JP2664078B2 - 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 outer space, near electronic furnaces, etc.
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
× is 10 ⁸ X-ray about ( "space development-related Symposium] 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 level (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.

On the other hand, "Physics of Sem
MESFET having two active layers is disclosed in "Iconductor Device SECOND EDITION" SMSze, p.334. According to this, the carrier concentration on the active layer surface becomes low,
The adverse effect of the surface depletion layer is eliminated, and a MESFET with excellent characteristics can be realized.

Therefore, the present invention has a simple configuration, in particular, a threshold voltage,
It is an object of the present invention to provide a semiconductor device capable of further improving radiation resistance in terms of at least two changes of a source-drain saturation current or a mutual inductance in a saturation region.

[Means for solving the problem]

The present inventors have found that when irradiating GaAs MESFETs with radiation,
The change amount ΔV _th of the threshold voltage V _th of AsMESFET, the change rate α = I _dssA / I _{dss of} the source-drain saturation current I _dss , and the change rate β = g _mA / g _{m of the} transconductance g _m in the saturation region , The effective thickness of the active layer a = a ₁ + a ₂ and the carrier concentration N _1D ,
In terms of focusing on the fact that the N _2D of variation .DELTA.N _D has a constant relationship, discovered that the above variation .DELTA.N _D has a constant quantitative relationship between the total dose R, the present invention has been completed.

That is, the present invention includes an upper layer and the carrier concentration of the effective thickness of the carrier concentration in the N _1D is a ₁ is the effective thickness in N _2D is a
₂ The active layer consisting of the lower layer (where N _1D <N _2D ) is G
It is formed by doping impurity and threshold voltage and the MESFET of V _th in GaAs, 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, or the saturation current I _dss When the change rate is within the allowable change rate α _L ,
Or one of the three conditions when the rate of change of mutual conductance g _m beta is within the allowable change rate "beta _L, is constructed in combination with the MESFET to operate correctly when at least two conditions are met And a signal processing circuit having a total dose R of 1 × 10 ^{9 to} 1 × 10 ¹⁰ X-rays.
_Assuming that the decrease amount of N _1D and N _2D is ΔN _D , the carrier mobilities before and after irradiation in the active layer are μ and μ _A respectively, the induction rate of the active layer is ε, and the electron charge is q, The carrier concentration N _1D and N2 _{D of the} active layer before irradiation is 1 × 10 ¹⁷ cm ⁻³
To 1 × is at 10 ¹⁹ cm ^-3 or less, decrease amount △ N _D of the carrier concentration, b, when a and c constants, a ^{_{△ N D = b · R c}} , further of the active layer Effective thickness a = a1 + a2
Is (1) a <｛(2ε △ V _thL ) / (q _△△ N _D )｝ ^1/2
That.

The carrier concentration N _2D of the previous irradiation of the active _{layer, (2) N D> △} N D / {1- [α L (μ / μ A)] 1/2} and that.

_{(3) N D> △ N} D / {1-β L (μ / μ A)} is to be.

It satisfies at least two of the three conditions (1) to (3).

[Action]

According to the present invention, the total dose R of the irradiated radiation is 1 ×
10 ⁹ X-ray or less, of course, total dose R is 1 × 1
0 Under the preset radiation dose within the range of ⁹ X-rays or more and 1 × 10 ¹⁰ X-rays or less, GaAsMESF
ET threshold voltage V _th , saturation current I _dss , transconductance g
_At least two of _m are within a predetermined range (permissible range determined by the signal processing circuit). Therefore, the semiconductor device including the GaAs MESFET and the signal processing circuit cooperating with the GaAs MESFET has the original design value. It will work normally.

〔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 combination circuit has a predetermined threshold voltage V _th , a source-drain saturation current I _dss, and a transconductance g _m in a saturation region. However, it is conventionally known that these values change under a radiation irradiation environment. Have been. And
If the changed threshold voltage V _thA , saturation current I _dssA and transconductance g _mA fall outside the ranges required in advance by the signal processing circuit, the combination circuit does not operate normally. Hereinafter, in this specification, the allowable amount of the change amount ΔV _th of the threshold voltage V _th in the allowable range is the allowable change amount ΔV _thL , the change rate of the saturation current I _dss α = I _dssA / I _dss is the allowable change rate. α _L , the rate of change β of the transconductance g _m
= G _mA / g _m is defined as an allowable change rate β _L for explanation.

