JP2610658B2 - Semiconductor device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a semiconductor device used for equipment requiring radiation resistance.

[Conventional technology]

Devices used in space or near reactors are:
So-called radiation resistance is required. Radiation includes gamma (γ) rays, as well as neutrons and protons, and the exposure limit for these is generally one silicon (Si) device.
× 10 ⁶ x-ray, 1 for gallium arsenide (GaAs) device
× 10 ⁸ x-rays ("Symposium on Space Development", p.35-38).

[Problems to be solved by the invention]

As a technique for improving the radiation resistance of a GaAs device, for example, the following has been conventionally proposed.
First, a p-type layer is buried under the n-type active layer, thereby reducing leakage current to the substrate, thereby improving radiation resistance, particularly in terms of threshold voltage. Second
The third is to shorten the gate length, and the third is to increase the carrier concentration by thinning the n-type active layer.

However, the conventional one has radiation resistance up to a total dose R of about 1 × 10 ⁸ x-rays, but cannot be said to be a sufficiently practical x-ray (1 × 10 ⁹ x-rays). Further, regarding the third type, although thinning of the active layer is generally required, no study has been made from the viewpoint of obtaining transistor characteristics required for circuit design. Therefore, a practical transistor having radiation resistance of about 1 × 10 ⁹ x-rays has not been realized so far.

Therefore, an object of the present invention is to provide a semiconductor device that can further improve radiation resistance with a simple configuration, particularly in terms of a change in threshold voltage.

[Means for solving the problem]

The present inventors have found that when irradiating GaAs MESFETs with radiation,
On the change amount [Delta] V _th of the threshold voltage V _th of AsMESFET have focused on the fact that with a variation .DELTA.N _D and constant relationship the effective thickness a and the carrier concentration N _D of the active layer, the amount of change in the Δ
N _D is found to have a certain quantitative relation between the total dose R, the present invention has been completed.

That is, the present invention, the active layer of the carrier concentration N _D is GaAs
And MESFET of V _th substantially formed uniformly doped with an impurity and the threshold voltage in the depth direction is in, when the change amount [Delta] V _th of V _th of the threshold voltage of the MESFET is within the allowable change amount [Delta] V _thL And a signal processing circuit configured in combination with the MESFET so as to operate normally, and a semiconductor device that can be used in a radiation irradiation environment in which the total dose R is equal to or more than 1 × 10 ⁹ x-rays and equal to or less than 1 × 10 ¹⁰ x-rays. a is decreased amount .DELTA.N _D of the carrier concentration N _D by irradiation of total dose R, the dielectric constant of the active layer epsilon, the electron charge is taken as q, the carrier density before irradiation of the active layer N _D is 1 × 10 ¹⁷ cm ^-3 or more 1 ×
10 ¹⁹ cm ⁻³ or less, and the effective thickness a of the active layer is a <{(2ε · ΔV _thL ) / (q · ΔN _D )} ^1/2 . At this time, the carrier concentration N _D is approximately N _D = {[2ε / (q · a ² )] · (V _bi −V _th )}. Here, V _bi is the built-in voltage of the MESFET.

[Operation] According to the present invention, the total dose R of the irradiated radiation is 1 ×
The threshold voltage _{Vth of the} GaAs MESFET is within a predetermined range (permissible range determined by the signal processing circuit) under a preset radiation dose, not to mention less than 10 ⁹ X-rays but also from 1 × 10 ⁹ X-rays to 1 × 10 ¹⁰ X-rays. Therefore, the semiconductor device including the GaAs MESFET and the signal processing circuit cooperating with the GaAs MESFET operates normally as originally designed.

〔Example〕

Hereinafter, the principle and configuration of the present invention will be described in detail with reference to the drawings.

The semiconductor device of the present invention has a MESFET made of GaAs and a signal processing circuit combined so as to cooperate therewith.
The MESFET and the signal processing circuit constitute a combination circuit such as an amplifier circuit, an inverter, and an oscillation circuit. The MESFET in this combinational circuit has a predetermined threshold voltage V _th, and it is conventionally known that the threshold voltage V _th changes under a radiation irradiation environment. If the changed threshold voltage V _thA falls outside the range required in advance by the signal processing circuit, the combination circuit does not operate normally. Hereinafter, in this specification, the allowable value of the variation ΔV _th of the threshold voltage V _th in this allowable range is referred to as the allowable variation ΔV
_It is defined and described as _thL .

