Abstract: A discrete electroabsorption modulator for optical signals has an electrical blocking region (12a, 12b) which is large as compared with the absorber region (13). The height of the blocking region (12a, 12b) is 3-20 .mu.m, e.g. 4-6 .mu.m and preferably about 5 .mu.m. In addition, the blocking region (12a, 12b) is 2-250 times, e.g. 15-25 times the height of the absorber region (13). The modulation depth is substantially increased, e.g. to values above 40 dB, by a thick electrical blocking region. It is believed that the thick blocking layer (12a, 12b) works because parasitic signals which escape into the blocking region are spatially removed from the vicinity of the absorber region; thus they are not acquired at the output.

Abstract: A semiconductor device, preferably a laser device such as a signal generator, a signal amplifier or a signal detector e.g. a distributed feedback laser, which is implemented in III/V semiconductors. Such devices often require a barrier layer to encourage current flow to pass through the localised p/n-interface and this invention provides the barrier layer in the form of a layer of hole trapping semiconductor material located between and in contact with two p-type layers. III/V semiconductors contain at least one of indium, gallium and aluminum and at least one of phosphorus and arsenic but the preferred devices are laser devices implemented in various types of indium phosphide except for the active zone wherein photons are generated. The active zone is preferably formed of ternary and/or quaternary semiconductors. In the preferred structures the barrier layer is formed of chromium doped indium phosphide which is located between two layers of p-type indium phosphide.