As described above, it is known that the threshold voltage V _th and the like of the MESFET change under the radiation irradiation environment. The causes of the change are firstly the decrease in the carrier concentration of the active layer due to the radiation, and the second Reports that the electron mobility is reduced by radiation. The present inventor has conducted a detailed study on the above first point, and has found that the carrier concentration N
_1D, between the decrease amount △ N _D and total dose R of irradiation radiation of N _D2, found that the relationship of ^{_{△ N D = b · R c}} ··· (1) is satisfied. Here, b, c is a both constant, (1) a carrier concentration N _D of the initial active layer (before irradiation with a radiation) is ^{1 × 10 17 cm~1 × 10 19} cm -3, and The total dose R of the irradiated radiation 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 , c = 0.678, and therefore the carrier concentration reduction amount △ N
_A ^typical value of _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 the drawing, the upper layer 21 of the lower layer 2 and the thickness of a ₁ thick a ₂ forming the n-type active layer 2 on the GaAs substrate 1 of semi-insulating is formed, further thereon n ⁺ The contact layer 3 is formed by an epitaxial growth method, and a part of the n-type active layer 2 and the n ⁺ -type contact layer 3 in the gate region is removed by etching to form a recess structure. Next, by vacuum evaporation method
A source electrode 4 and a drain electrode 5 are formed on the n ⁺ -type contact layer 3 with an ohmic metal, and a gate electrode 6 is formed on the n-type active layer 2 with a Schottky metal. Here, the n-type active layer 2 immediately below the gate electrode 6 is sufficiently thinner than the conventional product, and specifically, the effective thickness a is about 450 ° (about 1000 ° in the conventional product). 22 to a ₂ = 150
Å, the upper layer 21 is set to a ₁ = 300 °. Further, the n-type active layer 2
The carrier concentration on the lower side is higher than that of the conventional product. Specifically, the lower layer 22 is N _2D = 3 × 10 ¹⁸ cm ⁻³ , and the upper layer 21 is N _1D =
It is 2 × 10 ¹⁷ cm ⁻³ .

The theoretical value of the threshold voltage V _th of such a GaAs MESFET is the one-dimensional Poisson equation d ² φ ₁ / dχ ² = −qN _1D / ε (0 ≦ χ ≦ a ₁ ) d ² under the full depletion approximation. φ ₂ / dχ ² = −qN _2D / ε (a ₁ ≦ χ ≦ a ₁ + a ₂ ) with boundary conditions χ = a ₁ + d (where d> a ₂ ), dφ / dχ = 0 χ = 0 and φ = By solving under − (V _bi −V _G ), 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, the n-type active layer 2
The carrier concentration N _1D, N _2D is N _1DA, When becomes N _2DA,
The threshold voltage V _thA after irradiation is Becomes Therefore, the deviation amount ΔV _th of the threshold voltage V _th due to the radiation irradiation is Becomes However, there is a _{N D -N DA = N 1D -N} 1DA = N 2D -N 2DA. Accordingly, when the decrease of the carrier concentration by radiation irradiation and ΔN _D = N _D -N _DA, because it is _{a = a 1 + a 2,} ΔV th = {(q · a 2) / 2ε} · ΔN D ... (5)

