JP2664076B2 - Semiconductor device - Google Patents

Description

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

[Prior art] Devices used in outer space, near nuclear reactors, 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
× 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-rays has not been realized so far.

On the other hand, "Physics of Semicon 2nd edition"
ductor Device SECOND EDITION) SMSze, P.334,
A MESFET having two active layers is disclosed. According to this, since the carrier concentration on the surface of the active layer becomes low, the adverse effect of the surface depletion layer is eliminated, and a MESFET having excellent characteristics can be realized.

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 invention provides a method for irradiating GaAs MESFETs with GaAs.
Change rate α of source-drain saturation current I _dss of MESFET
_I dssA / I _dss (I dssA is after the change I _dss) the carrier concentration of the active layer N _1D, in terms of focusing on the fact that it has a constant relationship between the variation .DELTA.N _D of N _2D, said variation .DELTA.N The inventors have found that _D has a certain quantitative relationship with the total dose R, and completed the present invention.

That is, the present invention includes an upper layer effective thickness of a ₁ in carrier concentration △ N _1D, effective thickness at a carrier concentration N _2D is a
₍₂ ) An MESFET in which an active layer composed of a lower layer is formed by doping impurities into GaAs, and a source and a source of the MESFET.
_Composed with the MESFET so as to operate normally when the rate of change α = I _dssA / I _dss is within the allowable change rate α _L when the value after change of the drain-to-drain saturation current I _dss is I _dssA. Signal processing circuit, and the total dose R is 1 × 10 ⁹
A semiconductor device that may be used at 1 × 10 under ¹⁰ roentgens following radiation environment or roentgen, before irradiation of the active layer by irradiation of total dose R, respectively carrier movement after mu, and the mu _A Sometimes, the carrier concentration N _1D of the active layer before irradiation is 1 × 10 ¹⁷ cm ⁻³ or more.
× is the 10 ¹⁹ cm ^-3 or less, decrease amount △ N _D of the carrier concentration,
When b and c are constants, ΔN _D = b · R ^c , and the carrier concentration N _{2D of the} active layer before irradiation is 1
Wherein the × 10 ¹⁷ cm ^-3 or more A 1 × 10 ¹⁹ cm ^-3 or less, and a _{N 2D> △ N D / {} 1- [α L (μ / μ A)] 1/2} And At this time, the effective thickness a (= a ₁ + a ₂ ) of the active layer is approximately a = {[2ε / (q · N ₂ )] · (V _bi −V _th )} ^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
Under a preset radiation dose in the range of not less than ⁹ X-rays and not more than 1 × 10 ¹⁰ X-rays, GaAsME
The source-drain saturation current I _dss of the SFET falls within a predetermined range (permissible range determined by the signal processing circuit).
A semiconductor device including a GaAs MESFET and a signal processing circuit cooperating therewith 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 description will be made by defining the allowable change rate α _L of I _dss .

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. 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, between the total dose R of the carrier concentration N _1D in the active layer, irradiation decreased the amount .DELTA.N _D of N _2D radiation, ΔN _D = b · It was found from the experimental value of the change in the threshold voltage V _th that the relationship of R ^c ... (1) holds, and it was confirmed that this relationship explains the experimental value of the change in the saturation current I _dss quite appropriately. Where b, c
A both the constant, (1) a carrier concentration N _D of the initial active layer (before irradiation with a radiation) is ^{1 × 10 17 ~10 19 cm -3} , and total dose R of radiation irradiated Is 1 × 10 ⁸ -1
× 10 It is established in the range of ¹⁰ X-rays. in this case,
The constants b and c have a certain range due to variations in the initial carrier concentration of the active layer, the radiation energy, the quality of the substrate, and the like. According to the experiment of the inventor, the constants b and c
Has 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, a lower layer 22 having an effective thickness a ₂ forming an n-type active layer 2 on a semi-insulating GaAs substrate 1 is formed.
And the effective thickness a ₁ of the upper layer 21 is formed, further an n ⁺ -type contact layer 3 is formed by epitaxial growth thereon, one n-type active layer 2 and the n ⁺ -type contact layer 3 in the gate region The portion is removed by etching to form a recess structure. Next, the source electrode 4 is formed on the n ⁺ -type contact layer 3 by vacuum evaporation.
The drain electrode 5 is formed of an ohmic metal, and the 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
It is about 450 mm (about 1000 mm for the conventional product), and the breakdown is that a ₂ = 150 mm for the lower layer 22 and a ₁ = 300 mm for the upper layer 21. Also, the carrier concentration on the lower side of the n-type active layer 2 is higher than that of the conventional product, and specifically, the lower layer 22 is N _2D = 3 × 10 ¹⁸ c
m ⁻³ , and the upper layer 21 has N _1D = 2 × 10 ¹⁷ cm ⁻³ .

