Heterojunction semiconductor device

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG.
1 shows the thermal expansion coefficient .
alpha.
of an alloy of formula: Ga.
sub.
x In.
sub.
1-x As.
sub.
6 P.
sub.
1-y as a function of x, with x varying between 0 and 0.
47, the relationship y/x being constant and substantially equal to 2.
2.
On the ordinate is plotted the coefficient .
alpha.
, giving: .
alpha.
=(.
DELTA.
a/a) a being the size of a crystal mesh.
It should be noted that the gradient of the curve is high when x is between 0 and 0.
25 approximately and low when x is between 0.
35 and 0.
47.
It is known that by liquid phase epitaxy a quaternary mixture of type Ga.
sub.
x In.
sub.
1-x As.
sub.
y P.
sub.
1-y can be grown on an indium phosphide substrate, this operation being performed at a temperature of about 600.
degree.
C.
As a result if there is a good mesh matching 600.
degree.
C.
this will disappear at ambient temperature which is generally the temperature at which the semiconductor is used.
Thus, for InP the expansion coefficient is 4,5.
10.
sup.
-6 and for x=0.
25 that of the quaternary material exceeds 5.
10.
sup.
-6.
However, the latter varies slightly for 0.
30<x<0.
47.
Thus, there are constraints at ambient temperature between a layer having a parameter x close to 0.
45 and an indium phosphide layer, which reduces the reliability of the device.
A first embodiment is shown in FIG.
2 which is a cross-section of a photoluminescent diode of the laser type along the longitudinal axis of its cavity resonator.
It comprises an indium phosphide substrate 1 on the lower face of which is placed an ohmic contact 2.
This substrate of the n.
sup.
+ conductivity-type with a doping of the order of 10.
sup.
19 at/cm.
sup.
3 has a thickness of 10 to 100 .
mu.
m.
A first layer 3 of formula Ga.
sub.
x3 In.
sub.
1-x3 As.
sub.
y3 P.
sub.
1-y3 of thickness 1 to 10 .
mu.
m is deposited on this substrate.
On layer 3 is deposited active layer 4 having a thickness of only 0.
1 to 1 .
mu.
m of the same quaternary alloy of type Ga.
sub.
x4 In.
sub.
1-x4 As.
sub.
y4 P.
sub.
1-y4.
Finally layer 4 is surmounted by a layer 5, which is identical to layer 3 and by an ohmic contact 6