Semiconductor device with quantum well resonance states

An object of the invention to vary a light absorption coefficient within wider limits in a light absorption control semiconductor device. The device includes at least three quantum wells Q1, Q2, Q3. The width of the respective quantum wells and barriers is set such that wave functions of electrons in the respective quantum wells interact in a resonance state where the quantized energy levels in either one of conduction and valence bands are matched. In addition, the width and material of the respective quantum wells are set so that one of the bands is brought into the resonance state where the quantized energy levels at the respective quantum wells are matched in a state where no electric field is applied or a state where a suitable electric field is applied in a direction perpendicular to the junctions. The light absorption is changed by controlling components of the electric field perpendicular to the junctions. Since only one band is brought into the resonance state, the light absorption coefficient can be made greater.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a semiconductor device capable of controlling 
electrically a light absorption coefficient using a quantum well and, more 
particularly to a light absorption control semiconductor device for 
changing characteristics such as frequency modulation, intensity 
modulation, switching, and filtering a propagating light by controlling 
electrically the light absorption coefficient using the quantum well. 
2. Description of the Related Art 
There has been conventionally known a semiconductor device including 
quantum wells enclosed by energy barriers of junctions of different 
semiconductor materials (heterojunctions). By narrowing the width of this 
quantum well, quantized energy levels are formed at the quantum wells. 
There have been proposed a variety of devices taking advantage of the 
discreteness of the quantized energy levels. A resonant tunneling diode, a 
resonant tunneling transistor, and the like are known as examples of these 
devices. These devices take advantage of the fact that the quantized 
energy level is changed by applying an electric field in a direction 
perpendicular to the junction to thereby produce a resonant state which 
brings about a tunnel effect between two layers holding each barrier 
therebetween. 
Further, concerning the light absorption characteristic, the light 
absorption under the influence of direct transition is being studied 
between the quantized energy level in a valence band and the quantized 
energy level in a conduction band using two quantum wells formed of the 
same material and having the same width (Appl. Phys. Lett. 50(16), 20 Apr. 
1987, P1098). According to this reference, the energy of two quantum wells 
are the same in both the conduction and valence bands when the electric 
field is not applied thereto, thus realizing the resonance state where the 
wave functions of electrons interact. 
However, in a state where the quantized energy levels at the two quantum 
wells match in the conduction and valence bands, these two quantum wells 
are equivalent to a single quantum well having width equal to a sum of the 
widths of the two quantum wells. The light absorption coefficient is not 
necessarily large in this state. The above reference analyzes the fact 
that light absorption is unlikely to occur since the electrons localize in 
one of the quantum wells when the electric field is applied to these two 
quantum wells, which are identically structured. However, this mechanism 
is not always clear. 
On the other hand, there has been known a semiconductor device including a 
semiconductor layer having a function obtained by growing a compound 
semiconductor on a substrate by an epitaxy method, and then forming lower 
and upper electrode layers holding the semiconductor layer in parallel 
from opposite sides (in parallel with the substrate surface). For example, 
a light emitting diode, a laser diode, and a resonance tunnel diode are 
known as examples of these devices. In these semiconductor devices, the 
lower electrode layer is formed of a conductive compound semiconductor of 
a thickness of about 1 .mu.m in which impurities of a concentration of 
1.times.10.sup.17 cm.sup.-3 to 1.times.10.sup.18 cm.sup.-3 are included. 
However, in these semiconductor devices, it is necessary to grown an 
epitaxial semiconductor layer having a device function on the lower 
electrode layer. This degrades the crystallinity of the conductive 
semiconductor layer in which the impurities are doped, and accordingly the 
important semiconductor layer having the device function formed on the 
doped layer by the epitaxy method exhibits poor crystallinity. 
According to experiments conducted by the inventors of the present 
invention, in the case where a GaAs semiconductor layer has a device 
function on an n.sup.+ -GaAs layer having an impurity concentration of 
3.times.10.sup.18 cm.sup.-3 and a thickness of about 1 .mu.m and a 
multiple quantum well, it was found that the crystallinity of the multiple 
quantum well was poor based on the measured half value width of a 
photoluminescence spectrum. Since the crystallinity of the GaAs 
semiconductor layer having the device function is poor, the device 
characteristics are also poor. 
The spectrum of the photoluminescence intensity of the n.sup.+ -GaAs layer 
is as shown in FIG. 11. Specifically, there are obtained peaks at 805 nm 
(indicated at a), 820 nm (indicated at b), and 832 nm (indicated at c). 
The peak c is an absorption peak due to the transition of carriers between 
a donor level and an acceptor level; the peak b is an absorption peak due 
to the direct transition of carriers between the bottom of the conduction 
band and the top of the valence band; and the peak a is an absorption peak 
due to the transition of carriers between a higher position in the 
conduction band and the valence band resulting from an improvement in a 
Fermi level caused by the doping of the high concentration impurities. It 
is not desirable that the light absorption occurs in the band shorter than 
800 nm when the light absorption control device is constructed using the 
multiple quantum well of the present invention to be described later. 
SUMMARY OF THE INVENTION 
The invention solves the above problems, and an object thereof is to 
realize a highly efficient light absorption control device. 
Another object of the invention is to improve the crystallinity of a 
functional semiconductor layer. 
Still another object of the invention is to provide a semiconductor device 
which can be used as a light modulator for use in an optically integrated 
circuit, as a variable wavelength semiconductor laser, or the like as will 
be clear from the specific examples of the invention to be described 
later. 
In order to solve the aforementioned problems, a first mode of the 
invention is directed to a semiconductor device including a quantum well 
which is formed by a junction of different types of semiconductors having 
different band gaps and has a quantized energy level enclosed by energy 
barriers comprising at least three quantum wells, the width of the 
respective quantum wells and barriers being set such that wave functions 
of electrons in the respective quantum wells interact in a resonance state 
where the quantized energy levels in either one of the conduction and 
valence bands are matched, the width and material of the respective 
quantum wells being set so that one of the bands is brought into the 
resonance state where the quantized energy levels at the respective 
quantum wells are matched in a state where no electric field is applied or 
in a state where a suitable electric field is applied in a direction 
perpendicular to the junction, and the light absorption characteristic 
being changed by controlling components of the electric field 
perpendicular to the junction. 
A second mode of the invention is directed to a semiconductor device 
including a semiconductor layer having a device function formed on a 
substrate through the epitaxy method, lower and upper layers (first and 
second electrodes) formed to hold the semiconductor layer therebetween in 
parallel from opposite sides and to apply an electric field 
perpendicularly to the semiconductor layer, characterized by a first 
semi-insulating layer formed by growing undoped semi-insulating compound 
semiconductor on the substrate through the epitaxy method, a conductive 
.delta. doped layer of a thickness of about one atom formed by doping 
impurity atoms sparsely on a surface of the first semi-insulating layer, 
and a second semi-insulating layer formed by growing undoped 
semi-insulating compound semiconductor on the .delta. doped layer through 
the epitaxy method, and the .delta. doped layer being used as the second 
electrode. 
In the first mode of the semiconductor device according to the invention, 
when the respective quantum wells are formed of the same material in the 
device including at least three quantum wells, the quantized energy levels 
formed at the respective quantum wells change according to the width of 
the quantum wells. Accordingly, by designing the width of the three 
quantum wells properly, the quantized energy levels at the respective 
quantum wells are not equalized, but rather they are increased or 
decreased according to an arrangement order wherein the quantum wells 
whose quantized energy levels are closest are arranged in the state where 
no electric field is applied. When the electric field is applied 
perpendicularly to the junction, the close quantized energy levels at the 
respective quantum wells can be brought to the same level. When the 
quantized energy levels of the quantum wells are at the same level in the 
presence of the electric field and the wave functions of electrons 
mutually overlap at the three quantum wells, the three quantized energy 
levels are separated by a minute energy difference and bring about a state 
where the three quantum wells are connected continuously. This state is 
referred to as a resonance state. The light absorption coefficient can be 
increased remarkably by bringing only one band, e.g. the conduction band, 
into the resonance state while holding the valence band in a non-resonance 
state. At this time, light absorption occurs under the influence of direct 
transitions between a quantized energy level in the valence band at the 
respective quantum wells (levels mainly contributing to the light 
absorption are base levels) and three quantized energy levels in the 
conduction band in the resonance state. 
The quantized energy levels formed at the respective quantum wells can be 
changed according to the material forming the quantum wells. Accordingly, 
the resonance state may be generated in only one band when the suitable 
electric field is applied, by changing the materials forming the 
respective quantum wells instead of changing the width of the quantum 
wells. Further, the resonance state may be generated in only one band when 
the suitable electric field is applied by changing the width and materials 
of the quantum wells. 
In the case where the materials of the quantum wells are changed, the 
resonance state may be set in only one band where no electric field is 
applied. 
The resonance state may be changed to the non-resonance state by changing 
the applied electric field or by applying an electric field where the 
resonance state is generated in the absence of an electric field. 
In this way, the light absorption coefficient and the absorption wavelength 
can be varied by controlling the electric field acting in the direction 
perpendicular to the junction. 
Thus, the invention pertains to a semiconductor device including at least 
three quantum wells whose width is determined together with the width of 
the barriers such that the wave functions of electrons at the respective 
quantized energy wells interact in the resonance state, and which generate 
the resonance state where the quantized energy levels at the respective 
quantum wells are continuous and which generate the non-resonance state in 
only one band by controlling components of the electric field acting in 
the direction perpendicular to the junction so as to control the light 
absorption. 
Therefore, there can be obtained a characteristic of changing the light 
absorption greatly according to a change in the electric field, which 
enables the control of the absorption coefficient and absorption 
wavelength by means of an electric field. 
On the other hand, in the second mode, the first semi-insulating layer 
formed on the substrate through the epitaxy method is semi-insulating 
undoped compound semiconductor. Being undoped (free from impurities), the 
crystallinity of the first semi-insulating layer is preserved. On the 
first semi-insulating layer is formed the conductive .delta. doped layer 
of the thickness of one atom which is obtained by doping the impurity 
atoms sparsely. On the .delta. doped layer is further formed the second 
semi-insulating layer obtained by growing the undoped compound 
semiconductor through the epitaxy method. At this time, since the impurity 
atoms are sparsely doped on the planar surface in the thickness of one 
atom in the .delta. doped layer, the crystallinity of the lower located 
first semi-insulating layer is not destroyed. Since the regularity of 
crystal lattices of the lower located first semi-insulating layer is found 
on the surface of the .delta. doped layer, the second semi-insulating 
layer grown on the .delta. doped layer through the epitaxy is allowed to 
have exceedingly high crystallinity succeeding the regularity of crystal 
lattices of the first semi-insulating layer. Thus, the crystallinity of 
the functional semiconductor layer formed on the second semi-insulating 
layer is highly satisfactory. Particularly, in the case where the 
functional semiconductor layer is constituted by a multiple quantum well, 
a semiconductor layer of exceedingly high quality is obtained. Further, 
being conductive, the .delta. doped layer is allowed to function as an 
electrode layer for the functional semiconductor layer by etching the 
semiconductor layer formed thereon to form a window to thereby provide a 
lead. 
The invention is characterized in that the first semi-insulating layer is 
formed on the substrate by growing the undoped compound semiconductor 
through the epitaxy method, the .delta. doped layer is formed on the first 
semi-insulating layer by doping the impurity atoms sparsely in the 
thickness of one atom, the second semi-insulating layer is formed on the 
.delta. doped layer by growing the undoped compound semiconductor through 
the epitaxy method, the functional semiconductor layer is formed on the 
second semi-insulating layer through the epitaxy method, and the .delta. 
doped layer is used as an electrode layer. 
Since the .delta. doped layer is used as the electrode layer, the 
semiconductor layer formed thereon is not influenced by the impurity atoms 
and can be grown through the epitaxy method while maintaining the high 
crystallinity. As a result, good crystallinity can be obtained despite the 
presence of the conductive layer between the semiconductor layers, thereby 
improving a device characteristic of the semiconductor device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereafter, specific examples according to the invention will be described 
with reference to the accompanying drawings. 
First Embodiment 
FIG. 1 is a sectional view showing the construction of a semiconductor 
device pertaining to an embodiment of a first mode of the invention. An 
illustrated semiconductor device 100 is formed by a junction of different 
types of semiconductors having different band gaps and includes quantum 
wells having a quantized energy level enclosed by energy barriers. The 
semiconductor device 100 includes at least three quantum wells Q1, Q2, Q3. 
The widths of the respective quantum wells and barriers are set such that 
wave functions of electrons in the respective quantum wells interact in a 
resonance state where the quantized energy levels of either one of 
conduction and valence bands are matched. In addition, the width and 
material of the respective quantum wells are set so that one of the bands 
is brought into the resonance state where the quantized energy levels at 
the respective quantum wells are matched in at least one state selected 
from a state where no electric field is applied and in a state where a 
suitable electric field is applied in a direction perpendicular to the 
junction. The illustrated semiconductor device 100 is a light absorption 
control semiconductor device using the quantum wells constructed so that 
the light absorption is changed by controlling components of the electric 
field perpendicular to the junction. 
More specifically, on a semi-insulating GaAs substrate 24 is grown a first 
unit epitaxial GaAs semiconductor layer having a thickness of 500 nm and 
in which no impurity is doped (hereinafter referred to as "i-GaAs") 
through the epitaxy method, thereby forming an i-GaAs layer 23 which is a 
first unit semiconductor layer. Thereafter, the crystal growth is 
interrupted and Si is doped in the surface density of 1.times.10.sup.12 
cm.sup.-2 to form a .delta. doped layer 22 which is a second unit 
semiconductor layer. The i-GaAs is grown again to a thickness of 100 nm to 
form an i-GaAs layer 21 which is a third unit semiconductor layer. Then, 
i-AlGaAs (Al composition ratio=0.3) is grown to a thickness of 100 nm to 
form an i-AlGaAs layer 20 which is a fourth unit semiconductor layer. 
