Optical semiconductor device for neural network

An optical semiconductor device is disclosed which includes a semiconductor laser having at least an active layer, reflecting means formed on the semiconductor laser for reflecting internal feedback light generated from the semiconductor laser and at least two phototransistors formed on the reflecting means for detecting light having a wavelength substantially identical to that of laser light oscillated from the active layer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical semiconductor device, and more 
specifically, to an optical semiconductor device for forming a neural 
network. 
2. Description of the Related Art 
An information processing system using a neural network is one of 
sophisticated parallel distributed information processing systems having 
learning capability which simulates the information processing in the 
brain. Such an information processing system having a neural network is 
excellent in high-speed pattern recognition and knowledge processing based 
on incomplete data. On the contrary, Neumann type serial information 
processing systems which are in major use at present are inferior for 
conducting such types of information processing. The information 
processing system having a neural network is therefore expected to be a 
system which can overcome the above and other disadvantages of the Neumann 
type serial information processing systems, and is one which has been 
intensely studied. 
FIG. 8 schematically shows the operation of a neuron 100 to be used in a 
neural network. The operation of the neuron 100 is represented by formula: 
##EQU1## 
The neuron 100 receives an input signal Si (i=1 to N) and outputs an output 
signal x. The input signal Si is synaptically interconnected with the 
neuron 100 with a strength of wi (i=1 to N) which is a weight indicating 
the strength of the synaptic interconnection. When the weight wi is a 
positive value, the synaptic interconnection is excitatory. When the 
weight wi is a negative value, the synaptic interconnection is inhibitory. 
When the weight wi is zero, there is no synaptic interconnection. When the 
sum .SIGMA. (Siwi) of the products of the input signal Si and the weight 
wi exceeds a threshold level h, the neuron 100 is made excited and outputs 
the output signal x. Varying the weight wi is called learning. When the 
weight wi varies in response to the input signal Si, the learning is 
called self-learning. 
In order to form a neural network to complete an information processing 
system, a number of neurons identical to the neuron 100 are required, and 
they must be mutually connected. More concretely, an output signal x from 
another neuron not shown must be supplied to the neuron 100 as the input 
signal Si. As the number of neurons constituting the neural network 
increases, higher-level information processing is possible. However, if a 
number of neurons are to be mutually connected through conventional 
electrical wirings, the number of electrical wirings required is so 
enormous that it is difficult to complete the neural network using such 
electrical wirings. This is especially true when the neural network is 
composed of neurals arranged with high density. 
In order to solve the above problem, the use of light for the 
interconnection among a plurality of neurons has been studied. For 
example, IEEE Photonics Technology Letters, vol. 4 (1992), pp. 247-249 
describes an optical neurochip made of semiconductor material having a 
light emitting device and a photodetector. FIG. 9 schematically shows such 
an optical neurochip 200 including a light emitting diode (LED) array 201 
and a photodetector array 202. The LED array 201 consists of LEDs 203 
arranged in a matrix with eight lines and eight rows. Each of the LEDs 203 
includes a multi quantum-well active layer 204 and a distributed bragg 
reflector 205. The photodetector array 202 consists of photodetectors 206 
each of which is arranged at a position corresponding to each of the LEDs 
203. The photodetectors 206 having an MSM (metal-semiconductor-metal) 
structure are fabricated by evaporating aluminium on a GaAs substrate 207 
to form electrodes 208. The electrodes 208 are connected to wire bonding 
pads 209 formed on the GaAs substrate 207. Bumps 210 are formed on the 
GaAs substrate. 
In the optical neurochip 200, the eight LEDs 203 in each line 
simultaneously emit light having an identical intensity as the input 
signal Si shown in FIG. 8. This corresponds to supplying an output from 
one neuron to other neurons simultaneously as signals having an identical 
intensity. The light from the LEDs 203 is received by the corresponding 
photodetectors 206. In each of the photodetectors 206, a voltage is 
applied to one of the electrodes 208 thereof from an external source 
through the wire bonding pad 209. The sensitivity of the photodetectors 
206 is adjustable by varying the level and the polarity of the applied 
voltage. This corresponds to adjusting the weight wi of the synaptic 
interconnection shown in FIG. 8. Each eight of the photodetectors 206 are 
mutually connected in a row so that the sum of photocurrents flowing in 
the eight photodetectors 206 can be taken out. This corresponds to 
obtaining the sum .SIGMA.(Siwi) of the products of the input signal Si 
supplied from neurons and the weight wi of the synaptic interconnection. 
Thus, the optical neurochip 200 has realized the synaptic interconnection. 
However, the optical neurochip 200 has disadvantages as follows: First, 
threshold processing is not available. It is required, therefore, to 
provide an external operation circuit to conduct the threshold processing 
of the signals output from the optical neurochip 200 before being supplied 
to another optical neurochip. Second, since the input signals and the 
output signal of the optical neurochip 200 are electrical signals, 
electrical wirings are required for the formation of a neutral network. 
Thus arises the same trouble as described above. 
