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The present invention relates to semiconductor devices, more particularly light-emitting diodes (LEDs) and laser diodes (LDs).
According to the present invention there is provided a semiconductor device having a window layer, a current spreading layer below the window layer and a cladding layer below the current spreading layer, wherein the band gap energy of the spreading layer is higher than that of the window layer and lower than that of the cladding layer and the carrier concentration of the spreading layer is lower than that of the window layer and higher than that of the cladding layer.
The present invention will now be described, by way of example, with reference to the accompanying drawing, which is a section through an example of an LED according to the invention.
Referring to the drawing, a light-emitting diode comprises: an ohmic n-electrode 1 on a rear surface of a GaAs substrate 2; a distributed AlGaAs Bragg reflector layer 3 grown on the substrate 2 (in the form of a multi layer lamination) to improve luminous efficiency; a first, lower AlGaInP cladding layer 4 grown on the layer 3; an active layer 5 (which could be a double hetero-junction layer or a multi-quantum well layer) grown on the layer 4; a second, upper AlGaInP cladding layer 6 grown on the layer 5; an AlGaInP current spreading layer 7 grown on the layer 6; a GaInP window layer 8 grown on the layer 7; and an ohmic p-electrode 9 on the window layer 8.
The band gap energy of spreading layer 7 is higher than that of the window 4 layer 8 and lower than that of the upper cladding layer 6 and the carrier concentration of the spreading layer 7 is lower than that of the window layer 8 and higher than that of the upper cladding layer 6. From the upper cladding layer 6 to spreading layer 7 one can use a linear composition gradient, a parabolic composition gradient or a staged composition gradient layer to alleviate lattice mismatching. From the spreading layer 7 to window layer 8, one can use a linear composition gradient, a parabolic composition gradient or a staged composition gradient via a transfer layer to transfer band gap energy smoothly from the higher band gap spreading layer to the lower band gap window layer.
Compositions of a typical example are set out below.
1. Distributed Bragg reflector layer 3
Al.sub.x Ga.sub.1-x As/Al.sub.y Ga.sub.1-y As
where 0.ltoreq.x.ltoreq.1; 0.ltoreq.y.ltoreq.1; x.noteq.y
2. Cladding layers 4 and 6
(Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P
where 0.5.ltoreq.x.ltoreq.1; 0.4.ltoreq.y.ltoreq.0.6
Thickness 0.5 .mu.m.ltoreq.D.ltoreq.1.5 .mu.m
3. Current spreading layer 7
(Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P
where 0.ltoreq.x.ltoreq.0.5; 0.ltoreq.y.ltoreq.0.2
Thickness 0.ltoreq.D.ltoreq.1 .mu.m
4. The transfer layer
(Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P
where 0.ltoreq.x.ltoreq.1; 0.4.ltoreq.y.ltoreq.0.6
using a linear composition gradient, a parabolic composition gradient or a staged composition gradient.
5. Multi-quantum well
(Al.sub.x1 Ga.sub.1-x1).sub.1-y1 In.sub.y1 P/(Al.sub.x2 Ga.sub.1 x2).sub.1-y2 In.sub.y2 P
Thickness D.ltoreq.20 nm
where 0.5.ltoreq.x1.ltoreq.1; 0.4.ltoreq.y1.ltoreq.0.6/0.ltoreq.x2.ltoreq.0.4; 0.ltoreq.y2.ltoreq.0.4
6. Window layer 8
Ga.sub.x In.sub.1-x P
where 0.9.ltoreq.x.ltoreq.1
Thickness 5 .mu.m.ltoreq.D.ltoreq.15 .mu.m
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