JP2664073B2 - 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 to reduce 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). Although the third type generally requires a high carrier concentration in the active layer, 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-ray has not been realized so far.

Therefore, an object of the present invention is to provide a semiconductor device which can further improve radiation resistance with a simple configuration, particularly in terms of a change in transconductance in a saturation region.

[Means for solving the problem]

The present inventor has found that when irradiating GaAs MESFET with radiation,
transconductance g _m change rate β = g _mA / g _m in the saturation region of the GaAs MESFET (g _mA is g _m after the change) has a constant relationship with the variation .DELTA.N _D of the carrier concentration N _D of the active layer in terms of focusing on the fact that, to discover 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 a MESFET which is formed by doping a substantially uniform impurity in the depth direction while GaAs, the value after the change in the transconductance g _m in the saturation region of the MESFET g _mA
And a signal processing circuit configured in combination with the MESFET to operate correctly when a change rate β = g _mA / g _m when the is within the allowable rate of change beta _L, total dose R 1 × 10
⁹ A semiconductor device which may be used at 1 × 10 under ¹⁰ roentgens following radiation environment or Roentgen, .DELTA.N carrier concentration decrease of the active layer by irradiation of total dose R _D,
Before irradiation in the active layer, the carrier mobility after each mu, when the mu _A, decrease in the carrier concentration △
N _D is, b, when a and c constants, a △ N _D = b · R ^c, the carrier concentration N _D of the previous irradiation of the active layer is 1 ×
10 ¹⁷ cm ^-3 or more 1 × 10 ¹⁹ A at cm ^-3 or less, and characterized in that it is a _{N D> △ N D / {} 1-β L (μ / μ A)}. In this case, the effective thickness a of the active layer is approximately a = {[2ε / (q · N D)] · (V bi -V th)} becomes ^1/2. Here, V _bi is the built-in voltage of the MESFET.

[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, GaAs MES
The transconductance g _m in the saturation region of the FET falls within a predetermined range (an allowable range determined by the signal processing circuit). Therefore, the semiconductor device including the GaAs MESFET and the signal processing circuit cooperating therewith was initially manufactured. It will operate normally as 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 combination circuit has a predetermined transconductance g _m in the saturation region, and this transconductance g _m
Has been known to change under radiation irradiation environments. When the transconductance g _mA after the change becomes a value outside the range required in advance by the signal processing circuit, the combination circuit does not operate normally.
Hereinafter, in this specification, the tolerance of the mutual conductance g _m of the change rate β = g _mA / g _m in the allowable range, the allowable rate of change beta
_It is defined and described as _L.

As described above, the transconductance g _m of the MESFET in the saturation region is known to change by irradiation, but the threshold voltage V _th is also known to change in the irradiation environment. Regarding the causes of these changes, firstly, the carrier concentration of the active layer is reduced by radiation,
Second, it has been reported that the electron mobility is reduced by radiation. The present inventors have conducted detailed studies for the first of the above, reduction .DELTA.N _D of the carrier concentration N _D of the active layer
It is found from the experimental value of the change in the threshold voltage V _th that the relationship ΔN _D = b · R ^c is established between the total dose R and the total dose R of the irradiation radiation, and this relationship shows the experimental value of the change in the transconductance g _m It was confirmed that the explanation was extremely appropriate. Here, b, c is a both constant, (1) a carrier concentration N _D is 1 × 10 ¹⁷ ~1 × 10 initial active layer (before irradiation with a radiation)
¹⁹ cm ^-3 and the total dose R of the irradiated radiation is 1 ×
It is established in the range of 10 ⁸ to 1 × 10 ¹⁰ radiographs. In this case, the constants b and c are determined by variations in the initial carrier concentration of the active layer, the energy of radiation, or the quality of the substrate.
Has a certain width. According to our experiments,
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 decrease ΔN
_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 figure, semi-insulating GaAs
An n-type active layer 2 and an n ⁺ -type contact layer 3 are formed on a substrate 1 by an epitaxial growth method, and a part of the n-type active layer 2 and the n ⁺ -type contact layer 3 in a gate region is removed by etching to form a recess structure. I do. 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 a conventional product.
Specifically the effective thickness a is about 500Å is (conventional products 1000Å so) and, the carrier concentration N _D of the n-type active layer 2 is in a high concentration as compared with the conventional products, in particular 1 × 10 ¹⁸ cm ^{- Assume 3} .

