  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 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 electron and holes of a pnp device using MoS$_2$ for
the 10-nm base and WSe$_2$ for emitter and collector. Our three-terminal device
simulations confirm the working principle and a large current modulation
I$_\text{ON}$/I$_\text{OFF}\sim 10^8$ for $\Delta V_{\rm EB}=0.5$ V. Assuming
ballistic transport, we are able to achieve a current gain $\beta\sim$ 10$^4$
over several orders of magnitude of collector current and a cutoff frequency up
to the THz range. Exploration of the rich world of bipolar nanoscale device
concepts in 2D materials is promising for their potential applications in
electronics and optoelectronics.
