Semiconductors have been fundamental to various devices that are typically operated with electric field, such as transistors, memories, sensors, and resistive switches. There is growing interest in the development of novel inorganic materials for use in transistors and semiconductor switches, which can be operated with a temperature gradient. Here, we show that a crystalline semiconducting noble metal sulfide, AgCuS, exhibits a sharp temperature dependent reversible p-n-p type conduction switching, along with a colossal change in the thermopower (Delta S of similar to 1757 mu V K-1) at the superionic phase transition (T of similar to 364 K). In addition, its thermal conductivity is ultralow in 300-550 K range giving AgCuS the ability to maintain temperature gradients. We have developed fundamental understanding of the phase transition and p-n-p type conduction switching in AgCuS through temperature dependent synchrotron powder X-ray diffraction, heat capacity, Raman spectroscopy, and positron annihilation spectroscopy measurements. Using first-principles calculations, we show that this rare combination of properties originates from an effective decoupling of electrical conduction and phonon transport associated with electronic states of the rigid sulfur sublattice and soft vibrations of the disordered cation sublattices, respectively. Temperature dependent p-n-p type conduction switching makes AgCuS an ideal material for diode or transistor devices that operate reversibly on temperature or voltage changes near room temperature.