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The common phenomenological explanation of the forward active transistor operation is (e.g. in case of a NPN transistor) that when the base-emitter junction is forward biased holes can diffuse from the base side to the emitter side and electrons from the emitter side to the base side. Because the base is thin and there is an electron concentration gradient to the collector space charge region they diffuse in this direction and are then swept away to the collector by the space charge field.

This means the base current is just the normal hole current of the base-emitter diode and the collector current is just the normal electron current of the base-emitter diode. (Electron-hole recombination in the base neglected.)

This explanation would imply that when the collector is left open the whole emitter current is steered to the base, so the base current with open collector would be beta times larger.

From physics point of view (not electrical engineering point of view), why does this not happen? What impedes the electron diffusion even for large base widths when the collector is open?

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This means the base current is just the normal hole current of the base-emitter diode and the collector current is just the normal electron current of the base-emitter diode.

This isn't the way it's usually understood.

The collector current in forward active (FA) operation is still determined by minority carriers on either side of the C-B junction. That is, for an NPN device, holes on the collector side and electrons on the base side.

The collector current in FA operation is produced by the electrons injected into the base from the collector, not by the hole component of the base-emitter junction current. Because the C-B junction is reverse biased, there are very few holes on the base side of the junction and their contribution to the collector current is minimal. For the transistor to work well, the base region must be short enough that only a small fraction of the injected electrons recombine before reaching the C-B junction.

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  • $\begingroup$ The collector current is not produced by electrons injected from the collector but by electrons injected from the emitter. Of course the collector current is not caused by any hole current, I have not written this in my post. So ignore the holes for the moment, they are just "flowing" from base to emitter. My question is regarding the electrons. The emitter electron current is a function of the base-emitter voltage, not of the collector-emitter voltage (neglecting base width effects). So why is the emitter current reduced when the colector is left open? $\endgroup$
    – user3224237
    Mar 16 at 15:22

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