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For my high-school physics class, we are studying the theoretical basics of transistors. I have not been able to understand which side of the transistor is the collector and which side is the emitter by looking the current flow in the circuit. I know that the doping and the size are different, but that doesn't help me when I try to calculate the base current / current at the emitter.

Can you explain me which how I should name Currents I1, I2 and I3 in the drawing? Thanks! enter image description here

Skkk
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  • The emitter is much more heavily doped than the collector, though going into much more detail would take more time than I have right now. – Hearth May 27 '18 at 18:25
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    [This explanation](https://electronics.stackexchange.com/questions/78366/why-cant-two-series-connected-diodes-act-as-a-bjt/83268#83268) may also help somewhat. Books on "Microelecronics", such as Jacob Millman's 1979 edition, will also help a lot. Looking at how they are constructed also may help: [BJT Technology](http://mgh-courses.ece.gatech.edu/ece4430/ECE4430/Unit3/BJTTechnology.pdf). – jonk May 27 '18 at 18:34
  • I3 is base current, I1 is collector, I? is emitter. Collector is more lightly doped than emitter, but there are lots of other subtle geometry differences. The 'two diodes' model tells you what your 'diode testing' multimeter will read when you check two leads at a time, but it will tell you nothing about how it works as an amplifier. – Neil_UK May 27 '18 at 19:28
  • [from the "this explanation" of JONK]I'd thought of the difference as being one of geometry (by my understanding, BJTs are generally not symmetric, but have the emitter in the middle, surrounded by the collector which is in turn surrounded by the base), but it makes sense that transitor-ish behavior can't penetrate metal. – supercat Sep 24 '13 at 15:19 – analogsystemsrf May 28 '18 at 04:16
  • @analogsystemsrf This might also be a useful history: [transistor physics history](https://aip.scitation.org/doi/pdf/10.1063/1.1354371). – jonk May 28 '18 at 05:04
  • See also https://electronics.stackexchange.com/questions/227319/why-does-the-transistor-work-with-collector-and-emitter-swapped – dim Jun 29 '18 at 14:11

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Short answer is their doping level different.

Firstly, emitter doping is heavily doped relative to collector since good amplification needed. That is also why there is two gain parameters \$\beta_F\$ (forward current gain) and \$\beta_R\$ (reverse current gain).

The reason why we doped emitter heavily is decreasing the hole component of the base current not increasing electron component of emitter current.

All purpose is increasing emitter efficiency which defined as:

$$\gamma = \frac{I_{E_n}}{I_{E_n}+I_{E_p}}$$

\$\gamma\$ becomes larger with smaller \$I_{E_p}\$. Increase in \$\gamma\$ increase current transfer ratio and the current gain.

$$\alpha = B \gamma$$ $$\beta = \frac{\alpha}{1-\alpha}$$

SamGibson
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Nail Tosun
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For a transistor to work in the standard design, the emitter and base should be forward biased and collector base shoul be reverse biased.

Here, I1 is named as Iec. I2 is named as Ieb.

Sagnik Sarkar
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