Hereafter, illustrated unit semiconductor layers 19 to 12 are formed one 
after another. Specifically, i-GaAs, i-AlGaAs, i-GaAs, i-AlGaAs, and 
i-GaAs are grown in this order respectively to thicknesses of 6 nm, 2 nm, 
15 nm, 2 nm, and 5 nm to thereby form an i-GaAs layer 19, an i-AlGaAs 
layer 18, an i-GaAs layer 17, an i-AlGaAs layer 16, and an iGaAs layer 15. 
Next, i-AlGaAs is grown to a thickness of 100 nm to thereby form an 
i-AlGaAs layer 14. 
Layers 20 to 14 form a triple quantum well structure TQW which is an 
example of a multiple quantum well. The i-GaAs layer 15 is a first quantum 
well Q1; the i-GaAs layer 17 is a second quantum well Q2; and the i-GaAs 
layer 19 is a third quantum well Q3. The i-AlGaAs layers 14, 20 are 
potential barriers Vu, Vd at opposite ends. The i-AlGaAs layers 16, 18 are 
intermediate potential barriers V1, V2 for separating the respective 
quantum wells. 
Thereafter, on the i-AlGaAs layer 14 are grown i-GaAs and n-GaAs 
respectively to thicknesses of 10 nm and 100 nm to thereby form an i-GaAs 
layer 13 and an n-GaAs layer 12. In this way, an epitaxial film structure 
is completed. Next, in a region of the epitaxial film, the unit 
semiconductor layers 12 to 20 and and a portion of the semiconductor layer 
21 are removed by etching to expose the i-GaAs layer 21 on the doped layer 
22. In the exposed area is formed a double layer structure of AuGe alloy 
and Au (50 nm, 200 nm respectively) by vacuum deposition. An alloying 
process is applied to the thus formed structure in a hydrogen and nitrogen 
atmosphere at a temperature of 400 degrees for 2 minutes to obtain an 
electrode 25 having an ohmic contact. Further, on the n-GaAs layer 12, 
namely, the uppermost unit semiconductor layer, an Au layer of a thickness 
of 20 nm if formed to obtain a semi-transparent electrode 11 which is a 
first electrode. By holding this semi-transparent electrode 11 at a 
positive potential relative to the second electrode 25, an electrode field 
can be applied in a direction perpendicular to the junction of the triple 
quantum well TQW. 
The .delta. doped layer 22 is formed by sparsely accumulating a layer of 
silicon atoms on the i-GaAs layer 23. Thus, the .delta. doped layer 22 
acts as an electrode for applying an electric field uniformly to the 
surfaces of the three quantum wells. In addition, since the .delta. doped 
layer 22 does not disorder the lattice of the lower located i-Gas layer 
23, the layers 21 to 12 formed thereon are allowed to have the good 
crystallinity. 
As is clear from the construction of the above semiconductor device, a 
semiconductor device in a first mode of the invention is characterized in 
that the multiple quantum well TQW including at least three quantum wells 
is formed on the specified substrate 24 through the epitaxy method as a 
functional semiconductor layer P. 
In order to apply the electric field in the direction normal to the layer 
forming direction of the functional semiconductor layer P, there are 
provided the first electrode 11, i.e., the semi-transparent electrode 11, 
which is formed in contact with the surface of the unit semiconductor 
layer 12 located most outward from the substrate over a multitude of 
layers 12 to 23 constituting the functional semiconductor layer P, and the 
second electrode 25 which is formed at least partially in contact with the 
unit semiconductor layer 21. 
The semiconductor device 100 in the above specific example causes a light 
to be incident upon the triple quantum well TQW to which a bias voltage is 
applied between the semi-transparent electrode (first electrode) 11 and 
the second electrode 25 perpendicularly from the side of the electrode 11, 
and changes the absorption of the incident light in the triple quantum 
well TQW by changing the bias voltage. 
There will next be described a light absorbing mechanism by the 
semiconductor device 100. 
FIG. 2 is a diagram showing energy bands in a state where no electric field 
is applied to the triple quantum well TQW. 
In each of the conduction and valence bands are formed the potential 
barriers Vu, V1, V2, Vd including the AlGaAs layers 14, 16, 18, 20, and 
the first, second, and third quantum wells Q1, Q2, Q3 including the GaAs 
layers 15, 17, 19. 
There are formed a ground state quantized energy level Lc1.sub.o 
corresponding to the thickness of 5 nm in the conduction band at the first 
quantum well Q1; a ground state quantized energy level Lc2.sub.o 
corresponding to the thickness of 15 nm and a first excitated state energy 
level Lc2.sub.1 in the conduction band at the second quantum well Q2; and 
a ground state energy level Lc3.sub.0 corresponding to the thickness of 6 
nm in the conduction band at the third quantum well Q3. 
Further, there are formed a ground state quantized energy level Lv1.sub.o 
corresponding to the thickness of 5 nm in the valence band at the first 
quantum well Q1; a ground state quantized energy level Lv2.sub.o 
corresponding to the thickness of 15 nm in the valence band at the second 
quantum well Q2; and a ground state quantized energy level Lv3.sub.o 
corresponding to the thickness of 6 nm in the valence band at the third 
quantum well Q3. Other excitated state energy levels are not illustrated 
since they are not controlled by the light absorption. 
In this way, electrons are confined at certain discrete levels at the 
quantum wells when the widths of the wells are narrowed. 
In this state, the quantized energy levels are not at the same levels (not 
continuous) in the conduction band at the three quantum wells Q1, Q2, Q3. 
Accordingly, the electrons cannot drift through the intermediate barriers 
V1, V2 between the respective quantum wells. In other words, the light 
absorption occurs under the influence of direct transitions between the 
quantized energy level Lv1.sub.0 of the valence band and the quantized 
energy level Lc1.sub.0 of the conduction band at the first quantum well 
Q1, between the quantized energy level Lv2.sub.0 of the valence band and 
the quantized energy level Lc1.sub.0 of the conduction band at the second 
quantum well Q2, and between the quantized energy level Lv3.sub.0 of the 
valence band and the quantized energy level Lc3.sub.0 of the conduction 
band in the third quantum well Q3. 
More specifically, the three quantum wells Q1, Q2, Q3 do not interact, and 
the electrons transit from the valence band to the conduction band at the 
individual quantum wells upon the incidence of light having energy between 
the quantized energy levels in the valence and conduction bands at the 
respective quantum wells. Thus, the light absorption coefficient is small 
in this state. 
However, when the bias voltage is applied perpendicularly to the junction 
surface, an electric field is generated which equalizes the ground state 
energy level Lc1.sub.0, the first excitated energy level Lc2.sub.1, and 
the ground state energy level Lc3.sub.0 in the conduction band at the 
respective quantum wells Q1, Q2, Q3 as shown in FIG. 4A. To say that in 
the opposite way, the widths of the respective quantum wells Q1, Q2, Q3 
are designed such that three quantized energy levels match at the same 
level in response to a certain value of the electric field. At this time, 
as shown in FIG. 4B, the quantized energy levels do not match in the 
valence band with the electric field where the quantized energy levels are 
matched in the conduction band. When a negative electric field is applied, 
the quantized energy levels match in the valence band at the three quantum 
wells. If the quantized energy levels at the respective quantum wells are 
matched in the conduction band in this way, wave functions of the 
electrons at the quantum wells interact and accordingly the electrons are 
permitted to exist at the quantized energy level of a desired quantum well 
through the intermediate barriers V1, V2. In the case of three quantum 
wells, there exist three separate levels E1, E2, E3 which differ by a 
minute energy difference in this state (the degeneracy is released and 
three separate levels exist). 
In this state, the light absorption occurs under the influence of the 
direct transitions TE1, TE2, TE3 between the ground state quantized energy 
level Lv1.sub.0 in the valence band at the first quantum well Q1 and the 
quantized energy levels E1, E2, E3 commonly spread in the conduction band 
at the three quantum wells as shown in FIG. 3. Accordingly, the electrons 
are permitted to transit from the valence band to the quantized energy 
level of any quantum well in the conduction band by the light absorption. 
This state is referred to as a resonance state. In this resonance state, 
the light absorption coefficient becomes exceedingly large. 
According to the observation of the inventors, the light absorption 
coefficient is improved about 100 times compared to a semiconductor device 
including two quantum wells formed of the same material and having the 
same width. 
To be more precise, since the resonance conditions of the three quantized 
energy levels E1, E2, E3 differ slightly, it is difficult to observe three 
absorption spectra simultaneously and the wavelengths at the peaks of the 
absorption spectra change according to the intensity of the electric 
field. In other words, the electron transition which is the major 
controlling factor in the light absorption can be selected to be a desired 
one of the transitions TE1, TE2, TE3 by changing the intensity of the 
applied electric field slightly. Taking advantage of this characteristic, 
using a light source with a broad spectrum, the output light wavelength 
can be changed according to the intensity of the electric field. If a 
light of a single wavelength is to be controlled, the light absorption 
coefficient can be changed greatly according to the intensity of the 
electric field. Thus, the amplitude of the light can be modulated using 
the electric field as a modulation signal. 
FIG. 5 is a graph showing a characteristic of the photoconductivity based 
on the electrons excited by making the light incident upon the 
semiconductor device 100 of this embodiment, which characteristic is 
measured by changing the applied voltage and the wavelength of the 
incident light. 
The light is incident upon the semitransparent electrode (first electrode) 
11 and is transmitted through the quantum wells Q1, Q2, Q3. The bias 
voltage is changed from 2.5 V to 2.9 V. At 2.5 V, a photoconductivity peak 
P1 appears at 760 nm. At 2.9 V, the peak P1 disappears and a 
photoconductivity peak P2 appears at 765 nm. At an intermediate voltage of 
2.7 V, both the peaks P1 and P2 are observed weakly. 
As seen from the above, such an excellent characteristic was observed that 
the wavelength at the photoconductivity peak, i.e., the wavelength at the 
peak of the light absorption, could be controlled by changing the bias 
voltage by only 0.4 V. When the wavelength at a particular peak is taken 
note of, the light absorption coefficient can be controlled. 
Second Embodiment 
FIG. 6 is a diagram showing the structure of energy bands of a 
semiconductor device pertaining to a second embodiment showing the first 
mode of the invention. In the second embodiment, semiconductors formed of 
different materials are used for first, second and third quantum wells Q1, 
Q2, Q3. Specifically, the first well Q1 is formed of InGaAs; the second 
quantum well Q2 is formed of AlGaAs; the third quantum well Q3 is formed 
of GaAs; and barriers Vu, V1, V2, Vd are formed of AlGaAs having a large 
Al composition ratio. By regulating the width of the respective quantum 
wells Q1, Q2, Q3, ground state quantized energy levels coincide (match) 
only in a conduction band at the respective quantum wells in the absence 
of an electric field. The quantized energy levels do not coincide in a 
valence band. In this structure, the conduction bands of the three quantum 
wells can be brought into a resonance state in the state no electric field 
is applied. In the resonance state of the conduction band, the matched 
quantized energy levels are separated into three quantized energy levels 
E1, E2, E3 which differ by a minute energy, thereby releasing the 
degeneracy. 
In this structure, the direct transitions of electrons by light absorption 
include: transitions T1.sub.1, T1.sub.2, T1.sub.3 between a ground state 
quantized energy level Lv1.sub.o in the valence band at the first quantum 
well Q1 and the three quantized energy levels E1, E2, E3 commonly spread 
in the conduction band at the respective quantum wells, transitions 
T2.sub.1, T2.sub.2, T2.sub.3 between a ground state quantized energy level 
Lv2.sub.o in the valence band at the second quantum well Q2 and the three 
quantized energy levels E1, E2, E3 commonly spread in the conduction band 
at the respective quantum wells; and transitions T3.sub.1, T3.sub.2, 
T3.sub.3 between a ground state quantized energy level Lv3.sub.o in the 
valence band at the third quantum well Q3 and the three quantized energy 
levels E1, E2, E3 commonly spread in the conduction band at the respective 
quantum wells. 
Accordingly, the light absorption occurs at three wavelengths corresponding 
to the transitions T1, T2, T3 from the different quantized energy levels 
in the valence band at the three quantum wells Q1, Q2, Q3. Thus, the peak 
wavelength and the absorption coefficient at the peak wavelength in the 
absorption characteristic having a single light absorption peak at a 
different wavelength for each quantum well can be changed by changing the 
intensity of the applied electric field. In other words, this device has 
three simultaneous absorption lines in a single structure and is capable 
of electrically switching between the three absorption lines which differ 
slightly from one another. 
Third Embodiment 
FIG. 7 is a sectional view showing the structure of a semiconductor device 
101 pertaining to a third embodiment showing another example of the first 
mode of the invention. The device 101 is applicable to a light-light 
modulator capable of varying the amplitude of a carrier beam according to 
the intensity of a modulated signal light. If the carrier beam is a light 
having a broad spectrum, the device 101 is applicable to a light-light 
modulator capable of modulating a wavelength (frequency) of the carrier 
beam according to the intensity of the modulated signal light. 
The semiconductor device 101 of FIG. 7 is structured such that a p-InGaAs 
layer 30 is formed between the i-GaAs layer 21 of the triple quantum well 
structure TQW of FIG. 1 and the semi-insulating GaAs substrate 24. The 
carrier beam is incident upon a first electrode 11, modulated in the 
semiconductor device 101, and output from the GaAs substrate 24. 
