JP2610659B2 - 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 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 that can further improve radiation resistance with a simple configuration, particularly in view of a change in source-drain saturation current.

[Means for solving the problem]

The present inventors have found that when irradiating GaAs MESFETs with radiation,
Change rate α of source-drain saturation current I _dss of AsMESFET
_{= I dssA / I dss (I} dssA the I _dss after the change) on which is focused on the fact that with a variation .DELTA.N _D and certain relationship of the carrier concentration N _D of the active layer, the amount of change .DELTA.N _D is The present inventors have found that they have a certain quantitative relationship with the total dose R, and have completed the present invention.

That is, the present invention includes a MESFET which is formed by doping impurities substantially uniform in the depth direction during GaAs, the value after the change in the source-drain saturation current I _dss of the MESFET I _DSSA
And a signal processing circuit configured in combination with the MESFET to operate normally when the change rate α = I _dssA / I _dss is within the allowable change rate α _L when the total dose R is 1
A semiconductor device which can be used in a radiation irradiation environment of not less than × 10 ⁹ X-rays and not more than 1 × 10 ¹⁰ X-rays, wherein a carrier concentration reduction amount of an active layer due to irradiation of a total dose R is Δ
N _D, before irradiation of the active layer, the carrier mobility after each mu, when the mu _A, the carrier concentration N _D of the previous irradiation of the active _{layer, N D> ΔN D / {} 1- [α _L (μ / μ _A )] ^1/2 ｝. At this time, the effective thickness a of the active layer is approximately a = {(2ε / q · a ² ) · (V _bi −V _th )}. 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 ×
Of course, if the ¹⁰⁹ roentgens or less, under 1 × 10 ⁹ roentgen least 1 × 10 ¹⁰ Roentgen following preset dose, the source-drain saturation current I _dss of GaAsMESFET a predetermined range (signal processing circuit 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 combination circuit has a predetermined source-drain saturation current I _dss, and it is conventionally known that this saturation current I _dss changes under a radiation irradiation environment. Then, when the changed saturation current I _dssA becomes a value outside the range previously required by the signal processing circuit, the combination circuit does not operate normally. Hereinafter, in this specification, the change rate α of the saturation current I _dss in this allowable range α = I _dssA /
The tolerance of the I _dss, is described by defining the allowable rate of change alpha _L.

As described above, the source-drain saturation current of MESFET
It is known that I _dss changes by irradiation, but it is known that the threshold voltage V _th also changes under the irradiation environment. For the cause of this change,
First, the carrier concentration of the active layer is reduced by radiation,
Reports that the electron mobility is reduced by radiation. 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}} ... It was found from the experimental value of the change in the threshold voltage V _th that the relationship (1) holds, and it was confirmed that this relationship explained the experimental value of the change in the saturation current I _dss quite appropriately. A where b, c are both constants, equation (1) is a carrier concentration N _D is ^{1 × 10 17 ~1 × 10 19} cm -3 in the initial active layer (before irradiation with a radiation), and irradiated Total radiation R is 1 × 10 ⁸ -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 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 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}}} = { 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 = ｛(q · a ² ) / 2ε｝ · N _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 × 18 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.
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, the intrinsic FET ignoring source resistance _{R S I dss = {(W} g · μ · q 2 · N D 2 · a 3) / (6ε · L g _{)} × {1-3 (V bi} -V G) / V p +2 [V bi -V G) / V p] 3/2} ... a (6). Here, W _g is the electron mobility in the gate width, mu active layer 2, L _g is the gate length, V _G is the gate voltage, V _p is the pinch-off voltage. When the saturation current I _dss when the V _G = V _bi in order to simplify the calculations and I _DSS, (6) Equation _{I DSS = (W g · μ} · q 2 · N D 2 · a 3) / (6ε · L _g ) (7) Therefore, the rate of change α due to irradiation is (7)
= Formula from _{α = I DSSA / I DSS (} μ A · N DA 2) / (μ · N D 2) ... a (8). Here, N _DA is the carrier density after irradiation, since it is _{N DA = N D -ΔN D ...} (9), (8) formula _{α = {μ A (N D} -N D) 2} will become / ^{_{(μ · N D 2) ...}} (10).

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 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 considerations, firstly, the main cause of the deterioration of MESFET characteristics due to radiation damage is a 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 is the determined by depending on the radiation dose (1), the active layer only by setting the initial carrier concentration N _D it was found to be set a saturation current change rate α to a predetermined value. Specifically, 2 × 10 the carrier concentration N _D of the active layer 2 as conventional
¹⁷ cm ^-3 when the front and rear is the radiation resistance has become inadequate as FIG. 6, 1 × carrier concentration N _D
When set to 10 ¹⁸ cm ^-3 , 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 carrier concentration of the active layer 2. That is, GaAs
MESFET is combined with the signal processing circuit said semiconductor device is formed, when the source-drain saturation current I _dss acceptable rate of change of the MESFET is alpha _L, circuit combination according to the semiconductor device as designed To operate, the active layer 2
Initial carrier concentration N _D must be (10) from the _{N D> ΔN D / {1-} [α L (μ / μ A)] 1/2} ... (11), in this case the active effective thickness a of the layer 2 a = a {[(2ε / (q · N D)] · (V bi -V th)} ... (12). here, the total dose R = 1 × 10 ^{For 9} x-rays, the permissible change rate α _L of the saturation current I _DSS = 0.9 (I _DSSA > 0.9I
More specifically calculated as _DSS), 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) 1.45 × from the equation
10 ¹⁸ 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 417 ° according to the equation (12). 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 between the product of the present invention and the conventional product regarding radiation resistance
Shown in the figure. In the figure, (I), (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 saturation electric power change rate suddenly becomes a high value with the total dose R = 1 × 10 ⁹ X-ray as a boundary. 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 leakage current to the substrate are as shown by the curve (e) in FIG. Although not shown, no improvement in saturation current characteristics can be obtained. 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 characteristics shown in the figure), the change rate α 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 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, the source 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.・ Drain saturation current I
_{The dss} falls within a predetermined allowable range, so that a semiconductor device including a GaAs MESFET and a 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 sectional view of a GaAs MESFET illustrating the principle of the present invention, and FIG. 2 is a threshold change amount 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 rate of change of the source-drain saturation current I _dss of MESFET according to the present invention alpha, Figure 6 is a source-drain saturation current I _dss conventional MESFET The figure which shows the dependence of the change 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] An MESFET formed by doping impurities in GaAs substantially uniformly in a depth direction, and a source / source of the MESFET.
When the rate of change α = I _dssA / I _dss when the value after the change of the drain-to-drain saturation current I _dss is I _dssA is within the allowable change rate α _L , the MESFET operates properly.
And a signal processing circuit configured in combination with a radiation device having a total dose R of 1 × 10 ^{9 to} 1 × 10 ¹⁰ X-rays. .DELTA.N _D carrier concentration decrease of the active layer in which the MESFET according to irradiation with the radiation before in the active layer, the carrier mobility after each mu,
when the mu _A, the carrier concentration N _D of the previous irradiation of the active layer, 1 × 10
¹⁷ cm ⁻³ or more and 1 × 10 ¹⁹ cm ⁻³ or less, and N _D > ΔN _D / {1- [α _L (μ / μ _A )] ^1/2 }, and the carrier concentration decrease ΔN _D is a semiconductor device which is a ΔN _D = b · R ^C when b, and c are constants.