JPH03276642A - Semiconductor device - Google Patents

Abstract

PURPOSE:To enhance a radiation-resistant property by a method wherein the semiconductor device can be used under surroundings irradiated with a radiation whose total dose is a prescribed value or higher and the carrier concentration before an active layer is irradiated with the radiation is specified. CONSTITUTION:A signal processing circuit which is constituted by being combined with a MESFET is provided so as to be operated normally when a change rate alpha=IdssA/Idss is within a permissible change rate alphaL, where the value after a change in the saturation current Idss between a source and a drain of the MESFET is IdssA. This semiconductor device can be used under surroundings irradiated with a radiation whose total dose R is 1X10<9> roentgens or higher, and the carrier concentration ND before an active layer is irradiated with the radiation is formed as expressed by the formula when the reduction quantity of the carrier concentration before the active layer by being irradiated with the radiation having a total dose of R is DELTAND and carrier mobilities before and after the active layer is irradiated with the radiation are mu and muA, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor device used in equipment requiring radiation resistance.

[Conventional technology]

Devices used in outer space or near nuclear reactors are required to have so-called radiation resistance. Radiation has gamma (γ
) rays, neutron beams, proton beams, etc., but the exposure limits for these are generally I× for silicon (Sl) devices.
106 Roentgen, and about I×108 Roentgen for gallium arsenide (GaAs) devices (``Space Development Related Symposium Abstracts JP, 35-38).

[Problem to be solved by the invention]

As techniques for improving the radiation resistance of GaAs devices, the following techniques have been proposed in the past, for example. The first is to embed a P-type layer under the n-type active layer, thereby reducing leakage current to the substrate, thereby improving radiation resistance, particularly in terms of threshold voltage. The second method is to shorten the gate length, and the third method is to increase the carrier concentration by making the n-type active layer thinner.

However, although the conventional type has radiation resistance up to a total dose R of about 1×10 8 roentgens, it cannot be said to be at a sufficiently practical level (1×10 9 roentgens). Also,
Regarding the third type, although it is generally necessary to increase the carrier concentration in the active layer, no consideration has been made from the perspective of obtaining the transistor characteristics required for circuit design. Therefore, a practical transistor having radiation resistance of about 1×10 9 Roentgen has not been realized to date.

On the other hand, it is known that when GaAs is irradiated with gamma rays, traps that capture carriers are generated due to the total dose effect, leading to a decrease in carriers. Research on this is also available, for example, in J, ^pp1. Phys.
, , Vol. 83. No, 5. (March 1, 1988
B, Klano on pages 1678-1686
Published by Usek et al. However, no attempt has yet been made to quantify the influence of the total dose effect.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a semiconductor device that can further improve radiation resistance with a simple configuration, particularly in terms of changes in source-drain saturation current.

[Means to solve the problem]

The present inventor has determined that when a GaAs MESFET is irradiated with radiation, the rate of change α-I /dss of the source-drain saturation current I of the GaAs MESFET
Focusing on the fact that dssA 1 (I is 1 after change) has a certain relationship with the amount of change ΔN in the active layer's concentration N, the above amount of change ΔN is the total dose. The present invention was completed by discovering through Hall effect measurement that there is a certain quantitative relationship with R.

In other words, the present invention is based on a MESFET formed by doping impurities substantially uniformly in the depth direction in GaAs, and dss, where I is the value after the change in the source-drain saturation current I of this MESFET.
The rate of change α-1/I of dssA is the allowable rate of change α.

dssA dss so that the MESFE operates normally.
A semiconductor device comprising a signal processing circuit configured in combination with T, and which can be used in a radiation irradiation environment with a total dose R of 1XIO9 Roentgen or more, wherein the amount of decrease in carrier concentration in an active layer due to radiation irradiation with a total dose R When ΔN is the carrier mobility in the active layer before and after radiation irradiation is μ and μ, respectively, the carrier concentration ND in the active layer before radiation irradiation is 1/2 N 〉ΔN/+1−[α ( μ/μ)])DD
It is characterized by being LA. At this time, the effective thickness a of the active layer is approximately a-t(2ε/q-N) (y-y)
It becomes 11/2D bi th. Here, Vbl is the built-in voltage of the MESFET.

