A semiconductor laser to be fabricated on a semi-insulating substrate is expected to result in advantageous high frequency characteristics because parastici capacitance produced by its electrodes is extremely small. Such a type of a conventional semiconductor laser is described on pages 410 to 412 of "Applied Physics Letters, 32(7)" published on Apr. 1, 1978. The semiconductor laser comprises a lower n-cladding layer formed on a semi-insulating substrate, an active layer formed on the lower n-cladding layer, an upper n-cladding layer formed on the active layer, a Zn-diffused p+ region formed across the upper and lower n-cladding layers and active layer, n and p-electrodes respectively positioned on the upper n-cladding layer and Zn-diffused p+region, and an insulating layer formed between the n and p-electrodes. The active layer includes an active region in which electrons and holes are recombined and which is limited transversely to be narrowly striped.
In operation, the current confining is effected in accordance with the difference of built-in potentials between pn-junctions at the n-cladding layer and active layer so that electrons are injected to the active region through the n-cladding layers and active layer from the n-electrode, while holes are injected thereto through the Zn-cladding region from the p-electrode. As a result, the semiconductor laser oscillates in the active region in a transverse mode dependent upon the difference of refractive indexes at pn-junctions. The parasitic capacitance thereof is produced in accordance with pn-junctions at the n-cladding layers, while a resonator is structured therein between cleaved facets thereof.
In the semiconductor laser, however, there is a disadvantage that current leakage is large in a case where a high power is output therefrom for the reason why the current confining is effected in accordance with the difference of built-in potentials between pn-junctions at the n-cladding layers and active layer as mentioned before. Further, there is another disadvantage that a high frequency characteristic is lowered due to the parasitic capacitance produced by pn-junctions at the n-cladding layers. Still further, manufacturing yield is not so high as expected in fabricating semiconductor lasers for the reason why it is very difficult to diffuse Zn to a predetermined extent.
The U.S. Pat. No. 4,636,821 also discloses such a type of another conventional semiconductor laser which comprises n and p-conductive layers formed to have a predetermined width of a groove therebetween on a semi-insulating substrate, an active region formed in the groove, a semi-insulating cap layer formed on the active region in the groove, n and p-electrodes provided respectively on the n and p-conductive layers, and an insulating layer positioned between the n and p-electrodes.
In operation, electrons are injected to the active region through the n-conductive layer from the n-electrode, while holes are injected thereto through the p-conductive layer from the p-electrode. As a result, the semiconductor laser oscillates in the active region in a transverse mode dependent upon the difference of refractive indexes between the active region, and the n and p-conductive layers, semi-insulating cap layer and semi-insulating substrate.
In the latter semiconductor laser, however, there is a disadvantage that the electric resistance thereof is as large as several 10.OMEGA. for the reason why the active region is actually 0.1 .mu.m in accordance with the requirement of a single transverse mode so that a considerable amount of heat is generated in 0.1 .mu.m slit-shaped semiconductor layer.