||Silicon-based microdevices for NO2 detection and DNA amplification have been developed using bulk and surface micromachining technology. Thin film gas sensor has many advantages such as low production cost and possible integration with microelectronics. WO3 was chosen as the sensing material in this study due to its high sensitivity toward NO2. A thin film gas sensor consists of a pair of Au electrodes and WO3 sensing film has been fabricated on silicon substrate. The structural, compositional and electrical characteristics of the NO2 sensor have been studied in detail. The WO3 sensing film was found to be in triclinic phase and close to its stoichiometric ratio. On one hand, sensitivity of the sensor increases sharply with decreasing annealing time due to smaller grain size at short annealing time. On the other hand, it increases slowly with increasing annealing time for annealing time longer than 4 hr as a result of surge in oxygen vacancy concentration. Furthermore, higher sensitivity could be achieved at lower operating temperature, and the optimum was found at 200°C. The presence of NO2 could be detected at concentration as low as 1 ppm, and its high selectivity enabled the detection of NO2 to be distinguished from other interfering gases such as NO, CO, and H2. The sensor fabricated in this work is relatively stable and presents no compositional change after experiment. The sensing film can be ultimately integrated with micro-hot plate (MHP) for in-situ gas measurement. Prototypical microreactors with integrated heaters and temperature sensors, either Pt-based or polysilicon-based, have been established on silicon substrates for DNA amplification. The reaction chamber is capped from the top by either silicon or silicon nitride (Si3N4) membrane, and anodic bonded from the bottom by a borosilicate glass. The Si3N4 membrane provides an excellent thermal isolation for the reaction chamber so that thermal budget of the device is minimized. We have also implemented a digital PI control algorithm to ensure a precise temperature control, and rapid heating and cooling for fast thermal cycling of reagents. Chip-based polymerase chain reaction (PCR) has been successfully demonstrated using 10 μl Pt microreactor with silicon membrane. Similar amplification performance has been achieved using this microreactor compared with the conventional thermal cycler. Finally, the microreactor can be used as a platform for integration with other analytical modules to achieve "lab-on-a-chip" concept for highly integrated and efficient DNA analysis.