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

Bipolar junction transistors (BJTs) are designed to provide the optimum performance when they are correctly connected. In reverse connection, it behaves as a poor performance (low hFE) transistor. However, the voltage that can be applied to the collector-base voltage is very low. This voltage cannot exceed the absolute maximum rating (VEBO) (Table. 1). 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 hFE. 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, VCBO). 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.
A normal transistor maintains a high breakdown voltage (VCBO / VCEO) due to a low-concentration collector. If connected in reverse, the high-concentration emitter will act as the collector, and the voltage that can be applied (VEBO) will be extremely low.
If the connections of the collector and emitter terminals are reversed, the above relationship is not satisfied. In this case, hFE 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 VCEO, 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.

* At present, Toshiba does not provide muting transistors.

Table. 1: Absolute Maximum Ratings（2SC2712）
Table. 1: Absolute Maximum Ratings（2SC2712）

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