Next, the present inventors set the effective thickness a of the active layer 2 to 450 (= 150
+300) Å, and using the MESFET of FIG.
1 × 10 ^8, 1 × irradiated with ¹⁰⁹ and 3 × 10 ⁹ roentgens gamma were 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 =
When obtaining the ^{1 × 10 8, 1 × 10} 9, 3 × 10 9 variation of carrier concentration in the case of X-ray (decrease) .DELTA.N _D, now black point of FIG. 3, .DELTA.N indicated above _D = 3.06 × 10 ¹⁰ · R ^0.678 (1) It was found that the relational expression (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. Further, in FIG. 2, the change amount ΔV _th of the threshold voltage is a low value of about 0.04 V even for radiation of R = 1 × 10 ⁹ X-rays. Therefore, it was confirmed that when the total thickness a of the active layer 2 was about 450 °, 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 further proposes that the active layer 2 has substantially the same structure in the geometrical shape except for the active layer, and the active layer 2 is a single layer.
An experiment was performed using a MESFET as shown in FIG. here,
The effective thickness a of the active layer 2 is 1130 ° and the carrier concentration is
N _D = 2.09 × 10 ¹⁷ cm ⁻³ . In the case of such a gamma ray irradiation experiment using cobalt 60 using a conventional type GaAs MESFET by ion implantation, the total dose is R = 1 × 10 ⁶ , 1
× 10 ⁷ , 1 × 10 ⁸ , 3 × 10 ⁸ , 1 × 10 ⁹ , 2 × 10 ⁹ , 3 × 10 ⁹
And The obtained change in the threshold voltage _Vth is as shown by the black dot in FIG. 4, which is 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 experiments on these threshold voltages, firstly, the main cause of the deterioration of the threshold characteristics of the MESFET due to radiation damage is a decrease in the carrier concentration in the active layer, and the relational expression (1)
Showed that the decrease in carrier concentration under irradiation was explained very well. Second, in equation (5), q,
ε is a constant, set because .DELTA.N _D is the determined by depending on the radiation dose (1), a threshold amount of change [Delta] V _th only by setting the thickness of a = a ₁ + a ₂ of the active layer to a predetermined value I knew I could do it. Specifically, when the thickness a of the active layer 2 is set to about 1000 ° as in the conventional product, the radiation resistance is 4th.
Although the thickness is insufficient as shown in the figure, the thickness a = a ₁ +
When you set a ₂ to 450Å, the radiation resistance as in the second figure is significantly improved.

Next, the inventor has determined that the threshold voltage V _{th in} FIG. 2 and FIG.
The change of the source-drain saturation current I _dss due to radiation irradiation was measured using the same MESFET as the GaAs MESFET having the characteristics described above. As a result, the characteristic of the change rate α of the saturation current I _dss is obtained as shown in FIG. 5 corresponding to the characteristic of FIG. 2, and the saturation current I _ds as shown in FIG. The characteristics of _dss change were obtained. In FIGS. 5 and 6, the black points are experimental values, and the dotted lines are theoretical values obtained by applying the above-described equation (1) to theoretical equation (10) shown below.

Therefore, a theoretical formula of the change rate α = I _dssA / I _dss of the saturation current I _dss is obtained next. First, the source-drain saturation current I _dss of the MESFET is obtained by solving the above Poisson equation, and for an intrinsic FET ignoring the source resistance R _S Becomes Here, W _g is the electron mobility in the gate width, mu is the active layer 2, L _g is the gate length, V _G is the gate voltage. Get saturation current I _dss when the V _G = V _bi and I _DSS for ease of, when from N _1D << N _2D and N _1D / N _2D = 0, the d ₁ = a _1. Therefore, equation (6) is Becomes Therefore, the rate of change α due to irradiation is (7)
From the formula Becomes Here, N _2DA is the carrier density after irradiation, since it is _{N 2DA = N 2D -ΔN D ...} (9), (8) formula Becomes

Here, when examining equation (10), it can be seen that the rate of change α is also affected by the change in electron mobility μ due to irradiation (μ → μ _A ), but the carrier concentration before irradiation is 1 ×
In the case of about 10 ¹⁸ cm ^−3, μ _A / μ is about 0.95, and the change becomes smaller as the concentration becomes higher. Therefore, μ _A / μ =
When the calculation was performed at 0.95, the results were as shown by the dotted lines in FIGS. 5 and 6, and the agreement with the experimental value was confirmed as described above.

As a result of experiments and considerations on these source-drain saturation currents, firstly, the main cause of deterioration of the saturation current characteristics of the MESFET due to radiation damage is also the decrease in the carrier concentration in the active layer. It was found that the decrease in carrier concentration under irradiation was explained very well. Second, the value of mu is a constant mu _A / mu in (10) can be estimated approximately, yet because .DELTA.N _D is the determined from depending on the radiation dose (1), the active layer It has been found that the saturation current change rate α can be set to a predetermined value only by setting the initial carrier concentration _N2D of the lower layer 22. More specifically, when the carrier concentration N _D of the active layer 2 as in the conventional of 2 × 10 ¹⁷ cm ^-3 before and after, but the radiation resistance has become inadequate as FIG. 6 When the carrier concentration N _2D is set to 3 × 10 ¹⁸ cm ⁻³ , the radiation resistance is significantly improved as shown in FIG.

