Transistors and current direction

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SUMMARY

Current can flow in either direction in bipolar junction transistors (BJTs), metal-oxide-semiconductor field-effect transistors (MOSFETs), and junction field-effect transistors (JFETs), but practical applications dictate specific terminal usage. JFETs and MOSFETs are theoretically symmetric, meaning there is no inherent difference between drain and source; however, manufacturers design these devices to optimize performance, making them asymmetric in practice. Operating a BJT in reverse can lead to significantly reduced performance due to its lower beta in the reverse active region compared to the forward active region.

PREREQUISITES
  • Understanding of bipolar junction transistors (BJTs)
  • Knowledge of metal-oxide-semiconductor field-effect transistors (MOSFETs)
  • Familiarity with junction field-effect transistors (JFETs)
  • Basic principles of transistor operation and characteristics
NEXT STEPS
  • Research the differences between forward active and reverse active regions in BJTs
  • Explore the impact of lightly-doped drain design in MOSFETs
  • Study the performance characteristics of JFETs and MOSFETs in practical applications
  • Learn about the implications of transistor symmetry in circuit design
USEFUL FOR

Electrical engineers, electronics students, and professionals involved in circuit design and semiconductor technology will benefit from this discussion.

jaydnul
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Can current go either direction in a bjt, mosfet, and jfet? Or are they all usually just one direction?

(Especially the jfet. Given the symmetry, I don't see what the difference is between drain and source)

Thanks
 
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The current can go in either direction in any of those devices (by direction, I mean from drain/collector to source/emitter. JFETs and MOSFETS are *in principle* symmetric so there isn't any difference between drain and source in first-order theory. In that sense you're correct. HOWEVER, in practice they are not symmetric. Manufacturers play a lot of tricks to increase the breakdown voltage, improve performance etc (for example look up lightly-doped drain). For this reason you should use the device based on which terminals are labeled drain and source. The take away is that MOSFETs and JFETs that you buy are not symmetric.

For a BJT, while you can operate the device "backwards" keep in mind that performance will be greatly reduced if you do this. Doing so puts the transistor into what is called the "reverse active region" and has much lower beta than the device does in its intended "forward active region".

Hope this clears it up.
 
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Perfect, thanks!
 

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