LOCOS with sidewall spacer for transistors and other devices

An integrated circuit (IC) includes a first field-plated field effect transistor (FET), and a second field-plated FET, and functional circuitry configured together with the field-plated FETs for realizing at least one circuit function in a semiconductor surface layer on a substrate. The field-plated FETs include a gate structure including a gate electrode partially over a LOCOS field relief oxide and partially over a gate dielectric layer. The LOCOS field relief oxide thickness for the first field-plated FET is thicker than the LOCOS field relief oxide thickness for the second field-plated FET. There are sources and drains on respective sides of the gate structures in the semiconductor surface layer.