We propose and investigate the intrinsically thinnest transistor concept: a monolayer ballistic heterojunction bipolar transistor based on a lateral heterostructure of transition metal dichalcogenides. The device is intrinsically thinner than a field effect transistor because it does not need a top or bottom gate, since transport is controlled by the electrochemical potential of the base electrode. As is typical of bipolar transistors, the collector current undergoes a tenfold increase for each 60 mV increase of the base voltage over several orders of magnitude at room temperature, without sophisticated optimization of the electrostatics. We present a detailed investigation based on self-consistent simulations of electrostatics and quantum transport for both electrons and holes of a p− n− p device using MoS 2 for the 10-nm base and WSe 2 for the emitter and collector. Our three-terminal device simulations confirm the working …