Semiconductor device including a MESFET

Abstract: This invention is for improving the radiation hardness or radiation resistance of GaAs MESFETs. According to this invention, an n-type active layer (2) is formed by doping in a GaAs crystal (1). The n-type active layer (2) includes an upper layer (21) and a heavy doped lower layer (22). A Schottky gate electrode (6) is provided on the active layer (2), so that the carrier concentration in the active layer (2) and the thickness of the active layer (2) are set to required values. According to this invention, not only in the case of a total dose of exposure radiation of R=1x10⁹ roentgens but also in the case of a higher total dose, at least one of the threshold voltage V th of the GaAs MESFET, the saturated drain current I dss thereof, and the transconductance g m are in their tolerable range. Consequently, a semiconductor device comprising the GaAs MESFET and a signal processing circuit cooperatively combined therewith can operate normally as initially designed, with a result of considerably improved radiation hardness.

Description: (Background of the Invention) This invention relates to a semiconductor device using a GaAs metal-semiconductor field effect transistor (MESFET), specifically to that which can be used in machines and instruments requiring radiation hardness or radiation resistance. (Related Background Art) The devices which are used in aerospace systems and near nuclear reactors require a high radiation hardness. The radiation includes gamma (γ) rays, neutron rays, proton rays, etc. Generally the gallium arsenide (GaAs) MESFETs and ICs based upon these FETs will withstand total exposure dose of 1×10⁸ roentgens with little if any change in characteristics. By contrast, silicon (Si) MOS circuits have failed at dose of 1×10⁶ roentgens (Proceedings of Symposium of Space Development, 1987, ps. 35 ∼ 38). For improving the radiation hardness of the GaAs MESFET, the following art have been proposed. In a first one, a p-type layer is buried below an n-type active layer to thereby decrease leakage current to the substrate, and the threshold voltage of a GaAs FET is improving in the radiation hardness. In a second one, the Schottky gate of a GaAs FET is shortened. But these prior art have improved the radiation hardness up to a total exposure dose R of about 1 x 10⁸ roentgens, but it cannot be said that these prior art have succeeded in attaining the sufficiently practical level (1 x 10⁹ roentgens). Under these circumstances, no practical transistor having a radiation hardness level of about 1 x 10⁹ roentgens has been realized. On the other hand, Physics of Semiconductor Device, 2nd Edition, S.M. Sze., P. 334, describes a MESFET having two active layers. This MESFET has realized superior characteristics since the carrier concentration in the surface of the active layer decreases and consequently the MESFET is free from the affection of a deletion layer. International Electron Device Meeting, 7th-10th December 1986, pages 829-831 discloses a GaAs MESFET comprising a highly doped thin active channel layer, a thin barrier channel with reduced doping level and an AlGaAs buffer layer, wherein the highly doped active layer confines the depletion depths both laterally and vertically into the channel. This decreases the effective electrical length of the device which minimizes the parasitic gate capacitance and increases the device transconductance through enhanced charge control of the channel by the gate electrode. There is no teaching about a MESFET-structure providing a radiation hardness. Therefore, an object of this invention is to provide a GaAs MESFET which has a simple structure and a high radiation hardness which has been improved by especially making use of changes of at least one of the threshold voltage, the saturated drain current, and the transconductance. Summary of the Invention The inventors have noticed that when radiation is applied to a GaAs MESFET, the change amount ΔV th of the threshold voltage V th in the