AU643780B2 - A semiconductor device - Google Patents

Description

i 64 3 78 0'~ COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION (Original) FOR OFFICE USE 0r E- 6

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C 0 NAME ADDRESS OF APPLICANT: SUMITOMO ELECTRIC INDUSTRIES, LTD.

5-33, Kitahama 4-chome, Chuo-ku Osaka

JAPAN

NAME(S) OF INVENTOR(S): Masanori NISHIGUCHI Naoto OKAZAKI ADDRESS FOR SERVICE: DAVIES COLLISON, Patent Attorneys, 1 Little Collins Street, Melbourne, 3000.

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1 in 0 0r COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED: "A Semiconductor Device" The following statement is a full description of this invention, including the best method of performing it known to us: -1- 910419,gqcpd&h64,41791.poa,1 la 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 tilerance, radiation hardness or radiation resistance.

The devices which are used in aerospace systems and near nuclear furnaces are required high radiation hardness. The radiation includes gamma rays, neutron rays, proton rays, etc. Generally the gallium arsenide (GaAs) MESFETs and ICs based upon these FETs will withstand total exposure dose of Ixl10 roentgens with little if any change in characteristics. By contrast, silicon (Si) MOS circuits have failed at dose of 1x10 6 roentgens (Proceedings of Symposium of Space Development, 1987, ps. 35 to 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 1xl0 8 roentgens, but it cannot be said that these prior art have succeeded :in attaining the sufficiently practical level (1x10 9 roentgens). Under these circumstances, :i 20 practical transistors having a radiation resistance of about Ix10 9 roentgens have not been realized.

*i~ 93O81O\PApenCnmimhLco0Mj -2- An object of this invention is to provide a GaAs MESFET in which radiation hardness is impmroved such that at least one of the threshold voltage, the saturated drain current, and the transconductance remains within an operational tolerance after a total radiation exposure dose of 1xlO 9 roentgens.

The inventors have noticed that when radiation is applied to a GaAs MESFET, the change amount AV of the threshold voltage V, in the saturation region, the change rate a I o/I of saturated drain current at normal gate bias and the change rate P=gmA/g of a transconductance g has a constant relationship with the effective thickness t, of the active layer and the change amount AN D of the carrier concentration ND, and has found that the change amount AND has a constant quantitative relationship with the total exposure dose R.

In accordance with the present invention there is r ovided a semiconductor device including a MESFET having an active layer comprising GaAs crystal that is substantially evenly doped in a depth direction to have a carrier concentration ND and a threshold voltage Vth, and which normally operates when at least one of three following conditions are satisfied: a change AVth in the threshold voltage Vth of the MESFET is within a tolerable Schange amount AVth 20 a change rate a of saturated drain current Idss is within a tolerable rate aL; and a change rate p of transconductance gm is within a tolerable rate p; ego

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o 93O6WIO'.\op"4cm=AUMIIOI=M, -3where AND represents a decrease amount of the carrier concentration in the active layer due to radiation exposure of a total dose R equal to or higher than 1x10 9 roentgens, (I and PA represent carrier mobilities in the active layer respectively before and after the radiation exposure, e represents a dielectric constant of the active layer, and q represents an electron charge; wherein the decrease amount AND of the carrier concentration is given by ND bRc where b and c are constants in the range 5x10 5 b IxlO 6 1.0 s c s 1.3 wherein the de ice is constructed such that at least one of the following conditions is met: an effective thickness ta of the active layer is given by ta {(2e AVthL) (q-AND)} 2 and S: v a carrier concentration ND of the active layer before the radiation exposure is given by ND AND {1 [aL 1 and ND ND D PL Further and alternative forms of the invention may be determined from the pd* cl appended claims.

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tt 930R1p:\opexmsmilOlco,3 -4- 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. However, it should be unrderstood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and 9*

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o9 9 9 te r 93081OMpAOpcj*MAuMIl0=m.4 1 scope of the invention will become apparent to those skilled in the art from this detailed description.