An absorbing mechanism of this device will be described with referenced to 
FIG. 8 showing a band structure. In this example, the conduction band is 
brought into a resonance state at a specified bias voltage V.sub.o, and 
one of the transitions X1, X2, X3 is set to occur strongly. 
Accordingly, a transmitting light of the carrier beam is a light having a 
small amplitude. 
When the modulated signal light whose energy is smaller than a forbidden 
bandwidth of GaAs and larger than a forbidden bandwidth of InGaAs is made 
incident upon the GaAs substrate 24 from the outside, the modulated signal 
light is absorbed in the p-InGaAs layer 30 and holes are excited by the 
valence band. Thereby, an electric field is applied in such a direction as 
to hold the second electrode 25 at a positive potential relative to the 
first electrode 11. At his time, the band structure is as shown in FIG. 9. 
Accordingly, the resonance state cannot be maintained in the conduction 
band at the three quantum wells Q1, Q2, Q3, and the carrier beam cannot be 
absorbed in the quantum wells. As a result, the transmitting light becomes 
a light having a large amplitude. 
In this way, the modulated signal light provides a light-light switching 
device for binary control of the intensity of the transmitting light of 
the carrier beam. 
If the transitions X1, X2, X3 are selectively utilized while making an 
electromotive force generated by the modulated light smaller, a 
light-light amplitude modulator device is realized for carrying out 
amplitude modulation by the modulated signal light of the carrier light. 
Further, if the light having a broad spectrum is used as a carrier beam, 
the wavelength of the absorption peaks in the quantum wells can be shifted 
by the selection of the transitions X1, X2, X3 by the modulated light, 
thereby changing the spectrum of the transmitting light of the carrier 
beam. Therefore, light-light frequency modulation and light-light 
frequency-shift keying (FSK) modulation can be carried out in analog and 
digital technologies respectively. 
In all the foregoing embodiments, it is possible to increase a working 
temperature by narrowing the width of the three quantum wells and by using 
different semiconductor materials. 
Next will be described a specific example of a semiconductor device 
pertaining to a second mode of the invention. An object of the second mode 
of the invention is to improve the crystallinity of the functional 
semiconductor layer P constituting the light absorption control device as 
mentioned in the description of the first mode, and accordingly the second 
mode is directed first mode, and accordingly the second mode is directed 
to a semiconductor device in which the .delta. doped layer serving as a 
second electrode is formed entirely in the interior of the unit 
semiconductor layer formed in contact with the substrate. 
Fourth Embodiment 
In this embodiment showing the second mode of the invention, a .delta. 
doped layer is applied to a light absorption control semiconductor device 
in which a functional semiconductor layer is used as an asymmetric triple 
quantum well. FIG. 12 is a sectional view showing the construction of this 
semiconductor device. 
Specifically, the semiconductor device 102 in this specific example is 
characterized in that a unit semiconductor layer formed in contact with a 
substrate 24 incudes a first semi-insulating layer 23 formed by growing 
undoped semi-insulating compound semiconductor on the substrate 24 through 
the epitaxy method, a conductive .delta. doped layer 22 formed by sparsely 
doping impurity atoms on one main surface of the first semi-insulating 
layer 23 in the thickness of about one atom, and a second semi-insulating 
layer 21 formed by growing undoped semi-insulating compound semiconductor 
on one main surface of the .delta. doped layer 22 through the epitaxy 
method. The .delta. doped layer 22 is used as the second electrode 25. 
Hereinafter, there will be described an exemplary method of fabricating the 
semiconductor device 102 in the above specific example. 
First of all, GaAs in which no impurity is doped (hereinafter referred to 
as "i-GaAs") is grown to 500 nm on the semi-insulating GaAs substrate 24 
through the epitaxy method using an MBE method to thereby form an i-GaAs 
layer (first semi-insulating layer) 23. Thereafter, the crystal growth is 
interrupted by closing a shutter for a Ga supply source while pressurizing 
As, and a shutter for a supply source of Si which is n-type dopant is 
opened and Si is doped sparsely on the surface to a thickness of about one 
atom at a surface density of 1.times.10.sup.12 cm.sup.-2 to thereby form 
the .delta. doped layer 22 constituting the second electrode layer. Then, 
the shutter for the Si supply source is closed while that for the Ga 
supply source is opened so as to grow the i-GaAs to a thickness of 100 nm 
to form an i-GaAs layer (second semi-insulating layer) 21. Thereafter, the 
i-AlGaAs (Al composition ratio=0.3) is grown to a thickness of 100 nm to 
thereby form an i-AlGaAs layer 20. 
In the above example, it may be deemed that the first semi-insulating layer 
23, the 6 doped layer 22, and the second semi-insulating layer 21 form a 
single unit semiconductor layer. 
Thereafter, unit semiconductor layers 21 to 12 are formed in accordance 
with the procedure similar to the one described in the first embodiment. 
Next, in a region of the epitaxial film, the unit semiconductor layers 12 
to 20 and a portion of the unit semiconductor layer 21 up to some depth 
are removed by etching to expose the i-GaAs layer 21 on the .delta. doped 
layer 22. In the exposed area is formed a double layer structure of AuGe 
alloy and Au (50 nm, 200 nm respectively) by vacuum deposition. An 
alloying process is applied to the thus formed structure in a hydrogen and 
nitrogen atmosphere at a temperature 400 degrees for 2 minutes to obtain 
an ohmic electrode 25 serving a second electrode in contact with the 
.delta. doped layer 22 by diffusing Ge. Further, on the n-GaAs layer 12, 
namely, the uppermost unit semiconductor layer, is formed an Au layer at a 
thickness of 20 nm to obtain a semi-transparent electrode 11 which is a 
first electrode. By holding this first semi-transparent electrode 11 at a 
positive potential relative to the second electrode 25, an electric field 
can be applied in a direction perpendicular to the junction of the triple 
quantum well TQW. 
The .delta. doped layer 22 is formed by sparsely accumulating about a layer 
of silicon atoms on the i-GaAs layer 23. Thus, the .delta. doped layer 22 
acts as an electrode for applying an electric field uniformly to the 
surfaces of the three quantum wells (TQW). In addition, since the .delta. 
doped layer 22 does not disorder the lattice of the lower i-GaAs layer 23, 
the layers 21 to 12 formed thereon are allowed to have high crystallinity 
since they succeeding grow the good crystallinity of the i-GaAs layer 23. 
In the semiconductor device 102 in this specific example, the light is 
incident upon the first electrode 11 perpendicularly to the triple quantum 
well TQW to which a bias voltage is applied between the first 
semi-transparent electrode 11 and the second electrode 25, and the 
absorption of the incident light in the triple quantum well TQW is changed 
by changing the bias voltage. 
In the case where the sheet carrier density of the .delta. doped layer 22 
lies in the range of 1.times.10.sup.11 to 3.times.10.sup.13 cm.sup.-2, a 
layer of Si may be doped sparsely. 
A photoluminescence spectrum of the triple quantum well was measured. A 
spectrum having a half value width was obtained which is sufficiently 
narrower than that of the spectrum of the triple quantum well formed on 
the high concentration n.sup.+ -layer of the thickness of 1 .mu.m as in 
the prior art. 
The inventors measured the photoluminescence spectrum in 77K of the .delta. 
doped layer 22 at the stage where the respective unit semiconductor 
layers, namely the i-GaAs layer 23, .delta. doped layer 22, and i-GaAs 
layer 21, are formed. At this time, the doping time for the silicon atoms 
forming the .delta. doped layer 22 is changed. The measurement results are 
shown in FIG. 10. Peaks are found in the vicinity of about 825 nm 
(indicated at a) and about 870 nm (indicated at b). The illustrated peak a 
is an absorption peak caused by the carrier transition between the bottom 
of the conduction band and the top of the valance band. No peak is found 
at the wavelength shorter than the wavelength at the absorption peak. This 
is significant since the light absorption control semiconductor device in 
this embodiment controls the light absorption coefficient in a narrower 
wavelength range than the wavelength at the peak a as is clear from the 
description made later. 
Although the .delta. doping is applied to the devices in which a relatively 
small amount of current flows in the foregoing first to fourth 
embodiments, a multitude of i-GaAs layers and .delta. doped layers may be 
formed at intervals of about 1 nm to several tens of nm in the case of 
devices requiring a large amount of current such as laser diodes. In this 
case, it is possible to increase the current density without deteriorating 
the crystallinity. 
There will be described a third mode of a semiconductor device according to 
the invention. As described above, the semiconductor device 100 according 
to the invention includes a semiconductor layer P having a light 
absorption controlling function. A resonance state changes according to 
the voltage level when the voltage is applied in a direction perpendicular 
to junction surfaces of a plurality of unit semiconductor layers 
constituting the semiconductor layer P. It has been pointed out that the 
semiconductor device can be used as a light modulator since the 
absorptivity changes as the resonance state changes. 
Incidentally, there has been known a light modulator for use in an optical 
integrated circuit in which a waveguide path is formed on a LiNbO.sub.3 
substrate. There has been also proposed a light modulator taking advantage 
of an electric field effect in an exciton absorption in a structure 
wherein eight 8 nm GaAs layers and eight 5 nm AlGaAs layer are accumulated 
alternately (JJAP, Vol. 24, No. 6, 1985, pp. L442 to L444). 
However, the light modulator having the waveguide path formed on the 
LiNbO.sub.3 substrate suffers from the problem in that the size of the 
device becomes large since the modulator includes the device based on the 
geometric optical principle. Further, in the modulator taking advantage of 
the electric field effect in the exciton absorption, the suddenness is 
merely changed in the transition absorption characteristic between a donor 
level and an acceptor level. Accordingly, the modulation efficiency cannot 
be increased since the suddenness is not satisfactory. The modulator also 
suffers from the problem in that a working wavelength range is restricted 
to the vicinity of an absorption end. 
In view of this, an object of the third mode of the invention is to realize 
a light absorption control semiconductor device which has a high 
efficiency and a sudden absorption characteristic by taking advantage of 
the light absorption based on a quite novel principle, i.e., a light 
absorption control semiconductor device having a high wavelength 
selectivity. 
In order to solve the above problems, a semiconductor device according to 
the third mode of the invention is constructed so that a waveguide layer 
112 for guiding the light is joined with a light absorption control 
semiconductor region A capable of changing a light absorption spectrum 
characteristic, and so that the light propagating in the waveguide layer 
112 leaks into the light absorption control semiconductor region A. The 
region A is constructed similarly to the multiple quantum well in the 
first mode, and is formed by joining different types of semiconductors 
having different band gaps. The region A includes at least three quantum 
wells Q1, Q2, Q3 each having a quantized energy level enclosed by energy 
barriers. The width of the respective quantum wells and barriers are set 
such that wave functions of electrons in the respective quantum wells 
interact in a resonance state where the quantized energy levels of either 
one of conduction and valence bands are matched. In addition, the width 
and material of the respective quantum wells are set so that one of the 
bands is brought into the resonance state where the quantized energy 
levels in the respective quantum wells are matched in a state where no 
electric field is applied or a state where a suitable electric field is 
applied in a direction perpendicular to the junction. The semiconductor 
device 103 changes the light absorption by controlling components of the 
electric field perpendicular to the junction. 
The semiconductor device 103 according to the third mode of the invention 
is constructed such that the waveguide layer 112 is joined with the light 
absorption control semiconductor region A and that the light beam 
propagating in the waveguide layer leaks to the light absorption control 
semiconductor region. The leaked light is absorbed by a specified 
absorption spectrum characteristic in the light absorption control 
semiconductor region. Thus, the light is subjected to a variety of 
modulations while propagating in the waveguide layer. 
The light absorption control semiconductor region A acts similar to the one 
described in the semiconductor device according to the first mode. 
Accordingly, in the third mode of the invention, the semiconductor device 
is allowed to have a light absorption characteristic having a certain 
absorption peak by controlling the electric field acting in the direction 
perpendicular to the junction. Thus, the spectrum of the light beam 
changes since the light beam is subjected to the light absorption by the 
light absorption spectrum characteristic in the light absorption control 
semiconductor region while propagating in the waveguide layer. In other 
words, a variety of modulations and filterings can be realized by changing 
the spectrum of the light beam. 
The third mode of the invention pertains to a semiconductor device 
including at least three quantum wells whose width is determined together 
with the width of barriers such that wave functions of electrons in the 
respective quantum wells interact in a resonance state, a light absorption 
control semiconductor region for generating a resonance state where 
quantized energy levels at the respective quantum wells are continuous and 
having a non-resonance state in only one of the bands by controlling 
components of an electric field acting in a direction perpendicular to a 
junction so as to control the light absorption, and a waveguide layer for 
guiding a light, the light absorption control semiconductor region being 
joined with the waveguide layer so that the light propagating in the 
waveguide layer leaks to the light absorption control semiconductor 
region. 
Accordingly, the light absorption spectrum changes greatly in the light 
absorption control semiconductor region according to the change in the 
electric field. Thus, the spectrum of light can be changed by controlling 
the electric field. This change in the spectrum of the light permits 
realization of devices for carrying out the frequency modulation 
(wavelength modulation), intensity modulation, on-off switches, binary 
modulation of "0" and "1" FSK modulation, filtering, and the like. The 
modulation efficiency is high since almost no current flows in the quantum 
wells. 
Fifth Embodiment 
FIG. 13 is a sectional view showing the construction of the semiconductor 
device 103 pertaining to a fifth embodiment showing the third mode of the 
invention, and FIG. 14 is a perspective view of the device shown in FIG. 
13. 
On a 450 .mu.m semi-insulating GaAs substrate 213 is formed a 500 nm n-GaAs 
layer 212 as an electrode layer through epitaxy using the MBE method. 