saturation region, the change rate α = I dssA /I dss of saturated drain current at normal gate bias I dss , and the change rate β=g mA /g m of a transconductance g m has a constant relationship with the effective thickness t a =t a1 +t a2 of the active layer and the change amount ΔN D of the carrier concentrations N 1D , N 2D and has found that the change amount ΔN D has a constant quantitative relationship with the total exposure dose R. Based on this finding, they have completed this invention as defined in claim 1. In this arrangement, under irradiation of a total exposure dose even above 1x10⁹ roentgens, at least any one of the threshold voltage V th , the saturated drain current I dss , and the transconductance g m are within their corresponding preset ranges (tolerable ranges determined by the signal processing circuit). Consequently, the semiconductor device according to this invention comprising the GaAs MESFET and a signal processing circuit can operate normally as initially designed. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view of a GaAs MESFET explaining the principle of this invention; Fig. 2 is a graph showing the total exposure dose R dependence of a change amount ΔV th of the threshold voltage of the MESFET involved in this invention; Fig. 3 is a graph showing the total exposure dose R dependence of a decrease amount ΔN D of the carrier concentration; Fig. 4 is a graph showing the result of the experiment on the total exposure dose R dependence of a change amount ΔV th of the threshold voltage of a conventional MESFET; Fig. 5 is a graph showing the total exposure dose R dependence of a change rate α of the saturated drain current of the MESFET involved in this invention; Fig. 6 is a graph showing the total exposure dose R dependence of a change of the saturated drain current I dss ; Fig. 7 is a graph showing the total exposure dose R dependence of a change rate β of the transconductance of the MESFET involved in this invention; Fig. 8 is a graph showing the results of the experiments on the total exposure dose R dependence of a change of the transconductance g m of the conventional MESFET; Figs. 9 to 11 are characteristic curves of the threshold voltage, saturated drain current, and transconductance of the MESFET involved in this invention and of the conventional FETs for comparison in radiation hardness; and Fig. 12 is a sectional view of a GaAs MESFET having an active layer in one layer. DESCRIPTION OF THE PREFERRED EMBODIMENT This invention will be explained in good detail with reference to the drawings showing the principle and structure of this invention. The semiconductor device according to this invention comprises a GaAs MESFET, and a signal processing circuit cooperatively combined with the MESFET. The MESFET and the signal processing circuit can provide combination circuits, e.g., amplifiers, inverters, oscillaters, digital logic arrays, etc. The MESFET in this combination circuit involved in this invention has a preset threshold voltage V th , a saturated drain current I dss at normal gate bias, and a transconductance g m in the saturation region. It has been known that their values change under radiation exposure. When a changed threshold voltage V thA , a changed saturated drain current I dssA , and a changed transconductance g mA of which the initial values have changed due to radiation exposure are out of their preset ranges by the signal processing circuit, this combination circuit does not normally operate. Hereinafter in this specification, a tolerable value of a change amount ΔV th of the threshold voltage V th is represented by a tolerable change amount ΔV thL . A tolerable value of a change rate α=I dssA /I dss of the saturated drain current I dss is represented by a tolerable change rate α L , and a tolerable value of a change rate α=g mA /g m of the transconductance is represented by a tolerable change rate β L . The values of ΔV thL , α L and β L vary depending on designs of the above described signal processing circuit, but