Theoretical value of the threshold voltage V _th of this GaAs MESFET, according to Baifukan "Ultrafast compound semiconductor device" _{P.63, V th = V bi -} (q · N D · a 2) / 2ε ... (2 ). 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}}} = { a since ^{(q · a 2) / 2ε} } · (N D -N DA) ... (4), when the decrease of the carrier density by irradiation and .DELTA.N _D, [Delta] V _th = become ^{{(q · a 2) /} 2ε} · ΔN D ... (5).

Next, the present inventor used the MESFET of FIG. 1 in which the effective thickness of the active layer 2 was 500 ° and the carrier concentration was 1 × 10 ¹⁸ cm ^−3, and the total dose R = 1 × 10 ⁸ , 1 × 10 ⁸ irradiating the ⁹ 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. Accordingly, (5) in accordance with equation from the second view of the results, determine the total dose ^{R = 1 × 10 8, 1} × 10 9, 3 × 10 9 variation of carrier concentration in the case of X-ray (decrease) .DELTA.N _D It can be ^{seen that} the relational expression of ΔN _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.

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.

Next, the inventor has determined that the threshold voltage V _{th in} FIG. 2 and FIG.
Using the same MESFET as the GaAs MESFET having the characteristics described _above , the change of the transconductance gm in the saturation region due to irradiation was measured. As a result, as shown in FIG.
Characteristics of the transconductance g the rate of change of _m beta as figure obtained, the characteristics of the change in mutual conductance g _m, such as FIG. 6 corresponding to the characteristic of FIG. 4 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.

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 in the saturation region of the MESFET is the intrinsic FET neglecting the source resistance R _s
For _{g m = {(W g ·} μ · q · N D · a) / L g × {1 - a _{[(V bi -V G) /} V p] 1/2 ... (6). Here, W _g is the gate width, mu is the electron mobility in the active layer 2, the L _g gate length, V _G is the gate voltage, the V _p is the pinch-off voltage. The transconductance g _m in the case of V _G = V _bi When g _mmax for simplicity of calculation, (6) is _{g mmax = (W g · μ} · q · N D · a) / L g ... (7) Therefore, the rate of change β due to irradiation is (7)
= Expression than _{β = g mmaxA / g mmax (} μ A · N DA) / (μ · N D) ... is (8). Here, N _DA is the carrier density after irradiation, since it is _{N DA = N D -ΔN D ...} (9), (8) equation _{β = {μ A (N D} -ΔN D)} / to become _{(μ · N D) ... (} 10).

Here, when Equation (10) 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 with 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 these experiments and ideas, firstly, the main cause of the MESFET characteristic deterioration due to radiation damage is the decrease in the carrier concentration in the active layer, and the relational expression (1) shows that the decrease in the carrier concentration under irradiation is very good. I understood that it was explaining. Second, the value of mu is a constant mu _A / mu in (10) can be estimated approximately, yet because .DELTA.N _D are those determined from depending on the radiation dose (1), the active layer the setting of the initial carrier concentration N _D only was able to be set a change rate of the transconductance β to a predetermined value. 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 setting the carrier concentration N _D in 1 × 10 ¹⁸ cm ^-3, radiation resistance as in the fifth view is significantly improved.

