AU643781B2 - A semiconductor device - Google Patents

Description

COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION (Original) FOR OFFICE US0 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.

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- 9104234cpdtL065,41792.poa,1 la This invention relates to a semiconductor device using a GaAs metalsemiconductor field effect transistor (MESFET), specifically to that which can be used in machines and instruments requiring 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 1x7 O' 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 lxl0 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, no practical transistor having a radiation hardness level of about 1x10 9 roentgens has not 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 improved characteristics since the carrier concentration in the surface of the active layer lowers, and consequently the MESFET is free from the affection of a deletion layer.

The inventors have noticed that when radiation is applied to a GaAs MESFET, the change AVth in threshold voltage Vth in the saturation region, the change rate a=IdssA/Idss of saturated drain current at normal gate bias I s and the change rate 3=gmA/gm of a transconductance gm has a constant relationship with the thickness tatal+ta2 of the active layers and the change AND of the carrier concentrations N1D, N2D and has found that the change amount AND has a constant quantitative relationship 30 with the total exposure dose R.

s *ee 93O81OAp:\pAC="uWcml 1 -2- In accordance with the present invention there is provided a semiconductor device including a MESFET having an active layer doped in a GaAs crystal and a threshold voltage Vth, the active layer comprising an upper layer having a carrier concentration N1D and an effective thickness tal, and a lower layer having a carrier concentration N2D (N1D N2D) and an effective thickness ta2, and which normally operates when at least one of the following conditions are satisfied: a change AVth in the threshold voltage Vth of the MESFET is within a tolerable amount AVthL a change rate a in the saturated drain current Idss is within a tolerable rate ajL and a change rate p in the transconductance gm is within a tolerable rate pL; wherein AND represents a decrease amount of the carrier concentrations NID, N2D in the active layer due to radiation exposure of a total dose R equal to or higher than 1x10 9 roentgens, IA and pA represent carrier nobilities 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, the semiconductor device being constructed in accordance with at least one of the following three conditions: an effective thickness ta=tal+ta2 of the active layer given by ta {2AVth) (q.AND)} 1 2 a carrier concentration N2D of the lower layer of the active layer before the *radiation exposure given by 0N2D AND {1 aL (PA)] 1 2 N2D AND {1 L and wherein the decrease amount AND of the carrier concentration is given by

AN

D bRc where b and c are constants in the range 5xl0 5 ab s 1x10 6 c S 13 30 Further and alternative forms of the invention can be determined from the appended claims.

9308 0,p.Aopucmrsumitf02= -3- The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are OV to to*

AGO,

3 to too **to.

930810,V.\opecmEmi&2-m3 1 given by way of illustration only, and thus are not to be considered as limiting the present invention.

Farther scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood 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 10 scope of the invention will become apparent to those S* skilled in the art from this detailed description.

4 4** 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; 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 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 AL I,4 4L y- 1 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 Idss' Fig. 7 is a graph showing the total exposure dose R dependence of a change rate 8 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 owe:10 change of the.transconductance gm of the conventional

"MESFET;

SFigs. 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 a Fig. 12 is a sectional view of a GaAs MESFET having an active layer in one layer.

WWH-Wfl i@rT B-I hD 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

U-.

BI

t :1 00 0 9 0.

0 t 1 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. The active layer 2 includes an upper layer 21 of a thickness tal ad 'a lower layer 22 of a thickness ta2. Parts of the n-type active layer 2 and .0 the n+-type 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'-type contact region 3 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 N 2 D, compared with the carrier concentrations of the conventional MESFETs.