generally ΔV thL equal to or is lower than 0.2 V, and α L and β L equal to or are higher than 0.8. As described above, it is known that the threshold voltage V th , etc. change under radiation exposure. As causes for these changes have been reported, firstly, decreases in a carrier concentration of the active layer due to radiation exposure, and secondly decreases in an electron mobility therein due to radiation exposure. The inventors discussed the first cause in good detail and found the relationship ΔN D = b·R c where b and c are constants holds between a decrease amount ΔN D of a carrier concentrations N 1D , N 2D and a total exposure dose R. Formula 1 holds when an initial carrier concentration N 2D (before radiation exposure) of the active layer is 1×10¹⁷ ∼ 1×10¹⁹cm⁻³, a total dose R of exposure radiation is 1×10⁸ ∼ 1×10¹⁰ roentgens. The constants b and c have some ranges depending on changes of an initial carrier concentration of the active layer, qualities of the substrates, etc. According to the experiments conducted by the inventors, the constants b and c have ranges of 1.99 × 10¹⁰ ≦ b ≦ 3.98 × 10¹⁰ 0.5 ≦ c ≦ 0.8 and the typical values are b=3.06×10¹⁰ , c=0.678. Therefore, the typical value of the decrease amount ΔN D of the carrier concentration is defined by ΔN D = 3.06 × 10¹⁰ · R 0.678 . The experiment in which the relationship of Formula 1 was found will be explained below in good detail. Fig. 1 shows a sectional view of a selectively grown n⁺ source/drain GaAs MESFET having a recessed gate. As shown in Fig. 1, an n-type active layer 2, and an n⁺ doped contact region 3 are formed on a semi-insulating GaAs substrate 1. The active layer 2 includes an upper layer 21 of a thickness t a1 and a lower layer 22 of a thickness t a2 . Parts of the n-type active layer 2 and the n⁺ doped contact region 3 for a gate region to be provided are etched off to form a recess structure. Then a source electrode 4 and a drain electrode 5 of ohmic metal are formed on the n⁺ doped contact region 3 by the vacuum evaporation. A gate electrode 6 of Schottky-barrier metal is formed on the n-type active layer 2. The part of the n-type active layer 2 directly below the gate electrode 6 has a sufficiently smaller thickness, an effective thickness of about 450 Å, compared with the n-type active layers of the conventional MESFETs (each having a thickness of about 1000 Å (10Å = 1nm)). The lower layer 22 has a thickness of t a2 = 150 Å, the upper layer 21 has a thickness of t a1 = 300 Å. The active layer 2 has a higher carrier concentration N 2D in the lower part thereof compared with the carrier concentrations of the conventional MESFETs, specifically the lower layer 22 has a carrier concentration of N 2D = 3×10¹⁸cm⁻³, the upper layer 21 has a carrier concentration of N 1D = 2×10¹⁷cm⁻³. The theoretical value of the threshold voltage V th of this GaAs MESFET is given by V th = V bi -(q·N 2D /2ε)t a2 ² - (q·N 2D /ε)t a1 ·t a2 - (q·N 1D /2ε)t a1 ² given by solving the one dimensional Poisson equation d²₁/dχ² = -qN 1D /ε (0≦χ≦t a1 ) d²₂/dχ² = -qN 2D /ε (t a1 ≦χ≦t a1 +t a2 ) under the boundary conditions of d/dχ = 0 at χ = t a1 + d (d>t a2 )  = -(V bi - V G ) at χ = 0. In Formula 2, V bi represents a built-in voltage of the MESFET; q, an electron charge; and ε, a dielectric constant of the n-type active layer 2. When the carrier concentrations N 1D , N 2D of the n-type active layer 2 becomes N 1DA , N 2DA due to radiation exposure, a threshold voltage V thA after the radiation exposure is given by V thA = V bi - (q·N 2DA /2ε)t a2 ² - (q·N 2DA /ε)t a1 ·t a2 - (q·N 1DA /2ε)t a1 ² Then, a change amount of the threshold voltage V th due to the radiation exposure is given by In Formula 4 Therefore when a decrease amount of the carrier concentration due to radiation exposure is given by ΔN D = N D - N DA , with t a =t a1 +t a2 , ΔV th = {(q·t a ²)/2ε}·ΔN D The inventors further studied the change amount ΔV th of the threshold voltage V th by irradiating gamma rays in total exposure doses R=1×10⁸ roentgens, 1×10⁹ roentgens and 3×10⁹ roentgens to a MESFET of Fig. 1 having the active layer 2 of a thickness of 450 Å (=150+300). The result is shown in Fig. 2 by black points. Based on the results in Fig. 2, using Formula 5, change amounts (decrease amounts) ΔN D of the carrier concentration are indicated by the black points for total doses of R=1×10⁸ roentgens, 1×10⁹ roentgens, and 3×10⁹ roentgens in Fig. 3, which shows that the above described formula ΔN D = 3.06 × 10¹⁰ · R 0.678 (the dot line of Fig. 3) holds. When Formula 1 is applied to Fig.2, the dot line in Fig. 2 is obtained. This shows that the experimental result and the theoretical values well agree. In Fig. 2, with a total dose of R=1×10⁹ roentgens, a change amount ΔV th of the threshold voltage is as low as about 0.04V. Therefore, it is confirmed that the radiation hardness is conspicuously improved when the active layer 2 has a thickness of about 450 Å. Formula 1 described above was derived from actually measured values for three total doses of R=1×10⁸ roentgens, 1x10⁹ roentgens and 3×10⁹ roentgens. It can be said that these values are insufficient data to derive a general formula. Then the inventors conducted a further experiment of irradiating gamma rays from cobalt 60 to the GaAs MESFET of Fig. 12 which had the same geometrical structure except that in the active layer as the GaAs MESFET involved in this invention and has the active layer 2 in one layer. The active layer 2 had an effective thickness t a of 1130 Å so that the carrier concentration N D is 2.09×10¹⁷cm⁻³. In this experiment, total exposure doses were R=1×10⁶ roentgens, 1×10⁷ roentgens, 1×10⁸ roentgens, 3×10⁸ roentgens, 1×10⁹ roentgens, 2×10⁹ roentgens and 3×10⁹ roentgens. The resultant change amounts ΔV th of the threshold voltage are shown in Fig. 4 by the black points. These points well agree with the theoretical values indicated by the dot line. The change amounts ΔV th after a radiation exposure of R=1×10⁹ roentgens, however, was about 0.4V, which was remarkably inferior to that in Fig.2. The results of these experiments on the threshold voltage show the following. Firstly, a major cause for the degradation of the threshold voltage of the MESFET due to radiation damage is a decrease in the carrier concentration of the active layer, and it was found that Formula 1 well expresses a decrease in the carrier concentration under the radiation exposure. Secondly, it has found that the change amount ΔV th of the threshold voltage V th can be set at a required value by setting only the thickness t a =t a1 +t a2 of the active layer. Specifically, as shown in Fig. 4, when the thickness t a of the active layer 2 is set at about 1000 Å as in the conventional MESFETs, the radiation hardness is insufficient, but as shown in Fig. 2, when the thickness t a =t a1 +t a2 is set at 450 Å, the radiation hardness is conspicuously improved. Then, the inventors actually measured changes of the saturated drain current I dss due to the radiation exposure of the same GaAs MESFETs as those which exhibited the threshold voltages V th of Figs. 2 and 4. As a result, the characteristic of the change rate α of the saturated drain current I dss of Fig. 5 was obtained in use of same MESFET of Fig. 2. The saturated drain current I dss of Fig. 6 was obtained in use of same MESFET of Fig. 4. In Figs. 5 and 6, the black points indicate the experimental values, and the dot lines indicate the theoretical values given by applying Formula 10 described below to Formula 1. The theoretical formula for the change rate α=I dssA /I dss of the saturated drain current I dss will be derived below. The saturated drain current I dss of the MESFET is given for an intrinsic FET with a source resistance R s kept out of consideration, by I dss = {(W g ·µ·q²·N 2D ²)/(ε·L g )} × {t a1 ·t a2 (t a2 + t a1 -d₁) + (t a2 -t a1 )·[t a2 ² - (-t a1 + d₁)²]/2 - [t a2 ³ - (-t a1 + d₁)³]/3} where d₁ = {t a1 ² - N 1D · t a1 ²/N 2D + 2ε · (V bi - V G )/(q·N 2D )} 1/2 In Formula 6, W g represents a gate width; µ, an