The theoretical value of such a GaAs MESFET threshold voltage V _th is obtained by calculating the one-dimensional Poisson equation d ² φ ₁ / dχ ² = −qN _1D / ε (0 ≦ χ ≦ a ₁ ) d ² φ under the full depletion approximation. ₁ / 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, by irradiation, n-type active layer 2 having a carrier concentration N _1D, N _2D is N _1DA, When becomes N _2DA, the threshold voltage V _thA after Irradiation 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, 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.

In the relationship of the above equation (1), R = 1 × 10 ⁸ , 1 × 10 ⁹ ,
It is obtained from actual measurement values at three radiation doses of 3 × 10 ⁹ X-rays, and it can be said that data for deriving the general formula is somewhat insufficient. Therefore, the present inventor further proposes an eighth embodiment in which the geometrical shape of the portion leading the active layer is substantially the same, and the active layer is a single layer.
An experiment was performed using the MESFET shown in the figure. Here, 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 ,
The values were 1 × 10 ⁸ , 3 × 10 ⁸ , 1 × 10 ⁹ , 2 × 10 ⁹ , and 3 × 10 ⁹ . 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.

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 aforementioned Poisson equation, and for an intrinsic FET ignoring the source and resistance R _s. Becomes 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 saturation current I _dss when the V _G = V _bi and I _DSS for simplicity of calculation, N _1D «N _2D from N _1D / N _2D =
Assuming 0, 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 _DA 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 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 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.

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 lower layer 22 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 rate of the source-drain saturation current I _dss of the MESFET is α _L ,
In order for the combinational circuit according to the semiconductor device to operate as designed, the initial carrier concentration of the lower layer 22 of the active layer 2 is required.
From equation (10), N _2D must be N _2D > ΔN _D / {1− [α _L (μ / μ _A )] ^1/2 } (11). And the thickness a is: a = {[2ε / (q · N _2D )] · (V _bi −V _th )} ^1/2
(12) 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 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. In order to obtain a predetermined threshold voltage V _th at such a carrier concentration of the active layer 2, the effective thicknesses a ₁ and a ₂ of the active layer 2 are correlated with the carrier concentrations N _1D and N _2D. It will be determined by the relationship. 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 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 _2D of the lower layer 22 of the active layer 2 is set to 3 × 10 ¹⁸ cm ⁻³
In the structure according to the present invention (corresponding to the characteristic shown in FIG. 5), the change rate α is suppressed to a sufficiently low value even at R = 3 × 10 ⁹ X-ray as shown by the curve (f), and the radiation resistance is remarkably reduced. It can be seen that it has improved.

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 source-drain saturation current I _dss of the GaAsMESFET falls within a predetermined allowable range, and
A semiconductor device including an SFET and a signal processing circuit cooperating with the SFET 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 the conventional product, and FIG. 8 is a cross-sectional view of a 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 (1)

(57) [Claims] And the MESFET active layer made of the lower layer is formed by doping impurities into the GaAs in claim 1] upper layer and the carrier concentration of the carrier concentration N _1D is _{N 2D (N 2D> N 1D} ), this ME
The value after the change of the source-drain saturation current I _dss of the SFET
The MESFET to operate correctly when the change rate α = _I dssA / I _dss when the I _DSSA is within the allowable rate of change alpha _L
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. the carrier concentration N _1D of the active layer by irradiation, decrease the △ N _D of N _2D, the irradiated before in the active layer, the carrier mobility after each mu, mu _a
When 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 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, and characterized in that it is a _{N 2D> △ N D / {} 1- [α L (μ / μ A)] 1/2} Semiconductor device.