The MESFET in this combination circuit involved in this invention has a preset threshold voltage Vth, .a saturated drain current Idss at normal gate bias, 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 0 0 0 *ft' *00* 00 00 U 00 0 0 B 1 changed saturated drain current IdssA' and a changed transconductance gmA to which their 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 AVth of the threshold voltage Vth is represented by a tolerable change amount AVthL. A tolerable value of a change rate a=IdssA /Idss of the 10 saturated drain current Idss is represented by a tolerable change rate aL, and a tolerable value of a f change rate a=gmA /g of the transconductance is represented by a tolerable change rate 1

L

The values of AVthL, aL and RL vary depending on designs of the above described signal processing circuit, but generally So,: AVthL equal to or lower than 0.2 V, and aL and 8L 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 *9 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 AND bR (1) L where b and c are constants holds between a decrease Et) 0*Q- 1 amount AND of a carrier concentrations NID, N2D and a total exposure dose R. Formula 1 holds when an initial carrier concentration N2D (before radiation exposure) of the active layer is 1x1017 to 1xl0 1 9 cm 3 a total dose R of exposure radiation is 1x10 8 to 1x3010 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.

The values of the above-described constants b, c were 10 given by the following two methods. In a first method, Ssome 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 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 relatiouship between the total dose R and the carrier S. concentration decrease amount AND can be given empirically by measuring the total dose R of radiation and the change amount AVh. Based on this e* relationship, the constants b, c in the above-described 0.

Formula 1 could be given.

According to the experiments conducted by the inventors, the constants b and c have ranges of 1.99 X 101 0 b 5 3.98 x 1010 c 0.8 and the typical values are b=3.06x 1010 c=0.678.

1 Therefore, the typical value of the decrease amounts AND of the carrier concentration is defined by AND 3.06 x 1 0 0 R 7 8 This first method is for measuring a change amount Vth of the threshold voltage to calculate a carrier 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 i 10 relationship between the carrier concentration decrease amount AND 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 by Si doping was prepared. The n-type 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 5xl01 8 cm-3-carrier concentration were prepared. The Hall effect was measured on these samples to give the carrier concentration and Hall mobility.

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

According to these experiment, the constants b, c i0 a. a *4 0* 4 55,5.

al a S have an allowance of 5 b 1x10 6 1. 0 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 representative value of the carrier concentration decrease amount AND is AND 6.65xi0 5

R

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 accordance with the first method, on the assujnption of the following b) and the carrier concentration decrease amount AND was calculated based. ou 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, and the thickness remains unchanged for whole irradiations.

c) The mobility of carrier is not changed by 7-ray radiation.

In contrast to this, in the direct measure in see* 54 1 accordance with the second method, the carrier concentrition decrease amount AND was measured actually by the Hall effect, and the above-de -ibed 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 1 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, r 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

AN

D 6.65x10 5

R

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

The carrier concentration decrease amount AN, 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 1 Vth Vbi -(q*N 2 D/2e)ta 2 2 (q.N2D/ )tal'"a2 (q.N 1 D/2a )tal 2 (2) given by solving the one dimensional Poisson equation d 2 1/dx 2 -qNID/£ (0 x 1 tai) d 2 2/dx 2 -qN2D/4 (tal< tal+ta2) under the boundary conditions of do/dx 0 at X tal d (d>ta 2 *a 2 S* -(Vbi VG) at x 0.

In Formula 2, Vbi represents a built-in voltage of the MESFET; q, an electron charge; and a dielectric constant of the n-type active layer 2. When the carrier concentrations N1D, N 2 D of the n-type active layer 2 becomes N1DA, N2DA due to radiation exposure, a

S

threshold voltage V t A after the radiation exposure is given by VthA bi (qN2DA/2a)ta 2 2 (q-N2DA/ )tal ta2 (qNlDA/2 )tal 2 c Then, a change amount of the threshold voltage Vth due to the radiation exposure is given by AVth VthA Vth {Vbi -(q*N2DA/2E)ta22 (q.N2DA/a )tal'ta2 (q-NIDA/2 )tal 2 1 {Vbi (q-N2D/2 )ta 2 2 (q.N 2 )tal'ta2 (q.N 1 D/2e )tal2 =([q.(tal+ta)2]/2S }-(ND-NDA) In Formula 4 ND NDA N1D N1DA

N

2 D N2DA.

Therefore, when a decrease amount of the carrier concentration due to radiation exposure is given by S 10 AND= ND NDA o Cwith ta=tal+ta2 SAVth {(q-ta 2 AND In this Formula 5, the carrier concentration decrease amount AND can be given empirically by measuring Hall effect on the samples (Hall elements).

