Over the past decade, we have witnessed a renaissance in power and RF electronics -- enabled by breakthroughs in wide-band gap semiconductors made using III-nitride materials. Yet, an exciting but emerging application of these materials is their use in extreme temperature environments such as oil/gas, combustion and space, where traditional silicon electronics do not operate. In this thesis, I discuss how polarization fields in gallium nitride on silicon (GaN-on-Si) can be used to make thermoelectric energy harvesting and sensing devices to achieve this ambitious goal of "extreme environment" operation. In the first part of this thesis, I discuss measurements of the thermoelectric properties of the aluminum gallium nitride/gallium nitride (AlGaN/GaN) two-dimensional electron gas (2DEG) from room temperature to 300 degrees Celsius. Our experiments demonstrate state-of-the-art thermoelectric power factors and thermoelectric figures of merit ~4x better than doped III-nitride materials. These properties can enable a monolithic GaN-on-Si micro-thermoelectric generator with a power density of ∼1 mW for 1 cm by 1 cm footprint, which can be used to power an extreme environment IoT node. I follow this with a brief digression on the thermoelectric properties of AlGaN/GaN films on a pyramidal Si substrate, a strategy that can increase power density in micro-thermoelectric generators. Switching gears, I next discuss how the transfer of momentum from lattice phonons to electrons, a phenomenon called phonon drag, can be used to boost the low temperature thermoelectric performance in the AlGaN/GaN 2DEG. The measurements of the phonon drag Seebeck coefficient are conducted by varying the thickness of the underlying GaN layer. For large GaN thickness (∼1.2 micrometers), we find that ∼32% of Seebeck coefficient at room temperature can be attributed to phonon drag. At 50 K, the drag component increases significantly to ∼88%. In the last part of my thesis, I discuss how manipulation of the polarization fields in the AlGaN/GaN 2DEG can be used for high-performance sensing applications. I first discuss a model for studying electronic transport in AlGaN/GaN transistors under small applied strains, which may find use in pressure sensing and device packaging. Then, I present measurements of a novel ultraviolet (UV) photodetector employing the 2DEG formed at the AlGaN/GaN interface as an interdigitated transducer. This photodetector exhibits a record high normalized photocurrent-to-dark current ratio, which enables highly sensitive detection of UV optical stimuli. Overall, the techniques explored in this thesis are an important step towards the maturation of the AlGaN/GaN-on-Si platform as an extreme environment IoT node.