Are the collector and emitter terminals of a bipolar transistor interchangeable?

No. Transistors are designed to provide the optimum performance when they are correctly connected. Interchanging the collector and emitter terminals not only degrades the transistor performance but also might cause permanent damage to the device.　　

Generally, bipolar transistors are designed in such a manner as to achieve high h_FE. To accomplish this, numerous majority carriers (electrons in the case of npn transistors) in the emitter must diffuse into the base region and then be efficiently swept across the base region into the collector. It is also necessary to control the depletion layer in the collector region in order to increase a transistor’s withstand voltage (i.e., collector-base voltage, V_CBO). These conditions are fulfilled by:

1. making the dopant concentration in the emitter region higher than that in the base region;　
2. reducing the thickness of the base region;
3. reducing the dopant concentration in the collector region.

Therefore, the dopant concentrations in the three semiconductor regions have the following relationship: emitter >> base > collector.

If the connections of the collector and emitter terminals are reversed, the above relationship is not satisfied. In this case, h_FE decreases, making it impossible for a bipolar transistor to function as intended and possibly causing the base current to exceed its specified rating. Moreover, because of the decreased V_CEO, the transistor might break down when it turns off. As a result of the foregoing, a reverse-connected transistor might be permanently damaged.

Typical bipolar transistors must be connected in the normal direction, except for muting transistors^* that are designed for reverse connection.