A process provides a gate dielectric layer of a first thickness for a memory array and for certain peripheral circuits on the same substrate as the memory array. High-voltage peripheral circuits are provided with a gate dielectric layer of a second thickness. Low-voltage peripheral circuits are provided with a gate dielectric layer of a third thickness. The process provides protection from subsequent process steps for a gate dielectric layer. Shallow trench isolation allows the memory array cells to be extremely small, thus providing high storage density.