Thereafter, n-Al.sub.x Ga.sub.1-x As (x=0.3), i-Al.sub.x Ga.sub.1-x As, 
i-GaAs, i-Al.sub.x Ga.sub.1-x As, i-GaAs, i-Al.sub.x Ga.sub.1-x As, 
i-GaAs, i-Al.sub.x Ga.sub.1-x As, and n-Al.sub.x Ga.sub.1-x As are grown 
by 100 nm, 10 nm, 6 nm, 2 nm, 15 nm, 2 nm, 5 nm, 100 nm, 20 nm to thereby 
form an n-AlGaAs layer 211, an i-AlGaAs layer 120, an i-GaAs layer 119, an 
i-AlGaAs layer 118, an i-GaAs layer 117, an i-AlGaAs layer 116, an i-GaAs 
layer 115, an i-AlGaAs layer 114, and an n-AlGaAs layer 113 which are unit 
semiconductor layers respectively. 
The light absorption control semiconductor region A including the plurality 
of unit semiconductor layers 120 to 113 constitutes a triple quantum well 
structure TQW. The i-GaAs layer 115 is a first quantum well Q1; the i-GaAs 
layer 117 is a second quantum well Q2; and the i-GaAs layer 119 is a third 
quantum well Q3. The i-AlGaAs layers 114, 120 are potential barriers Vu, 
Vd at opposite ends. The i-AlGaAs layers 116, 118 are intermediate 
potential barriers V1, V2 for separating the respective quantum wells. 
Thereafter, on the i-AlGaAs layer 113 are grown i-Al.sub.y Ga.sub.1-y As 
(y=0.2) to a thickness of 1 .mu.m to thereby form an i-AlGaAs layer 112. 
This i-AlGaAs layer 112 functions as a waveguide layer. In this way, an 
epitaxial film structure is completed in which a light absorption control 
semiconductor region A (layers 213 to 113) is joined with the waveguide 
layer 112. 
Then, the waveguide layer 112 is etched to expose a part of the n-AlGaAs 
layer 113 in the form of a strip, so that the width of the waveguide layer 
112 is narrower than that of the light absorption control semiconductor 
region A. Further, the layers 113 to 212 are etched in the form of a strip 
so as to expose the n-GaAs layer 212. An Au layer and a double layer 
structure of AuGe alloy and Au (50 nm, 200 nm respectively) are formed on 
the exposed n-AlGaAs layer 113 and the exposed n-GaAs layer 212 by vacuum 
deposition. The alloying process is applied to the thus formed layers in a 
hydrogen and nitrogen atmosphere at a temperature of 400 degrees for 2 
minutes to thereby obtain a first electrode 111 having an ohmic contact 
and a second electrode 214. 
The light beam propagates in an X-axis direction in FIG. 14 in the 
waveguide layer 112. At this time, a distribution of cross-sectional 
intensity of the light beam is as shown in FIG. 14, and the light is 
leaked to the lower light absorption control semiconductor region A. The 
leaked light is absorbed in the light absorption control semiconductor 
region A, and is completely subjected to a desired modulation while the 
light beam is propagating in the waveguide layer 112 of the specified 
length. 
An operation of a light absorbing mechanism in the light absorption control 
semiconductor region A is shown in FIG. 15, but it is specifically 
identical to the contents described with reference to FIG. 2. 
The operation of the semiconductor device 103 shown in FIG. 13 differs from 
that of the semiconductor device shown in FIG. 2 in that, upon the 
application of such a voltage as to hold the electrode 111 at a positive 
potential relative to the electrode 214, the n-GaAs layer 212 and the 
n-AlGaAs layer 113 function as electrode layers and an electric field is 
applied uniformly in a direction perpendicular to the junction surfaces of 
the triple quantum well TQW. Then, effects similar to the ones shown in 
FIGS. 4A and 4B are demonstrated. 
The light absorption and the change in the resonance state in the third 
specific example according to the invention are shown in FIG. 16, but an 
operation thereof is similar to the one described with reference to FIG. 
3. 
FIG. 5 is a graph showing a characteristic of the photoconductivity based 
on the electrons excited by making the light incident upon the light 
absorption control semiconductor region A in the fifth embodiment showing 
the third mode, which characteristic is measured by changing the applied 
voltage and the wavelength of the incident light. Specifically, this graph 
shows a light absorption spectrum in the light absorption control 
semiconductor region A. The bias voltage is changed from 2.5 V to 2.9 V. 
At 2.5 V, a photoconductivity peak P1 appears at 760 nm. At 2.9 V, the 
peak P1 disappears and a photoconductivity peak P2 appears at 765 nm. At 
an intermediate voltage of 2.7 V, both the peaks P1 and P2 are observed 
weakly. In this way, such an excellent characteristic can be observed so 
that the wavelength at the photoconductivity peak, i.e., the wavelength at 
the peak of the light absorption, can be controlled by changing the bias 
voltage by only 0.4 V. When the wavelength at a particular peak is to be 
controlled, the light absorption coefficient can be controlled. 
On the other hand, as will be understood from the distribution of the 
cross-sectional intensity of the propagating light shown in FIG. 14, a 
part of the propagating light leaks to the quantum wells Q3, Q2, Q1 in the 
light absorption control semiconductor region A having this light 
absorption characteristic. Accordingly, taking advantage of this light 
absorption characteristic, the wavelength components shown by the peak in 
FIG. 5 can be removed from the spectrum of the propagating light. The 
spectrum of the light transmitting through this filter region changes 
according to the voltage. This enables the modulation of changing the 
spectrum of the propagating light (filtering, frequency modulation, 
intensity modulation when a certain wavelength is taken note of). 
Particularly by suitably selecting the wavelength of the incident light, 
the semiconductor device is permitted to function as an optical switch 
device for switching between a transmitting state and a state where the 
light is completely absorbed and is not propagating. 
In the case where the light absorption control semiconductor region is used 
as an optical switch device, the light can be modulated into binary states 
of "0" and "1". Further, the FSK modulation is enabled since the 
absorption peak wavelength shifts when the voltage is changed minutely. 
Sixth Embodiment 
In a sixth embodiment showing the third mode of the invention, the 
construction of the light absorption control semiconductor region A is 
modified. FIG. 17 is a diagram showing the structure of energy bands in 
the light absorption control semiconductor region A pertaining to the 
sixth embodiment. In the sixth embodiment, semiconductors formed of 
different materials are used for first, second, and third quantum wells 
Q1, Q2, Q3. the first quantum well Q1 is formed of InGaAs; the second 
quantum well Q2 is formed of AlGaAs; the third quantum well Q3 is formed 
of GaAs; and barriers Vu, V1, V2, Vd are formed of AlGaAs having a large 
Al composition ratio. By regulating the width of the respective quantum 
wells Q1, Q2, Q3, ground state quantized energy levels coincide (match) 
only in a conduction band at the respective quantum wells in the absence 
of an electric field. The quantized energy levels do not coincide in 
valence bands. In this structure, the conduction band at the three quantum 
wells can be brought into a resonance state only where no electric field 
is applied. In the resonance state in the conduction band, the matched 
quantized energy levels are separated into three quantized energy levels 
E1, E2, E3 which differ by a minute energy, thereby releasing the 
degeneracy. 
The action and effects of this structure are identical to the contents 
described with reference to FIG. 6, thus no description will be given 
thereon. 
Seventh Embodiment 
FIG. 18 is a sectional view showing the structure of a light absorption 
control semiconductor device 104 pertaining to a seventh embodiment 
showing another specific example to the third embodiment of the invention. 
In this embodiment, an i-GaAs layer 222 is formed in place of the n-GaAs 
layer 212 in FIG. 13 showing the fifth embodiment, and a .delta. doped 
layer 221 in which silicon atoms are sparsely doped on a surface in the 
thickness of one atom is formed halfway in the layer 222. The .delta. 
doped layer 221 acts as an electrode for applying an electric field 
uniformly to the surfaces of the three quantum wells. In addition, since 
the .delta. doped layer 221 does not disorder the lattice of the lower 
located i-GaAs layer 222a, an i-GaAs layer 222b grown on the layer 222a 
and layers 221, 120 to 113 grown on the layer 222b are allowed to have 
high crystallinity since they succeedingly grow the good crystallinity of 
the i-GaAs layer 222a. 
In the case where the sheet carrier density of the .delta. doped layer 221 
lies in the range of 1.times.10.sup.11 to 3.times.10.sup.13 cm.sup.-2, a 
layer of Si may be doped sparsely. 
It may be appropriate to form a plurality of .delta. doped layers in the 
i-GaAs layer at specified intervals. 
Eighth Embodiment 
An eighth embodiment showing still another specific example of the third 
mode of the invention pertains to a semiconductor device constructed 
similar to the light absorption control semiconductor device 103 in the 
fifth embodiment. In this embodiment, electrodes 311a, 311b, 311c, 311d, 
311e are formed sufficiently long to obtain a desired absorption 
characteristic and are provided as shown in FIG. 19 in place of the 
electrode 111. If a voltage is applied between the five electrodes 311a to 
311e and the electrode 214, the light absorption spectra in a light 
absorption control semiconductor region under the five electrodes differ 
from one another. Accordingly, light propagating in a waveguide layer 112 
is subjected to modulation by a composite spectrum of the light absorption 
spectra in the respective regions, thereby enabling the modulation or 
filtering to be more complicated than those obtained by the spectrum of 
the light. 
Ninth Embodiment 
In a ninth embodiment showing another specific example of the third mode of 
the invention, the waveguide layer 112 in the fifth embodiment is embedded 
in a clad layer 230 formed of Al.sub.2 Ga.sub.1-3 As (where z&gt;y for y in 
Al.sub.y Ga.sub.1-y As forming the waveguide layer 112). A refractive 
index of the waveguide layer 112 is larger than that of the clad layer 
230, and accordingly the light is confined in the waveguide layer 112 
excluding the lower located light absorption control semiconductor region 
A. Although the waveguide layer 112 is formed on the light absorption 
control semiconductor region A in the foregoing embodiment, it may be 
appropriate to form the waveguide layer 112 on the substrate 213 and to 
form the light absorption control semiconductor region A on the layer 112. 
Further, while the waveguide layer 112 is formed of AlGaAs compound 
semiconductor, it may be formed of other compound semiconductor, for 
instance LiNbO.sub.3, or of optical fiber. 
There will next be described a light absorption control semiconductor 
device which is highly efficient in modulating a light spectrum as a 
fourth mode of the invention. Specifically, a first basic feature in the 
construction of the semiconductor device for realizing the fourth mode of 
the invention is a light absorption control semiconductor region capable 
of changing a light absorption spectrum characteristic, and a photovoltaic 
semiconductor layer which is joined with a semiconductor layer on at least 
one end face of the light absorption control semiconductor region through 
energy barriers for separating pairs of electrons and holes to be excited, 
and which applies an electric field perpendicularly to the respective 
layers of the light absorption control semiconductor region by the excited 
electrons and holes. 
The light absorption control semiconductor region includes at least three 
quantum wells formed by a junction of different types of semiconductor 
materials having different band gaps and having quantized energy levels 
enclosed by energy barriers. The width of the respective quantum wells and 
barriers are set such that wave functions of electrons in the respective 
quantum wells interact in a resonance state where the quantized energy 
levels of either one of the conduction and valence bands are matched. In 
addition, the width and material of the respective quantum wells are set 
so that one of the bands is brought into the resonance state where the 
quantized energy levels in the respective quantum wells are matched where 
no electric field is applied or where a suitable electric field is applied 
in a direction perpendicular to the junction. 
The electric field to be applied to the light absorption control 
semiconductor region is controlled by a voltage generated by a control 
beam incident upon the photovoltaic semiconductor layer to thereby 
modulate a carrier beam incident upon the light absorption control 
semiconductor region. 
The second feature in the fourth mode of the invention is that the 
semiconductor device includes at least three quantum wells formed by a 
junction of different types of semiconductor materials having different 
band gaps and having quantized energy levels enclosed by energy barriers 
in an energy diagram, the width of the respective quantum wells and 
barriers are set such that wave functions of electrons in the respective 
quantum wells interact in a resonance state where the quantized energy 
levels of either one of the conduction and valence bands are matched, the 
width and material of the respective quantum wells are set so that one of 
the bands is brought into the resonance state where the quantized energy 
levels in the respective quantum wells are matched where no electric field 
is applied or where a suitable electric field is applied in a direction 
perpendicular to the junction and no control beam exists. 
The control beam is made incident upon the quantum wells to generate pairs 
of electrons and holes. The absorption spectrum is changed by the electric 
field generated by an unbalanced distribution of the electrons and holes, 
thereby modulating the carrier beam present in the quantum wells. 
The third feature in this mode is, in addition to the aforementioned first 
feature, that a waveguide layer is provided which is joined with the 
semiconductor layer on at least one end face of the light absorption 
control semiconductor region or the photovoltaic semiconductor layer. The 
waveguide layer is constructed such that the carrier beam propagating 
therein leaks to the light absorption control semiconductor region, so as 
to modulate the carrier beam with the control beam. 
The fourth feature in this mode is, in addition to the aforementioned 
second feature, that a waveguide layer is provided which is joined with 
the semiconductor layer at one end of the quantum well. The waveguide 
layer is constructed such that the carrier beam propagating therein leaks 
to the quantum well, so as to modulate the carrier beam with the control 
beam. 