As described above, it is known that the threshold voltage V _th of the MESFET changes under a radiation irradiation environment. The causes of this change are firstly a decrease in the carrier concentration of the active layer due to radiation, and secondly. Reductions in electron mobility due to radiation have been reported. The present inventors have conducted a detailed study about the points first described above, between the total dose R of reduction .DELTA.N _D irradiation radiation of carrier concentration N _D of the active ^{_{layer, ΔN D = b · R C}} ... (1) was found to hold. Here, b, c is a both constant, equation (1) is the carrier concentration N _D is ^{1 × 10 17 ~1 × 10 19} cm -3 in the initial active layer (before irradiation with a radiation), and radiation The total dose R of radiation to be applied is 1 × 10 ⁸ to 1 × 1
0 It is established in the range of ¹⁰ X-rays. In this case, the constants b and c have a certain width due to variations in the initial carrier concentration of the active layer, the energy of the radiation, the quality of the substrate, and the like. According to the experiments of the present inventors, the constants b and c have a width of about 1.99 × 10 ¹⁰ ≦ b ≦ 3.98 × 10 ¹⁰ 0.5 ≦ c ≦ 0.8, and a typical value is b = 3.06 ×
10 ¹⁰ a c = 0.678, therefore characters concentration decrease .DELTA.N _D
^Can be expressed as ΔN _D = 3.06 × 10 ¹⁰ · R ^0.678 .

Hereinafter, an experiment which led to the discovery of the relationship of the equation (1) will be described.

First, GaAs having a recess gate structure shown in a sectional view in FIG.
Prepare MESFET. As shown in FIG. 1, an n-type active layer 2 and an n ⁺ -type contact layer 3 are formed on a semi-insulating GaAs substrate 1 by an epitaxial growth method, and an n-type active layer 2 in a gate region is formed.
And a part of n ⁺ -type contact layer 3 is removed by etching to form a recess structure. Next, the source electrode 4 and the drain electrode 5 are formed of an ohmic metal on the n ⁺ -type contact layer 3 by a vacuum deposition method, and the gate electrode 6 is formed of a Schottky metal on the n-type active layer 2. Here, the n-type active layer 2 immediately below the gate electrode 6 is sufficiently thinner than the conventional product, specifically, the effective thickness a is about 500 ° (about 1000 ° in the conventional product). the carrier concentration N _D in a high concentration as compared with the conventional products, in particular to 1 × 10 ¹⁸ cm ^-3.

Theoretical value of the threshold voltage V _th of such GaAsMESFET, according to Baifukan "Ultrafast compound semiconductor device" _{P.63, V th = V bi -} (q · N D · a 2) / 2ε ... (2) Can be obtained as Here, V _bi is the built-in voltage of the MESFET, q is the electron charge, and ε is the dielectric constant of the n-type active layer 2. Therefore, irradiation of the n-type active layer 2
When the carrier concentration N _D of the became N _DA, the threshold voltage V _thA after Irradiation V _thA = V _bi - a ^{_{(q · N DA · a 2}} ) / 2ε ... (3). For this reason, the shift amount ΔV _th of the threshold voltage V _th due to irradiation is ΔV _th = V _thA −V _th = ｛V _bi − (q · N _DA · a ² ) / 2ε｝ − ｛V _bi − (q · N _D · a ² ) / 2ε｝ = {(q · a ² ) / 2ε｝ · (N _D −N _DA ) (4) Therefore, the amount of decrease in carrier concentration due to radiation irradiation is Δ
Assuming that N _D , ΔV _th = ｛(q · a ² ) / 2ε｝ · N _D (5)

