a tuning element,
This invention relates to a variable capacitance diode device with a high figure of merit Q.

[Conventional technology]

FIGS. 5a and 5b show a plan view and a sectional view of a conventional variable capacitance diode device.

In FIGS. 5a and 5b, the semiconductor substrate is of the N conductivity type and has a high impurity concentration (N" 10) and the semiconductor substrate v
i10 and a semiconductor layer consisting of an epitaxial layer (N-) 20 of N conductivity type and relatively high resistivity. N from the main surface of the semiconductor substrate to the epitaxial layer 20
A conductive type diffusion layer (N'') 30 with high impurity concentration is diffused and formed so as to reach the semiconductor substrate 10.Diffusion layer 3
Diffused J of P conductivity type and high impurity concentration so as to cover 0
i (P”) 40 is diffused to form a junction J2. The silicon dioxide film covering the central main surface of the element formed in the diffusion process is removed, and a PN junction J3 is exposed on the main surface.
The silicon dioxide film 50 covering is left as a protective film for the junction. A conductor film 60 made of aluminum or the like is formed on the exposed main surface of the semiconductor substrate.

A gold wire 70 is wire-bonded to this conductor film 60 to form a variable capacitance diode device.

In variable capacitance diode devices for high frequencies, the maximum capacitance value C
vamx must be set to several pF or less. That is, it is necessary to set the maximum bonding area extremely small. Therefore, the area of the conductor film 60 for wire bonding needs to be extremely large compared to the bonding area, which is unnecessary for electrical characteristics. Normally, the size of the gold ball 71 of the gold wire 70 is about 3 to 5 times the surface shape of the gold wire, so if the diameter of the gold wire 70 is 25 μm, the diameter will be 75 to 125 μm. The area of the conductor film 60 is set correspondingly.

[Problem to be solved by the invention]

In recent years, in variable capacitance diode devices for high frequencies,
As the Q becomes higher, the maximum capacitance value is required to be smaller and smaller. For example, in a variable capacitance diode device used in AM radio, etc., the maximum capacitance value C++ax
is about 350 to 650 pF, and becomes smaller as the frequency band used becomes higher, such as the UHF TV band, and a maximum capacitance value C and X of about 0.1 to 15 pF are required. The bonding area becomes extremely small. Therefore, it is necessary to prevent the gold balls from protruding from the conductor film 60, and the area of the conductor film for wire bonding is set to be extremely large compared to the bonding area. Therefore, unnecessary stray capacitance may occur between the epitaxial layer 20 and the conductive film 60 extending in the shape of a brim from the joint, which has the drawback of adversely affecting high frequency characteristics.

Furthermore, in a high frequency variable capacitance diode device, as explained above, the area of the outer peripheral wall of the columnar diffused diffusion layer 30 to obtain the required maximum capacitance value Cvaax is extremely small. , there is a drawback that the figure of merit Q decreases due to the occurrence of electric field concentration explained by the skin effect of high frequencies.

The present invention has been made to improve the above-mentioned drawbacks, and its main purpose is to provide a variable capacitance diode device with good high frequency characteristics.

Another object of the present invention is to provide a variable capacitance diode device with improved figure of merit Q.

[Means to solve problems]

The variable capacitance diode device of the present invention is intended to solve the above-mentioned problems. A first diffusion layer of a first conductivity type is formed with an impurity concentration higher than that of the first diffusion layer, and a second diffusion layer of a second conductivity type is formed so as to cover the first diffusion layer of the first conductivity type. In this variable capacitance diode device, the first diffusion layer is formed in the shape of a hollow column or a hollow square column in order to increase the area of the outer peripheral wall.

[Effect]

In the variable capacitance diode device of the present invention, impurities of the same conductivity type are diffused and formed in a high concentration epitaxial layer in a high-resistivity epitaxial layer, and the first diffusion layer is expanded into a hollow cylinder shape or square pillar shape. This prevents a decrease in high-frequency resistance, improves the figure of merit Q, and improves high-frequency characteristics by reducing stray capacitance generated by the conductive film that is unnecessarily extended to connect the gold balls.

