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I am new in BJT. Why is the hFE of a BJT less than 1 in reverse active mode?

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  • \$\begingroup\$ A BJT used in reverse (collector and emitter swapped) will work as a BJT, but usually with reduced gain. The gain is still usually above 1, but lower than when the transistor is used as intended. \$\endgroup\$ Dec 29, 2014 at 14:56

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The hfe is the ratio of collector current to base current. Due to asymmetry in the doping of collector and (more highly doped) emitter regions, the hfe will be less in the reverse active mode compared to forward active mode. See this answer, for example.

There's no reason that I can think of that the hfe might not be less than one under some conditions, though it's typically something more like 5-20 for an ordinary 'jellybean' transistor (in comparison to several hundred in the forward region). For example, hfe at very low or very high currents, or temperature extremes, tends to be significantly less than the hfe under optimal conditions.

There are special symmetrical transistors that have more symmetrical gain and breakdown voltages.

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follow NPTEL documents for explanation and study of BJT.
There are few Lecture Series also available.

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