A light absorption control semiconductor region pertaining to the first 
feature of the fourth mode of the invention can demonstrate effects 
identical to those described with respect to the first mode of the 
invention. By controlling the electric field acting in the direction 
perpendicular to the junction, the light absorption control semiconductor 
region is allowed to have a light absorption characteristic having a 
certain absorption peak. The electric field is changed by the control beam 
incident upon the photovoltaic semiconductor layer joined with the light 
absorption control semiconductor region. More specifically, the 
photovoltaic semiconductor layer is formed of the same compound 
semiconductor as is the semiconductor layer, for example, at one end of 
the light absorption control semiconductor region, and constitutes PI, NI, 
PIN, PN, NP junctions. Accordingly, upon the incidence of the control 
beam, the electric field generated by the photoelectromotive force is 
applied to the light absorption control semiconductor region, thereby 
generating in the light absorption control semiconductor region the 
photovoltaic electric field or an electric field which is a sum of a bias 
electric field applied from an external power supply and the photovoltaic 
electric field. Thus, the electric field is changed by the control beam 
and the resonance and non-resonance states are changed by this changed 
electric field, with the result being that the absorption spectrum is 
changed. Therefore, the carrier beam incident upon the light absorption 
control semiconductor region has the spectrum thereof modulated with the 
control beam. 
According to the second feature in the fourth mode of the invention, the 
photovoltaic semiconductor layer does not exist unlike the first feature. 
The structure and material of the triple quantum well are designed such 
that only one of the bands is brought into the resonance state where a 
specified electric field (including an electric field of zero) is applied 
when no control beam exists. Upon the incidence of the control beam of a 
specified wavelength in this resonance state, pairs of electrons and holes 
are generated. At this time, carriers (e.g., electrons) which have been 
transited to one band (e.g., conduction band) in the resonance state are 
permitted to tunnel into other quantum wells since the quantized energy 
levels are continuous. However, carriers (e.g., holes) in the other band 
(e.g., valence band) are confined in the one quantum well since the 
quantized energy levels are not continuous. Accordingly, there is 
generated a difference in the distribution of the electrons and holes, 
thereby generating an internal electric field. This internal electric 
field releases the resonance state. In other words, the resonance and 
non-resonance states can be controlled with the control beam. 
When the carrier beam is incident upon the light absorption control 
semiconductor region in a state where the resonance and non-resonance 
states are controlled, it is subjected to the light absorption based on 
the light absorption spectra in the respective states. In other words, the 
spectrum of the carrier beam is modulated with the control beam. The 
meaning of the modulation differs according to the use of the device. For 
example, it is possible to transmit or shield the carrier beam with the 
control beam. 
According to the third feature in the fourth mode of the invention, the 
waveguide layer for guiding the carrier beam is joined with the light 
absorption control semiconductor region of the first feature or the 
photovoltaic semiconductor layer, and the carrier beam propagating in the 
waveguide layer leaks to the light absorption control semiconductor 
region. This leaked carrier beam is modulated with the control beam. All 
the carrier beams are modulated while propagating in the waveguide layer 
for only a specified distance. 
According to the fourth feature in the fourth mode of the invention, the 
waveguide layer for guiding the carrier beam is joined with the 
semiconductor layer at one end of the quantum well, and the carrier beam 
propagating in the waveguide layer leaks to the light absorption control 
semiconductor region. The leaked carrier beam is modulated with the 
control beam. All the carrier beams are modulated while propagating in the 
waveguide layer for only a specified distance. 
According to the first feature in the fourth mode of the invention, the 
control beam is made incident upon the photovoltaic semiconductor layer to 
generate a photoelectromotive force, which is applied to the light 
absorption control semiconductor region of an asymmetric triple quantum 
well. Accordingly, a light-light modulator capable of modulating the 
carrier beam is realized with the control beam. Further, the modulation 
efficiency is high since the resonance and non-resonance states can be 
controlled by a minute electric field. 
According to the second feature in the fourth mode of the invention, only 
one band of the asymmetric triple quantum well is brought into the 
resonance state in the state where no control beam exists. The pairs of 
electrons and holes are generated by the control beam which is incident in 
the resonance state, so that the electrons and holes are distributed 
asymmetrically due to the fact that only the one band is brought into the 
resonance state. This distribution generates the internal electric field, 
which in turn changes the resonance state into the non-resonance state. 
Accordingly, a light-light modulator capable of modulating the carrier 
beam with the control beam is realized. Further, the time required to 
generate the internal electric field is exceedingly short since it is 
determined by a time during which the carriers excited by the band in the 
resonance state move to the other quantum wells through the tunnel effect 
by the control beam, thereby enabling the exceedingly high speed 
modulation of the carrier beam. 
This modulation enables frequency modulation (wavelength modulation), 
intensity modulation, on-off switches, binary modulation of "0" and "1" 
FSK modulation, filtering, and the like. 
Next, the fourth mode of the invention with respect to specific examples 
thereof will be described. 
Tenth Embodiment 
FIG. 21 is a perspective view showing the construction of a semiconductor 
device 105 as a specific example of the fourth mode of the invention. 
On a 450 .mu.m semi-insulating GaAs substrate 413 is formed a 500 nm n-GaAs 
layer 412 by epitaxy using the MBE method. Thereafter, a plurality of unit 
semiconductor layers are formed one after another. Specifically, 
n-Al.sub.x Ga.sub.1-x As (x=0.3), i-Al.sub.x Ga.sub.1-x As, i-GaAs, 
i-Al.sub.x Ga.sub.1-x As, i-GaAs, i-Al.sub.x Ga.sub.1-x As, i-GaAs, 
i-Al.sub.x Ga.sub.1-x As, and p-Al.sub.x Ga.sub.1-x As are grown by 100 
nm, 10 nm, 5 nm, 2 nm, 15 nm, 2 nm, 6 nm, 10 nm, 100 nm to thereby form an 
n-AlGaAs layer (photovoltaic semiconductor layer) 411, an i-AlGaAs layer 
320, an i-Ga As layer 319, an i-AlGaAs layer 318, an i-GaAs layer 317, an 
i-AlGaAs layer 316, an I-GaAs layer 315, an i-AlGaAs layer 314, and a 
p-AlGaAs layer (photovoltaic semiconductor layer) 313 respectively. 
A multitude of layers 320 to 314 form a triple quantum well TQW. The i-GaAs 
layer 315 is a first quantum well Q1; the i-GaAs layer 317 is a second 
quantum well Q2; and the i-GaAs layer 319 is a third quantum well Q3. The 
i-AlGaAs layers 314, 320 are potential barriers Vu, Vd at opposite ends. 
The i-AlGaAs layers 316, 318 are intermediate potential barriers V1, V2 
for separating the respective quantum wells. 
Thereafter, on the p-AlGaAs layer 313 are grown i-Al.sub.y Ga.sub.1-y As 
(y=0.2) by 1 to 3 .mu.m to thereby form an i-AlGaAs layer 312. This 
i-AlGaAs layer 312 functions as a waveguide layer. In this way, an 
epitaxial film structure is completed in which the photovoltaic 
semiconductor layer 411 (n-layer), the light absorption control 
semiconductor region A (layers 314 to 320), the photovoltaic semiconductor 
layer 313 (p-layer), and the waveguide layer 312 are joined one after 
another. The epitaxial film structure forms a pin photovoltaic device if 
the triple quantum well TQW is assumed as an i-layer. 
The control beam is incident upon the waveguide layer 312 perpendicularly, 
and reaches a junction surface C1 between the photovoltaic semiconductor 
layer 313 and the i-AlGaAs layer 314 and a junction surface C2 between the 
i-AlGaAs layer 320 and the photovoltaic semiconductor layer 411. The 
wavelength of this control beam is selected at a value where the light 
absorption occurs under the influence of the direct transition from the 
valence band to the conduction band only on the above junction surfaces. 
In other words, the wavelength of the control beam is set such that it is 
not absorbed in the waveguide layer 312 and the triple quantum well TQW. 
On the other hand, the carrier beam propagates in an x-axis direction in 
the waveguide layer 312 while repeating the multiple reflection in a 
z-axis direction in FIG. 21. At this time, the carrier beam leaks to the 
lower located light absorption control semiconductor region A since the 
thickness of the light absorption control semiconductor region A is 
exceedingly smaller than that of the waveguide layer 312. A distribution 
of cross-sectional intensity of the carrier beam is as shown in FIG. 21. 
This leaked carrier beam is absorbed in the light absorption control 
semiconductor region A, and all the carrier beams are subjected to a 
desired modulation while propagating in the waveguide layer 312 of a 
specified length in the X-axis direction. 
There will be next described a light absorbing mechanism in the light 
absorption control semiconductor region A. 
FIG. 22 is a diagram showing energy bands in a state where no electric 
field is applied to the triple quantum well (the band is inclined because 
of the pin structure), and FIG. 24 is a diagram showing energy bands in an 
entire device including the photovoltaic semiconductor layers 313, 411. 
An operation of the light absorbing mechanism shown in FIG. 22 is similar 
to the one described with reference to FIG. 2 showing the first mode, and 
accordingly no description will be given thereof. 
Since the pin structure is adopted in this specific example, the conduction 
and valence bands are inclined in the light absorption control 
semiconductor region A corresponding to the i-layer as illustrated. 
However, as shown in FIG. 25, when the control beam is incident 
perpendicularly upon the junction surfaces C1 and C2 through the waveguide 
layer 312, an electric field is generated in such a direction as to hold 
the p-AlGaAs layer 313 at a positive potential relative to the n-AlGaAs 
layer 411 by the distribution of electrons and holes excited on the pi 
junction surface C1 between the p-AlGaAs layer (p-type photovoltaic layer) 
313 and the i-AlGaAs layer (and barrier layer vu of the light absorption 
control semiconductor region) 314 and the ni junction surface C2 between 
the n-AlGaAs layer (n-type photovoltaic layer) 411 and the i-AlGaAs layer 
(and barrier layer of the light absorption control semiconductor region) 
320. This electric field is applied to the light absorption control 
semiconductor region A making the inclination of the conduction and 
valence bands gentle. As a result, the quantized energy levels in the 
conduction band become continuous thereby bringing the conduction band 
into the resonance state. 
By changing the electric field acting perpendicular to the junction surface 
of the triple quantum well TQW, the action and effects similar to those 
shown in FIGS. 4A and 4B are obtainable. In this specific example, the 
wavelength of the carrier beam is set such that the electrons are 
permitted to transit between the quantized energy levels in the valence 
band and the quantized energy levels in the conduction band in this 
resonance state. The light absorption of the carrier beam occurs under the 
influence of direct transitions TE1, TE2, TE3 between a ground state 
energy level Lv1.sub.0 in the valence band at the first quantum well Q1 
and quantized energy levels E1, E2, E3 commonly spread in the conduction 
band at the three quantum wells as shown in FIG. 23. Accordingly, the 
electrons are permitted to transit from the valence band to the quantized 
energy levels at any quantum well in the conduction band by the light 
absorption. This state is referred to as a resonance state. In the 
resonance state, the light absorption coefficient becomes exceedingly 
large. 
According to the observation of the inventors, the light absorption 
coefficient is improved about 100 times compared to the semiconductor 
device including two quantum wells formed of the same material and having 
the same width. 
To be more precise, it is difficult to observe three absorption spectra 
simultaneously since the resonance conditions of the three quantized 
energy levels E1, E2, E3 differ slightly, and the wavelengths at the 
absorption peaks of the absorption spectra change according to the 
intensity of the electric field. In other words, the electron transition 
which is the major controlling factor in the light absorption can be 
selected to be a desired one of the transitions TE1, TE2, TE3 by changing 
the intensity of the applied electric field slightly. 
FIG. 5 is a graph showing a characteristic of the photoconductivity based 
on the electrons excited by making the light incident upon the light 
absorption control semiconductor region A in this specific example, which 
characteristic is measured by changing the applied voltage and the 
wavelength of the incident light. This graph shows the light absorption 
spectrum in the light absorption control semiconductor region A. The 
voltage applied to the opposite ends of the light absorption control 
semiconductor region A is changed from 2.5 V to 2.9 V. At 2.5 V, a 
photoconductivity peak P1 appears at 760 nm. At 2.9 V, the peak P1 
disappears and a photoconductivity peak P2 appears at 765 nm. At an 
intermediate voltage of 2.7 V, both the peaks P1 and P2 are observed 
weakly. In this way, an excellent characteristic was observed in that the 
wavelength at the photoconductivity peak, i.e., the wavelength at the 
light absorption peak, could be controlled by changing the voltage by 0.4 
V. When the wavelength at a particular peak is to be controlled, the light 
absorption coefficient can be controlled. 
As will be understood from the distribution of cross-sectional intensity of 
the carrier beam shown in FIG. 21, a part of a laser beam leaks to the 
quantum wells Q3, Q2, Q1 in the light absorption control semiconductor 
region A having this light absorption characteristic. Accordingly, taking 
advantage of this light absorption characteristic, the wavelength 
components shown by the peak in FIG. 5 can be removed from the spectrum of 
the carrier beam. The spectrum of the carrier beam changes according to 
the electric field of the light absorption control semiconductor region A, 
i.e., the intensity of the control beam. This enables the modulation of 
changing the spectrum of the carrier beam (filtering,, frequency 
modulation, wavelength modulation, intensity modulation when a certain 
wavelength is taken note of). Particularly by suitably selecting the 
wavelength of the carrier beam, the semiconductor region is permitted to 
function as an optical switch device for switching between a transmitting 
state and a state where the beam is not propagating while being completely 
absorbed. 
In the case where the light absorption control semiconductor region is used 
as an optical switch device, the beam can be modulated into binary states 
of "0" and "1". Further, the FSK modulation is enabled since the 
absorption peak Wavelength shifts when the voltage is changed minutely. 