BIR-IEF DB9RIPTT O THE toRAWiS:N 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 AVth of the threshold voltage of the MESFET involved in this invention; **10 Fig. 3 is a graph showing the total exposure dose R dependence of a decrease amount AND of the carrier concentration; Fig. 4 is a graph showing the result of the experiment on the total exposure dose R dependence of a 15 change amount AVth of the threshold voltage of a conventional MESFET; Fig. 5 is a graph showing the total exposure dose R dependence of a change rate a of the saturated drain current of the MESFET involved in this invention; 20 Fig. 6 is a graph showing the total exposure dose R dependence of a change of the saturated drain current Idss; Fig. 7 is a graph showing the total exposure dose R dependence of a change rate 3 of the transconductance of the MESFET involved in this invention; Fig. 8 is a graph showing the results of the J-EULI, experiments on the total exposure dose R dependence of a 1 change of the transconductance gm of the conventional MESFET; and Figs. 9 to 11 are cheracteristio curves of the threshold voltage, saturated drain current, and 6 transconductance of the MESPET involved in this invention and of the conventional PETs for comparison in radiation hardness.

19 This invention will be explained in-good detail with Osseo.: reference to the drawings showing the principle and structure of this invention.

set The semiconductor device according to this invention comprises a GaAs MESFET, and a signal processing circuit cooperatively combined with the MESPET. The MESFET and :.so the signal processing circuit can provide combination circuits, amplifiers, inverters, oscillators,* digital logic arrays, etc.

Fig. 1 shows a sectional view of a GaAs MESFET having a recess gate structure. As shown in, Fig. 1, an n-type active layer 2, and a heavily doped n-type (n -type) contact region 3 are formed on a semi-insulating GaAs substrate 1. Parts of the n-type active layer 2 and the n "-type clontact 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 e1-type contact region 3 1 by the vacuum evaporation. A gate electrode 6 of Schottky 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, compared with the n-type active layers of the conventional MESFETs, and the active layer 2 has a higher carrier concentration ND, compared with the *carrier concentrations of the conventional MESFETs.

The MESFET in this combination circuit involved in "10 this invention has a preset threshold voltage Vth, a Ssaturated drain current at normal gate bias Idss and a "transconductance gm in the saturation region. It has been known that their values change under radiation exposure. When a changed threshold voltage VthA, a 15 changed saturated drain current IdssA, and a changed transconductance gmA to which their initial values have Schanged due to radiation exposure are out of their preset ranges by the signal processing circuit, this combination circuit does not normally operate.

4* :20 Hereinafter in this specification, a tolerable value of a change amount AVth of the threshold voltage Vth is represented by a tolerable change amount AVthL. A tolerable value of a change rate a=IdssA /dss of the saturated drain current Idss is represented by a tolerable change rate aL, and a tolerable value of a change rate a=gmA /gm of the transconductance is represented by a tolerable change rate B

L

The values 1 of AVthL, aL and RL vary depending on designs of the above described signal processing circuit, but generally AVthL equal to or lower than 0.2 V, and aL and 3L equal to or higher than 0.8.

As described above, it is known that the threshold voltage Vth, 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 1 0 electron mobility therein due to radiation exposure.

The inventors discussed the first cause in good detail and found the relationship AND b.Rc (1) where b and c are constants holds between a decrease 15 amount of a carrier concentration ND and a total exposure dose R. Formula 1 holds when an initial r carrier concentration ND (before radiation exposure) of the active layer is 1x10 17 to 1x0 19 cm 3 a total dose R of exposure radiation is 1xl08 to 1x10 1 0 roentgens. The a 0 .20 constants b and c have some ranges depending on changes of an initial carrier concentration of the active layer, qualities of the substrates, etc.

The values of the above-described constants b, c were given by the following two methods. In a first method, some samples of the GaAs MESFETs were prepared, and those samples were exposed to radiation to measure the change amounts AVth of the threshold voltages. That 1 is, since the relationship of Formula 5 which will be described below is given between the change amount AVth and the carrier concentration decrease amount AND, the relationship between the total dose R and the carrier concentration decrease amount AN D can be given empirically by measuring the total dose R of radiation and the change amount AVth. Based on this relationship, the constants b. c in the above-described For.j.ula 1 could be given.

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10 According to the experiments conducted by the inventors, the constants b and c have ranges of 1.99 x 101 0 b g 3.98 x 1010 0.5 c S 0.8 and the typical values are b=3.06x10 10 c=0.678.

Therefore, the typical value of the decrease amounts AND of the carrier concentration is defined by AND 3.06 x 10 1 0

R

0 6 7 8 This first method is for measuring a change amount AVth of the threshold voltage to calculate a carrier 20 concentration decrease amount AND and accordingly for indirectly giving the relationship between the carrier concentration decrease amount AND and the total dose R.

In contrast to the first method, in a second method, the relationship between the carrier concentration decrease amount AN D and the total dose R is directly given. To this end, the Hall effect was measured.

First, a GaAs Hall element which has been made n-type 1 by Si doping was prepared. The n-tyoe GaAs layer was formed by epitaxial growth, and the carrier concentration distribution in the n-type GaAs layer is constant in the directions of the depth, length and width of the Hall element. A plurality of such sample having a 100 A to 2 um-thickness n-type GaAs layer and a x10 16 cm- 3 to 5xO 18 cm- 3 -carrier concentration were prepared. The Hall effect was measured on these samples to give the carrier concentration and Hall mobility.

10 Then when a carrier concentration is represented by N, I and a carrier concentration change amount is indicated by AN, a carrier extinction number is given by S"AN N(before radiation) -N(after radiation).

According to these experiment, the constants b, c have an allowance of 5X10 5 5 b 5 1x10 6 c 1.3.

The constant b is represented by b=6.65x10 5 and the constant c is represented by c=1.17. Therefore, the 20 representative value of the carrier concentration *o S: decrease amount AND is AND 6.65x10 5

R

1 7 As seen from the above, the first indirectly measuring method and the second directly measuring method give different values of the constants b, c. The reason for the occurrence of such difference is presumed as follows. That is, in the indirect measurement in 1 accordance with the first method, on the assumption of the following b) and the car concentration decrease amount AND was calculated based on the actually measured values of the change amount AVth.

a) The depth profile of the effective carrier concentration is constant over the active layer of the GaAs MESFET.

b) The decrease in the effective carrier concentration occurs uniformly over the active layer, 0S 0 10 and the thickness remains unchanged for whole irradiations.

au c) The mobility of carrier is not changed by y-ray radiation.

In contrast to this, in the direct measure in accordance with the second method, the carrier concentration decrease amount AND was measured actually

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by the Hall effect, and the above-described assumption was not used. It is considered that due to this the difference in the values of the constants b, c between the first and the second methods took place. But anyway it is doubtless that the above-described Formula I is given. In the following description, the result of the second method, the measurement of the Hall effect, is taken as the values of the constants b, c.

The result of the measurement of the Hall effect is expressed in a two-dimensional logarithmic graph where y-ray exposure dose is taken on the horizontal axis, 1 and the carrier extinction number is taken on the vertical axis as shown by the dots in Fig. 3. The above-described Formula 1 expressed by AND 6.65x10 5

R

1 1 7 as described above is plotted as indicated by the dot line in Fig. 3.

The carrier concentration decrease amount AND can be given by the above-described Hall effect measurement, but the general relationship between the change amount AVth of the threshold voltage Vth of the GaAs MESFET and the carrier concentration decrease amount AND can be given theoretically as follows.

The theoretical value of the threshold voltage Vth of this GaAs MESFET is given by :15 Vth Vbi (q*ND.ta2)/2. S. Sze, "Physics of Semiconductor Devices," 2nd ed. John Wiley and Sons, 1981, ps. 312 to 361. In Formula 2, Vbi represents a built-in voltage of the MESFET; q, an electron charge; and s a dielectric constant of the n- 20 type active layer 2. When the carrier concentration ND of the n-type active layer 2 becomes NDA due to radiation exposure, a changed threshold voltage VthA after the radiation exposure is given by VthA Vbi (qNDA.ta 2 )/2s A change amount AVth of the threshold voltage Vth due to the radiation exposure is given by AVth VthA Vth 1 Vbi -(qNDA'ta 2 {Vbi -(q.ND'ta 2 {(q*ta 2 NDA) When a decrease amount of the carrier concentration due to radiation exposure is represented by AND AVth {(qta2)/2 AND In this Formula 5, the carrier concentration decrease amount AN, can be given empirically by measuring Hall effect on the samples (Hall elements).

Accordingly the theoretical value of the change amount 10 AVth of the threshold voltage can be given. Reversely, the change amount AVth can be given experimentally s using samples of the MESFET.

The inventors further studied the change amount AVth of the threshold voltage Vth by irradiating gamma rays in total exposure doses R=lxl0 8 roentgens, 1x10 9 0 roentgens and 3x10 9 roentgens to a MESFET of Fig. 1 having the active layer 2 of a thickness of 500 A. The s*e* result is shown in Fig. 2 by black points.

Then, the above-described relationship given by 20 measuring the Hall effect,

AN

D =6.05xl05 R 1 1 7 is adapted to the MESFET of Fig. 1 to give the change amount AVth of the threshold voltage Vth using the above-described Formula 5. The result is as indicated by the dot line in Fig. 2. The theoretical value of the change amount AVth of the threshold voltage well agrees with the experimental value thereof.

1 In Fig. 2, with a total dose of R=lxl0 9 roentgens, a change amount AVth of the threshold voltage is as low as about 0.075V. Therefore, it is confirmed that the radiation hardness is conspicuously improved when the active layer 2 has a thickness of about 500 A.

Formula 1 described above was derived from actually measured values for six total doses of R=lx10 8 roentges, 3x10entgens, 6x10 roentgens, 1x10 9 9 roentgens, 2x10 and 3x10 9 roentgens. It can be said 10 that these values are insufficient data to derive a 00*00: general formula. Then the inventors conducted a further *0 experiment of irradiating'gamma rays from cobalt 60 to a conventional GaAs MESFET having the same geometrical structure as the GaAs MESFET according to this embodiment, having the active layer 2 in an effective thickness ta of 1130 A so that the carrier concentration ND is 2.09x10 17 cm 3 In this experiment, total doses 6 7 8 were R=lxlO 6 roentgens, 1x10 7 roentgens, 1x10 8 roentgens, 3x10 8 roentgens, xl10 9 roentgens, 2xl0 9 o20 roentgens and 3x10 9 roentgens. The resultant change amounts AVth 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 AVth after a radiation exposure of R=lxl0 9 roentgens, however, was about 0.4V, which was remarkably inferior to that in Fig.2.

The results of these experiments on the threshold 1 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 AVth of the threshold voltage Vth can be set at a required value by setting only the thickness ta of the active 'layer.

**l*e 10 Specifically, as shown in Fig. 4, when the thickness ta *00* of the active layer 2 is set at about 1000 A as in the conventional MESFETs, the radiation hardness is insufficient, but as shown in Fig. 2, when the thickness ta is set at 500 A, the radiation hardness is conspicuously improved.

Then, the inventors actually measured changes of the saturated drain current Idss due to the radiation exposure of the same GaAs MESFETs as those which exhibited the threshold voltages Vth of Figs. 2 and 4.

:20 As a result, the characteristic of the change rate a of the saturated drain current Idss of Fig. 5 was obtained in use of same MESFET of Fig. 2. The change rate a of the saturated drain current Idss 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.

1 The theoretical formula for the change rate A=IdssA /Idss of the saturated drain current Idss will be derived below. The saturated drain current Idss of the MESFET is given for an intrinsic FET with a source resistance R s kept out of consideration, by Idss (Wg.q2ND2.ta 3 -Lg)} x {l-3(Vbi VG)/Vp 2[(Vbi VG)/Vp]3/2} (6) where Wg represents a gate width; an electron S 10 mobility in the active layer 2; Lg, a gate length; VG, a I S gate voltage; and V a pinch-off voltage. When a saturated drain current Idss for VG =Vbi is represented by IDSS for simplifying the computation, Formula 6 is rewritten into 15 IDS (Wg .q 2

.ND

2 .ta 3 Lg) (7) when a saturated drain current after the radiation exposure is represented by IDSSA, a change rate a due to the radiation exposure is given based on Formula 7 by a IDSSA

IDSS

2 2 (PA.NDA) (8) C where NDA is a carrier concentration after the radiation exposure and is given by NDA ND AND Then Formula 8 is substituted by Formula 9 into a {(PA(ND AND) 2

ND

2 Then Formula 10 will be discussed below. It is found that the change rate a is influenced by changes 1 PA of the electron mobility p due to the radiation exposure. But is around 0.95 0.98 when the carrier concentration before the radiation exposure is about lxl018cm 3 The change becomes smaller as the carrier concentration becomes higher. Then the computation was made with PA/'A =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. J O These experiments on the saturated drain current Idss 0 t* and the studies of their results show the following.

Firstly, a major cause for the degradation of the Idss of the MESFET as total dose radiation effects is decreases 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 a of the saturated drain current can be set at a required value by setting only the initial carrier S"20 concentration (before the radiation exposure) of the 0* active layer, because Ai is a constant in Formula the value of P.A/P can be approximated, and AND can be determined depending on a, total radiation dose in Formula 1. Specifically, when the carrier concentration ND of the active layer 2 is set at about 2x10 17 cm 3 as in the conventional ones, the radiation hardness is insufficient as seen in Fig. 6. When the carrier 1 6sS* 4S 10 0 0

S

15 0 concentration ND is set at 1xlQ 1 8 cz- 3 the radiation hardness is outstandingly improved as seen in Fig. Then, the inventors actually measured changes of the transconductances gm due to the radiation exposure in the saturation regions of the same GaAs MESFETs as those which exhibited the threshold voltages Vth of Figs. 2 and 4 and the saturated drain current Idss characteristics of Figs. 5 and 6. As results, the change rate fS of the transconductance gm of Fig. 7 was obtained in use of same MESFET of Fig. 2 and 5, and the change rate of the transconductance gm 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 described below to Formula 1.

The theoretical formula for the change rate B=gmA/g m of the transconductance will be derived below. A transconductance gm in the saturation region of the MESFET is given for an intrinsic FET with a source resistance R s kept out of consideration, by m {(Wg -q-.ND.ta)/Lg x {1 -E(Vbi VG Vp 3 1/2 (11).

When a transconductance gm for VG =Vbi is represented by gmmax for simplifying the computation, Formula 11 is rewritten into gmmax (Wg''q-NDta)/Lg (12).

1 A change rate R due to the radiation exposure is given based on Formula 12 by gmmaxA gmmax (ANDA) ND) (13) where NDA is a carrier concentration after the radiation exposure and given by NDA ND AND (14).

Then Formula 13 is substituted by Formula 14 into 0 1 f {#A(ND AND)}/(.N

D

S 10 Then Formula 15 will be discussed below. It is found O**tes that the change rate 3 is influenced by changes (u A) of the electron mobility u due to the radiation see exposure. But AA/ is around 0.95 when the carrier concentration before the radiation exposure is about *15 1x10 cm 3 The change becomes smaller as the carrier concentration becomes higher. Then the computation was made with uA/'u=0.95. The results were the dot lines 9s** of Figs. 7 and 8. As described above, it was confirmed that the results agreed with the experimental values.

a *20 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 decreases 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 6 of the transconductance can 1 be set at a required value by setting only the initial carrier concentration of the active layer, because p is a constant in Formula 15, the value of pA/l can be approximated, and AND 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 2x10 17 cm-3 as in the conventional ones, the radiation hardness is insufficient as seen in Fig.

8. When the carrier concentration ND is set at 1xl0 18 cm-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 1x10 9 roentgens but also a total dose 1 above Ix10 9 roentgens can be specified based on a thickness ta of the active layer 2 and a carrier concentration ND. That is, in order that a GaAs MESFET and a signal processing circuit are combined into ,20 such semiconductor device, and the signal processing circuit can operate as designed when a tolerable change amount of the threshold voltage Vth of the MESFET is VthL, an effective thickness ta of the active layer 2 must be t a {(2E .AVthL)/(q.AND)}1/ 2 (16), based on Formula 5. In this case, the carrier concentration ND of the active layer 2 is given as 1 C .69 ao a 0 Cas a 420 2a 05.9 'p Soc follows based on Formula 2 by ND {[2/(q.ta 2 ).(Vbi Vth)) (17) Here, when a tolerable change amount AVthL of the threshold voltage Vth for a total exposure dose of R=lxlO 9 roentgens is specifically computed by AVthL O.1V(AVth 0.1V), a change amount AND of the carrier concentration is given by AND 3.87 x 10 16 cm 3 An effective thickness t a of the active layer 2 is given below 767 A (500 A for the MESFET of Fig. based on Formula 16. Further, with a thickness of the active layer 2 set at below 767 A, when the threshold voltage Vth is given Vth -1.2V, a carrier, concentration ND of the active layer 2 is given 4.28x1017cm 3 by Formula 17, where a dielectric constant of the active layer S= &s'O 12.0 x 8.85 X 10- 12 F/m an electron charge q 1.602 x 10 19 C, and a built-in voltage Vbi 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 Idss of the MESFET is represented by aL, an initial 1 carrier concentration ND of the active layer 2 must be, based on Formula 10 by ND AND 1 2 (18).

In this case, an effective thickness t a of the active layer 2 is given by ta {[2s/(q.ND)](Vbi-Vth) (19).

Here, for a total exposure dose of R=1x10 9 roentgens, with aL (a tolerable change rate of the saturated drain current IDSS)=0.

9 (IDSSA>0.9IDSS), a change amount AND tO of the carrier concentration is given based on Formula 1 0«ee e by O- 1f SAND 3.87 x 1016cm 3 for and the carrier concentration of the active layer 2 becomes above 8.84x10 18 cm 3 based on Formula 10. With this carrier concentration, when the threshold voltage 0 Vth is given Vth -1.2V.

the effective thickness of the active layer 2 is given 546 A based on Formula 19.

2O Further, in order that the GaAs MESFET and a signal 0 O 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 gm in the saturation region of the MESFET is BL, an initial carrier concentration ND of the active layer 2 must be, based on Formula ND AND RL (21).

1 In this case, an effective thickness ta of the active layer 2 must be ta (Vbi-Vth)}1/ 2 (22).

Here, for a total exposure dose of R=lxl0 9 roentgens, with L (a tolerable change rate of the transconductance gmax 09 (gmmaxA >0.9 gmmax) a change amount AND is given by AND 3.87 x 10 16 cm 3 based on Formula 1. The carrier concentration of the 17 active layer 2 is given above 4.50xl0 1 7 cm'3 by Formula 15. With the carrier concentration of the active layer 2 above this value and with the threshold voltage Vth given Vth -1.2V, :15 an effective thickness t of the active layer 2 is given 52P A by Formula 22.

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 S20 AVth of the threshold voltage Vth due to the radiation exposure. Fig. 10 shows change rates a of the saturated drain current Idss. Fig. 11 shows change rates .I of the transcinductance gm. In Figs. 9 to 11, curves and 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 A effective thickness 1 ta and a carrier concentration of 2.09x10 17 cm- 3 The curve shows the characteristics of a conventional HEMT (high electron mobility transistor). As evident from Fig. 9, these conventional semiconductor devices have change amounts AVth of the threshold voltage as high as 0.2 to 0.3V for a total dose of R=1x10 9 roentgens. The curve in Fig. 9 shows the characteristic of a MESFET having a p-type layer buried S* below an n-type active layer for decreasing leakage 0 current to the substrate, and the change amount AVth is too**: .suppressed to about 0.12V for R=1x10 9 roentgens. In contrast to this, in the MESFET according to this invention having the active layer 2 of an effective thickness ta of 500 A (corresponding to the :15 characteristic of Fig. the change amount AVth is suppressed to a value lower than 0.1V even for R=1x10 9 roentgens as indicated by the curve and it is found 0@ that the radiation hardness is much improved. It is evident from Figs. 10 and 11 that such improvement in 0 *20 the radiation hardness is also exhibited in the saturated drain current Idss and the transconductance gm' In this invention, even under the radiation exposure of a total exposure dose equal to or higher than R=1x10 8 roentgens, the values of the threshold voltage Vth, saturated drain current Idss and transconductance gm remain within their tolerable ranges. A GaAs MESFET 1 which is acknowledged as superior in radiation hardness characteristic must have radiation hardness to a total exposure dose of about 1.4x10 8 to 4.3x10 9 roentgens.

For this exposure dose, the absorbed dose of GaAs is totally 1xl0 8 to 3x10 9 rad (1 roentgen 0.7 rads. in GaAs). On the other hand, the tolerable range of the change (positive shift) amount AVth of the threshold voltage Vth is 0.2 V, and the tolerable ranges of the change rates a=Idss/Idssg, =gmA/gm of the saturated drain current Idss and the transconductance gm are about 80%. Specifically, when the change amount AVth 9 0.15V with a total exposure dose R=1.5x0 9 roentgens, it *e *OS see can be said that the GaAs MESFET has superior radiation hardness.

5 But what has to be noted here is that the above described tolerable change amount AVthL, and the tolerable change rates aL, jL greatly vary depending on circuits combined with the GaAs MESFET. Specifically, one example is SCFL (Source coupled FET Logic) circuits 20 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 Vth, Idss and gm vary due to a radiation exposure, the operation speed greatly varies. But the influence on the operation speed by the changes of values of Vth, Idss and gm can be reduced by 1/3 to 1/4 1 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 Vth (VthL=0.2V), and the tolerable changes for the saturated drain current Idss and the transconductance gm are about 20% (aL=0.8, RL= 0 By contrast, another example is a memory cell for a memory IC which have high integrity on a semiconductor goose: S 10 chip, the tolerable range for those changes are 0 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, ;15 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 "20 the threshold voltage Vth is only 50 mV (AVthL 0.05V), and the tolerable change rates of the saturated drain current Idss and the transconductance gm are only (aL=0.9, 3t= 0 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 1 ion implantation. The recess structure of Fig. 1 is not essential.

From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

'10 0 0 0 4 0 0*

S

Claims (6)

1. A semiconductor device including a MESFET which has an active layer comprising GaAs crystal that is substantially evenly doped in a depth direction to have a carrier concentration ND and a threshold voltage and which normally operates when a change AV, in the threshold voltage is within a tolerable change amount AVL, an effective thickness t, of the active layer being t. (q'AN)} where AND represents a decrease in the carrier concentration ND due to radiation exposure of a total exposure dose R equal to or higher than 1x10 9 roentgens, s represents a dielectric constant of the active layer, and q represents an electron change, and wherein the decrease amount AND of the carrier concentration ND is given by AND b-Rc where b and c are constants in the range 5x10 5 s bs Ix10 6 sct 1.3 I2. A semiconductor device including a MESFET which has an active layer comprising GaAs crystal that is substantially evenly doped in a depth direction, and 20 which normally operates when a change rate a=IdssA/Idss of a saturated drain current Idss of the MESFET, where IdssA represents the value to which the saturated drain current Idss has changed, is within a tolerable rate aL, a carrier concentration ND of the active layer before radiation exposure is given by 25 ND>AND -aL 1 /2 where AND represe.n a decrease in the carrier concentration of the active layer due to radiation exposure of a total exposure dose R equal to or higher than x109 roentgens, and L and tA represent carrier mobilities in the active layer respectively before and after the radiation exposure, and wherein the decrease amount AND of the carrier concentration is given by AND b'Rc 93081%pciPcmAmuris1.conm,2 29 where b and c are constants in the range 5x10 5 s b s sc! 1.3

3. A semiconductor device including a MESFET ,hich has an active layer comprising GaAs crystal that is substantially evenly doped in a depth direction, and which normally operates when a change rate P=gmA/g m of a transconductance gm in the saturation region of the MESFET, where gmA represents the value to which the transconductance gm has changed, is within a tolerable rate pL, a carrier concentration ND of the active layer before radiation exposure is given by ND AND {1 (L A)} where AND represents a decrease in the carrier concentration of the active layer due to radiation exposure of a total exposure dose R equal to or higher than x10 9 roentgens, and i and tA represent carrier mobilities in the active layer respectively before and after the radiation exposure, and wherein the decrease amount AND of the carrier concentration is given by AND b.Rc where b and c are constants in the range 20 5x10 ab bs Ix1O 6 s c 1.3

4. A semiconductor device including a MESFET which has an active layer S. comprising GaAs crystal that is substantially evenly doped in a depth direction to have 25 a carrier concentration ND and a threshold voltage Vth, and which normally operates Swhen a change AV in the treshold voltage is within a tolerable amount AVth, and a change rate a=IdssA/dss of a saturated drain current Idss of the MESFET, where IdssA represents the value to which the saturated drain current Ids s has changed, is within a tolerable change rate a D where AND represents a decrease in the carrier concentration of the active layer due to radiation exposure of a total exposure dose R equal to or higher than 1x10 9 j roentgens, ut and pA represent carrier mobilities in the active layer respectively before 93O1'P-.pjcsAUIflcOIM.29 30 and after the radiation exposure, e represents a dielectric constant of the active layer, and q represents an electron charge, an effective thickness ta of the active layer being ta {(2e AVthL)/(q AND)}1/ 2 a carrier concentration ND of the active layer before the radiation exposure is given by ND AND {1 [aL /A)]11} and wherein the decrease amount AND of the carrier concentration is given by ND b.Rc where b and c are constants in the range 5x10 5 b; I sg c g 1.3 A semiconductor device including a MESFET which has an active layer comprising GaAs crystal that is substantially evenly doped in a depth direction to have S: a carrier concentration ND and a threshold voltage Vth, and which normally operates 0 9 when a change AVth in the threshold voltage is within a tolerable amount AVthL, and a change rate P=gnA/gm of a transconductance gm in the saturation region of the MESFET, where gmA represents a transconductance to which the transconductance gm 20 has changed, is within a tolerable rate 3 L, where a decrease AND in the carrier concentration due to radiation exposure of total exposure dose R equal to or higher than 1x109 roentgens, p and gA represent .carrier mobilities in the active layer respectively before and after the radiation exposure, e represents a dielectric constant of the active layer, and q represents an electron charge, S 25 an effective thickness ta of the active layer being ta {(2s AVthL) (q AND)l/ 2 a carrier concentration ND of the active layer before the radiation exposure being ND AND {1 L and wherein the decrease amount AND of the carrier concentration is given by ND b-Rc 9381pb0,petocmbm'minhcm.C30 31 where b and c are constants in the range 5 b i 1x10 6 s: c s 1.3

