Semiconductor device

In a semiconductor device constituting a GaAs MESFET, a GaAs substrate is prepared from a base material containing boron ions as a dopant impurity having a total impurity concentration of 2.times.10.sup.17 atoms/cm.sup.3 or more. The boron ions are introduced into the GaAs substrate during crystal growth so that a uniform distribution of boron ions in the substrate results. Electrode layers are formed at predetermined portions on the GaAs substrate, and an active layer is formed to be adjacent to the electrode layers by ion implantation. Source and drain electrodes are formed on the electrode layers respectively, and a gate electrode is formed on the active layer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an improvement of a semiconductor device 
constituting a GaAs MESFET. 
2. Description of the Related Art 
Conventionally, Cr-doped semi-insulating GaAs or non-doped LEC (Liquid 
Encapsulated Czochralski) GaAs has been used as a base material of a GaAs 
Metal-Semiconductor Field-Effect-Transistor (to be referred to as a GaAs 
MESFET hereinafter). In order to form a GaAs MESFET from this base 
material, the following manufacturing steps are conventionally required. 
When a non-doped LEC GaAs substrate is used as a base material, a proper 
pretreatment is performed for the substrate. Thereafter, an SiOx layer 
having a thickness of, e.g., 5,000 .ANG. is deposited on the substrate by 
chemical vapor deposition. Openings are formed on desired portions of the 
substrate through the SiOx layer using a photolithography technique. Si 
ions are implanted through the openings at an accelerated energy of 180 
keV and a dose of 5.times.10.sup.13 ions/cm.sup.2, to form electrode 
layers corresponding to to the openings in the substrate. After the resist 
pattern including the SiOx layer is removed, a proper pretreatment is 
performed again on the surface of the substrate, and another SiOx layer is 
deposited by chemical vapor deposition. An opening is formed on the 
desired portion of the substrate through the SiOx layer by using a 
photolithography technique. Si ions are implanted through the opening at 
an accelerated energy of 100 keV and a dose of 3.times.10.sup.-12 
ions/cm.sup.2, to form an active layer just under the opening in the 
substrate. Then, the resist pattern including the SiOx layer is removed. 
Thereafter, a capless annealing process is performed in an arsenic 
atmosphere at 850.degree. for 15 minutes and the implanted Si ions are 
electrically activated. Finally, gate, source, and drain electrodes are 
formed on the substrate by a lift-off method. 
As described above, it is known that a GaAs base material used in a GaAs 
MESFET obtained by conventionally-known manufacturing steps normally 
contains boron and carbon atoms, and the activation ratio of the implanted 
Si ions during a capless annealing process after an active layer is formed 
depends upon the concentrations of boron and carbon ions. For this reason, 
it has been attempted to reduce the concentrations of residual impurities 
such as boron and carbon ions. However, it is known that when the total 
concentration of boron and carbon ions contained in the base material is 
about 2.times.10.sup.17 atoms/cm.sup.3 or less, the drain current-drain 
voltage (I.sub.D -V.sub.D) characteristic curve of the manufactured GaAs 
MESFET represents a nonlinear operation, as shown at a point A in FIG. 1, 
at a predetermined voltage value, e.g., 3 V or more and a drain current 
value of 100 .mu.AM or more. It is considered that these nonlinear 
characteristics are caused because the interface between the active layer 
formed by implantation of Si ions and the base material in contact with 
the active layer is not clear but is blurred, and a depth profile is 
formed loosely extending downward from the portion near the lower end of 
the active layer. More specifically, in a conventional GaAs MESFET, if the 
drain voltage, V.sub.D, is increased, a leakage current flowing through a 
portion other than the active layer and the electrode layer formed in the 
substrate, in particular, a current flowing through the above-mentioned 
blurred portion starts oscillating at or above a predetermined threshold 
value. It is considered that because this current serves as a gate, the 
above-mentioned nonlinear characteristics are generated. The nonlinear 
operation of the drain current adversely affects the noise characteristics 
of the GaAs MESFET. In addition, it is confirmed that the above problem is 
similarly posed on a GaAs MESFET manufactured by forming an epitaxial 
layer on a base material 
As a means for solving the above problem, it has been already disclosed 
that high-concentration Cr ions serving as an impurity doped in a 
substrate can prevent the above-mentioned nonlinear operation, in "IEEE 
Transaction on Electron Devices., Vol. ED-34, No. 6, June 1987, pp. 
1239-1244". However, in this disclosed technique, the problem of a large 
change in drain current, ID, over time is also pointed out. 
SUMMARY OF THE INVENTION 
The present invention has been made in consideration of the above 
situation, and has as its principal object to provide an improved 
semiconductor device constituting a GaAs MESFET having one or more 
advantages, such as, e.g., nonlinear operation can be suppressed and/or a 
change in drain current does not occur over a period of time. 
Additional objects and advantages of the invention will be set forth in 
part in the description which follows and in part will be obvious from the 
description, or may be learned by practice of the invention. The objects 
and advantages of the invention may be realized and attained by means of 
the instrumentalities and combinations particularly pointed out in the 
appended claims. 