[Effect]

According to the present invention, the total dose R of the irradiated radiation is 1×1
Not only when the radiation level is below 09 Roentgen, but even above this level, under a preset radiation dose, GaAs
The source-drain saturation current I of MESFET falls within a predetermined range (tolerable range determined by the signal processing circuit), so GaAs
A semiconductor device including a MESFET and a signal processing circuit that cooperates with the MESFET will operate normally according to the originally designed values.

〔Example〕

Hereinafter, the principle and configuration of the present invention will be explained in detail with reference to the drawings.

The semiconductor device of the present invention is a MESFET made of GaAs.
The MESFET and the signal processing circuit are combined to form a combination circuit such as an amplifier circuit, an inverter, and an oscillation circuit.

Here, the basic configuration of one embodiment of the above MESFET is as follows:
This will be explained with reference to FIG. This MESFET is a GaAs MESFE with a reset gate structure shown in the cross-sectional view in FIG.
It is T. As shown in the figure, an n-type active layer 2 and an n+-type contact layer 3 are formed on a semi-insulating GaAs substrate 1 by epitaxial growth, and the n-type active layer 2 in the gate region is
Then, a part of the n+ type contact layer 3 is removed by etching to form a recessed structure. Next, a source electrode 4 and a drain electrode 5 are formed of ohmic metal on the n-type contact layer 3 by vacuum evaporation, and a gate electrode 6 is formed on the n-type active layer 2.
is made of Schottky metal. Here, the n-type active layer 2 directly under the gate electrode 6 is sufficiently thinner than the conventional product, and the carrier concentration ND of the n-type active layer 2 is higher than that of the conventional product.

The MESFET in this combinational circuit has a predetermined source
It has been known that this saturation current I changes under the radiation irradiation environment. If the changed saturation current I becomes a value outside the range previously requested by the signal processing circuit, this combinational circuit will no longer operate normally. Hereinafter, in this specification, the permissible value of the dss change rate α-I /I of the saturation current I in this permissible range is defined as the permissible change d
This will be explained by defining the ssA dss rate α.

As mentioned above, it is known that the source-drain saturation current I of MESFET changes due to radiation irradiation. 2, a decrease in electron mobility due to radiation has been reported.

The present inventor conducted a detailed study on the above first point, and found that the difference between the amount of decrease ΔN of the carrier concentration N in the active layer and the total dose R of the irradiation radiation is ΔND-b-R... It was found through Hall measurements that the relationship (1) holds true, and it was confirmed that this relationship adequately explains the experimental value of the change in saturation current I. Here, b, c
are both constants, and equation (1) shows that the initial carrier concentration ND of the active layer (before radiation irradiation) is I x 7 10 to 1 x 1019 cITI-3, and the total dose R of Salel radiation is I This holds true in the range of ×108 to 1×101O Roentgen. In this case, the constants s and c have a certain range due to variations in the initial carrier concentration of the active layer, the energy of the radiation, the quality of the substrate, etc.

The constants S and c mentioned above were determined by the following experiment.

First, a GaAs Hall element doped with Si to make it n-type is prepared. Here, the n-GaAs layer is fabricated by an epitaxial growth method, and therefore the carrier concentration distribution in the n-GaAs is constant in the depth direction, length direction, and depth direction of the Hall element. Such a sample was prepared with an n-GaAs layer thickness of 100 A to 2
urn, carrier concentration is I X 1016C111-3
5 X i 018 cm-''. Then, when Hall measurements are performed to find the carrier concentration and mobility, the carrier extinction number is as follows, where the carrier concentration is N1 and the amount of carrier change is ΔN. It can be determined as ΔN - N (before irradiation) - N (after irradiation).

According to such experiments, the constant S,c is 5×105≦b
≦I
5×10 c−1,17, and therefore, the representative value of the carrier concentration reduction amount ΔN can be expressed as 5 1.17 ΔN −6,65×10 φ R.

