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There is a rising demand for harsh-environment integrated circuits and sensors for a wide variety of applications, ranging from structure health monitoring and process control to space navigation. The ability to continually obtain information in situ in high temperature environment such as a jet engine or a deep oil well can potentially save millions of dollars and even human lives. It also opens doors to space missions to locations with extreme conditions such as Venus, where devices would be required to operate around 500 °C. Wide bandgap materials are well suited for these applications due to their superior electrical and mechanical properties compared to the silicon incumbents. 4H-SiC, a polytype of silicon carbide (SiC), for instance, has a bandgap (3.2 eV) that is almost 3 times of that of silicon (1.12 eV). The wider bandgap results in a much lower intrinsic carrier concentration compared to that of silicon, which makes it an ideal candidate for high temperature (> 300°C) applications. These high temperature capabilities will be the main subject of investigation in this thesis. The overarching objective of the thesis is to develop 4H-SiC technologies for both transistor and sensor devices. Specifically, we investigate and develop the design, simulation, fabrication and characterization of 4H- SiC bipolar junction transistor (BJT) that is capable of operating at elevated temperatures up to 400°C. In addition, a high-performance temperature sensor based on 4H-SiC pn diode which can stably operate in a temperature range from 20°C to 600°C is demonstrated. This type of temperature sensor can be integrated with supporting circuitries to create a sensing module that is capable of working at extremely high temperatures.