Although a bias voltage is not applied in this embodiment, a suitable bias 
voltage may be applied between the p-AlGaAs layer 313 and the n-AlGaAs 
layer 411. The intensity and polarity of the bias voltage differ depending 
upon the thickness of the respective layers of the light absorption 
control semiconductor region A and whether the resonance or non-resonance 
state is set when the bias voltage is applied. If the construction is such 
that the resonance state is set when the bias voltage is applied, the 
resonance state is changed to the non-resonance state upon the incidence 
of the control beam. 0n the contrary, if the non-resonance state is set 
close to the resonance state when the bias voltage is applied, the 
non-resonance state is changed to the resonance state upon the incidence 
of the control beam. 
Eleventh Embodiment 
This embodiment is another specific example of the fourth mode of the 
invention. In this example, the construction of the light absorption 
control semiconductor region A is modified. FIG. 26 is a diagram showing 
the structure of energy bands in the light absorption control 
semiconductor region A pertaining to the eleventh embodiment. In this 
embodiment, semiconductors formed of different materials are used for 
first, second, and third quantum wells Q1, Q2, Q3. The first quantum well 
Q1 is formed of GaAs; the second quantum well Q2 is formed of AlGaAs; the 
third quantum well Q3 is formed of InGaAs; and barriers Vu, V1, V2, Vd are 
formed of AlGaAs having a large Al composition ratio. By regulating the 
width of the respective quantum wells Q1, Q2, Q3, grounds state energy 
levels coincide (match) only in a conduction band at the respective 
quantum wells in the absence of an electric field (the bands are inclined 
because of the PIN structure). The quantized energy levels do not coincide 
in a valence band. In this structure, only the conduction band at the 
three quantum wells can be brought into a resonance state in the state 
where no electric field generated by the photoelectromotive force exists. 
In the resonance state in the conduction band, the matched quantized 
energy levels are separated into three quantized energy levels E1, E2, E3 
which differ by a minute energy, thereby releasing the degeneracy. In this 
structure, the direct transitions of electrons by the light absorption 
include: transitions T1.sub.1, T1.sub.2, T1.sub.3 between a ground state 
energy level Lv1.sub.0 in the valence band of the first quantum well Q1 
and three quantized energy levels E1, E2, E3 commonly spread in the 
conduction band at the respective quantum wells; transitions T2.sub.1, 
T2.sub.2, T2.sub.3 between a ground state energy level Lv2.sub.0 in the 
valence band at the second quantum well Q2 and the three quantized energy 
levels E1, E2 E3 commonly spread in the conduction band at the respective 
quantum wells; and transitions T3.sub.1, T3.sub.2, T3.sub.3 between a 
ground state energy level Lv3.sub.0 in the valence band at the third 
quantum well Q3 and the three quantized energy levels E1, E2, E3 commonly 
spread in the conduction band at the respective quantum wells. 
Accordingly, the light absorption occurs at three wavelengths corresponding 
to the transitions T1, T2, T3 from the different quantized energy levels 
in the valence band at the three quantum wells Q1, Q2, Q3. Thus, the peak 
wavelength and the absorption coefficient at the peak wavelength in the 
absorption characteristic having a single light absorption peak at a 
different wavelength for each quantum well can be changed according to the 
intensity of the control beam. In other words, this device is allowed to 
have three absorption lines simultaneously in a single structure and to 
switch electrically the three absorption lines which differ slightly from 
one another. 
This structure can be used in quantum dots or quantum wires. In these 
structures, the light absorption coefficient is improved. Furthermore, by 
choosing appropriate sizes of well thickness, wire width, and dot 
diameter, the resonant condition can be achieved with no bias voltage. In 
this structure, signal light can be modulated by a modulation light beam 
and thus the bias voltage electrode is not necessary. 
Twelfth Embodiment 
FIG. 27 is a sectional view showing the structure of a light absorption 
control semiconductor device 106 pertaining to a twelfth embodiment which 
is still another example of the fourth mode of the invention. In this 
embodiment, the photovoltaic semiconductor layers 313, 411 in the tenth 
embodiment are not formed. On a semi-insulating substrate 520 is formed a 
500 nm i-GaAs layer 521. On this layer 521 is formed a 100 nm n-AlGaAs 
layer 523 as an electrode layer. On the layer 523 is formed an asymmetric 
triple quantum well TQW structured similar to the one in the tenth 
embodiment. On the triple quantum well TQW is formed a 20 nm n-AlGaAs 
layer 524 as an electrode layer. On the layer 524 is formed an i-AlGaAs 
layer 512 (waveguide layer). 
A .delta. doped layer 522 in which silicon atoms are sparsely doped on a 
surface in the thickness of one atom is formed halfway in the i-GaAs layer 
521. The .delta. doped layer 522 acts as an electrode for applying an 
electric field uniformly to the surfaces of the three quantum wells. In 
addition, since the .delta. doped layer 522 does not disorder the lattice 
of the lower located i-GaAs layer 521a, an i-GaAs layer 521b grown on the 
layer 521a and layers 523, 520 to 514 grown on the layer 521b are allowed 
to have the high crystallinity since they succeedingly grow the good 
crystallinity of the i-GaAs layer 521a. 
In the case where the sheet carrier density of the .delta. doped layer 221 
lies in the range 1.times.10.sup.11 to 3.times.10.sup.13 cm.sup.-2 about a 
layer of Si may be doped sparsely. 
It may be appropriate to form a plurality of .delta. doped layers in the 
i-GaAs layer 521 at specified intervals. Then, the waveguide layer 512 is 
etched to expose a part of the n-AlGaAs layer 524 in the form of a strip, 
so that the width of the waveguide layer 512 is narrower than that of the 
triple quantum well TQW. Further, the layers 514 to 521 are etched in the 
form of a strip so as to expose the i-GaAs layer 521. An Au layer and a 
double layer structure of AuGe alloy and Au (50 nm, 200 nm respectively) 
are formed on the exposed n-AlGaAs layer 524 and the exposed i-GaAs layer 
521 by vacuum deposition. The alloying process is applied to the thus 
formed layers in a hydrogen and nitrogen atmosphere at a temperature of 
400 degrees for 2 minutes to thereby obtain first and second electrodes 
511, 525 having an ohmic contact. 
The carrier beam leaks to the triple quantum well TQW similarly as in the 
tenth embodiment. An advancing direction of the carrier beam and an 
incident direction of the control beam are the same as in the tenth 
embodiment. Further in this embodiment, a bias voltage is applied between 
the electrodes 511 and 525. Only the conduction band is in the resonance 
state as shown in FIG. 28 in a state where the voltage is applied. 
In this embodiment, the wavelength of the control beam is selected to be a 
value corresponding to an energy difference between the ground state level 
Lv2.sub.0 in the valence band at the second quantum well Q2 and the 
quantized energy level E2 in the conduction band. Only the conduction band 
is in the resonance state while no control beam is incident, thereby 
increasing the light absorption by the absorption characteristic of a 
specified spectrum. Accordingly, the carrier beam is modulated greatly. 
The carrier beam is shielded in the case of the modulation in the binary 
states of "0" and "1". 
On the other hand, in the state where the control beam is incident upon the 
triple quantum well TQW, pairs of electrons and holes are generated by the 
control beam in the second quantum well as shown in FIG. 29. At this time, 
the holes cannot move to other wells since the valance band is not in the 
resonance state and accordingly localize in the second quantum well Q2. On 
the contrary, the electrons are permitted to move to the other wells since 
the conduction band is in the resonance state. In other words, the 
electrons drift under the influence of the bias voltage. Since the holes 
are distributed only in the second quantum well Q2, an internal electric 
field is generated in the triple quantum well TQW. Thereby, the conduction 
band is brought out of the resonance state and the carrier beam is no 
longer absorbed. In other words, the carrier beam is brought into a 
transmitting state. In this way, the carrier beam can be modulated 
according to the intensity of the control beam. 
In this embodiment, the responsiveness determined only by a moving velocity 
due to a tunnel effect of the electrons excited by the control beam and is 
not influenced by an electrostatic capacity unlike the existing 
transistors, thereby enabling an exceedingly high speed modulation. 
Further, the wavelength of the control beam is selected to a value 
corresponding to the one between Lv2.sub.o and E1, E2, between Lc2.sub.o 
and E1, E2, E3, or between Lv2.sub.o and Lc2.sub.o. 
Thirteenth Embodiment 
This embodiment pertains to an optical integrated circuit (IC) 107 
constructed by integrating the devices of the tenth, eleventh, or twelfth 
embodiment showing the fourth mode of the invention. 
A waveguide layer 612 is formed as shown in FIG. 30. Portions L1, L2, L3, 
L4 of the waveguide layer 612 having a specified length are device 
portions corresponding to the devices of the foregoing embodiments. The 
carrier beam is modulated in the respective portions by projecting control 
beams onto the respective portions. Each portion functions as a gate to 
the control beam, thus this IC functions as an optical logic IC having 
functions of an equivalent circuit shown in FIG. 31. 
In this way, this embodiment demonstrates the effects of modulating the 
carrier beam directly with the control beam. 
There will next be described a fifth mode of the invention. 
The fifth mode of the invention pertains to realization of a high 
performance semiconductor laser utilizing the aforementioned light 
absorption control semiconductor according to the invention. 
Specifically, this mode pertains to a semiconductor laser in which an 
oscillating wavelength is made variable by controlling electrically a 
light absorption coefficient using quantum wells. 
Conventionally, a method of making the oscillating wavelength of the 
semiconductor laser variable is known, according to which a refractive 
index in a resonator is changed by injecting a current into the resonator 
to thereby change an optical resonant period thereof. 
However, the current injection method suffers from the defect of a large 
current consumption. 
An object of this mode of the present invention is to provide a variable 
wavelength semiconductor laser taking advantage of a light absorption 
characteristic based on quite a novel power-saving principle. 
In order to solve the above problem, a semiconductor laser according to the 
fifth mode of the invention includes a light absorption control 
semiconductor region joined therewith and is formed such that a partial 
cross-section of a laser beam normal to an optical path thereof leaks and 
exists in the light absorption control semiconductor region. 
The light absorption control semiconductor region includes at least three 
quantum wells formed by a junction of different types of semiconductor 
materials having different band gaps and having quantized energy levels 
enclosed by energy barriers. The width of the respective quantum wells and 
barriers is set such that wave functions of electrons in the respective 
quantum wells interact in a resonance state where the quantized energy 
levels of either one of conduction and valence bands are matched. In 
addition, the width and material of the respective quantum wells are set 
so that one of the bands is brought into the resonance state where the 
quantized energy levels at the respective quantum wells are matched where 
no electric field is applied or where a suitable electric field is applied 
in a direction perpendicular to the junction. The light absorption is 
changed by controlling components of the electric field acting in the 
direction perpendicular to the junction. 
The constructed semiconductor laser oscillates under the influence of the 
induced radiation resulting from the multiple reflection of a laser beam 
between opposite end faces. A partial cross-section of this laser beam 
normal to an optical path thereof leaks and exists in the light absorption 
control semiconductor region joined with the semiconductor laser. This 
light absorption control semiconductor region includes at least three 
quantum wells as described above. 
The light absorption control semiconductor region used in the fifth mode of 
the invention includes the aforementioned light absorption control 
semiconductor. The action thereof is substantially identical to that of 
the light absorption control semiconductor used in the first to fourth 
modes of the invention, and accordingly no description will be given 
thereof. 
In this mode as well, the light absorption control semiconductor is allowed 
to have a light absorption characteristic including a certain absorption 
peak by controlling the electric field acting in the direction 
perpendicular to the junction. Accordingly, the laser beam is subjected to 
the light absorption by the light absorption spectrum characteristic in 
the light absorption control semiconductor region while reflecting a 
multitude of times between resonators, with the result being that the 
spectrum of an oscillating wavelength changes. In other words, the 
wavelength of the laser beam can be made variable. 
The fifth mode of the invention pertains to a semiconductor device 
including at least three quantum wells whose width is determined together 
with the width of barriers such that the wave functions of electrons in 
the respective quantum wells interact in a resonance state, and a light 
absorption control semiconductor region for generating a resonance state 
where quantized energy levels at the respective quantum wells are 
continuous and a non-resonance state only in one band by controlling 
components of an electric field acting in a direction perpendicular to a 
junction so as to control the light absorption, the light absorption 
control semiconductor region being joined with a semiconductor laser so 
that a partial cross-section of a laser beam leaks into the light 
absorption control semiconductor region. carolyn The light absorption 
spectrum changes greatly in the light absorption control semiconductor 
region according to a change in the electric field, thus the wavelength of 
the laser beam can be made variable by controlling the electric field. 
Since almost no current flows in the quantum wells, a power loss for 
making the wavelength variable is small. 
Hereafter, specific examples of the fifth mode of the invention will be 
described with reference to the drawings. 
Fourteenth Embodiment 
FIG. 32 is a perspective view showing the construction of a semiconductor 
device 108 as an embodiment in the fifth mode of the invention. 
On a 100 .mu.m p-GaAs substrate 814 are formed a 1 .mu.m p-Al.sub.y 
Ga.sub.1-y As layer 813, a 0.2 .mu.m i-Al.sub.x Ga.sub.1-x As layer 812 
(x.noteq.y), a 100 .mu.m n-Al.sub.y Ga.sub.1-y As layer 811, and a 20 
.mu.m n-GaAs layer (an uppermost semiconductor layer of a semiconductor 
laser) 810 one after another. The layers 811 to 813 constitute a 
semiconductor laser B of a double heterojunction structure, the layer 812 
is an active layer; and the layers 813, 811 are clad layers. The layer 810 
is an electrode layer. The clad layer 811 is made exceedingly thin to 
lessen a confinement effect of the laser beam by the clad layer 811. 