If this is shown in a double logarithmic graph with the horizontal axis representing the gamma ray irradiation dose and the vertical axis representing the carrier extinction number, it will be shown by the black dots in FIG. 17 ΔN-6.65X10 fists R It becomes like the dotted line in the same figure.

Next, as a consideration regarding the characteristics of the invention, the present inventor used a Ga As MESFET having the structure shown in FIG.
We actually measured the change in the source-drain saturation current I due to radiation irradiation. As a result, the characteristics of the rate of change α of the saturation current I as shown in FIG. 3ss were obtained, and the characteristics of the change of the saturation current I as shown in FIG. 4 were obtained. In FIGS. 3 and 4, the black dots are experimental values, and the dotted lines are theoretical values obtained by applying the above-mentioned equation (1) to the following theoretical equation (10).

Therefore, next, a theoretical formula for the rate of change α-ss 1 /I of the saturation current I is determined. First, MESd
The source-drain saturation current of a ssA dss FET is 2 2 for an intrinsic FET ignoring the dss source resistance R.
3 1 -1(W ・μ・q −N −a )/ds
s g D (6ε·L)1 X (1-3(Vbl-VG)/V.

3/2 +2[(V-V)/V] 1bi
G p ... (6) becomes. Here, W is the gate width, μ is the electron mobility in the active layer 2, L is the gate length, Vc is the gate voltage,
is the pinch-off voltage. To simplify the calculation, V
. If the saturation current I at -Vbi is I, then (
6) The formula is dss DSS 2 2 3 I − (W φμ q −N ・a)
/DSS g D (6ε·L) (7). Therefore, from equation (7), the rate of change α due to radiation irradiation is α −I /I D5SA DSS 2 − (μ ・N ) / (μ ・N )
... (8) A, DA
It becomes D. Here, NDA is the carrier concentration after radiation irradiation, NDA - ND - ΔND ・
... (9), so equation (8) is 2 α−(μ (N −ΔN ) l/(μ・N
)ADDD...(10)

Here, if we consider equation (10), we can see that the rate of change α is also affected by the change in electron mobility μ due to radiation irradiation (μ−μA), but the carrier concentration before radiation irradiation is I
In the case of X 1018cm-”, μA/μm0.9
It is about 5, and the change becomes smaller as the concentration becomes higher. Therefore, when we performed calculations using μA/μ-〇595, the results were as shown by the dotted lines in Figures 3 and 4.
As explained above, agreement with experimental values was confirmed.

As a result of these experiments and considerations, firstly, the main cause of the deterioration of MESFET characteristics due to radiation damage is a decrease in the carrier concentration in the active layer, and the relational expression (1) based on Hall effect measurements is based on the decrease in carrier concentration under radiation irradiation. It was found that the decrease was explained very well. Second, (1
In Equation 0), μ is a constant and the value of μ^/μ can be approximately estimated, and ΔND is determined from Equation (1) depending on the radiation dose. Therefore, the initial carrier concentration N in the active layer, It has been found that the saturation current change rate α can be set to a predetermined value only by setting . Specifically, when the carrier concentration N of the active layer 2 is set to around 2X1017cIn-3 as in the conventional product, the radiation resistance is insufficient as shown in Fig. 4, but the carrier concentration N I x 1
When the radiation resistance is set to 0.018 cm-'', the radiation resistance is significantly improved as shown in FIG.

Based on the above knowledge, we have developed a structure for a semiconductor device that operates normally not only when the total dose R is 1 x 109 Roentgens or less, but also under radiation irradiation environments greater than that, especially with regard to the carrier concentration in the active layer 2. It can be specified from

That is, the semiconductor device is formed by combining a GaAs MESFET with a signal processing circuit, and the allowable rate of change of the source-drain saturation current I of the MESFET is α,
In order for the combinational circuit related to the semiconductor device to operate as designed, the initial carrier concentration ND of the active layer 2 must be 1/2 N > Δ N/ll−[ from equation (10). α (μ / μ
) ko) D D
L A... (11) In this case, the effective thickness a of the active layer 2 is 1/2 a-([2ε/(q-N)]・(Vbi-Vth")
... (12:) Here, for the total dose R-I
DSS When calculated, the amount of change in carrier concentration ΔN is ΔN −2,25×1016(1)−3 from equation (1), and the carrier concentration in active layer 2 is 8.44×10′ from equation (10).
(to) -3 or more. When the threshold voltage Vth is set to Vth--1, 2V at such a carrier concentration in the active layer 2, the effective thickness of the active layer 2 is 546 A from equation (12). However, the dielectric constant ε−ε ・ε.