Further, the present inventor has found that the threshold voltage V _th shown in FIGS.
And characteristics of, using a fifth diagram and the same MESFET and GaAsMESFET to give the characteristics of the six views saturation current I _dss, was measured the change in mutual conductance g _m in the saturation region by irradiation. As a result, obtained from the characteristics of the rate of change β of the transconductance g _m as Figure 7 in correspondence to the characteristic of FIG. 2 and FIG. 5, first in response to the characteristics of FIG. 4 and FIG. 6 characteristics of changes in the transconductance g _m as Figure 8 were obtained. In FIGS. 7 and 8, black points are experimental values, and dotted lines are theoretical values obtained by applying the above-described equation (1) to theoretical equation (15) shown below.

Accordingly, then the rate of change in the mutual conductance g _m β = g _mA /
Calculate the theoretical formula of g _m . First, the transconductance g _m of the MESFET in the saturation region is obtained by solving the above Poisson equation, and for an intrinsic FET ignoring the source resistance R _S Becomes Get transconductance g _m in the case of V _G = V _bi and g _mmax To simplify, N from _{N 1D «N 2D 1D / N 2D} = 0
Then, d ₁ = a ₁ . Therefore, equation (11) Becomes Therefore, the rate of change β due to irradiation is (12)
From the equation, β = g _mmaxA / g _mmax = (μ _A · N _2DA ) / (μ · N _2D ) (13) Here, N _2DA is the carrier density after irradiation, since it is _{N 2DA = N 2D -ΔN D ...} (14), (13) equation _{β = {μ A (N 2D} -ΔN D)} / (Μ · N _2D ) ... (15)

Here, when Equation (15) is examined, it can be seen that the rate of change β is also affected by the change in electron mobility μ due to irradiation (μ → μ _A ), but the carrier concentration before irradiation is 1 ×
In the case of about 10 ¹⁸ cm ^−3, μ _A / μ is about 0.95, and the change becomes smaller as the concentration becomes higher. Therefore, μ _A / μ =
When the calculation was performed at 0.95, the results were as shown by the dotted lines in FIGS. 7 and 8, and the agreement with the experimental value was confirmed as described above.

As a result of experiments and considerations on these transconductances, first, the main cause of the deterioration of the transconductance characteristics of the MESFET due to radiation damage is the decrease in the carrier concentration in the active layer. It was found that the decrease in carrier concentration was explained very well. Second, the value of mu in (15) is a constant mu _A / mu may be estimated approximately, yet because .DELTA.N _D is the determined from depending on the radiation dose (1), the active layer It has been found that the transconductance change rate β can be set to a predetermined value only by setting the initial carrier concentration _N2D of the lower layer 22. More specifically, when the carrier concentration N _D of the active layer 2 as in the conventional of 2 × 10 ¹⁷ cm ^-3 before and after, but the radiation resistance has become inadequate as FIG. 8 When the carrier concentration N _2D is set to 3 × 10 ¹⁸ cm ⁻³ , the radiation resistance is remarkably improved as shown in FIG.

Based on the above findings, when the total dose R is less than 1 × 10 ⁹ Roentgen course, total dose R to operate normally even under the following radiation environment 1 × 10 ¹⁰ Roentgen 1 × 10 ⁹ roentgen or The structure of the semiconductor device can be specified particularly from the viewpoint of the thickness a of the active layer 2 and the carrier concentration _N2D . That is, the GaAs MESFET is combined with the signal processing circuit to form the semiconductor device, and the threshold voltage V of the MESFET is
_When the allowable variation of _th is ΔV _thL , the effective thickness a = a ₁ + a ₂ of the active layer ₂ is (5) in order for the combinational circuit according to the semiconductor device to operate as designed. According to the equation, a <｛(2ε ・ ΔV _thL ) / (q ・ ΔN _D )｝ ^1/2 (16), and the carrier concentration N _2D of the lower layer 22 of the active layer 2 in this case is ( From equation (2), N _2D = {[2ε / (q · a ² ) · (V _bi −V _th )} (17) Here, for the total dose R = 1 × 10 ⁹ X-ray, the threshold voltage V When the allowable variation of _th specifically calculated as _{ΔV thL = 0.1V (ΔV th <} 0.1V), the variation .DELTA.N _D of the carrier concentration _{(1) ΔN D = 3.87 ×} 10 16 cm -3 next from equation The effective thickness a = a ₁ + a ₂ of the active layer 2 is 585 ° or less (450 ° in the case of the MESFET corresponding to FIG. 2) according to the equation (16). to obtain a threshold voltage V _th . And when the carrier concentration in the active layer 2 N _1D, and thus determined by the correlation between N _2D thickness a _1, a _2, however, 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.