An optical semiconductor device having a photodetector and a light emitting 
device is described in the Institute of Electronics, information and 
communication Engineers Technical Report, OQE-91-53 (1991), pp. 45-50. 
Referring to FIG. 10, an optical semiconductor device 250 includes a 
semiconductor laser 251, an absorbing layer 252 formed on the 
semiconductor laser 251, and heterojunction phototransistors 253, 254, and 
255 formed on the absorbing layer 252. The semiconductor laser 251 
includes an undoped active layer 256 having a band gap of 1.3 .mu.m. Each 
of the phototransistors 253 to 255 includes a collector layer 257, a base 
layer 258, and an emitter layer 259, which are doped with impurities at 
concentrations of 1.times.10.sup.17 cm.sup.-3, 5.times.10.sup.16 
cm.sup.-3, and 1.times.10.sup.18 cm.sup.-3 respectively. The band gap of 
the base layer 258 is 1.2 .mu.m. The phototransistors 253 and 255 are 
formed at positions shifted from the center of emission of the 
semiconductor laser 251 by a horizontal distance of 185 .mu.m opposite to 
each other. The phototransistor 254 is formed above the center of emission 
of the semiconductor laser 251. The absorbing layer 252 includes a first 
absorbing layer 260 having a thickness of 1 .mu.m and a band gap of 1.2 
.mu.m and a second absorbing layer 261 having a thickness of 1 .mu.m and a 
band gap of 1.3 .mu.m. 
In the optical semiconductor device 250 having the above-described 
structure, when the phototransistor 253 is irradiated with an input beam 
262, a photocurrent is generated. The photocurrent then flows into the 
semiconductor laser 251 and causes laser oscillation. In general, a 
semiconductor laser emits weak light even when the current flowing therein 
is too small to cause laser oscillation. In the optical semiconductor 
device 250, therefore, if the semiconductor laser 251 emits weak light 
without generating laser oscillation, the emitted light will be absorbed 
into the phototransistors 253 to 255 as feedback light, resulting in 
production of a large photocurrent. This large photocurrent will cause the 
semiconductor laser 251 to emit light more intensely. With this positive 
optical feedback, the semiconductor laser 251 will finally oscillate, 
which will prevent the semiconductor device 250 from conducting normal 
optical amplification operations. 
In order to solve the above problem, the active layer 256 of the 
semiconductor laser 251 is formed of a semiconductor material different 
from that for the base layers 258 of the phototransistors 253 to 255. 
Thus, the wavelength of light oscillated by the semiconductor laser 251 is 
different from the detection peak wavelength of the phototransistors 253 
to 255. Further, the absorbing layer 252 is provided in order to minimize 
the influence of internal feedback light. 
With the above structure, however, the phototransistor 254 formed above the 
center of emission of the semiconductor laser 251 still receives internal 
feedback light. Therefore, in order to substantially eliminate the 
influence of internal feedback light, the phototransistor 253 (or 255) 
which is formed at a position shifted from the center of emission of the 
semiconductor laser 251 is used as the photodetector. 
Thus, in the optical semiconductor device 250, the phototransistor 253 is 
used as the photodetector, and the current is biased by dark current to a 
level slightly lower than that at which the semiconductor laser 251 starts 
oscillating. Under these conditions, when an input beam 262 is incident to 
the phototransistor 253, the semiconductor laser 251 oscillates and 
outputs an output beam 263. Since the semiconductor laser 251 has an 
output light-current characteristic with good linearity, an output beam 
263 with high intensity can be obtained even when the input beam 262 is 
weak, allowing the optical semiconductor device 250 to conduct the optical 
amplification operation. 
However, the optical semiconductor device 250 is still disadvantageous for 
use as a neuron constituting a neutral network for the following reasons: 
The wavelength of the light emitted from the semiconductor laser 251is 
different from the detection peak wavelength of the phototransistor 253. 
Accordingly, when the output beam 263 is introduced to a phototransistor 
of another optical semiconductor device as an input beam, the detection 
sensitivity lowers, and therefore it is difficult to form a neural network 
with effective signal transmission. Further, since the phototransistor 254 
formed above the center of emission of the semiconductor laser 251 cannot 
be used as the photodetector, high-density integration of the optical 
semiconductor device 250 cannot be realized. 
SUMMARY OF THE INVENTION 
The optical semiconductor device of this invention includes: a 
semiconductor laser having at least an active layer; reflecting means 
formed on the semiconductor laser for reflecting internal feedback light 
generated from the semiconductor laser; and at least two phototransistors 
formed on the reflecting means for detecting light having a wavelength 
substantially identical to that of laser light oscillated from the active 
layer. 
Alternatively, the optical semiconductor device of the present invention 
includes: a plurality of semiconductor lasers each having at least an 
active layer; reflecting means formed on each of the plurality of 
semiconductor lasers for reflecting internal feedback light generated from 
the semiconductor laser; and at least two phototransistors formed on the 
reflecting means for detecting light having a wavelength substantially 
identical to that of laser light oscillated from the active layer. 
