The precise voltage at which the Collector ceases to be an effective 'collector of electrons' depends on the temperature and the manufacturing details of the transisor. In general we can expect most Bipolar Transistors to work efficiently provided that we arrange for a V CE value of at least two or three volts - and preferrably five volts or more. Such a device can be used as an effective amplifier. Lower voltages may prevent it from working correctly. Note that the graphs shown on this page are only meant as a general guide.
Some transistors can work with much higher currents, or have much higher current gains, etc.
However, the general pattern of behaviour of all Bipolars is essentially the same as described in these pages. University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.