〔Example〕

FIGS. 1a and 1b are diagrams showing one embodiment of a variable capacitance diode device of the present invention, and FIG. 1a is a plan view schematically showing the variable capacitance diode device, and FIG. FIG.

In Figure 1, an epitaxial layer (N-) 2 of N conductivity type and high resistivity is grown in vapor phase on a semiconductor substrate (N'') 1 of which the semiconductor substrate is of N conductivity type and has a high impurity concentration. A semiconductor layer is formed. From the main surface of the semiconductor substrate of the epitaxial layer 2, an annular or hollow cylindrical diffusion layer (N'') 3 of N conductivity type and having a high impurity concentration reaches the semiconductor substrate 1. form like this. The diffusion layer (P~) 4, which is of P conductivity type and has a high impurity concentration, covering the diffusion layer 3 is diffused to form a PNN junction with the diffusion layer 3. The silicon dioxide film formed on the main surface of the semiconductor substrate in the diffusion process is removed from the center of the device, leaving the silicon dioxide film 5 covering the PN junction exposed on the main surface of the device as a protective film. The removed exposed portion of the main surface of the element is covered with a conductive film 6 of aluminum or the like to form an electrode. This conductor film 6 is gold! 7 is wire-bonded to form a variable capacitance diode device.

Generally, the maximum capacitance value Ca+a of a variable capacitance diode device
Since x is determined by the product of the junction area S and the impurity concentration coefficient α of the junction surface, the maximum capacitance value Ca+a is
To reduce x, junction area S or impurity concentration coefficient α
All you have to do is make it smaller. Incidentally, the maximum capacitance value (, +ax is,
The product of the junction area S and the impurity concentration coefficient α of the junction surface is expressed as follows.

Cwax cCS a ・----(11 this(1
According to the relationship in Equation 1, if it is desired to have a small maximum capacitance value Ctaax and a large capacitance change ratio, it is necessary to reduce the junction area S or the impurity concentration coefficient α. However, there is a limit to reducing the impurity concentration coefficient α due to the impurity concentration of the semiconductor substrate. In other words, by adjusting the impurity concentration of the epitaxial layer, which determines the minimum capacitance value Ca1n, to a low value, the sheet resistance of the epitaxial layer can be reduced. There is a limit to setting the impurity concentration of the semiconductor substrate low because attempting to increase the thickness of Ctsax causes a decrease in the figure of merit Q.From this point of view, conventional methods for reducing the maximum capacitance value Ctsax , reducing the bonding area S is an effective means.

For example, when trying to obtain a variable capacitance diode device with a maximum capacitance value Cmax of several pF in a manufacturing process adjusted to a variable capacitance diode device for an FM radio receiver, according to the conventional example shown in Fig. 5, the diameter of the junction area S, is reduced to about 80 μm. The radius is r
o, the approximate junction area S is S1=πr,,t,, −・−・−−−−−−−
−−−−−−−−−−−−−(2).

In the embodiment of the present invention, the sheet resistance of the epitaxial layer 2 is made relatively high, and the hollow columnar diffusion layer 3 is formed in the epitaxial layer 2 in consideration of the skin effect. In order to make the maximum capacitance value Cmax of the variable capacitance diode device shown in FIG. 1 equal to that of the conventional device, the junction area of the diffusion layer 3 should be set to the junction area S, which is equal to the junction area Sl shown in FIG. You can set it as follows. That is, the surface area S0 of the diffusion layer 3 of the variable capacitance diode device shown in FIG. 1 is expressed as follows.