electron mobility in the active layer 2; L g , a gate length; and V G , a gate voltage. In Formula 6, N 1D /N 2D =0 based on N 1D <N 2D . Therefore, Formula 6 is rewritten d₁=t a1 . When a saturated drain current I dss for V G =V bi is represented by I DSS for simplifying the computation, Formula 6 is rewritten into I DSS = {(W g ·µ·q²·N 2D )/(ε·L g )} ·[t a1 ·t a2 ² + (t a2 - t a1 )t a2 ²/2 - t a2 ³/3] when a saturated drain current after the radiation exposure is represented by I DSSA , a change rate α due to the radiation exposure is given based on Formula 7 by where N 2DA is a carrier concentration of the lower layer after the radiation exposure and is given by N 2DA = N 2D - ΔN D Then Formula 8 is substituted by Formula 9 into α = {µ A (N 2D - ΔN D )²}/(µ·N 2D ²) Then Formula 10 will be discussed below. It is found that the change rate α is influenced by changes (µ → µ A ) of the electron mobility µ due to the radiation exposure. But µ A /µ is around 0.95 - 0.98 when the carrier concentration before the radiation exposure is about 1×10¹⁸cm⁻³. The change becomes smaller as the carrier concentration becomes higher. Then the computation was made with µ A /µ=0.95. The results were the dot lines of Figs. 5 and 6. As described above, it was confirmed that the results agreed with the experimental values. These experiments on the saturated drain current I dss and the studies of their results show the following. Firstly, a major cause for the degradation of the I dss of the MESFET as total dose radiation effects is a decrease of the carrier concentration of the active layer, and it was found that Formula 1 is very explanatory of the decrease of the carrier concentration due to the radiation exposure. Secondly, the change rate α of the saturated drain current can be set at a required value by setting only the initial carrier concentration (before the radiation exposure) N 2D of the lower layer 22 of the active layer, because µ is a constant in Formula 10, the value of µ A /µ can be approximated, and ΔN D can be determined depending on a total radiation dose in Formula 1. Specifically, when the carrier concentration N D of the active layer 2 is set at about 2×10¹⁷cm⁻³ as in the conventional ones, the radiation hardness is insufficient as seen in Fig. 6. When the carrier concentration N 2D of the lower layer is set at 1×10¹⁸cm⁻³, the radiation hardness is outstandingly improved as seen in Fig. 5. Then, the inventors actually measured changes of the transconductances g m due to the radiation exposure in the saturation regions of the same GaAs MESFETs as those which exhibited the threshold voltages V th of Figs. 2 and 4 and the saturated drain current I dss characteristics of Figs. 5 and 6. As results, the change rate β of the transconductance g m of Fig. 7 was obtained in use of same MESFET of Fig. 2 and 5, and the transconductance g m of Fig. 8 was obtained in use of same MESFET of Fig. 4 and 6. In Figs. 7 and 8, the black points indicate the experimental values, and the dot lines indicate the theoretical values given by applying Formula 15 described below to Formula 1. The theoretical formula for the change rate β=g mA /g m of the transconductance will be derived below. A transconductance g m in the saturation region of the MESFET is given for an intrinsic FET with a source resistance R s kept out of consideration, by g m = {(W g ·µ·q·N 2D )/(d₁·L g )} × {-t a1 ·t a2 (t a2 + t a1 ) + (t a2 - t a1 )· [t a2 ² - 2(d₁ - t a1 )²]/2 + [t a2 ³ - (d₁ - t a1 )³]/3} In this Formula, d₁ = {t a1 ² - N 1D ·t a1 ²/N 2D + 2ε ·(V bi - V G )/(q·N 2D )} 1/2 In Formula 11, N 1D /N 2D =0 based on N 1D <N 2D . Therefore, Formula 11 is rewritten d₁=t a1 . When a transconductance g m for V G =V bi is represented by g mmax for simplifying the computation, Formula 11 is rewritten into g mmax = {(W g ·µ·q·N 2D )/(t a1 ·L g )}· [-t a1 ·t a2 ²(t a1 + t a2 ) - (t a2 - t a1 ) t a2 ²/2 + t a2 ³/3] A change rate β due to the radiation exposure is given based on Formula 12 by where N 2DA is a carrier concentration of the lower layer after the radiation exposure and given by N 2DA = N 2D - ΔN D Then Formula 13 is substituted by Formula 14 into β = {µ A (N 2D - ΔN D )}/(µ·N 2D ) Then Formula 15 will be discussed below. It is found that the change rate β is influenced by changes (µ → µ A ) of the electron mobility µ due to the radiation exposure. But µ A / µ is around 0.95 when the carrier concentration before the radiation exposure is about 1×10¹⁸cm⁻³. The change becomes smaller as the carrier concentration becomes higher. Then the computation was made with µ A /µ=0.95. The results were the dot lines of Figs. 7 and 8. As described above, it was confirmed that the results agreed with the experimental values. These experiments and the studies of their results show the following. Firstly, a major cause for the degradation of the transconductance of the MESFET as total dose radiation effect is a decrease of the carrier concentration in the active layer, and it was found that Formula 1 is very explanatory of the decrease of the carrier concentration due to the radiation exposure. Secondly, the change rate β of the transconductance can be set at a required value by setting only the initial carrier concentration N 2D of the lower layer 22 of the active layer, because µ is a constant in Formula 15, the value of µ A / µ can be approximated, and ΔN D is determined based on Formula 1, depending on a radiation dose. Specifically, when the carrier concentration N D of the active layer 2 is set at about 2×10¹⁷cm⁻³ as in the conventional ones, the radiation hardness is insufficient as seen in Fig. 8. When the carrier concentration N 2D in the lower layer is set at 1×10¹⁸cm⁻³, the radiation hardness is outstandingly improved as seen in Fig. 7. It is possible that, based on the above described findings, a structure of a semiconductor device which is able to operate normally under the radiation exposure of a total dose R not only below 1×10⁹ roentgens but also a total dose R above 1×10⁹ roentgens can be specified based on a thickness t a of the active layer 2 and a carrier concentration N 2D in the lower layer 22. That is, in order that a GaAs MESFET and a signal processing circuit are combined into such semiconductor device, and the signal processing circuit can operate as designed when a tolerable change amount of the threshold voltage V th of the MESFET is ΔV thL , an effective thickness t a =t a1 +t a2 of the active layer 2 must be t a < {(2ε·ΔV thL )/(q·ΔN D )} 1/2 based on Formula 5. In this case, the carrier concentration N 2D of the lower layer 22 of the active layer 2 is given as follows based on Formula 2 by N 2D = {[2ε/(q·t a ²)]·(V bi - V th )} Here, when a tolerable change amount ΔV thL of the threshold voltage V th for a total exposure dose of R=1×10⁹ roentgens is specifically computed by ΔV thL = 0.1V(ΔV th < 0.1V), a change amount ΔN D of the carrier concentration is given by ΔN D = 3.87 × 10¹⁶cm⁻³ . An effective thickness t a =t a1 + a2 of the active layer 2 is given below 585 Å (450 Å for the MESFET of Fig. 2), based on Formula 16. Further, with a thickness of the active layer 2 set at below 585 Å, the carrier concentrations N 1D , N 2D of the active layer is determined by their interrelationships with the thickness t a1 , t a2 . In this case, a dielectric constant of the active layer an electron charge q = 1.602 × 10⁻¹⁹C, and a built-in voltage V bi = 0.7V. In order that the combination circuit relates to the semiconductor device operates as required when a tolerable change rate of the saturated drain current I dss of the MESFET is represented by α L , an initial carrier concentration N 2D of the lower layer 22 of the active layer 2 must be, based on Formula 10 by N 2D > ΔN D /{1-[α L (µ/µ A )] 1/2 } In this case, an effective thickness t a =t a1 +t a2 of the active layer 2 is given by t a ={[2ε/(q·N 2D )](V bi -V th )} Here, for a total exposure dose of R=1×10⁹ roentgens, with α L (a tolerable change rate of the saturated drain current I DSS )=0.9 (I DSSA >0.9I DSS ), a change