Accordingly the theoretical value of the change amount 0 AVth of the threshold voltage can be given. Reversely, the change amount AVth can be given experimentally 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=xl10 8 roentgens, 1x10 9 roentgens and 3x10 9 roentgens to a MESFET of Fig. 1 having the active layer 2 of a thickness of 450 (150 300) A. The result is shown in Fig. 2 by black points.

Then, the above-described relationship given by measuring the Hall effect, 1 1-'IS -49 t 1 AND =6.65x105 R1.17 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.

In Fig. 2, with a total dose of R=1x10 9 roentgens, a change amount AVth of the threshold voltage is as low 49 *i10 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 A.

Formula 1 described above was derived from actually measured values for six total doses of R=lx10 8 roentgens, 3x10 8 roentgens, 6x10 8 reoengens, 1x10 roentgens, 2x1b 9 roentgens and 3x10 9 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 A so that the carrier concentration ND is 2.09x10 17 cm 3 In this experiment, total exposure doses were R=lxl0 6 roentgens, /J N .ixl0 7 roentgens, 1x10 8 roentgens, 3x10 8 roentgens, 1x10 9 1 roentgens, 2x10 9 roentgens and 3xl0 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=lxlOQ 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 *doe*: *10 the degradation of the threshold voltage of the MESFET due to radiation damage is a decrease in the carrier Sconcentration 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 t=tal+ta2 of the active layer. Specifically, as shown in Fig. 4, when the thickness ta 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=tal+ta2 is set at 450 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.

1 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.

The theoretical formula for the change rate a=IdssA 10 /Idss of the saturated drain current Idss will be derived below. The saturated drain current Ids s of the s s MESFET is given for an intrinsic FET with a source resistance R s kept out of consideration, by Idss (Wg..q2.4N 2

D

2 X {talta2(ta2 tal -d) S+ (ta2 -tal)'ta2 2 (-tal di) 2 ]/2 [ta2 3 (-tal di)3]/3} 2 al 2 where d {tal ND tal 2 /N 2a S(Vbi- VG)/(q.N 2

D)}

1 2 In Formula 6, Wg represents a gate width; an o 0 electron mobility in the active layer 2; Lg, a gate length; and VG a gate voltage. In Formula 6, NID/N 2

D=O

based on NlD<N 2 D. Therefore, Formula 6 is rewritten dl=tal. When a saturated drain current Idss for VG =Vbi is represented by IDSS for simplifying the computation, Formula 6 is rewritten into S DSS q2N2D)( .Lg)} 1 Ctal'ta2 (ta2 tal)ta2/2 ta23/3] (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

(AN2DA 2 (u N 2

D

2 (8) where N2DA is a carrier concentration of the lower layer after the radiation exposure and is given by 1 0 NDA N 2 D AND g6 S Then Formula 8 is substituted by Formula 9 into Sa{(AA (N 2 D AND) 2

}/(AN

2 D) Then Formula 10 will be discussed below. It is found that the change rate a is influenced by changes (AL AuA) of the electron mobility p due to the radiation Sexposure. But AA/L is around 0.95 0.98 when the 4 e carrier concentration before the radiation exposure is about 1x10 1 8 cm 3 The change becomes smaller as the carrier concentration becomes higher. Then the computation was made with PA/Ju=0.