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 carrier concentration of the active layer 2. That, in combination GaAs MESFET is a signal processing circuit said semiconductor device is formed, when the allowable rate of change of the transconductance g _m in the saturation region of the MESFET is beta _L, the combination circuit is designed according to the semiconductor device to the operating value as the carrier concentration of the active layer 2 initial N _D is (10) from the _{N D> ΔN D / {1} -βL (μ / μ a)} ... (11) unless Narazu, the effective thickness a of the active layer 2 in this case is a = {[ε / (q · N D)] · (V bi -V th)} 1/2 ... (12). 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 variation .DELTA.N _D of the carrier concentration _{(1) ΔN D = 3.87 ×} 10 16 cm -3 next from the equation, the carrier concentration in the active layer 2 (10) 7.35 ×
10 ¹⁷ cm ^-3 or more. When the threshold voltage V _{th is set} to V _th = −1.2 V in such a carrier concentration of the active layer 2, the effective thickness of the active layer 2 is 585 ° according to the equation (12). Here, the permittivity 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 between the product of the present invention and the conventional product regarding radiation resistance
Shown in the figure. In the figure, (a), (b), (c) the curve shows the characteristics of commercially available MESFET conventional, in particular (b) of the curve of the active layer 2 carrier concentration N _D of 2.09 × 10 ¹⁷
corresponds to the characteristic of Figure 6 which is a cm ^-3. 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 change rate suddenly becomes high with the total dose R = 1 × 10 ⁹ X-ray as a boundary. On the other hand, a p-type layer was buried under the n-type active layer to reduce leakage current to the substrate.
The characteristics of the MESFET are as shown by the curve (e) in the figure, and although the characteristics of the threshold voltage are improved (not shown), the characteristics of the transconductance are not improved. In contrast,
The carrier concentration N _D of the active layer 2 was set to 1 × 10 ¹⁸ cm ^-3 (5
In the structure of the present invention (corresponding to the characteristic shown in the figure), the rate of change β is suppressed to a sufficiently low value even at R = 1 × 10 ⁹ X-ray as shown by the curve (f), and the radiation resistance is significantly improved. I understand.

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

For example, the formation 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, the transconductance g _m in the saturation region of the GaAs MESFET is within a predetermined allowable range, and
A semiconductor device including As MESFET and a signal processing circuit cooperating therewith will operate normally as originally designed. Therefore, the radiation resistance can be significantly improved.

[Brief description of the drawings]

FIG. 1 is a sectional view of a GaAs MESFET for explaining the principle of the present invention, and FIG. 2 is a threshold variation ΔV _{th of the} MESFET according to the present invention.
Shows the dose dependence of R, FIG. 3 is a diagram showing a dependency on dose R of carrier concentration decrease .DELTA.N _D, Fig. 4, the radiation amount of change in the threshold V _th of the conventional MESFET FIG. 5 shows the results of an experiment examining the dependence on R, FIG.
Figure is a diagram showing a dependency on dose R of the transconductance g the rate of change of _m beta in the saturation region of the MESFET according to the present invention, Figure 6 is the change in mutual conductance g _m in the saturation region of a conventional MESFET FIG. 4 shows the dependence of the radiation dose on the radiation dose R;
FIG. 7 is a characteristic diagram comparing the radiation resistance of the product of the present invention with that of a conventional product. 1 ...... GaAs substrate, 2 ...... n-type active layer, 3 ...... n ⁺ -type contact layer.

Claims (1)

(57) [Claims] A MESFET which is formed by doping impurities substantially uniform in claim 1] depth direction during GaAs, when the value after the change in the transconductance g _m in the saturation region of the MESFET was g _mA and a signal processing circuit configured in combination with the MESFET to operate correctly when the change rate β = g _mA / g _m is within the allowable rate of change beta _L, total dose R is 1 × 10 ⁹ Roentgen a semiconductor device which can be used under 1 × 10 ¹⁰ Roentgen following radiation environment more, the decrease in the carrier concentration N _D of the active layer by irradiation of the total dose R △ N _D, in the active layer the radiation before, the carrier mobility after each mu, when the mu _a, decrease amount △ N _D of the carrier concentration, b, when a and c _{constants, △ N D = b · R} c , and the carrier concentration N _D of the previous irradiation of the active layer is 1 × 10 ¹⁷
A is cm ^-3 to 1 × 10 ¹⁹ cm ^-3 or less, and a semiconductor device which is a _{N D> △ N D / {} 1-β L (μ / μ A)}.