amount ΔN D of the carrier concentration is given based on Formula 1 by ΔN D = 3.87 × 10¹⁶cm⁻³ , and the carrier concentration N 2D of the lower layer 22 of the active layer 2 becomes above 1.45×10¹⁸cm⁻³ based on Formula 10. Further, in order that the GaAs MESFET and a signal processing circuit are combined into this semiconductor device, and the signal processing circuit can operate as designed when a tolerable change rate of the transconductance g m in the saturation region of the MESFET is β L , an initial carrier concentration N 2D of the lower layer 22 of the active layer 2 must be, based on Formula 15, N 2D > ΔN D /{1 - β L (µ/µ A )} In this case, an effective thickness t a =t a1 +t a2 of the active layer 2 must be t a = {[ε/(q·N 2D )}] (V bi -V th )} 1/2 Here, for a total exposure dose of R=1×10⁹ roentgens, with β L (a tolerable change rate of the transconductance g mmax ) = 0.9 (g mmaxA >0.9 g mmax ), a change amount ΔN D is given by ΔN D = 3.87 × 10¹⁶cm⁻³ based on Formula 1. The carrier concentration N 2D of the lower layer of the active layer 2 is given above 7.35×10¹⁷cm⁻³ by Formula 15. The semiconductor device according to this invention and the conventional ones will be compared in radiation hardness in Figs. 9 to 11. Fig. 9 shows change amounts ΔV th of the threshold voltage V th due to the radiation exposure. Fig. 10 shows change rates α of the saturated drain current I dss . Fig. 11 shows change rates β of the transconductance g m . In Figs. 9 - 11, curves (a), (b) and (c) show characteristics of the conventional commercial MESFETs. The curve (b) corresponds to the characteristics of Figs. 4, 6 and 8 for the active layer 2 of a 1130 Å effective thickness t a and a carrier concentration of 2.09×10¹⁷cm⁻³. The curve (d) shows the characteristics of a conventional HEMT (high electron mobility transistor). As evident from Fig. 9, these conventional semiconductor devices have change amounts ΔV th of the threshold voltage as high as 0.2 to 0.3V for a total dose of R=1×10⁹ roentgens. The curve (e) in Fig. 9 shows the characteristic of a MESFET having a p-type layer buried below an n-type active layer for decreasing leakage current to the substrate, and the change amount ΔV th is suppressed to about 0.12V for R=1×10⁹ roentgens. In contrast to this, in the MESFET according to this invention having the active layer 2 of an effective thickness t a =t a1 +t a2 of 450 Å (corresponding to the characteristic of Fig. 2), the change amount ΔV th is suppressed to a value lower than 0.1V even for R=1×10⁹ roentgens as indicated by the curve (f), and it is found that the radiation hardness is much improved. It is evident from Figs. 10 and 11 that such improvement in the radiation hardness is also exhibited in the saturated drain current I dss and the transconductance g m . In this invention, even under the radiation exposure of a total exposure dose equal to or higher than R=1×10⁸ roentgens, the values of the threshold voltage V th , saturated drain current I dss and transconductance g m remain within their tolerable ranges. A GaAs MESFET which is acknowledged as superior in radiation hardness characteristic must have radiation hardness to a total exposure dose of about 1.4×10⁸ ∼ 4.3×10⁹ roentgens. For this exposure dose, the absorbed dose of GaAs is totally 1×10⁸ ∼ 3×10⁹ rad.(1 roentgens = 0.7 rads. in GaAs). On the other hand, the tolerable range of the change (positive shift) amount ΔV th of the threshold voltage V th is 0.2 V, and the tolerable ranges of the change rates α=I dss /I dss , β=g mA /g m of the saturated drain current I dss and the transconductance g m are about 80%. Specifically, when the change amount ΔV th = 0.15V with a total exposure dose R=1.5×10⁹ roentgens, it can be said that the GaAs MESFET has superior radiation hardness. But what has to be noted here is that the above described tolerable change amount ΔV thL , and the tolerable change rates α L , β L greatly vary depending on circuits combined with the GaAs MESFET. Specifically, one example is SCFL (Source coupled FET Logic) circuits which have low integrity but have enabled high speed operation. In theses circuits, the operation speed is substantially determined by a current flowing two transistors in the buffer stages. Accordingly, when values of the V th , I dss and g m vary due to a radiation exposure, the operation speed greatly varies. But the influence on the operation speed by the changes of values of V th , I dss and g m can be reduced by 1/3 ∼ 1/4 by setting the values of resistors of the SCFL circuit at suitable values. Consequently, even in a SCFL circuit which allows for a change of the operation speed of only 10%, the tolerable change is 200 m/V for the threshold voltage V th (V thL =0.2V), and the tolerable changes for the saturated drain current I dss and the transconductance g m are about 20% (α L =0.8, β L =0.8). By contrast, another example is a memory cell for a memory IC which have high integrity on a semiconductor chip, the tolerable range for those changes are narrowed. Specifically, in this IC, a time in which one small memory cell charges and discharges the data lines occupies a large part of a total access time. Furthermore, each memory cell has the transistors, resistors, etc. miniaturized for reducing power consumption. Consequently, the operation speed greatly varies depending on changes of the parameters. Specifically, in order to keep the change of the memory access time within 20%, the tolerable change amount of the threshold voltage V th is only 50 mV (ΔV thL = 0.05V), and the tolerable change rates of the saturated drain current I dss and the transconductance g m are only 10% (α L =0.9, β L =0.9). This invention is not limited to the above described embodiment and covers various modifications. For example, the active layer is not necessarily formed by the epitaxial growth but may be formed by the ion implantation. The recess structure of Fig. 1 is not essential.

Claims: A radiation resistive semiconductor device comprising a signal processing circuit combined with a MESFET having a semi-insulating GaAs substrate (1) with an active layer (2) said active layer comprising an upper layer (21) having a carrier concentration N 1D and an effective thickness t a1 , and a lower layer (22) having a carrier concentration N 2D and an effective thickness t a2 , wherein N 1D < N 2D , said active layer being directly formed on said substrate (1), and a gate electrode (6) formed on said active layer (2), said MESFET having a threshold voltage V th with a change amount ΔV th , a saturated drain current I dss with a change rate α and a transconductance g m with a change rate β, these parameters being changed upon exposure of the device to a radiation exposure of a total dose R equal to or higher than 1 x 10⁹ roentgens within admissible change amounts ΔV thL and α L and β L , respectively, wherein ΔV thL ≦ 0.2 V, α L and β L ≧0.8, the effective thickness t a = t a1 +t a2 of the active layer (2) being given by t a < {(2ε·ΔV thL ) / (q · ΔN D )} 1/2 and the carrier concentration N 2D of the lower layer (22) before the radiation exposure being given by N 2D > ΔN D / {1 - [α L (µ/µA)] 1/2 } and/or N 2D > ΔN D / {1 - β L (µ/µ A )}, where µ and µ A represent the carrier mobilities in the active layer (2) respectively before and after the radiation exposure, ΔN D the decrease of carrier concentrations N 1D or N 2D of the upper and lower layers (21,22) due to the radiation exposure, ε the dielectric constant of the active layer (2), and q the electron charge. A semiconductor device according to claim 1, wherein a decrease amount ΔN D of the carrier concentration is given by ΔN D = b · R C where b and c are constants. A semiconductor device according to claim 2, wherein the constants b and c are 1.99 x 10¹⁰ ≦ b ≦ 3.98 x 10¹⁰ 0.5 ≦ c ≦ 0.8.

Inventors: NISHIGUCHI MASANORI YOKOHAMA W, OKAZAKI NAOTO, NISHIGUCHI, MASANORI YOKOHAMA WORKS OF SUMITOMO, OKAZAKI, NAOTO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1989
Main classification: H01L29
IPC codes: H01L29