9 5 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 Idss 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 1 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 concentration (before the radiation exposure) N2D of the lower layer 22 of the active layer, because p is a constant in Formula 10, the value of pA/' can be 10 approximated, and AND can be determined depending on a total radiation dose in Formula 1. Specifically, when Sthe carrier concentration ND of the active layer 2 is set at about 2x10 17 cm-3 as in the conventional ones, see the radiation hardness is insufficient as seen in Fig.

6. When the carrier concentration N2D of the lower layer is set at 1x10 1 8 cm the radiation hardness is 0 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 Vand 4 and the saturated drain current Idss characteristics of Figs. 5 and 6. As results, the .change rate 3 of the transconductance gm of Fig. 7 was obtained in use of same MESFET of Fig. 2 and 5, and the change rate P of the transconductance gm of Fig. 8 was i,,T obtained in use of same MESFET of Fig. 4 and 6. In 1 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 -=gmA/gm 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 gm ={(Wg'*gq'N 2 D)/(dl-Lg)} Sx {-talta2(ta2 tal) (ta2 tal) [ta2 2 2(d tal) 2 ]/2 0*e [ta2 (d tal)3]/3} In this Formula, d {tal 2 NIDtal 2

/N

2 D 2a .(Vbi VG)/(q.N 2 D)1/ 2 (11).

In Formula 11, N1D/N 2 D=O based on NlD<N2D. Therefore, Formula 11 is rewritten d=tal. When a transconductance gm for VG =Vbi is represented by gmmax for simplifying 0 the computation, Formula 11 is rewritten into 0 gmmax {(Wg'-qN 2 D)/(talLg)} [-talta22(tal ta2) (ta2 tal) ta22/2 ta2 3 (12).

A change rate 8 due to the radiation exposure is ,given based on Formula 12 by gmmaxA gmmax as 0 to

C

9 *i a gad.

a CO a Ca..

S

asee *edt a Sq as ai 1 A'N2DA) N 2 D) (13) where N2DA is a carrier concentration of the lower layer after the radiation exposure and given by N2DA N 2 D AND (14).

Then Formula 13 is substituted by Formula 14 into 6 {-A(N2D 2 D) Then Formula 15 will be discussed below. It is found that the change rate 3 is influenced by changes (u uA) of the electron mobility p due to the radiation exposure. But pA/p is around 0.95 when the carrier concentration before the radiation exposure is about 18 cm-3 The change becomes smaller as the carrier concentration becomes higher. Then the computation was made with 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 20 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 8 of the transconductance can be set at a required value by setting only the initial carrier concentration N2D of the lower layer 22 of the 2 A active layer, because p is a constant in Formula 1 &0

LO

-el 1 e~aa S. *10 .5 S '0 a 4ta 'SOS 4 6p* £0 S)

'Y,

56 00 0.Js oaaa 0 *005 o soa S. 5 0455 the value of JA/4 can be approximated, and AND is determined based on Formula 1, depending on a radiation dose. 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. 8. When the carrier concentration N 2 D in the lower layer is set at 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 dcse R not only below 1x10 9 roentgens but also a total dose R above 1xl0 9 roentgens can be specified based on a thickness ta of the active layer 2 and a carrier concentration N2D 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 Vth of the MESFET is AVthL, an effective thickness taal+ta2 of the active layer 2 must be t a {(2a.AVthL)/(q.AND)1 1 2 (16), ,based on Formula 5. In this case, the carrier concentration N 2 D of the lower layer 22 of the active layer 2 is given as follows based on Formula 2 by

N

2 D {[2E/(q.t a 2 Vth)) (17) See.

*0 d1 S

S

S S c 00 ilC Tt 1 Here, when a tolerable change amount AVthL of the threshold voltage Vth for a total exposure dose of 9 roentgens is specifically computed by AVthL 0.1V(AVth 0.1V), a change amount AND of the carrier concentration is given by AND 2.25 x 10 16 cm 3 An effective thickness ta=tal+t a2 of the active layer 2 is givsn below 529 A based on Formula 16. Further, with 10 a thickness of the active layer 2 set at below 529 A, the carrier concentrations NlD N 2 D of the active layer 6 is determined by their interrelationships with the thickness tal ta2. In this case, a dielectric constant o* of the active layer a Se 0 12.0 x 8.85 x 10- 1 2 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 o• semiconductor device operates as required when a tolerable change rate of the saturated drain current ldss of the MESFET is represented by aL, an initial carrier concentration N 2 D of the lower layer 22 of the active layer 2 must be, based on Formula 10 by