S o =” (r t” r z”) −=−−
---=(31 (However, the radius of the outer circumferential circle of the diffusion layer 3 is r,
Let the radius of the inner circumferential circle be r2. ) Therefore, from equations (2) and (3), the radius rO+rl+r!
The bonding area can be made equal by setting the relationship as follows.

ir, ""π(rl"-rt") r@=r, small -r, Z, --...- (4) Also in the example of Fig. 1, the relationship as shown in equation (4) By doing so, a capacitor having the same maximum capacitance value Cmax as that shown in FIG. 5 can be formed. Moreover, the surface area of the outer peripheral edge of the cylindrical diffusion layer 3 is greatly expanded. Therefore, the high frequency resistance based on the skin effect becomes small, and the figure of merit Q does not deteriorate. Moreover, by further expanding the area of the outer peripheral wall of the diffusion layer 3, it is possible to further reduce the high frequency resistance, and it is also possible to further improve the figure of merit Q.

FIG. 2 is a plan view showing another embodiment of the variable capacitance diode device of the present invention. In contrast to the single diffusion region N3 in the embodiment of FIG. This is an example in which □ was formed.

Further, although not shown, the diffusion N3 may be formed in double or more layers. In this way, the diffusion area 3. ．． By forming 3□, the area of the outer peripheral wall can be expanded, so that high frequency resistance can be further reduced and the figure of merit Q can be improved.

Thermal theory, as shown in FIG. 3, the diffusion layer 3 shown in FIG.
It is clear that the area of the outer peripheral wall can be expanded even if it is made into a rectangular shape when viewed from a plan view. Further, as in the embodiment shown in FIG. 2, a rectangular diffusion region 3. It is clear that double or more may be formed.

Furthermore, as shown in FIG. 4, it is clear that the diffusion layer 34 may be formed so that substantially circular diffusion regions are scattered on the circumference when viewed from the top.

〔effect〕

In the variable capacitance diode device of the present invention, the diffusion layer that approximately determines the variable capacitance ratio is not cylindrical as in the conventional case, but the area of the outer peripheral wall must be made large in consideration of the skin effect of high frequencies.
, it is possible to expand the area of the outer peripheral wall very easily by forming it by diffusion so that it has a hollow cylindrical shape, which makes it possible to further reduce high frequency resistance and improve the figure of merit Q.
Therefore, it is extremely effective as a variable capacitance diode device for high frequencies.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1a is a plan view showing an embodiment of the variable capacitance diode device of the present invention, FIG. 1a is a sectional view showing an embodiment of the variable capacitance diode device of the present invention, 2 to 4 are plan views showing other embodiments of the diffusion layer shown in the embodiment in FIG.
FIG. 5a is a plan view showing an example of a conventional variable capacitance diode device, and FIG. 5 is a sectional view showing an example of a conventional variable capacitance diode device. 1: Semiconductor substrate. 3.4: Diffusion layer. 6: Conductor film. 2: Epitaxial layer 5: Silicon dioxide film. 7: Gold wire

Claims (4)

[Claims] (1) In a variable capacitance diode device, a high resistivity semiconductor layer of a first conductivity type formed on a semiconductor substrate and a first conductivity type semiconductor layer formed on the semiconductor layer of the first conductivity type are provided. a diffusion layer, and a second diffusion layer formed to cover the first diffusion layer.
and a second diffusion layer of a conductivity type, the first diffusion layer having a higher concentration of impurity elements diffused than the semiconductor layer of the first conductivity type has a hollow cylindrical shape. Variable capacitance diode device. (2) The variable capacitance diode device according to claim 1, wherein the first diffusion layer is formed in the shape of a double or more hollow cylinder. (3) A patent claim characterized in that the first diffusion layer is formed with a higher impurity concentration than the semiconductor layer of the first conductivity type, and the first diffusion layer has a hollow quadrangular prism shape. The variable capacitance diode device according to item 1. (4) The first diffusion layer is formed with a higher impurity concentration than the semiconductor layer of the first conductivity type, and is formed in the semiconductor layer of the first conductivity type so as to be distributed in dots on the circumference. A variable capacitance diode device characterized by: