BJT transistor, collector base junction
In a way, the collector-base junction IS the transistor.
A bipolar junction transistor is basically a reverse polarized diode (the CB junction) whose inverse saturation current (you know, the asymptotic line of the seemingly flat portion that sits just under the negative voltage axis in the characteristic of a diode) is altered by the carriers injected by the base-emitter junction.
Consider the negative voltage portion of a diode and draw it for different values of the Isat current. You will see curves that resembles - albeit in reverse - the output characteristic of a transistor.
The missing part of the picture is a link between the collector and emitter current and that is also what allows a transistor to work as an amplifier: the diffusion current that takes place in the narrow base region allows for the decoupling of the current crossing the CB junction and the current crossing the BE junction from the respective voltages.
Note that you cannot have diffusion current in the conductor between two discrete diodes back-to-back. That's why you cannot build a transistor with two separate diodes, but need to use the same semiconductor in common with the two junctions.
So, in a common base configuration (the configuration usually employed to demonstrate transistor action) you can have a moderate input IE current associated with a small voltage, while on the output nearly the same current (IC is approx IE) is associated with a much bigger voltage. This decoupling also explains why you can use a transistor to match impedances.