N

2 D AND 1 2 (18).

In this case, an effective thickness ta al+ta2 of the 1 active layer 2 is given by /(q.N2D) (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 (IDsA>O.

9 IDSS), a change amount AND of the carrier concentration is given based on Formula 1 by AND 3.25 x 10 6 cm 3 and the carrier concentration N 2 D of the lower layer 22 of the active layer 2 becomes above 8.44x10 1 7 cm 3 based

*S

on Formula 9 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 gm in the saturation region of the *go* MESFET is RL, an initial carrier concentration N 2 D of the lower layer 22 of the active layer 2 must be, based on Formula S20

N

2 D AND L (21).

9* t In this case, an effective thickness t=tal+ta2 of the of th active layer 2 must be ta (Vbi-Vth)1/ 2 (22).

Here, for a total exposure dose of R=lxl0 9 roentgens, with 1L (a tolerable change rate of the transconductance gmmax) 0.9 (gmmaxA >0.9 emmax a change amount AND is given by ?23 0c_ -4 1 AND Z.25 x 10 16 cm-3 based on Formula 1. The carrier concentration N 2 D of the lower layer of the active layer 2 is given above 4.50x10 17 cm 3 by Formula The semiconductor device according to this invention and the conventional ones will be compared in radiation hardness in Fig 9 to 11. Fig. 9 shows change amounts AVth of the threshold voltage Vth due to the radiation exposure. Fig. 10 shows change rates a' of the 10 saturated drain current Idss Fig. 11 shows change Srates 8 of the transconductance gm. In Figs. 9 to 11,

C

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 t and a carrier concentration of 2.09x0 1 7 cm 3 The curve shows the characteristics of a conventional HEMT (high electron mobility transistor). As evident from Fig. 9, these conventional semiconductor devices 00.0 20 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,(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 AVth is suppressed to about 0.12V for R=1xl0 9 roentgens. In NN} contrast to this, in the MESFET according to this invention having the active layer 2 of an effective thickness ta tal+ta2 of 450 A (corresponding to the 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 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 Idss and the transconductance Sgm.

In this invention, even under the radiation exposure a 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 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 Ixl0 8 to 3x10 9 rad. (1 roentgens 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/Idss, 6=gmA/g m of the saturated drain current Idss and the transconductance gm are about 80%. Specifically, when the change amount S AVth 0.15V with a total exposure dose R=1.5xl0 t=.hx0 1 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 AVthL, and the tolerable change rates aL', 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 :10 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 g can be reduced by 1/3 to 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 *20 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, 6L= 0 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 J'i' small memory cell charges and discharges the data lines 1 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 Vth is only 50 mV AVthL 0.05V), and the tolerable change rates of the saturated drain 0* 10 current Idss and the transconductance gm are only (aL=0.

9 8L=0.9).

S 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 S" ion implantation. The recess structure of Fig. 1 is not S* essential.

From the invention thus described, it will be obvious that the invention may be varied in many ways. Such 20 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.

