The rapid development of mobile and internet‐of‐thing devices demands continuous scaling of metal–insulator–semiconductor field‐effect transistors for high‐resolution and low‐power displays. However, such technology is limited by inadequate scaling of supply voltage and sophisticated dielectric engineering. Here, to enable continued scaling, indium–gallium–zinc‐oxide (InGaZnO) tunnel field‐effect transistor (TFET) and junction field‐effect transistor (JFET) are designed and fabricated based on vertically stacked black phosphorus (BP)/InGaZnO van der Waals heterojunctions. By varying BP thickness, BP/InGaZnO heterojunctions can be operated as forward rectifying diode, Zener diode, and backward rectifying diode, respectively. Room‐temperature negative‐differential‐resistance behavior with large peak‐to‐valley ratio of 2.1 and high tunneling current density of 160 mA mm−2 is obtained in thick‐BP/InGaZnO heterojunction. On this basis, sub‐thermionic subthreshold swing (SS) of 11 mV dec−1 is achieved in InGaZnO TFET. Meanwhile, the InGaZnO JFET based on thin‐BP/InGaZnO heterostructure exhibits good transistor performance of on/off ratio exceeding 105, high field‐effect mobility of 23.5 cm2 V s−1, negligible hysteresis, and improved SS of 83 mV dec−1. The BP/InGaZnO heterojunction is possibly the only device architecture so far to realize the amorphous metal‐oxide‐semiconductors TFET and JFET, thus providing promising pathways for further thin‐film transistor technology.