Also, when the source-drain saturation current I _dss acceptable rate of change of the MESFET is alpha _L, to the combination circuit to the operation as designed according to the semiconductor device, the lower layer of the active layer 2 2 From the equation (10), the initial carrier concentration N _2D must be N _2D > ΔN _D / {1− [α _L (μ / μ _A )] ^1/2 } (18), and the activity in this case is The effective thickness a = a ₁ + a ₂ of the layer 2 is as follows: a = {[2ε / (q · N _2D )] (V _bi −V _th )} (19) Here, the total dose R = 1 × 10 ⁹ Roentgen, permissible saturated current I _DSS change rate _{α L = 0.9 (I DSSA>} 0.9I
More specifically calculated as _DSS), the amount of change in carrier concentration .DELTA.N _D is (1) from equation ^{_{ΔN D = 3.87 × 10 16 cm}} -3 , and the carrier concentration N _2D of the lower layer 22 of the active layer 2 (1
According to the equation (0), the value is 1.45 × 10 ¹⁸ cm ⁻³ or more.

Furthermore, the transconductance g _m in the saturation region of the MESFET
In order for the combinational circuit according to the semiconductor device to operate as designed when the allowable change rate of is equal to β _L , the initial carrier concentration N _2D of the lower layer 22 of the active layer 2 is expressed by the following equation (15). Therefore, N _2D > ΔN _D / {1−β L (μ / μ _A )} (21), and the effective thickness a = a ₁ + a ₂ of the active layer 2 in this case is a = ｛ [Ε / (q · N _2D )] (V _bi −V _th )｝ ^1/2 (22) Here, for the total dose R = 1 × 10 ⁹ X-ray, the allowable change rate β _L = 0.9 of the transconductance g _mmax is obtained.
If _(g mmaxA> 0.9g _mmax) specifically calculated as the change amount .DELTA.N _D of the carrier concentration _{(1) ΔN D = 3.87 ×} 10 16 cm -3 , and the active layer 2 carrier of the lower layer 22 from the equation The concentration N _2D is (1
From equation (5), the value is 7.35 × 10 ¹⁷ cm ^-3 or more.

Comparison between the product of the present invention and the conventional product regarding radiation resistance
This is shown in FIGS. FIG. 9 shows the change amount ΔV _th of the threshold voltage V _th due to irradiation, FIG. 10 shows the change rate α of the source-drain saturation current I _dss , and FIG. 11 shows the change rate β of the transconductance g _m Is shown. In these figures,
The curves (a), (b), and (c) show the conventional MESFETs
Characteristics shows a particularly curve of (b) is the effective thickness a of the active layer 2 and 1130A, and the carrier concentration N _D 2.09
Figure 4 which is a × 10 ¹⁷ cm ^-3, and correspond to the characteristics of FIG. 6 and FIG. 8. Curve (d) shows the characteristics of a commercially available HEMT (high electron mobility transistor). As is apparent from FIG. 9, the total dose R of these conventional products is 1 × 10 ⁹
The threshold change amount ΔV _th in the X-ray is a high value of about 0.2 to 0.3 V or more. On the other hand, the characteristics of the MESFET in which the p-type layer is buried under the n-type active layer to reduce the leak current to the substrate are as shown by the curve (e) in FIG.
^The change amount ΔV _th is suppressed to about 0.12 V with ⁹ X-rays. On the other hand, the effective thickness a = a ₁ + a ₂ of the active layer 2 is set to 45
In the structure of the present invention in which the angle is set to 0 ° (corresponding to the characteristic shown in FIG. 2), the change amount ΔV _th can be suppressed to a value sufficiently lower than 0.1 V even if R = 1 × 10 ⁹ X-ray as shown by the curve (F). It can be seen that the radiation resistance was significantly improved. It should be noted that these improvements in radiation resistance also appear for the saturation current I _dss and the transconductance g _m similarly.
This can be understood from FIG. 10 and FIG.