Then, the present inventors using the MESFET of the first view and 500Å effective thickness of the active layer 2, the total dose ^{R = 1 × 18 8, 1} × 10 9
And 3 × 10 ⁹ was irradiated with gamma rays Roentgen, was examined variation [Delta] V _th of the threshold voltage V _th. As a result, the result shown by a black dot in FIG. 2 was obtained. Therefore, according to the equation (5), the total dose R = 1 × 10 ⁸ , 1 × 10 ⁹ , 3
× 10 ⁹ variation of carrier concentration in the case of X-ray when obtaining the (decrease) .DELTA.N _D, now black point of FIG. 3, .DELTA.N indicated above ^{_{D = 3.06 × 10 10 · R}} 0.678 ... (1) (Dotted line in FIG. 3) holds. When the relationship of the equation (1) is applied to FIG. 2, it becomes as shown by a dotted line in the figure, and the experimental results and the theoretical values are in good agreement. In the second diagram, the variation [Delta] V _th of the threshold voltage in irradiation of R = 1 × 10 ⁹ Roentgen 0.0
It is a low value of about 75V. Therefore, it was confirmed that when the thickness a of the active layer 2 was about 500 °, the radiation resistance was significantly improved.

The relationship of the above equation (1) is as follows: R = 1 × 10 ⁸ , 1 × 10 ⁹ , 3
It is those determined from the measured values in the three radiation dose of × 10 ⁹ X-ray, as the data for guiding the general formula can also be said to be somewhat inadequate. Therefore, The present inventor has a substantially identical structure in geometry, conventionally the effective thickness a of the active layer 2 and 1130Å and the carrier concentration N _D by ion implantation and 2.09 × 10 ¹⁷ cm ^-3 A gamma ray irradiation experiment with cobalt 60 was performed using a type GaAs MESFET. In this case, the total dose is R = 1
× 10 ⁶ , 1 × 10 ⁷ , 1 × 10 ⁸ , 3 × 10 ⁸ , 1 × 10 ⁹ , 2 × 1
0 ⁹ and 3 × 10 ⁹ . The change in the obtained threshold voltage V _th is the fourth
The result was like a black point in the figure, which was in good agreement with the theoretical value indicated by the dotted line. However, the threshold change amount ΔV _th after the irradiation of R = 1 × 10 ⁹ radiographs was about 0.4 V, which was considerably inferior to that of FIG.

As a result of these experiments, firstly, ME in radiation damage
The main cause of the deterioration of the characteristics of the SFET is the decrease in the carrier concentration in the active layer, and it has been found that the relational expression (1) explains the decrease in the carrier concentration under irradiation with radiation very well. Second, a q, epsilon is a constant in the equation (5), .DELTA.N _D
Is determined by the equation (1) depending on the radiation dose, the threshold change amount ΔV _th is determined only by setting the thickness a of the active layer.
Can be set to a predetermined value. Specifically, when the thickness a of the active layer 2 is set to about 1000 ° like a conventional product, the radiation resistance is insufficient as shown in FIG. 4, but the thickness a is reduced to 500 °. When set, the radiation resistance is significantly improved as shown in FIG.

Based on the above findings, when the total dose R is less than 1 × 10 ⁹ x-rays, of course, it is more than 1 × 10 ⁹ x-rays and 1 × 1
The structure of a semiconductor device that operates normally even under a radiation irradiation environment of ¹⁰ X-rays or less can be specified particularly from the viewpoint of the thickness of the active layer 2. That is, when the GaAs MESFET is combined with the signal processing circuit to form the semiconductor device, and the allowable change amount of the threshold voltage _Vth of the MESFET is ΔV _thL , the combination circuit according to the semiconductor device operates as designed. In order to achieve this, the effective thickness a of the active layer 2 must be a <｛(2ε ・ ΔV _thL ) / ( _{qΔΔN D} ) _{よ り} ^1/2 (6) according to equation (5). , The carrier concentration of the active layer 2 in this case
N _D is the (2) N _D = from equation {[2ε / (q · a 2)] · (V bi -V th)} ... (7). Here, the total dose R = 1 × 10 ⁹ Roentgen, specific calculating the allowable change amount of the threshold voltage V _th as _{ΔV thL = 0.1V (ΔV th <} 0.1V), the variation .DELTA.N _D of the carrier concentration From equation (1), ΔN _D = 3.87 × 10 ¹⁶ cm ⁻³ , and the effective thickness a of the active layer 2 is 585 from equation (6).
Å (500 で は for the MESFET corresponding to FIG. 2).
Then, the threshold voltage V
when the V _th = -1.2 V is the _th, the carrier concentration N _D of the active layer 2 becomes 7.353 × 10 ¹⁷ cm ^-3 from (7). Here, the dielectric constant ε = ε _S · ε _O = 12.0 × 8.85 × 10 ⁻¹² F / m Charge of electron q = 1.602 × 10 ⁻¹⁹ C Built-in voltage V _bi = 0.7V.

