Semiconductor structures and devices in III-nitride materials are described herein, including material structures comprising III-nitride material regions (e.g., gallium nitride material regions). In certain cases, the material structures comprise substrates having relatively high electrical conductivities. In other cases, the material structures comprise substrates having relatively high resistivities. Certain embodiments include one or more features that reduce the degree to which thermal runaway occurs, which can enhance device performance including at elevated flange temperatures. Some embodiments include one or more features that reduce the degree of capacitive coupling exhibited during operation. For example, in some embodiments, relatively thick III-nitride material regions and/or relatively small ohmic contacts are employed.