Thereafter, respective layers of the light absorption control semiconductor 
region A are accumulated one after another on the uppermost semiconductor 
layer 810 of the semiconductor laser B as follows. On the uppermost 
semiconductor layer 810 are grown i-Al.sub.2 Ga.sub.1-z As (z=0.3), 
i-GaAs, i-Al.sub.2 Ga.sub.1-z AS, i-GaAs, i-Al.sub.z Ga.sub.1-z As, 
i-GaAs, and i-Al.sub.z Ga.sub.1-z As by 10 nm, 6 nm, 2 nm, 15 nm, 2 nm, 5 
nm, 100 nm to thereby form an i-AlGaAs layer 719, an i-GaAs layer 718, an 
i-AlGaAs layer 717, an i-GaAs layer 716, an i-AlGaAs layer 715, an i-GaAs 
layer 714, and an i-AlGaAs layer 713 which are unit semiconductor layers 
respectively. 
The layers 719 to 712 form a triple quantum well TQW. The i-GaAs layer 714 
is a first quantum well Q1; the i-GaAs layer 716 is a second quantum well 
Q2; and the i-GaAs layer 718 is a third quantum well Q3. The i-AlGaAs 
layers 713, 719 are potential barriers Vu, Vd at opposite ends. The 
i-AlGaAs layers 715, 717 are intermediate potential barriers V1, V2 for 
separating the respective quantum wells. 
Thereafter, on the i-AlGaAs layer 713 are grown n-GaAs to 20 .mu.m to 
thereby form an n-GaAs layer 712. In this way, an epitaxial film structure 
is completed. A portion of the layers 712 to 719 of the light absorption 
control semiconductor region A corresponding to an electrode forming 
portion of the semiconductor laser B is removed by means of etching to 
expose a part of the n-GaAs layer 810. A double layer structure of AuGe 
alloy and Au (50 nm, 200 nm respectively) and an Au layer are formed on 
the exposed region of the layer 810 and the n-GaAs layer 712 by vacuum 
deposition. The alloying process is applied to the thus formed layers in a 
hydrogen and nitrogen atmosphere at a temperature of 400 degrees for 2 
minutes to thereby obtain electrodes 816, 711 having an ohmic contact. 
Further, on a rear surface of the p-GaAs substrate 814 is formed an 
electrode 815 made of AuZn alloy. 
The width and length of the electrode 816 are 10 .mu.m and 200 .mu.m, 
respectively. In the semiconductor layer B, a voltage is applied so that 
the electrode 815 is held at a positive potential relative to the 
electrode 816. The n-GaAs layer 810 functions as a common electrode for 
the semiconductor laser B and the light absorption control semiconductor 
region A. Upon the application of this voltage, the electrons are injected 
from the clad layer 811 to the active layer 812 while the holes are 
injected from the clad layer 813 to the active layer 812. As a result, a 
laser beam is radiated from the active layer 812. A distribution of 
cross-sectional intensity of this laser beam is as shown in FIG. 33. It is 
found from FIG. 33 that no beam leakage exists on the side of the p-GaAs 
substrate 814, but a partial cross-section of the laser beam leaks greatly 
to the triple quantum well TQW on the side of the light absorption control 
semiconductor region A since the clad layer 811 is thin. 
FIG. 34 is a diagram showing energy bands of a light absorbing mechanism in 
the light absorption control semiconductor region A in this embodiment in 
a state where no electric field is applied to the triple quantum well. 
In each of the conduction and valence bands, there are formed potential 
barriers Vu, V1, V2, Vd including AlGaAs layers 713,715, 717, 719, and 
first, second, and third quantum wells Q1, Q2, Q3 including GaAs layers 
714, 716, 718. 
The light absorbing mechanism of the light absorption control semiconductor 
structure in FIG. 34 is identical to that of the light absorption control 
semiconductor used in the foregoing first to fourth modes of the 
invention, and accordingly no description will be given thereof. 
When a voltage is applied so that the electrode 711 is held at the positive 
potential relative to the electrode 816, the n-GaAs layer 810 functions as 
an electrode layer, with the result being that the electric field is 
applied uniformly and perpendicularly to the junction surfaces of the 
triple quantum well TQW. Then, there occurs the phenomenon as shown in 
FIGS. 4A and 4B. Under these circumstances, the light absorption occurs 
under the influence of direct transitions TE1, TE2, TE3 between a ground 
state energy level Lv1.sub.0 in the valence band at the first quantum well 
Q1 and quantized energy levels E1, E2, E3 commonly spread in the 
conduction band at three quantum wells as shown in FIG. 16. Accordingly, 
the electrons are permitted to transit from the valence band to the 
quantized energy level at any quantum well in the conduction band by the 
light absorption. This state is referred to as a resonance state. In the 
resonance state, the light absorption coefficient becomes exceedingly 
large. 
FIG. 5 is a graph showing a characteristic suited to a case wherein the 
photoconductivity based on the electrons excited by the beam made incident 
upon the light absorption control semiconductor region A of this 
embodiment is measured by changing the applied voltage and the wavelength 
of the incident beam. 
As will be understood from the distribution of cross-sectional intensity of 
the laser beam shown in FIG. 33, a part of the laser beam leaks to the 
quantum wells Q1, Q2, Q3 of the light absorption control semiconductor 
region A having this light absorption characteristic. The spectrum of this 
laser beam is as shown in FIG. 35. Taking advantage of this light 
absorption characteristic, the wavelength components shown by the peak in 
FIG. 5 can be removed from the spectrum of the carrier beam. Thereby, the 
spectrum of the laser beam changes according to the voltage. Thus, the 
wavelength of the laser beam can be changed according to the voltage. 
Although the triple quantum well is arranged in the order Q3 (6 nm), V2 Q2 
(15 nm), V1 Q1 (5 nm) from the closest to the laser region in this 
example, this order may be reversed to thereby invert the polarity of the 
bias voltage. In this case, since the layer for absorbing the beam is 
closer to the laser waveguide layer, a stronger absorption can be 
obtained, providing a more desirable construction. 
The semiconductor laser B may adopt an index waveguide type structure 
instead of the gain waveguide type structure as in this embodiment. In the 
case where the laser beam is mode-locked at a signal spectrum by an 
increased current injection caused by the index waveguide type structure, 
a frequency to be mode-locked can be changed by the light absorption 
characteristic of the light absorption control semiconductor region A. 
Fifteenth Embodiment 
This embodiment pertains to a modification of the construction of the light 
absorption control semiconductor region A of the fourteenth embodiment. 
FIG. 36 is a diagram showing energy bands in a light absorption control 
semiconductor region A of the fifteenth embodiment. In this embodiment, 
semiconductors formed of different materials are used for the first, 
second and third quantum wells Q1, Q2, Q3. The first quantum well Q1 is 
formed of InGaAs; the second quantum well Q2 is formed of AlGaAs; the 
third quantum well Q3 is formed of GaAs; and barriers Vu, V1, V2, Vd are 
formed of AlGaAs having a large Al composition ratio. By regulating the 
width of the respective quantum wells Q1, Q2, Q3, ground state energy 
levels coincide (match) only in conduction band at the respective quantum 
wells in the absence of an electric field. The quantized energy levels do 
not coincide in valence bands. In this structure, only the conduction band 
at the three quantum wells can be brought into a resonance state in the 
state where no electric field is applied. In the resonance state in the 
conduction band, the matched quantized energy levels are separated into 
three quantized energy levels E1, E2, E3 which differ by a minute energy 
width, thereby releasing the degeneracy. 
This device is allowed to have three absorption lines simultaneously in a 
single structure and to switch electrically the three absorption lines 
which differ slightly from one another. Thus, the shape of an envelope of 
the spectrum of the laser beam and a set of frequencies of the spectrum to 
be mode-locked can be changed delicately. 
Sixteenth Embodiment 
FIG. 37 is a sectional view showing the structure of a semiconductor laser 
109 pertaining to a sixteenth embodiment. In this embodiment, an i-GaAs 
layer 820 is formed in place of the n-GaAs layer 810 which is uppermost 
semiconductor layer in the fifteenth embodiment. Halfway in the layer 820 
are formed .delta. doped layers 830, 831 obtained by doping silicon atoms 
sparsely on the surface in the thickness of one atom. The .delta. doped 
layers 830, 831 function as electrodes for applying an electric field 
uniformly onto the surfaces of the three quantum wells. In addition, since 
the .delta. doped layers 830, 831 do not disorder the lattice of the lower 
located i-GaAs layer 820, layers 719, 712 grown on the layer 820 are 
allowed to have the high crystallinity since they succeedingly grow the 
good crystallinity of the i-GaAs layer 820. 
In the case where the sheet carrier density of the .delta. doped layers 
830, 831 lies in the range of 1.times.10.sup.11 to 3.times.10.sup.13 
cm.sup.-2, a layer of Si may be doped sparsely. 
Although the light absorption control semiconductor region is formed on the 
semiconductor laser in the embodiment, it may be appropriate to form the 
light absorption control semiconductor region on the substrate and to grow 
the respective layers of the semiconductor laser thereon. In this case, it 
may be suitable to grow i-GaAs on the substrate, to form the .delta. doped 
layer, to grow i-GaAs, and to form the light absorption control 
semiconductor region on these layers. 
There will next be described a sixth mode of the invention. The sixth mode 
of the invention pertains to a realization of a high performance 
semiconductor light emitting diode utilizing the aforementioned light 
absorption control semiconductor according to the invention. 
Specifically, this mode pertains to a semiconductor light emitting diode 
capable of changing a visible color of a light emitted therefrom by 
controlling electrically a light absorption spectrum using quantum wells. 
Conventionally, there is a method using a resin filter mixed with pigments 
as a method of changing a luminescent color of the light emitting diode. 
There is also a method of combining a plurality of light emitting diodes 
having different luminescent colors. 
However, with the former method, a light whose color is different from the 
luminescent color of a luminous source can be obtained, but the 
luminescent color cannot be changed. The latter method is disadvantageous 
in integration since respective luminous source chips are integrated. This 
method changes the visible color of the mixed light by changing the 
luminous intensity of the respective light emitting diode chips, and 
accordingly is only capable of changing the luminescent color to a limited 
extent. Since the spectrum of the light emitted from the light emitting 
diode is normally broad, the peak wavelength of the light can be changed 
by changing the absorption spectrum. An object of the sixth mode of the 
invention is to realize a variable wavelength semiconductor light emitting 
diode capable of easily changing an emission spectrum by a voltage taking 
advantage of the light absorption characteristic based on a quite novel 
power-saving principle. 
In order to solve the above problems, a light emitting diode according to 
the invention for radiating a light from a junction surface of 
semiconductor layers includes a light emitting diode region having a 
junction surface of the semiconductor layers for radiating the light and a 
light absorption control semiconductor region upon which the light emitted 
in the light emitting diode region is incident. The light absorption 
control semiconductor region includes at least three quantum wells formed 
by a junction of different types of semiconductor materials having 
different band gaps and having quantized energy levels enclosed by energy 
barriers in an energy diagram. The width of the respective quantum wells 
and barriers is set such that wave functions of electrons in the 
respective quantum wells interact in a resonance state where the quantized 
energy levels of either one of conduction and valence bands are matched at 
the respective quantum wells. In addition, the width and material of the 
respective quantum wells are set so that one of the bands is brought into 
the resonance state where the quantized energy levels at the respective 
quantum wells are matched in a state where no electric filed is applied or 
a state where a suitable electric field is applied in a direction 
perpendicular to the junction. The light absorption is changed by 
controlling components of the electric field acting in the direction 
perpendicular to the junction. 
In the light emitting diode of this mode, the light emitting diode region 
and the light absorption control semiconductor region are joined with each 
other. The light emitted from the light emitting diode region is incident 
upon the light absorption control semiconductor region where the light has 
a spectrum thereof modulated. The light absorption control semiconductor 
region is capable of changing the absorption spectrum by the electric 
field. Thereby, the peak wavelength of the light transmitted through the 
light absorption control semiconductor region is controllably changed by 
the voltage. This light absorption control semiconductor region includes 
at least three quantum wells as described above. 
The light absorption control semiconductor region used in the sixth mode of 
the invention includes the aforementioned light absorption control 
semiconductor. The action thereof is substantially identical to that of 
the light absorption control semiconductor region used in the first to 
fifth modes of the invention, and accordingly no description will be given 
thereof. 
In this mode as well, the light absorption control semiconductor is allowed 
to have a light absorption characteristic having a certain absorption peak 
by controlling the electric field acting in the direction perpendicular to 
the junction. Thus, the light emitted from the light emitting diode region 
has the spectrum thereof modulated while transmitting through the light 
absorption control semiconductor region, with the result being that the 
peak wavelength of the light transmitted through the light absorption 
control semiconductor region is controllably changed by the voltage. 
The sixth mode of the invention pertains to a light emitting diode that 
includes a light absorption control semiconductor region consisting 
essentially of at least three quantum wells whose width is determined 
together with the widths of barriers such that wave functions of electrons 
in the respective quantum wells interact in a resonance state, and 
generating the resonance state where quantized energy levels at the 
respective quantum wells are continuous and a non-resonance state in only 
one of the bands by controlling components of an electric field acting in 
a direction perpendicular to a junction to control the light absorption, 
in conjunction with a light emitting diode region, a light emitted from 
the light emitting diode region being caused to transmit through the light 
absorption control semiconductor region. 
Thus, the light absorption spectrum is changed greatly according to the 
electric field in the light absorption control semiconductor region, and 
the spectrum of the light transmitting through the light absorption 
control semiconductor region, i.e., the peak wavelength of the light, can 
be made variable by controlling the electric field. Since a small amount 
of current flows through the quantum wells, a power loss for making the 
wavelength variable is small. 