Comparison of the product of the present invention and the conventional product with respect to radiation resistance
Shown in the figure. In the figure, the curves (a), (b) and (c) show the characteristics of a conventional MESFET, and especially the curve (b) shows that the effective thickness a of the active layer 2 is 1130 °. This corresponds to the characteristic shown in FIG. Curve (d) shows the characteristics of a commercially available HEMT (high electron mobility transistor). As is clear from the figure, in these conventional products, the threshold change amount ΔV _th at the total dose R = 1 × 10 ⁹ X-rays is 0.2 to
It is a high value of about 0.3V or more. On the other hand, the characteristics of MESFET with reduced leakage current to the substrate by burying the p-type layer below the n-type active layer is as shown in curve in the diagram (e), R = 1 × 10 ⁹ Roentgen Thus, the variation ΔV _th is suppressed to about 0.12 V. On the other hand, in the structure of the present invention in which the effective thickness a of the active layer 2 was set to 500 ° (corresponding to the characteristic shown in FIG. 2), even if R = 1 × 10 ⁹ X-ray as shown by the curve (f), It can be seen that the amount ΔV _th is suppressed to a value sufficiently lower than 0.1 V, and the radiation resistance is significantly improved.

The present invention is not limited to the above embodiment, and various modifications are possible.

For example, the type of the active layer is not limited to the epitaxial growth method, but may be an ion implantation method. Further, it is not essential to form a recess gate structure as shown in FIG.

〔The invention's effect〕

As described above in detail, in the present invention, the threshold value of the GaAs MESFET is not limited to the case where the total dose R of the irradiated radiation is 1 × 10 ⁹ X-rays or less, or even 1 × 10 ⁹ X-rays or more and 1 × 10 ¹⁰ X-rays or less. The voltage _Vth falls within a predetermined allowable range, so that the semiconductor device including the GaAs MESFET and the signal processing circuit cooperating therewith operates normally as originally designed. Therefore, the radiation resistance can be significantly improved.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a GaAs MESFET illustrating the principle of the present invention, and FIG. 2 is a threshold change amount ΔV _{th of} a MESFET having a structure of the present invention.
Shows the dependence of the relative radiation dose R, FIG. 3 is a diagram showing a dependency on dose R of carrier concentration decrease .DELTA.N _D, Fig. 4, the change in the threshold voltage V _th of the MESFET of the conventional structure FIG. 5 is a graph showing the results of an experiment for examining the dependence on the radiation dose R. FIG. 5 is a characteristic diagram comparing the radiation resistance of the product of the present invention with the conventional product. 1 ...... GaAs substrate, 2 ...... n-type active layer, 3 ...... n ⁺ -type contact layer

Claims (1)

(57) [Claims] And 1. A MESFET which substantially uniformly formed by doping impurity and threshold voltage active layers of the carrier concentration N _D is the depth direction in the GaAs is the V _th, the threshold voltage V of the MESFET _th change amount ΔV _th is allowable change amount Δ
The relevant MESFET to operate normally when it is within V _thL
And a signal processing circuit configured in combination with the radiation source, wherein the semiconductor device can be used in a radiation irradiation environment where the total dose R is not less than 1 × 10 ⁹ X-rays and not more than 1 × 10 ¹⁰ X-rays. decrease amount .DELTA.N _D of the carrier concentration N _D by irradiation, the dielectric constant of the active layer epsilon, the electron charge is taken as q, the carrier concentration N _D of 1 × 10 ¹⁷ cm ^-3 to 1 × 10 ¹⁹ cm ⁻³ or less, and the effective thickness a of the active layer is a <｛(2ε ・ ΔV _thL ) / (q ・ ΔN _D )｝ ^1/2 , and the amount of decrease in the carrier concentration .DELTA.N _D is a semiconductor device which is a ΔN _D = b · R ^C when b, and c are constants.