12-12. OX8.85X10 F/m19 Electron charge q=1.602xlo C Hilt-in voltage Vb, -0.7V.

A comparison between the product of the present invention and the conventional product regarding radiation resistance is shown in FIG. In the same figure, (a) and (b).

The curve (c) shows the characteristics of conventionally commercially available MESFETs, and especially the curve (b) shows the carrier concentration N of the active layer 2 of 2. 09 x 10 'cs-'' corresponds to the characteristics shown in Figure 6. Also, the curve (D) corresponds to the characteristics of the commercially available HE
It shows the characteristics of MT (high electron mobility transistor). As is clear from the figure, in these conventional products, the saturation electric rate change rate suddenly becomes a high value when the total dose R-1×10 9 roentgen is reached. On the other hand, M
The characteristics of the ESFET are as shown in the curve (e) in the figure, and although there is an improvement in the threshold voltage characteristics (not shown),
No improvement in saturation current characteristics can be obtained. On the other hand, in the structure of the present invention in which the carrier concentration ND of the active layer 2 is 1×10 18 cIn-3 (corresponding to the characteristic shown in FIG. 3),
It can be seen that the rate of change α is suppressed to a sufficiently low value even in the case of the curve <R-1×109 X-rays, and the radiation resistance is significantly improved.

The present invention is not limited to the above embodiments, and various modifications are possible.

For example, the formation of the active layer is not limited to epitaxial growth, and ion implantation may also be used. Furthermore, it is not essential to have a cess gate structure as shown in FIG.

〔Effect of the invention〕

As explained in detail above, in the present invention, the source-drain saturation current of the GaAs MESFET falls within a predetermined tolerance range, not only when the total dose R of irradiated radiation is less than 1 x 109 Roentgen, but even when it is more than this. Therefore, a semiconductor device configured with a Gas As MESFET and a signal processing circuit that cooperates therewith will operate normally according to the originally designed value. Therefore, it becomes possible to significantly improve radiation resistance.

[Brief explanation of drawings]

FIG. 1 shows a GaAs MES that can be used in an embodiment of the present invention.
A cross-sectional view of the FET, FIG. 2 is a diagram showing the dependence of the carrier concentration reduction amount ΔND on the radiation dose R, and FIG. 3 is a diagram showing the source-drain saturation current l of the MESFET according to the present invention.
Figure 4 shows the dependence of the rate of change α on the radiation dose Rss.
FIG. 5 is a characteristic diagram comparing the radiation resistance of the product of the present invention with that of a conventional product. 1... Ga As substrate, 2... n-type active layer, 3...
・N+ type contact layer.

Claims (1)

[Claims] 1. MESFET formed by doping impurities substantially uniformly in the depth direction in GaAs, and when the value after change of the source-drain saturation current I_d_s_s of this MESFET is I_d_s_s_A. Rate of change α=I_d
The total dose R
is a semiconductor device that can be used in a radiation irradiation environment of 1×10^9 Roentgen or more, and the carrier concentration reduction amount of the active layer due to radiation irradiation of the total dose R is ΔN_D
, when the carrier mobility in the active layer before and after the radiation irradiation is μ and μ_A, respectively, the carrier concentration N_DN_D in the active layer before the radiation irradiation is
>ΔN_D/(1-[α_L(μ/μ_A)]^1^/
^2) A semiconductor device characterized by: 2. The amount of decrease ΔN_D in the carrier concentration is ΔN_D=b·R^c when b and c are constants, and the constants b and c are 5×10^5≦b≦1×10^6 1. 2. The semiconductor device according to claim 1, wherein 0≦c≦1.3.