In order to achieve the above object, a semiconductor device comprising: a 
GaAs substrate prepared from a base material containing boron ions added 
to a crystal during growth of the crystal as a dopant impurity, and having 
an impurity concentration of more than 2.times.10.sup.17 atoms/cm.sup.3 
and of less than 1.times.10.sup.18 atoms/cm.sup.3, thereby attaining a 
uniform distribution of boron ions in the substrate; electrode layers on 
predetermined portions of the GaAs substrate; an active layer formed 
adjacent to the electrode layers on the GaAs substrate by ion 
impregnating; and source and drain electrodes respectively on the 
electrode layers, and a gate electrode on the active layer. With the above 
arrangement, i.e., with a semiconductor device constituting a GaAs MESFET 
using, as a base material, a GaAs substrate containing boron ions at a 
total concentration of 2.times.10.sup.17 atoms/cm.sup.3 or more, there is 
provided a semiconductor device in which nonlinear operation can be 
suppressed and/or a change in drain current does not occur over a period 
of time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of a semiconductor device according to the present invention 
will be described below with reference to FIGS. 2A to 2C and FIG. 3. 
As shown in FIG. 2A, a base material containing boron and carbon ions at a 
total impurity concentration of 2.times.10.sup.17 atoms/cm.sup.3 is used 
for a GaAs substrate 11. In the preferred embodiment of the present 
invention, boron is used as the exclusive dopant impurity ion. The boron 
ions are introduced into the base material of the substrate when the base 
material is prepared by crystal growth. Therefore, when the base material 
is grown by a Czochvalski technique, for example, the boron ions are 
uniformly distributed in the GaAs substrate. Preferably, the concentration 
of boron ions is more than 2.times.10.sup.17 atoms per cm.sup.3 and less 
than 1.times.10.sup.18 atoms per cm.sup.3. 
After a proper pretreatment is performed on the GaAs substrate 11, an SiOx 
layer 12 having a thickness of, e.g., 5,000 .ANG. is deposited on the 
substrate by chemical vapor deposition. Openings 21 and 22 are formed on 
desired portions of the substrate, through the SiOx layer, using resist 
pattern 13, by photolithography. Si ions are implanted in the openings 21 
and 22 at an accelerated energy of 180 keV and a dose of 5.times.10.sup.13 
ions/cm.sup.2 to form electrode layers 141 and 142. 
Then, as shown in FIG. 2B, the SiOx layer 12 and the resist pattern 13 are 
removed. Thereafter, a pretreatment is performed again on the surface of 
the GaAs substrate 11, and an SiOx layer 15 having a thickness of, e.g., 
5,000 .ANG. is deposited on the substrate by chemical vapor deposition. In 
addition, an opening 143 is formed on the GaAs substrate 11, through the 
SiOx layer 15, using resist pattern 16, by photolithography. Si ions are 
implanted through the opening 143 at an accelerated energy of 100 keV and 
a dose of 3.times.10-12 ions/cm.sup.2 to form an active layer 17 in the 
GaAs substrate between the electrode layers 141 and 142, and the active 
layer 17 are formed, a capless annealing process is performed in an 
arsenic atmosphere at 850.degree. for 15 minutes to electrically activate 
these ion-implanted layers. 
Thereafter, a source electrode 19 is formed on the electrode layer 141, a 
drain electrode 20 is formed on the electrode layer 142, and a gate 
electrode 18 is formed on the active layer 17, by a lift-off method. 
In the semiconductor device constituting a GaAs MESFET of the present 
invention manufactured as described above, a base material containing 
boron and carbon ions for serving as an acceptor at a total impurity 
concentration of 2.times.10.sup.17 atoms/cm.sup.3 or more is used for the 
GaAs substrate 11. Thus, the boron and carbon ions cancel the Si ions 
serving as a donor forming the active layer 17. As a result, the blur of 
the lower end of the active layer 17 is eliminated, and the interface 
between the active layer 17 and the GaAs substrate 11 is rendered clear, 
thus suppressing a nonlinear operation, as shown in FIG. 3. 
In the above embodiment, an example wherein a base material contains boron 
and carbon ions as impurities is exemplified, and the impurity 
concentration is represented as a total concentration of those ions. 
However, when the impurity concentration of only boron or carbon ions is 
2.times.10.sup.17 atoms/cm.sup.3 or more, the same effect can be obtained 
as in the above embodiment. As a result, a nonlinear operation of the GaAs 
MESFET can be suppressed and a change in drain current does not occur over 
time. 
The foregoing description of preferred embodiment of the invention has been 
presented for purposes of illustration and description. It is not intended 
to be exhaustive or to limit the invention to the precise form disclosed, 
and obviously many modifications and variations are possible in light of 
the above teachings or may be acquired from practice of the invention. The 
embodiment was chosen and described in order to best explain the 
principles of the invention and its practical application to thereby 
enable one skilled in the art to best utilize the invention in various 
embodiments and with various modifications as are suited to the particular 
use contemplated. It is intended that the scope of the invention be 
defined by the claims appended hereto.