6. A semiconductor device including a MESFET having an active layer comprising GaAs crystal that is substantially evenly doped in a depth direction, and which normally operates when a change rate a=IdssA/dss of a saturated drain current Idss of the MESFET, where Idss represents the value to which the saturated drain current Ids s has changed, is within a tolerable rate aL, and a change rate P=gmA/gm of a transconductance in the saturation region of the MESFET, where gmA represents the value to which the transconductance gm has changed, is within a tolerable rate p L a carrier concentration ND of the active layer before radiation exposure being ND> AND {1 [aL (PA)1/ 2 and ND >AN D L( A)} where AND represents a decrease in the carrier concentration of the active layer due to Sradiation exposure of a total exposure dose R equal to or higher than 1x10 9 roentgens, and pi and pA represent carrier mobilities in the active layer respectively before and after the radiation exposure, and wherein the decrease amount AND of the carrier concentration Sis given by AND =bRc where b and c are constants in the range 5x10 5 s b 1xl10 6 :5e c 1.3 25 7. A semiconductor device including a MESFET having an active layer comprising '.4 GaAs crystal that is substantially evenly doped in a depth direction to have a carrier concentration ND and a threshold voltage Vth, and which normally operates when a change AVth in the threshold voltage is within a tolerable amount AVthL, a change rate p-IdssA/Idss of a saturated drain current Idss of the MESFET, where IdssA represents the value to which the saturated drain current Idss has changed, is within a tolerable rate tL, and a change rate p=gmA/gm of a transconductance gm in the saturation region of the MESFET, where gmA represents the value to which the transconductance gm has 930e1o,popecmiimIKcom,31 32 changed, is within a tolerable rate P 1 L wherein AND represents a decrease in the carrier concentration ND due to radiation exposure of a total exposure dose R equal to or higher than Ix10 9 roentgens, It and pA represent carrier mobilities in the active layer respectively before and after the radiation exposure, e represents a dielectric constant of the active layer, and q represents an electron charge, an effective thickness t a of the active layer being ta {(2e-AVt) (qAND)}I/2; and a carrier concentration ND of the active layer before the radiation exposure being ND AND {1 [L and ND AD (1 L pAA)}; and wherein the decrease amount AND of the carrier concentration is given by AND b'Rc where b and c are constants in the range 5x10s b Ixl10 6 fT a1.0 caC 1.3

8. A semiconductor device including a MESFET having an active layer comprising GaAs crystal that is substantially evenly doped in a depth direction to have a carrier concentration ND and a threshold voltage Vth, and which normally operates when at least one of three following conditions are satisfied: Sa change AVth in the threshold voltage Vth of the MESFET is within a tolerable change amount AVthL a change rate a of saturated drain current Idss is within a tolerable rate aL; and a change rate p of transconductance gm is within a tolerable rate jL; where AND represents a decrease amount of the carrier concentration in the active layer due to radiation exposure of a total dose R equal to or higher than 1x10 9 roentgens, t and PA represent carrier mobilities in the active layer respectively before and after the radiation exposure, e represents a dielectric constant of the active layer, and q represents an electron charge; wherein the decrease amount AND of the carier concentration is given by SND b.Rc 93lOp-Apc*mnsumU0Icom32 33 where b and c are constants in the range 5x10 5 :r sb 1x10' :sc z1.3 wherein the device is constructed such that at least one of the following conditions is met: an effective thickness ta of the active layer is given by ta AVt]) (p-AND)} 112 and a carrier concentration ND of the active layer before the radiation exposure is given by ND AND aL 1 1 2 1; and ND AND P~L (4ILA)}*

9. A semiconductor device substantially as hereinbefore described with reference to the drawings. DATED this 10th day of August, 1993 SUMITOMO ELECTfRIC INDUSTRIES, LTD. By its Patent Attorneys DAVIES COLLISON CAVE 4u 9380 ~pamum0=*3