Semiconductor devices include one or more transistors having a floating gate and a control gate. In at least one embodiment, the floating gate comprises an intermediate portion extending between two end portions. The intermediate portion has an average cross-sectional area less than one or both of the end portions. In some embodiments, the intermediate portion may comprise a single nanowire. In additional embodiments, semiconductor devices have one or more transistors having a control gate and a floating gate in which a surface of the control gate opposes a lateral side surface of a floating gate that defines a recess in the floating gate. Electronic systems include such semiconductor devices. Methods of forming semiconductor devices include, for example, forming a floating gate having an intermediate portion extending between two end portions, and configuring the intermediate portion to have an average cross-sectional area less than one or both of the end portions.