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, according to the present invention, when the total dose R of the irradiated radiation is 1 × 10 ⁹ X-rays or less, the total dose R is 1 × 10 ⁹ X-rays or more and 1 × 10 ¹⁰ X-rays or less. Also, at least two of the threshold voltage V _{th of the} GaAs MESFET, the source-drain saturation current I _dss, and the transconductance g _m in the saturation region fall within a predetermined allowable range, and therefore, the signal cooperating with the GaAs MESFET is The semiconductor device including the processing circuit 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 shows the result of an experiment for examining the dependence on the radiation dose R. FIG. 5 shows the saturation current change rate α of the MESFET having the structure of the present invention.
FIG. 6 shows the dependence of the change in the saturation current I _dss of the conventional MESFET on the radiation dose R, and FIG. 7 shows the results of an experiment conducted. FIG. 8 shows the dependence of the transconductance change rate β on the radiation dose R of the MESFET of the invention structure. FIG.
Shows the results of experiments of examining the dependence on the radiation dose R of change in the mutual conductance g _m of the SFET, Figure 9, second
Figure 11 shows the radiation resistance of the product of the present invention, the threshold voltage,
FIG. 12 is a characteristic diagram of the drain-to-drain saturation current and the transconductance as compared with the conventional product, and FIG. 12 is a cross-sectional view of the conventional MESFET used in the experiment. 1 ...... GaAs substrate, 2 ...... n-type active layer, the upper layer of 21 ...... active layer, the lower layer of 22 ...... active layer, 3 ...... n ⁺ -type contact layer.

Claims (4)

