  We show that a quantum thermal transistor can also cause the transistor
effect - where one out of three terminals can control the flow of heat current
in the other two - with good amplification properties in the transient regime
for certain paradigmatic initial states. We find three broad classes of
transient quantum thermal transistors - the first having a smaller
amplification than the steady state quantum thermal transistor, the second with
better amplification but a smaller operating region in terms of temperature,
and the third that gives higher amplification with a larger operating region.
The last type is of particular interest as it also operates in the region where
the steady state thermal transistors lose the transistor effect. We discuss in
some detail certain initial states for which the cases of necessarily transient
transistors arise. We analyze the time variation of the amplification factor of
transient thermal transistors and estimate the preferable time and duration for
which they can work efficiently. Cumulative studies of the differences in
magnitudes of the amplifications of heat currents at the non-base terminals of
the quantum thermal transistor, first with respect to the base-terminal
temperature and next with time, are also presented.