Hereafter, specific examples of the sixth mode of the invention will be 
described with reference to the drawings. 
Seventeenth Embodiment 
FIG. 38 is a sectional view showing the construction of a semiconductor 
device pertaining to this embodiment. 
On a 450 .mu.m semi-insulating GaAs substrate 1030 is formed a 500 nm 
i-GaAs layer 1018 through the epitaxy method using the MBE method. On the 
layer 1018 is formed a .delta. doped layer 1017 obtained by doping silicon 
atoms sparsely on the surface in the thickness of one atom and having a 
surface density of 1.times.10.sup.12 cm.sup.-2. On the layer 1017 is 
formed a 100 nm i-GaAs layer 1016. The layers 1018, 1017 and 1016 function 
as electrode layers. The .delta. doped layer 1017 functions to apply an 
electric field uniformly to a light emitting diode region B. Since the 
layer 1016 is exceedingly thin, a current flows perpendicularly through 
the layer 1016. 
Thereafter, p-Al.sub.y Ga.sub.1-y As (y=0.15) and n-Al.sub.y Ga.sub.1-y As 
are grown to 1000 nm, 1000 nm on the layer 1016 to thereby form a p-AlGaAs 
layer 1015 and an n-AlGaAs layer 1014. The layers 1015, 1016 form a light 
emitting diode by a pn junction. When the current is injected, the light 
is radiated from this pn junction surface. On the layer 1014 is formed a 
100 nm i-GaAs layer 1013. On the layer 1013 is formed a .delta. doped 
layer 1012 obtained by doping silicon atoms sparsely on the surface at a 
thickness of one atom and having a surface density of 1.times.10.sup.12 
cm.sup.-2. On the layer 1012 is formed a 100 nm i-GaAs layer 1011. The 
layers 1013, 1012, 1011 function as electrode layers. The .delta. doped 
layer 1012 functions to apply an electric field uniformly to the light 
emitting diode region B and a light absorption control semiconductor 
region A. Since the layers 1013, 1011 are exceedingly thin, a current 
flows perpendicularly through the layer 1016. 
In this way, the layers 1011 to 1018 form the light emitting diode region 
B. 
Then, the light absorption control semiconductor region A is formed as 
follows. 
On the i-GaAs layer 1011 are grown i-Al.sub.x Ga.sub.1-x As, (x=0.3), 
i-GaAs, i-Al.sub.x Ga.sub.1-x As, i-GaAs, i-Al.sub.x Ga.sub.1-x As, 
i-GaAs, i-Al.sub.x Ga.sub.1-x As to 100 nm, 6 nm, 2 nm, 15 nm, 2 nm, 5 nm, 
100 nm to thereby form an i-AlGaAs layer 920, and i-GaAs layer 919, an 
i-AlGaAs layer 918, and i-GaAs layer 917, an i-AlGaAs layer 916, an i-GaAs 
layer 915, and in i-AlGaAs layer 914. 
A multitude of layers 920 to 914 form a triple quantum well TQW. The i-GaAs 
layer 915 is a first quantum well Q1; the i-GaAs layer 917 is a second 
quantum well Q2; and the i-GaAs layer 919 is a third quantum well Q3. The 
i-AlGaAs layers 914, 920 are potential barriers Vu, Vd at opposite ends. 
The i-AlGaAs layers 916, 918 are intermediate potential barriers V1, V2 
for separating the respective quantum wells. 
Then, on the i-AlGaAs layer 914 is grown i-GaAs to a thickness of 10 nm to 
form an i-GaAs layer 913. On the layer 913 is grown n-GaAs to a thickness 
of 100 nm to form an n-GaAs layer 912. The layers 913,912 function as 
electrode layers for the light absorption control semiconductor region A. 
In this way, an epitaxial film structure is completed. 
Thereafter, the layers 912 to 920 form the light absorption control 
semiconductor region A corresponding to a common electrode between the 
light emitting diode region B and the light absorption control 
semiconductor region A. The layers 912 to 920 of the light absorption 
control semiconductor region A corresponding to an electrode forming 
portion for the light emitting diode region B, and the layers 1011 to 1015 
of the light emitting diode region B are etched to expose the i-GaAs layer 
1011 and the i-GaAs layer 1016. Double layer structures of AuGe alloy and 
Au (50 nm, 200 nm respectively) are formed on those exposed regions, and 
an Au layer is formed on the n-GaAs layer 912 by vacuum deposition. The 
alloying process is applied to the thus formed layers in a hydrogen and 
nitrogen atmosphere at a temperature of 400 degrees for 2 minutes to 
thereby obtain a common electrode 1019 having an ohmic contact, an 
electrode 1020 for the light emitting diode region, and an electrode 911 
for the light absorption control semiconductor region. Since the electrode 
911 is 20 nm in thickness, the light is allowed to transmit through the 
electrode 911. 
A voltage is applied from a power supply 930 so that the electrode 1020 is 
held at a positive potential relative to the common electrode 1019 to 
inject a current to the pn junction, and thereby the light is radiated 
from the pn junction surface. This light transmits perpendicularly through 
the respective layers of the light absorption control semiconductor region 
A. 
FIG. 38 is a diagram showing energy bands of a light absorbing mechanism in 
the light absorption control semiconductor region A in this embodiment in 
a state where no electric field is applied to the triple quantum well. 
In each of the conduction and valence bands, there are formed potential 
barriers Vu, V1, V2, Vd including AlGaAs layers 913, 915, 917, 919, and 
first, second, and third quantum wells Q1, Q2, Q3 including GaAs layers 
914, 916, 918. 
The light absorbing mechanism of the light absorption control semiconductor 
structure in FIG. 39 is identical to that of the light absorption control 
semiconductor used in the foregoing first to fourth modes of the 
invention, and accordingly no description will be given thereof. 
When the voltage is applied from the power supply 931 so that the electrode 
911 is held at the positive potential relative to the common electrode 
1019, the n-GaAs layer 912 and the .delta. doped layer 1012 function as 
electrode layers, with the result being that the electric field is applied 
uniformly and perpendicularly to the junction surfaces of the triple 
quantum well TQW. Then, there occurs the phenomenon as shown in FIGS. 4A 
and 4B. 
Under these circumstances, the light absorption occurs under the influence 
of direct transitions TE1, TE2, TE3 between a ground state energy level 
Lv3.sub.0 in the valence band at the third quantum well Q2 and the 
quantized energy levels E1, E2, E3 commonly spread in the conduction band 
at the three quantum wells as shown in FIG. 40. Accordingly, the electrons 
are permitted to transit from the valence band to the quantized energy 
level at any quantum well in the conduction band by the light absorption. 
This state is referred to as a resonance state. In the resonance state, 
the light absorption coefficient becomes exceedingly large. 
FIG. 5 is a graph showing a characteristic suited to a case wherein the 
photoconductivity based on the electrons excited by the beam made incident 
upon the light absorption control semiconductor region A of this 
embodiment is measured by changing the applied voltage and the wavelength 
of the incident beam. This graph shows the light absorption spectrum in 
the light absorption control semiconductor region A. The bias voltage is 
changed from 2.5 V to 2.9 V. At 2.5 V, a photoconductivity peak P1 appears 
at 760 nm. At 2.9 V, the peak P1 disappears and a photoconductivity peak 
P2 appears at 765 nm. At an intermediate voltage of 2.7 V, both the peaks 
P1 and P2 are observed weakly. In this way, such an excellent 
characteristic was observed that the wavelength at the photoconductivity 
peak, i.e., the wavelength at the light absorption peak, could be 
controlled by changing the voltage by only 0.4 V. When the wavelength at a 
particular peak is taken note of, the light absorption coefficient can be 
controlled. 
As seen from above, when the light transmits through the quantum wells Q1, 
Q2, Q3 of the light absorption control semiconductor region A having this 
light absorption characteristic, the spectrum of the transmitting light is 
the spectrum of the light emitted in the light emitting diode region B 
minus the spectrum shown in FIG. 5. Taking advantage of the light 
absorption characteristic variable according to the voltage, the spectrum 
of the transmitting light can be changed according to the intensity of the 
voltage applied to the light absorption control semiconductor region A. 
Eighteenth Embodiment 
This embodiment pertains to a modification of the construction of the light 
absorption control semiconductor region A of the seventeenth embodiment. 
FIG. 41 is a diagram showing energy bands in a light absorption control 
semiconductor region A of the eighteenth embodiment. In this embodiment, 
semiconductors formed of different materials are used for first, second, 
and third quantum wells Q1, Q2, Q3. The first quantum well Q1 is formed of 
InGaAs; the second quantum well Q2 is formed of AlGaAs; the third quantum 
well Q3 is formed of GaAs; and barriers Vu, V1, V2, Vd are formed of 
AlGaAs having a large Al composition ratio. By regulating the width of the 
respective quantum wells Q1, Q2, Q3, ground state energy levels coincide 
(match) only in the conduction bands at the respective quantum wells in 
the absence of an electric field. The quantized energy levels do not 
coincide in valence bands. In this structure, only the conduction band at 
the three quantum wells can be brought into a resonance state in the state 
where no electric field is applied. In the resonance state in the 
conduction band, the matched quantized energy levels are separated into 
three quantized energy levels El, E2, E3 which differ by a minute energy, 
thereby releasing the degeneracy. 
This device has three absorption lines simultaneously in a single structure 
and switches electrically the three absorption lines which differ slightly 
from one another. Thus, the spectrum of the light transmitted through the 
light absorption control semiconductor region A can be controlled by 
changing the voltage minutely. Therefore, the spectrum of the light can be 
changed delicately according to the minute change in the voltage. 
Nineteenth Embodiment 
FIG. 42 is a sectional view showing the construction of a light emitting 
diode 201 pertaining to this embodiment. In this embodiment, on a 
semi-insulating GaAs substrate 1230 is formed a 500 nm i-GaAs layers 1218, 
a .delta. doped layer 1217, and a 100 nm i-GaAs layer 1216. The .delta. 
doped layer 1217 functions as an electrode layer for a light absorption 
control semiconductor region A. On the i-GaAs layer 1216 is formed the 
light absorption control semiconductor region A (layers 1220 to 1214 
corresponding to the layers 920 to 914 of the eighteenth embodiment 
including the potential barrier Vd, third quantum well Q3, potential 
barrier V2, second quantum well Q2, potential barrier V1, first quantum 
sell Q1, and potential barrier Vu) constructed identically to that of the 
first embodiment. 
Thereafter, on the i-AlGaAs layer 1214 is formed a 1000 nm i-Al.sub.y 
Ga.sub.1-y As (y=0.15) layer 1040. The layer 1040 functions as a waveguide 
layer. 0n the layer 1040 are formed a 100 nm i-GaAs layer 1311, a .delta. 
doped layer 1312, and a 100 nm i-GaAs layer 1313. These layers correspond 
to the layers 1111, 1112, 1113 in the eighteenth embodiment, and function 
as common electrode layers for the light absorption control semiconductor 
region A and the light emitting diode region B. 
Thereafter, on the i-GaAs layer 1311 are formed a 1000 nm p-AlGaAs layer 
1315, and a 1000 nm n-AlGaAs layer 1314. These layers correspond to the 
layers 1115, 1114 in the eighteenth embodiment. On the layer 1314 is 
formed a 100 nm n-GaAs layer 1316 (corresponding to the layer 912 in the 
eighteenth amendment). Then, the above layers are etched into a shape as 
illustrated. Double layer structures of AuGe and Au are formed on the 
n-GaAs layer 1316 and the exposed i-GaAs layers 1313 to obtain electrodes 
1318, 1319 for the light emitting diode region. Likewise, electrodes 1320, 
1322 for the light absorption control semiconductor region are formed on 
the exposed i-GaAs layer 1313 and the expose i-GaAs layer 1216. Then, a 
reverse mesa etching is carried out by which the lower the layers are 
positioned, the wider they are etched. A reflection film 1323 is formed by 
depositing an oxide film, nitride, or aluminum on lateral end faces of the 
layers. 
In a thus constructed light emitting diode, when a voltage is applied from 
a power supply 1032 so that the electrode 1319 is held at a positive 
potential relative to the electrode 1318, a current is injected into the 
pn junction and a light is radiated therefrom. This light advances 
perpendicular to the junction surface, is reflected by the reflection film 
1323, and advances in a direction parallel with the substrate 1230. At 
this time, the light propagates in the waveguide layer 1040 in parallel 
with the layer surfaces. However, since the light absorption control 
semiconductor region A joined with the waveguide layer 1040 is exceedingly 
thin, some light leaks to the light absorption control semiconductor 
region A. More specifically, the light advances while being reflected a 
multitude of times between upper and lower surfaces of the waveguide layer 
1040. The light transmitted through the lower surface leaks to the light 
absorption control semiconductor region A, is reflected by the respective 
layers, and returns to the waveguide layer 1040 again. Accordingly, the 
light is absorbed according to a specified absorption spectrum in the 
light absorption control semiconductor region A while propagating in the 
waveguide layer 1040. Thus, similar to the eighteenth embodiment, the 
spectrum of the light propagating in the waveguide layer 1040 can be 
modulated according to the intensity of the voltage applied between the 
electrodes 1320 and 1322. In other words, the spectrum of the light 
propagating in the waveguide layer 1040 can be changed by the voltage. 
While the foregoing embodiment is described with respect to the device of 
changing the emission spectrum, the invention may be applicable to a light 
receiving apparatus for measuring the intensity of the light having a 
specific wavelength by measuring a current flowing through the light 
absorption control semiconductor region A between the electrodes 911 and 
1019 and between the electrodes 1322 and 1320 in the resonance state.