(57) [Claims] An active layer comprising an upper layer having a carrier concentration of N _1D and an effective thickness of a1 and a lower layer of a carrier concentration of N _2D and an effective thickness of a2 (where N _1D <N _2D ) is formed by doping impurities into GaAs and the threshold voltage is set to V _th , and the variation ΔV of the threshold voltage V _th of this MESFET is
_th is within the permissible change amount ΔV _thL , and the rate of change α = I _dssA / I _dss when the value after the change in the source-drain saturation current I _dss is I _dssA is within the permissible change rate α _L A semiconductor which comprises a signal processing circuit configured in combination with the MESFET so as to operate normally at a certain time, and which can be used in a radiation irradiation environment having a total dose R of 1 × 10 ^{9 to} 1 × 10 ¹⁰ X-rays. An apparatus, wherein the carrier concentration is obtained by irradiation of the total dose R.
The amount of decrease in N _1D and N _2D was ΔN _D , the carrier mobilities before and after the irradiation of the active layer were μ and μ _A , the induction rate of the active layer was ε, and the electron charge was q. Sometimes, the carrier concentration N _{1D of the} active layer before irradiation is 1 × 10
¹⁷ cm ^-3 to 1 × is at 10 ¹⁹ cm ^-3 or less, decrease amount △ N _D of the carrier concentration, b, when a and c constants, a ^{_{△ N D = b · R c}} , said active effective thickness of the layer a = a1 + a2 is a <{(2ε · △ V thL) / (q · △ N D} ^1/2 and the carrier concentration N _2D of the previous irradiation of the active layer is 1 × Ten
¹⁷ cm ^-3 to 1 × is at 10 ¹⁹ cm ^-3 or less, and a semiconductor, which is a _{N 2d> △ N D / {} 1- [α L (μ / μ A)] 1/2} apparatus. 2. An active layer comprising an upper layer having a carrier concentration of N _1D and an effective thickness of a1 and a lower layer of a carrier concentration of N _2D and an effective thickness of a2 (where N _1D <N _2D ) is formed by doping impurities into GaAs and the threshold voltage is set to V _th , and the variation ΔV of the threshold voltage V _th of this MESFET is
_th is within the permissible change amount ΔV _thL , and the change rate β = g _mA / g _m when the value after the change of the transconductance g _m in the saturation region is g _mA is within the permissible change rate β _L A semiconductor which comprises a signal processing circuit configured in combination with the MESFET so as to operate normally at a certain time, and which can be used in a radiation irradiation environment having a total dose R of 1 × 10 ^{9 to} 1 × 10 ¹⁰ X-rays. An apparatus, wherein the carrier concentration is obtained by irradiation of the total dose R.
The amount of decrease in N _1D and N _2D was ΔN _D , the carrier mobilities before and after the irradiation of the active layer were μ and μ _A , the induction rate of the active layer was ε, and the electron charge was q. Sometimes, the carrier concentration N _{1D of the} active layer before irradiation is 1 × 10
¹⁷ cm ^-3 to 1 × is at 10 ¹⁹ cm ^-3 or less, decrease amount △ N _D of the carrier concentration, b, when a and c constants, a ^{_{△ N D = b · R c}} , said active effective thickness of the layer a = a1 + a2 is a <{(2ε · △ V thL) / (q · △ N D} ^1/2 and the carrier concentration N _2D of the previous irradiation of the active layer is 1 × Ten
¹⁷ cm ^-3 to 1 × is at 10 ¹⁹ cm ^-3 or less, and wherein a is _{N 2d> △ N D / {} 1-β L (μ / μ A)}. 3. An active layer comprising an upper layer having a carrier concentration of N _1D and an effective thickness of a1 and a lower layer of a carrier concentration of N _2D and an effective thickness of a2 (where N _1D <N a MESFET to _2D) is formed by doping impurities into the GaAs, this MESF
The value after the change of the source-drain saturation current I _dss of ET
Change rate α when _dssA is _assumed = I _dssA / I _dss is the allowable change rate α
_L and transconductance g _m in the saturation region
A signal processing circuit configured in combination with the MESFET to operate correctly when the change rate β = g _mA / g _m when the value after the change was g _mA of is within the allowable rate of change beta _L A semiconductor device which can be used in a radiation irradiation environment having a total dose R of not less than 1 × 10 ⁹ x-rays and not more than 1 × 10 ¹⁰ x-rays, wherein the carrier concentration by the irradiation of the total dose R is
N _1D, decrease the △ N _D of N _2D, the irradiated before in the active layer, the carrier mobility after each mu, when mu _A, the carrier concentration N _1D before irradiation of the active layer 1 × 10
¹⁷ cm ^-3 to 1 × is at 10 ¹⁹ cm ^-3 or less, decrease amount △ N _D of the carrier concentration, b, when a and c constants, a ^{_{△ N D = b · R c}} , said active The carrier concentration N _2D of the layer before irradiation is 1 × 10
¹⁷ cm ^-3 or more A 1 × 10 ¹⁹ cm ^-3 or less, an _{N 2D> △ N D / {} 1- [α L (μ / μ A)] 1/2}, and N _2D> △ N _D / {1−β _L (μ / μ _A )}. 4. An active layer comprising an upper layer having a carrier concentration of N _1D and an effective thickness of a1 and a lower layer of a carrier concentration of N _2D and an effective thickness of a2 (where N _1D <N acceptable and MESFET to _2D) is formed by doping a substantially uniform impurity in the depth direction in GaAs and the threshold voltage is the V _th, the amount of change △ V _th of the threshold voltage V _th of this MESFET is The change amount is within △ V _thL ,
The value after the change of the source-drain saturation current I _dss is expressed as I _dssA
A change rate when the _α = I dssA / I _dss is within the allowable change rate alpha _L, and mutual conductance g _m change rate beta = g _mA when the value after the change was g _mA in the saturation region / g _m is within the permissible change rate β _L so that the ME operates properly.
A signal processing circuit configured in combination with the SFET,
A semiconductor device that can be used in a radiation irradiation environment in which a 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, wherein the carrier concentration due to the total dose R is irradiated.
The amount of decrease in N _1D and N _2D was ΔN _D , the carrier mobilities before and after the irradiation of the active layer were μ and μ _A , the induction rate of the active layer was ε, and the electron charge was q. Sometimes, the carrier concentration N _{1D of the} active layer before irradiation is 1 × 10
¹⁷ cm ^-3 to 1 × is at 10 ¹⁹ cm ^-3 or less, decrease amount △ N _D of the carrier concentration, b, when a and c constants, a ^{_{△ N D = b · R c}} , said active effective thickness of the layer a = a1 + a2 is a <{(2ε · △ V thL) / (q · △ N D} ^1/2 and the carrier concentration N _2D of the previous irradiation of the active layer is 1 × Ten
¹⁷ cm ^-3 or more A 1 × 10 ¹⁹ cm ^-3 or less, an _{N 2D> △ N D / {} 1- [α L (μ / μ A)] 1/2}, and N _2D> △ N _D / {1−β _L (μ / μ _A )}.