Claims (5)

1. A semiconductor device including a MESFET which has an active layer doped in a GaAs crystal and a threshold voltage Vth, the active layer comprising an upper layer having a carrier concentration NiD and an effective thickness tal, and a lower layer having a carrier concentration N2D (N1D N2D) and an effective thickness ta2, and which normally operates when a change AVth in the threshold voltage Vth is within a tolerable amount AVthl, an effective thickness ta= tal ta2 of the active layer being ta {2e*AVthL) (q.AND)}/ 2 where AND represents a decrease amount of the carrier concentrations NiD, N2D due to radiation exposure of a total exposure dose R equal to or higher than 1x10 9 roentgens, e represents a dielectric constant of the active layer, and q represents an electron charge, and wherein the decrease amount AND of the carrier concentrations NID, N2D is given by AND b.Rc where b and c are constants in the range 5x10 5 b lx10 6 s c 1.3

2. A semiconductor device including a MESFET which has an active layer doped in a GaAs crystal, the active layer comprising an upper layer having a carrier concentration ND, and a lower layer having a carrier concentration N2D (ND N2D), and which normally operates when 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 Idss has changed, is withir. a. tolerable rate aL; the carrier concentration N2D of the lower Slayer before radiation exposure is given by N2D AND/ {i [aL A)] 1 2 where AND represents a decrease in the carrier concentrations N1D, N2D of the active 30 layer due to radiation exposure of a total exposure dose R equal to or higher than 1x10 9 roentgens, and I and "A represent carrier mobilities in the active layer respectively before and after the radiation exposure, and wherein the decrease amount AND of the 9308I1p pcipjcmsuw 1102.xmc.29 carrier concentration is given by AND b-Rc where b and c are constants in the range 5x10 s b Ix10 6 1.0 c g 1.3

3. A semiconductor device including a MESFET which has an active layer doped in a GaAs crystal, the active layer comprising an upper layer having a carrier concentration N1D, and a lower layer having a carrier concentration N2D (ND N2D), and which normally operates when 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 gn has changed, is within a tolerable rate the carrier concentration N2D of the lower layer before radiation exposure is given by N2D AND {1 3 L (tLA)} where AND represents a decrease in the carrier concentrations N1D, N2D of the active layer due to radiation exposure of a total exposure dose R equal to or higher than 1x10 9 roentgens, and t and IAA 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 care constants in the range 5x10 5 ab Ix10 6 1.0 c 1.3 Sii ii S' 25 4. A semiconductor device including a MESFET which has an active layer doped in a GaAs crystal and a threshold voltage Vth, the active layer comprising an upper layer Shaving a carrier concentration N1D and an effective thickness tal, and a lower layer having a carrier concentration N2D (ND N2D) and an effective thickness ta2, and which normally operates when a change AVth in the threshold voltage Vth is within a 30 tolerable amount AVth L and a change rate a=IdssA/dss of a saturated drain current Ids s of the MESFET, where IdssA represents the value to which the saturated drain current Idss has changed, is within a tolerable change rate aL; 93W8IOp.opcjcmmum2.om.3O

91- wherein AND represents a decrease in the carrier concentrations N1D, N2D of the active layer due to radiation exposure of a total exposure 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, an effective thickness ta=ta+ta2 of the active layer being ta {(2eAVthL) (q-AND)}/ 2 and the carrier concentrLtion N2D of the lower layer of the active layer before the radiation exposure is given by N2D AND {1 [a L (tA) 1/2} 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 5 b s 1x10 6 1.0 c 1.3 A semiconductor device including a MESFET which has an active layer doped in a GaAs crystal and a threshold voltage Vth, the active layer comprising an upper layer having a carrier concentration N1D and an effective thickness tal, and a lower layer having a carrier concentration N2D (N1D N2D) and an effective thickness ta2, and which normally operates when a change AVth in the threshold voltage Vth is within a tolerable amount AVth, and a change rate P=gnA/gm of a transconductance gm in the saturation region of the MESFET, where gA represents a transconductance to which the transconductance gm has changed, is within a tolerable rate BL, 25 wherein a decrease amount AND in the carrier concentrations NID, N2D due to radiation exposure of a total exposure dose R equal to or higher than Ix10 9 roentgens, and IA 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, 30 an effective thickness =t=tal+ta2 of the active layer being ta {(2sAVthL) (q-AND)2; and the carrier concentration N2D of the lower layer of the active layer before the

