Unipolar / Bipolar ?

[Electroenthusiast]

BJT Transistors are Bipolar...(current is through holes and electrons)
FET 's are Unipolar...(current is cos of Electron or Holes)

Can anyone pls justify how it is so?

 

[mneary]

Look at the relative doping levels.

 

[ke5frf]

You need to understand that in semiconductors, the doping of the silicon. P material has an impurity that creates an electron deficiency. N material has excess electrons from its donor material.

The electron deficiency in the P material creates what are known as "holes". And of course the N material has the "free" electrons.

Like magnetic POLES, P and N material attracts. The "poles" referring to the charge carriers (holes and electrons)
In bipolar devices, when the P-N junction is conducting, there is an exchange of the charge carriers in both directions. This leads us to conventional flow vs electron flow. Holes moving toward the N material is called conventional flow, Electrons moving toward the P material is called electron flow. A bipolar junction transistor works this way, and the region where the P-N materials meet behaves very much like the poles of a magnet.

In unipolar devices, only one charge carrier moves. Because there is only one carrier influencing the device, the ELECTRIC FIELD created is the means of control. This is why FETs are called voltage controlled devices.