Porous silicon membranes have been formed by electrochemical etching through wafers up to 500 μm in thickness. The results show that there is essentially no limit to the thickness of porous silicon layers and that self-supporting porous structures can be formed by this method. In addition, the membranes are structurally intact and can be removed from the wafer. The rate of porous layer formation, at an etching current of 50 mA cm−2, is in the range 20–50 nm s−1 and is dependent on the dopant type and concentration.