According to another aspect of the present invention, an optical neurochip 
is provided, which includes: a plurality of optical semiconductor devices 
each including; a plurality of semiconductor lasers each having at least 
an active layer; reflecting means formed on each of the plurality of 
semiconductor lasers for reflecting internal feedback light generated from 
the semiconductor laser; at least two phototransistors formed on the 
reflecting means for detecting light having a wavelength substantially 
identical to that of laser light oscillated from the active layer; and 
voltage application means for applying bias voltages to the 
phototransistors; and a bias control circuit optically connected to each 
of the semiconductor lasers and each of the phototransistors, for 
detecting light oscillated from the semiconductor laser and applying a 
voltage corresponding to the intensity of the detected light to the 
voltage application means. 
The optical semiconductor device of the present invention includes a 
plurality of phototransistors having a symmetrical heterojunction 
structure formed on the semiconductor laser. The phototransistors are 
capable of responding to output light emitted outside from the 
semiconductor laser, but are prevented from absorbing internal feedback 
light therefrom. 
The semiconductor laser is a semiconductor light emitting device which 
outputs coherent light when the current flowing in the device exceeds a 
threshold level specific to the device. The heterojunction phototransistor 
is a semiconductor photodetector which can obtain a photocurrent 
corresponding to the intensity of input light and has a gain. Accordingly, 
with the above structure of the present invention, when the 
phototransistors receive light inputs, photocurrents are generated 
corresponding to the respective light inputs and flow in the 
phototransistors. All of such photocurrents then flow into the 
semiconductor laser. When the sum of the photocurrents exceeds a 
predetermined threshold level, the semiconductor laser oscillates and 
outputs light. 
In the optical semiconductor device of the present invention, internal 
feedback light from the semiconductor laser is prevented from being 
absorbed by the heterojunction phototransistors. Accordingly, it is 
possible for the semiconductor laser to emit light with a wavelength 
identical to that of the input light incident to the phototransistors. 
This enables connection of a plurality of identical optical semiconductor 
devices to complete an optical neural network. This also makes it possible 
to dispose the phototransistors right above an active region of the 
semiconductor laser, allowing for high-density integration of the 
phototransistors on the semiconductor laser. 
Thus, the invention described herein makes possible the advantages of (1) 
providing an optical semiconductor device in which threshold processing by 
the device itself is possible, (2) providing an optical semiconductor 
device of which input light and output light have an identical wavelength, 
and (3) providing an optical semiconductor device in which the influence 
of internal feedback light is minimized and thus phototransistors can be 
integrated with high density. 
These and other advantages of the present invention will become apparent to 
those skilled in the art upon reading and understanding the following 
detailed description with reference to the accompanying figures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Example 1 
FIG. 1 shows a first example of the optical semiconductor device of the 
present invention. An optical semiconductor device 11 includes a 
semiconductor laser 12, a semiconductor reflecting mirror 13 formed on the 
semiconductor laser 12, and phototransistors 14a, 14b, 14c, and 14d formed 
on the semiconductor reflecting mirror 13. 
The semiconductor laser 12 includes a p-type semiconductor substrate 15, a 
p-type cladding layer 16 formed on the p-type semiconductor substrate 15, 
an active layer 17 formed on the p-type cladding layer 16, and an n-type 
cladding layer 18 formed on the active layer 17. The semiconductor laser 
12 has a stripe buried structure. More specifically, an area of the 
semiconductor laser 12 including the active layer 17, part of the p-type 
cladding layer 16, and part of the n-type cladding layer 18 is partly 
etched so that a stripe 21 is formed by the unetched portion thereof. An 
n-type current blocking layer 19 and a p-type current blocking layer 20 
are formed in this order in the etched portion. A p-type electrode 29 is 
formed on the bottom surface of the p-type semiconductor substrate 15. 
When a voltage is applied between the n-type cladding layer 18 and the 
p-type cladding layer 16, the semiconductor laser 12 oscillates an output 
beam 27 having a wavelength .lambda. from the active layer 17 thereof. 
The semiconductor reflecting mirror 13 is formed on a surface 18f of the 
n-type cladding layer 18 which is the uppermost layer of the semiconductor 
laser 12. The semiconductor reflecting mirror 13 is composed of a 
plurality of semiconductor thin films having n-type conductivity and which 
substantially reflects light with the wavelength .lambda.. Such a 
reflecting mirror is known to those skilled in the art as a Distributed 
Bragg Reflector (DBR). By selecting appropriate refractive indices and 
film thicknesses for the semiconductor thin films, a reflecting mirror 
capable of reflecting light having a desired wavelength can be fabricated. 
Each of the phototransistors 14a, 14b, 14c, and 14d is located above the 
stripe 21 of the semiconductor laser 12, and includes a collector layer 22 
formed on the semiconductor reflecting mirror 13, a base layer 23 formed 
on the collector layer 22, and an emitter layer 24 formed on the base 
layer 23. The base layer 23 substantially absorbs an input beam 28 having 
the wavelength .lambda.. A photocurrent is generated by this absorption of 
the input beam 28, and flows between the collector layer 22 and the 
emitter layer 24. The emitter layer 24 and the base layer 23 are 
preferably made of different materials from each other so as to prevent 
the input beam 28 from being absorbed by the emitter layer 24. 
Accordingly, the phototransistors 14a, 14b, 14c, and 14d are preferably 
heterojunction phototransistors. 
Each of the phototransistors 14a, 14b, 14c, and 14d has an electrically 
symmetrical structure. More specifically, the current-voltage 
characteristic of the phototransistors 14a, 14b, 14c, and 14d obtained 
when a forward bias voltage is applied between the collector layer 22 and 
the emitter layer 24 is substantially the same as that obtained when a 
reverse bias voltage is applied. This feature can be obtained by forming 
the collector layer 22 and the emitter layer 24 so that they have the same 
composition, impurity concentration, and thickness. Each of the 
phototransistors 14a, 14b, 14c, and 14d also includes an n-type electrode 
26 with a window 25 formed on the top surface of the emitter layer 24 
thereof. 
The operation of the optical semiconductor device 11 will be described. 
The optical semiconductor device 11 is operated by the application of a DC 
voltage between the n-type electrode 26 of the phototransistor 14a, 14b, 
14c, or 14d and the p-type electrode 29 of the semiconductor laser 12. 
Under the condition where a reverse bias voltage is applied between the 
n-type electrode 26 and the p-type electrode 29 so as to apply the emitter 
layer 24 with a negative bias voltage against the collector layer 22, when 
the input beam 28 having the wavelength .lambda. is received by the base 
layer 23 through the window 25 of the n-type electrode 26, a current flows 
from the collector layer 22 to the emitter layer 24 in the phototransistor 
14a, 14b, 14c, or 14d. The current also flows from the p-type cladding 
layer 16 to the n-type cladding layer 18 through the active layer 17 in 
the optical semiconductor laser 12, so that the semiconductor laser 12 
oscillates. Thus, in this case, the current flows from the p-type cladding 
layer 16 to the emitter layer 24 through the semiconductor reflecting 
mirror 13. On the other hand, under the condition where a forward bias 
voltage is applied between the n-type electrode 26 and the p-type 
electrode 29 so as to apply the emitter layer 24 with a positive bias 
voltage against the collector layer 22, when the input beam 28 having the 
wavelength .lambda. is received by the base layer 23 through the window 
25, a current flows from the emitter layer 24 to the collector layer 22 in 
the phototransistor 14a, 14b, 14c, or 14d. The current also flows from the 
n-type cladding layer 18 to the p-type cladding layer 16 through the 
active layer 17, so that the semiconductor laser 12 is prevented from 
oscillating. Thus, in this case, the current flows from the emitter layer 
24 to the p-type cladding layer 16 through the semiconductor reflecting 
mirror 13. 
Now, referring to FIGS. 1 to 3, the operation of the optical semiconductor 
device 11 will be described in more detail. 
FIG. 2 shows part of an equivalent circuit of the optical semiconductor 
device 11. The phototransistors 14a and 14b are connected in parallel to 
the semiconductor laser 12. In this equivalent circuit diagram, only the 
phototransistors 14a and 14b are shown. In practice, however, the 
phototransistors 14c and 14d are also connected in parallel with the 
phototransistors 14a and 14b to the semiconductor laser 12. As shown in 
FIGS. 1 and 2, the p-type electrode 29 of the semiconductor laser 12 is 
grounded, and a negative voltage, for example -5 V, is applied to the 
n-type electrode 26 of the phototransistor 14a, while a positive voltage, 
for example +5 V, is applied to the n-type electrode 26 of the 
phototransistor 14b. Since the semiconductor laser 12 can be considered to 
be a constant voltage source of 1.5 V, for example, the voltages applied 
across the phototransistors 14a and 14b are 3.5 V and -6.5 V, 
respectively, at the respective emitter layers 24. 
FIG. 3 shows the current-voltage characteristic of the phototransistors 14a 
and 14b. As is shown by curves A, B, and C, the current flowing in the 
phototransistors 14a and 14b increases as the intensity of the input beam 
28 increases. As is apparent from FIG. 3, for any levels of intensities of 
the input beam 28 incident to the phototransistors 14a and 14b, the 
photoelectric transfer gains are the same at the forward biasing and at 
the reverse biasing. For example, for the input beam 28 having the 
intensity represented by the curve B, when voltages v1 and -v1 are applied 
across the phototransistors 14a and 14b, currents i and -i having the same 
intensity flow therein, respectively, in opposite directions. Also, as is 
apparent from FIG. 3, the photoelectric transfer gain is substantially 
independent of the bias voltage. For example, for the input beam 28 having 
the intensity represented by the curve B, whichever voltage v1 or v2 is 
applied across the phototransistors 14a and 14b, the same current i flows. 
Accordingly, when voltages of -5 V and +5 V are applied to the n-type 
electrodes 26 of the phototransistors 14a and 14b, respectively, while 
they are irradiated with the input beams 28 having the same intensity, the 
currents i and -i having the same intensity flow in opposite directions. 
Referring to FIG. 1 again, the semiconductor laser 12 receives through the 
semiconductor reflecting mirror 13 the sum of the currents which have 
flown in the phototransistors 14a, 14b, 14c, and 14d. When the sum of the 
currents exceeds a predetermined threshold level, the semiconductor laser 
12 oscillates the output beam 27. In the above case shown in FIG. 2, 
provided that no current flows in the phototransistors 14c and 14d, the 
sum of the currents received by the semiconductor laser 12 is zero because 
the currents flowing in the phototransistors 14a and 14b have the same 
intensity but the directions thereof are opposite. Thus, no light is 
emitted from the semiconductor laser 12. 
The above-described operation of the phototransistors 14a, 14b, 14c, and 
14d corresponds to obtaining the product of the input signal Si and the 
weight wi represented in Formula (1). In other words, the bias voltage 
corresponding to the weight wi is applied across the phototransistors 14a, 
14b, 14c, and 14d, to which the input beam 28 corresponding to the input 
signal Si is incident. As a result, the current corresponding to the 
product of the input signal Si and the weight wi is obtained. Further, the 
operation of the semiconductor laser 12 corresponds to obtaining the sum 
.SIGMA.(Siwi) for the threshold operation. That is, the sum of the 
currents flowing in the phototransistors 14a, 14b, 14c, and 14d is 
received by the semiconductor laser 12, and only when the sum of the 
currents exceeds the threshold level h, the semiconductor laser 12 
oscillates. Therefore, this indicates that the optical semiconductor 
device 11 can effect the operation of the neurochip represented by Formula 
(1) by itself. 
In the optical semiconductor device 11, the weight wi showing the strength 
of the synaptic interconnection can be changed by changing the polarity of 
the bias voltage to be applied. This means that the optical semiconductor 
device 11 has learning capability. The bias voltage may be varied based on 
the output beam 27, or may be varied so that the output beam 27 with a 
desirable output can be obtained, which is called "learning with a 
teacher". 
The operation of the optical semiconductor device 11 as the neurochip will 
be concretely described with reference to Tables 1 and 2 below. 
TABLE 1 
______________________________________ 
Phototransistor 
14a 14b 14c 14d 
______________________________________ 
Bias voltage (wi) 
-5 5 5 5 
Optical signal (Si) 
O O X O 
Current (Siwi) -i i O i 
______________________________________ 
Sum of current received 
i 
by semiconductor laser 
(.SIGMA.Siwi) 
Threshold level (h) 
i &lt; h &lt; 2i 
.SIGMA.Siwi - h 0 
______________________________________ 
In Table 1, it is assumed that the bias voltages, -5, 5, 5, and 5 V are 
applied across the phototransistors 14a, 14b, 14c, and 14d, respectively, 
as the weight wi. When the phototransistors 14a, 14b, 14c, and 14d are 
irradiated with light as the input signal si (in Table 1, indicates that 
the phototransistor is irradiated with light, while X indicates that it is 
not irradiated with light), the currents (Siwi), -i, i, 0, and i flow in 
the phototransistors 14a, 14b, 14c, and 14d, respectively. As a result, 
the sum of the currents received by the semiconductor laser 12 as the sum 
.SIGMA.(Siwi) is i. Since the current i does not exceed the threshold 
level h of the semiconductor laser 12 which is i&lt;h&lt;2i, the semiconductor 
laser 12 does not emit light. 
TABLE 2 
______________________________________ 
Phototransistor 
14a 14b 14c 14d 
______________________________________ 
Bias voltage (wi) 
5 -5 5 5 
Optical signal (Si) 
O O O O 
Current (Siwi) -i -i i i 
______________________________________ 
Sum of current received 
by semiconductor laser 
2i 
(.SIGMA.Siwi) 
Threshold level (h) 
i &lt; h &lt; 2i 
.SIGMA.Siwi - h 1 
______________________________________ 
In Table 2, it is assumed that the bias voltages, 5, -5, 5, and 5 V are 
applied across the phototransistors 14a, 14b, 14c, and 14d, respectively, 
as the weight wi. When the phototransistors 14a, 14b, 14c, and 14d are 
irradiated with light as the input signal si, the currents (Siwi), i, -i, 
i, and i flow in the phototransistors 14a, 14b, 14c, and 14d, 
respectively. As a result, the sum of the currents received by the 
semiconductor laser 12 as the sum .SIGMA.(Siwi) is 2i. Since the current 
2i exceeds the threshold level h of the semiconductor laser 12 which is 
i&lt;h&lt;2i, the semiconductor laser 12 emits light. 
The optical semiconductor device 11 of the present invention is 
advantageous in that, as the neurochip, it can detect the input beam 28 
having the same wavelength .SIGMA. as that of the output beam 27. 
Accordingly, the output beam 27 from the optical semiconductor device 11 
can be used as input beams for other optical semiconductor devices 
identical to the optical semiconductor device 11 without the necessity of 
an external apparatus for converting the wavelength. Thus, a plurality of 
optical semiconductor devices can be mutually connected to form a neural 
network. This advantage has been obtained because the semiconductor 
reflecting mirror 13 which is electrically conductive is formed between 
the semiconductor laser 12 and the phototransistors 14a, 14b, 14c, and 
14d. The semiconductor reflecting mirror 13 reflects internal feedback 
light which propagates from the active layer 17 vertically with regard to 
the active layer 17, so as to prevent the internal feedback light from 
entering the phototransistors 14a, 14b, 14c, and 14d. At the same time, 
the semiconductor reflecting mirror 13 can reflect light transmitted 
through the phototransistors 14a, 14b, 14c, and 14d without being absorbed 
by the base layer 23. Therefore, even if part of light entering the 
phototransistors 14a, 14b, 14c, and 14d is transmitted without being 
absorbed by the base layer 23, the part of light is reflected by the 
semiconductor reflecting mirror 13 and propagates to the base layer 23 
again. This improves the photoelectric transfer efficiency. Thus, the 
semiconductor reflecting mirror 13 optically isolates the phototransistors 
14a, 14b, 14c, and 14d from the semiconductor laser 12. This enables 
location of the phototransistors 14a, 14b, 14c, and 14d above the active 
layer 17 of the semiconductor laser 12, so that the optical semiconductor 
device 11 can be made compact. 
The optical semiconductor device 11 can be fabricated by a known 
semiconductor fabrication technique. Referring to FIG. 1, the method for 
fabricating the optical semiconductor device 11 will be described. 
First, the semiconductor laser 12 having the p-type semiconductor substrate 
15 is fabricated by liquid phase epitaxy (LPE) or other known techniques. 
The p-type semiconductor substrate 15 is made of p-type InP with an 
impurity concentration of 1.times.10.sup.18 cm.sup.-3. The p-type cladding 
layer 16 is made of InP with an impurity concentration of 
5.times.10.sup.17 cm.sup.-3 and a thickness of 2 .mu.m, and the n-type 
cladding layer 18 is made of InP with an impurity concentration of 
5.times.10.sup.16 cm.sup.-3 and a thickness of 2.5 .mu.m. The active layer 
17 is made of undoped In.sub.0.72 Ga.sub.0.28 As.sub.0.6 P.sub.0.4 with an 
impurity concentration of 1.times.10.sup.16 cm.sup.-3 and a thickness of 
0.2 .mu.m and has a band gap wavelength of .lambda. =1.3 .mu.m. An area of 
the semiconductor laser 12 including the active layer 17, part of the 
p-type cladding layer 16, and part of the n-type cladding layer 18 is 
partly etched so that the stripe 21 having a width Wa of 2 .mu.m can be 
formed. In the etched portions are buried the n-type current blocking 
layer 19 and the p-type current blocking layer 20, both of which are made 
of InP having an impurity concentration of 5.times.10.sup.17 cm.sup.-3 and 
a thickness of 1 .mu.m. 
Then, the semiconductor reflecting mirror 13 is formed on the semiconductor 
laser 12. The semiconductor reflecting mirror 13 is designed to reflect 
light having a wavelength of 1.3 .mu.m. It is composed of, for example, 
each 15 of InP layers having a thickness of 101.6 nm, an impurity 
concentration of 1.times.10.sup.17 cm.sup.-3 and a refractive index of 
3.20 and In.sub.0.72 Ga.sub.0.28 As.sub.0.6 P.sub.0.4 layers having a 
thickness of 93.7 nm, an impurity concentration of 1.times.10.sup.17 
cm.sup.-3 and a refractive index of 3.47, alternately formed on the n-type 
cladding layer 18. In order to strictly control the thicknesses, the 
semiconductor reflecting mirror 13 is preferably formed by molecular beam 
epitaxy (MBE). 
Thereafter, the collector layer 22 is formed on the semiconductor 
reflecting mirror 13, the base layer 23 on the collector layer 22, and the 
emitter layer 24 on the base layer 23. The collector layer 22 and the 
emitter layer 24 both have a thickness of 1.5 .mu.m, and are made of 
n-type InP having an impurity concentration of 5.times.10.sup.17 cm.sup.-3 
and p-type InP having an impurity concentration of 5.times.10.sup.17 
cm.sup.-3, respectively. The base layer 23 is made of In.sub.0.72 
Ga.sub.0.28 As.sub.0.6 P.sub.0.4 having a thickness of 0.3 Nm and an 
impurity concentration of 1.times.10.sup.17 cm.sup.-3. Then the collector 
layer 22 the base layer 23, and the emitter layer 24 are partly etched so 
as to form the phototransistors 14a, 14b, 14c, and 14d having a size of 
L1=L2=60 .mu.m at positions above the stripe 21 of the semiconductor laser 
12. 
Finally, the n-type electrodes 26 having the windows 25 are formed on the 
respective emitter layers 24, and the p-type electrode 29 is formed on the 
p-type semiconductor substrate 15. The thus-fabricated structure is 
appropriately cut so as to obtain the semiconductor laser 12 having a size 
of L3=300 .mu.m and L4=150 .mu.m. 
