Semiconductor sub-assemblies for emitting modulated light

A method for fabricating a semiconductor device for emitting modulated light, comprising:

• providing a PIN junction on a semi-insulating substrate, wherein the PIN junction comprises a first doped layer on top of the substrate, an undoped active layer on top of the first doped layer, and a second doped layer on top of the undoped active layer;
• etching, at least partially, the second doped layer to define an electrically insulating section between a first section and at least a second section, wherein the second doped layer only remains unetched in the first section and in the second section;
• depositing a waveguide mask that covers partially the first section, the second section and the electrically insulating section;
• etching regions of the first section, the second section and the electrically insulating section that are not covered by the waveguide mask, so that:
• the first doped layer and the undoped active layer are removed in regions of the electrically insulating section that are not covered by the waveguide mask;
• the second doped layer and the undoped active layer are removed in regions of the first section and the second section that are not covered by the waveguide mask;
• and
• the remaining of the undoped active layer forms an optical waveguide;
• removing the waveguide mask in the electrically insulating section;
• providing a semi-insulating epitaxy layer in the electrically insulating section and in regions of the first section and the second section that are not covered by the waveguide mask;
• removing the waveguide mask to uncover, at least partially, the second doped layer in the first section and the second section;
• at least partially removing the semi-insulating epitaxy layer in the first section and the second section, in areas outside the waveguide, to uncover the first doped layer in the first section and the second section in the areas.