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A monolithic gas sensor array fabricated in industrial CMOS-technology combined with post-CMOS micromachining is presented, which comprises three metal-oxide-covered micro-hotplates and the necessary driving and signal-conditioning circuitry. The array approach enables effective detection and discrimination of several analyte gases by using different gas-sensitive materials. The operating temperature of the metal oxide resistors varies between 200 and 350°C. Individual temperature regulation is performed by three on-chip temperature controllers with a resolution of 0.5°C. The phase margin of the temperature controllers was improved by adding degeneration resistors in the sources of the power transistors. An implemented sequential operation mode reduces the current consumption by 15%. A ring counter provides the clock signal for sequential operation. A novel circular membrane design was developed to improve the power efficiency of the micro-hotplates.