Example 2 
FIG. 4 shows a second example of the optical semiconductor device according 
to the present invention. The same components as those in Example 1 are 
denoted by the same reference numerals. 
An optical semiconductor device 31 of this example includes a semiconductor 
laser 32, a semiconductor reflecting mirror 13 formed on the semiconductor 
laser 32, and phototransistors 14a, 14b, 14c, and 14d. formed on the 
semiconductor reflecting mirror 13. 
Unlike the optical semiconductor device 11 of Example 1, the optical 
semiconductor device 31 includes a 45.degree. mirror 34 for oscillating an 
output beam 35 in a direction parallel to an input beam 28 incident to the 
phototransistors 14a, 14b, 14c, and 14d. As in Example 1, the 
semiconductor laser 32 includes a p-type semiconductor substrate 15, a 
p-type cladding layer 16 formed on the p-type semiconductor substrate 15, 
an active layer 17 formed on the p-type cladding layer 16, and an n-type 
cladding layer 18 formed on the active layer 17. The semiconductor laser 
32 has a wedgeshaped cut portion formed by partialy etching the 
semiconductor laser 32 from the side of a p-type electrode 29 formed on 
the bottom surface of the p-type semiconductor substrate 15 so as to reach 
the active layer 17. The cut portion includes an end face 36 cut 
vertically to the active layer 17 and the 45.degree. mirror 34 cut with an 
inclination of 45.degree. with regard to the active layer 17. The output 
beam 35 output from the end face 36 is reflected by the 45.degree. mirror 
34 and emitted in a direction vertical to the active layer 17, i.e., in 
the same direction as the input beam 28. This structure is advantageous 
when a plurality of optical semiconductor devices identical to the optical 
semiconductor device 31 are integrated together, especially when they are 
stacked in the direction of the input beam 28 and the output beam 35. 
The 45.degree. mirror 34 can be formed by a known etching technique. A 
resonator having a length L3 is formed between the end face 36 and an end 
face 37. An area including the p-type semiconductor substrate 15, the 
p-type cladding layer 16, the active layer 17, and part of the n-type 
cladding layer 18 are partly etched so as to form the 45.degree. mirror 34 
with an inclination of 45.degree. with regard to the active layer 17. In 
this example, no reflecting film is formed on the 45.degree. mirror 34. 
However, a reflecting film made of metal, for example, may be formed to 
cover the 45.degree. mirror 34. 
Example 3 
FIG. 5A shows a third example of the optical semiconductor device according 
to the present invention. The same components as those in Example 1 are 
denoted by the same reference numerals. 
An optical semiconductor device 41 of this example includes a semiconductor 
laser 12, a semiconductor reflecting mirror 13 formed on the semiconductor 
laser 12, and phototransistors 44a, 44b, 44c, and 44d. formed on the 
semiconductor reflecting mirror 13. 
The optical semiconductor device 41 is different from the semiconductor 
device 11 of Example 1 in that the phototransistors 44a, 44b, 44c, and 44d 
of this example have a current-voltage characteristic different from that 
of the phototransistors 14a, 14b, 14c, and 14d of Example 1. 
FIG. 5B shows the current-voltage characteristic of the phototransistors 
44a, 44b, 44c, and 44d. The phototransistors 44a, 44b, 44c, and 44d have 
the current-voltage characteristic represented by curves D, E, and F. As 
is apparent from FIG. 5B, the photoelectric transfer gains are dependent 
on the bias voltage. For example, for the input beam 28 having the 
intensity represented by the curve E, when a voltage v1 is applied across 
the phototransistor 44a, 44b, 44c, or 44d, a current i1 flows while, when 
a voltage v2 is applied, a current i2 flows. Also, for any levels of 
intensities of the input beam 28 incident to the phototransistor 44a, 44b, 
44c, or 44d, the photoelectric transfer gains are the same at the forward 
biasing and at the reverse biasing. For example, for the input beam 28 
having the intensity represented by the curve E, when voltages v1 and -v1 
are applied across the phototransistor 44a, 44b, 44c, or 44d, currents i1 
and -i1 having the same intensity flow therein, respectively, in opposite 
directions. 
In the phototransistors 44a, 44b, 44c, and 44d having the above 
current-voltage characteristic, the current can be varied in response to 
the bias voltage. This indicates that the weight wi representing the 
strength of the synaptic interconnection shown in Formula (1) can be 
varied successively in response to the bias voltage. As a result, the 
input signal Si can be multiplied by variable weights wi to obtain an 
enhanced learning efficiency. 
