1
\$\begingroup\$

I read a tutorial about transistors and I reached the following sentence. "The Bipolar Junction Transistor (BJT) is a three layer device constructed form two semiconductor diode junctions joined together, one forward biased and one reverse biased."

It made me think that diodes are inherently forward biased or reverse biased but from what I know this depends on what is the direction of the voltage applied to the diode, so the same diode can be forward biased or can be reverse biased depending on the direction of the electric field (voltage).

Is this right?

\$\endgroup\$
  • 1
    \$\begingroup\$ Any diode can be forward or reversed biased but it depends on the external circuit not the device. \$\endgroup\$ – JIm Dearden Mar 26 '17 at 14:37
  • \$\begingroup\$ Tutorial is badly worded. Biasing is an external stimulus not a property of the device. \$\endgroup\$ – Trevor_G Mar 26 '17 at 14:49
  • \$\begingroup\$ thanks for the clarifications, do you know any good materials to learn semiconductors ? \$\endgroup\$ – yoyo_fun Mar 26 '17 at 14:50
  • 1
    \$\begingroup\$ You might get more answers and attention if you formally accepted answers to questions that you have raised. \$\endgroup\$ – Andy aka Mar 26 '17 at 14:54
  • \$\begingroup\$ @andy awarded best answers \$\endgroup\$ – yoyo_fun Mar 26 '17 at 19:41
4
\$\begingroup\$

The tutorial is worded very badly.

Biasing is an external stimulus not a property of the device.

The tutorial should read something like.. "The Bipolar Junction Transistor (BJT) is a three layer device constructed LIKE two semiconductor diode junctions joined together, cathode to cathode, or anode to anode."

\$\endgroup\$
4
\$\begingroup\$

The tutorial is plain wrong and you are right to question it. A good call on your part.

Biasing is an action and a consequence of applying a voltage. The BJT's PN junctions will conduct when forward biased and oppose conduction when reverse biased (until the reverse voltage is high enough to cause an electrical breakdown). The effect of one junction on another in a transistor leads to its useful application as a current-amplifying device.

Can I suggest that you pass feedback to the tutorial's author, if possible, so others don't get confused and misled.

\$\endgroup\$
  • \$\begingroup\$ i will send feedback.i will also post yhe link here when i am home \$\endgroup\$ – yoyo_fun Mar 26 '17 at 19:43
2
\$\begingroup\$

And to paint the picture

enter image description here

showing that the 'diodes' of the BJT can be biased in any direction simply by changing the polarity of the voltage across them.

Note the 'normal' way of setting up the BJT is (3) which is what the article seems to be alluding to. Vbe forward, Vcb reverse but failed to give the whole picture.

Also you can't make a functioning BJT by simply connecting two diodes connected back to back - they need to be constructed in the same crystal.

\$\endgroup\$
1
\$\begingroup\$

so the same diode can be forward biased or can be reverse biased depending on the direction of the electric field (voltage).

at any given point, a diode is either forward conducting and reverse conducting (shutting off ideally).

whether it is forward biased or reverse biased will depend on the design.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.