The design and performance of an electrochemical apparatus and, the process for the preparation of porous silicon with different controlled surface structures is described. The apparatus includes controlled rotation of the electrolyte vessel, which is in contact with a thermal bath. This permits the etching electrolyte to react with the silicon substrate at different temperatures and at different rates of renewal of the solution. Atomic force microscopy images show at least three different classes of samples according to the topographical features: samples with surface hillocks, samples with surface holes, and samples with a mixture of holes and hillocks. The different morphologies are important for various applications.