The optical semiconductor device 41 of this example is fabricated as in 
Example 1. The phototransistors 44a, 44b, 44c, and 44d having the 
abovedescribed current-voltage characteristic can be formed by using the 
Early effect of the bipolar transistor. The Early effect occurs when the 
thickness of a base layer substantially varies as the thickness of a 
collector depletion layer varies due to the change of the collector 
voltage. This effect is obtained by thinning the base layer of the bipolar 
transistor or by reducing the impurity concentration of the base layer. In 
this example, a collector layer 45 is formed on the semiconductor 
reflecting mirror 13, a base layer 46 on the collector layer 45, and an 
emitter layer 47 on the base layer 46. The collector layer 45 and the 
emitter layer 47 have an identical thickness of 1.5 .mu.m, and are made of 
n-type InP having an impurity concentration of 5.times.10.sup.17 cm.sup.-3 
and p-type InP having an impurity concentration of 5.times.10.sup.17 
cm.sup.-3 respectively. The base layer 46 has a thickness of 0.2 .mu.m and 
is made of In.sub.0.72 Ga.sub.0.28 As.sub.0.6 P.sub.0.4 having an impurity 
concentration of 5.times.10.sup.16 cm.sup.-3. 
Example 4 
FIG. 6 shows a fourth example of the optical semiconductor device according 
to the present invention. 
An optical semiconductor device assembly 51 includes optical semiconductor 
devices 61, 71, 81, and 91 which have substantially the same structure as 
the optical semiconductor device 11 of Example 1. The optical 
semiconductor device 61 includes a semiconductor laser 62, a semiconductor 
reflecting mirror 63 formed on the semiconductor laser 62, and 
phototransistors 64a, 64b, 64c, and 64d formed on the semiconductor 
reflecting mirror 63. The optical semiconductor device 71 includes a 
semiconductor laser 72, a semiconductor reflecting mirror 73 formed on the 
semiconductor laser 72, and phototransistors 74a, 74b, 74c, and 74d. 
formed on the semiconductor reflecting mirror 73. The optical 
semiconductor device 81 includes a semiconductor laser 82, a semiconductor 
reflecting mirror 83 formed on the semiconductor laser 82, and 
phototransistors 84a, 84b, 84c, and 84d formed on the semiconductor 
reflecting mirror 83. The optical semiconductor device 91 includes a 
semiconductor laser 92, a semiconductor reflecting mirror 93 formed on the 
semiconductor laser 92, and phototransistors 94a, 94b, 94c, and 94d, 
formed on the semiconductor reflecting mirror 93. 
A common p-type electrode 52 is formed on the bottom surfaces of the 
semiconductor lasers 62, 72, 82, and 92. The phototransistors 64a to 64d, 
74a to 74d, 84a to 84d , and 94a to 94d have n-type electrodes 66a to 66d, 
76a to 76d, 86a to 86d, and 96a to 96d on the top surfaces thereof, 
respectively. 
The semiconductor lasers 62, 72, 82, and 92 emit output beams 65, 75, 85, 
and 95 based on beams incident to the phototransistors 64a to 64d, 74a to 
74d, 84a to 84d, and 94a to 94d and bias voltages applied to the n-type 
electrodes 66a to 66d, 76a to 76d, 86a to 86d, and 96a to 96d, 
respectively, as described in Example 1. 
The optical semiconductor device assembly 51 is capable of forming by 
itself a neural network using light as the signal. For example, using an 
optical fiber, the output beam 95 is used as input beams incident to the 
phototransistors 64d, 74d, 84d, and 94d. Likewise, the output beam 85 is 
used as input beams incident to the phototransistors 64c, 74c, 84c, and 
94c, the output beam 75 as input beams incident to the phototransistors 
64b, 74b, 84b, and 94b, and the output beam 65 as input beams incident to 
the phototransistors 64a, 74a, 84a, and 94a. The structure accomplished by 
these connections corresponds to a structure where four neurons are 
optically interconnected. Thus, the information processing by an optical 
neural network can be realized by one optical semiconductor device 
assembly 51. 
The optical semiconductor device assembly 51 can also be provided with a 
self-learning capability. As shown in FIG. 7, the output beam 95 is input 
to a bias control circuit 53. A bias voltage based on the output beam 95 
is applied to the n-type electrodes 66d, 76d, 86d, and 96d of the 
phototransistors 64d, 74d, 84d, and 94d. Likewise, bias voltages are 
applied to the phototransistors 64c, 74c, 84c, and 94c, 64b, 74b, 84b, and 
94b, and 64a, 74a, 84a, and 94a based on the output beams 85, 75, and 65, 
respectively. With these connections, the weight wi can be varied based on 
the output signal x. Accordingly, the information processing by an optical 
neural network having self-learning capability can be realized by one 
optical semiconductor device assembly 51. 
In the above examples, the optical semiconductor device has four 
phototransistors. However, at least two phototransistors are sufficient 
for the optical semiconductor device of the present invention. Also in the 
above examples, the n-type semiconductor reflecting mirror was formed on 
the n-type semiconductor layer of the semiconductor laser, and the 
npn-type phototransistor was formed on the n-type semiconductor reflecting 
mirror. However, the optical semiconductor device of the present invention 
can also be formed of the semiconductor layers having the reverse 
conductivity types. 
Various other modifications will be apparent to and can be readily made by 
those skilled in the art without departing from the scope and spirit of 
this invention. Accordingly, it is not intended that the scope of the 
claims appended hereto be limited to the description as set forth herein, 
but rather that the claims be broadly construed.