Abstract: A thin film/MEMS electrochemical gas sensor includes a body having first and second joined subassemblies to form an interior portion of the body, and is composed of a semiconductor material. The body includes at least one opening configured to allow air to pass into the interior portion of the body. A membrane stack is located in the interior of the body, producing an electrical signal that represents a concentration of target gas in the air at the membrane stack. Conductive contacts are configured to provide electrical connection to the membrane stack to access the electrical signal produced by the membrane stack.