93010.p.Apc mumiO2.cofl,31 radiation exposure being N2D AN D PL 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 5x10 5 b s 1x10 6 c s 1.3 6. A semiconductor device including a MESFET having an active layer doped in a GaAs crystal, the active layer comprising an upper layer having a carrier concentration NiD, and a lower layer having a carrier concentration N2D (NID N2D), and which normally operates when a change rate a=IdssA/Idss of a saturated drain current Tdss of the MESFET, where IdssA 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 of a transconductance to which the transconductance gm has changed, is within a tolerable rate PjL the carrier concentration N2D of the lower layer of the active layer before radiation exposure being N2D AND aL A)] 11 and N2D AND 1 -PL A)} where AND represents a decrease in the carrier concentrations N1D, N2D of the active layer due to radiation exposure of a total exposure dose R equal to or higher than Ix10 9 roentgens, and g 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 is given by AND bR where b and c are constants in the range 5x10 5 b Ix10 6 1.0 :i c s 1.3 *i 04 7. A semiconductor device including a MESFET having an active layer doped in a kII, 930810qV'opcn*om mIm2lcom,32 33.- GaAs crystal and a threshold voltage Vth, the active layer comprising an upper layer having a carrier concentration NiD and an effective thickness ta, and a lower layer having a carrier concentration N2D (ND N2D) and an effective thickness ta2, and which normally operates when a change AVth in the threshold voltage Vth is within a tolerable amount AVth, a change rate a=IdssAIdss of a saturated drain current Ids s of the MESFET, where IdssA represents the value to which the saturated drain current Ids s has changed, is within a tolerable rate a, and a change rate P=gmA/gm of a transconductance gm in the saturation region of the MESFET, where gmA represents the value of a transconductance to which the transconductance gm has changed, is within a tolerable rate 3 L; wherein AND represents a decrease in the carrier concentration NID, N2D due to radiation exposure of a total exposure dose R equal to or higher than 1x10 9 roentgens, It and tt A 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 elctron charge, an effective thickness ta=tal+ta2 of the active layer being ta 2 AVthL) (q'AND)}1/ 2 and the carrier concentration N2D of the lower layer of the active layer before the radiation exposure being N2D> AND 1 [aL (l/A)1 2 and S" N2D> AND {1 PL (iP/IA)}; 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 5x10 b s Ixl0 c 1.3 8. A semiconductor device including a MESFET having an active layer doped in a GaAs crystal and a threshold voltage Vth, the active layer comprising an upper layer 30 having a carrier concentration NiD and an effective thickness ta, and a lower layer **1 Shaving a carrierconcentration N2D (N1D N2D) and an effective thickness ta, and which normally operates when at least one of the following conditions are satisfied: 930610,p:opeuu I m,33 a change AVth in the threshold voltage Vth of the MESFET is within a tolerable amount AVthL a change rate a in the saturated drain current Idss is within a tolerable rate aL and a change rate p in the transconductance gm is within a tolerable rate BP; wherein AND represents a decrease amount of the carrier concentrations N1D, N2D in the active layer due to radiation exposure of a total dose R equal to or higher than 1xl10 roentgens, Ip and IpA 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, the semiconductor device being constructed in accordance with at least one of the following three conditions: an effective thickness ta=tal+ta2 of the active layer given by ta 2 e-AVthL) (q-AN)} 1 2; a carrier concentration N2D of the lower layer of the active layer before the radiation expsure given by N2D AND /1 [aL (9I/IA)] 1 2 N2D AND {1 PL 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 5x10 5 b s x10 6 1.0 :5 C 13 9. A semiconductor device substantially as hereinbefore described with reference to 25 the drawings DATED this 10th day of August, 1993. SUMITOMO ELECTRIC INDUSTRIES, LTD. By its Patent Attorneys DAVIES COLLISON CAVE 93081%opaPcrcm~sum2.com34