Field Effect Transistor - High Input Impedance

[AnonymousPT]

I know FET offers a very high input impedance..but what's the reason behind it? Is it due to the reverse voltage applied at gate with respect to the source which makes the P-N junction reversed biased? If that the case, what happens when there is no voltage applied at gate?

 

[QuantumQuest]

The very high impedance is caused by the isolation layer between gate and channel. For the case of a MOSFET it is made of SiO₂ (resistance 10¹⁶ Ω m). So, because of this, applying a voltage to the gate gives a small current - leakage current. If you compare to a normal bipolar transistor, this needs considerably more current to its collector - depending on the application, in order to operate and this implies some bigger current on its base too.

 

[AnonymousPT]

What about JFET? There is no such insulating layer between the gate and the channel.. Then?

 

[QuantumQuest]

JFETs have a narrow piece of high resistivity semiconductor material. Thus, a channel is formed ( N-type or P-type Si), through which there is flow of majority carriers. The channel is doped with donor impurities for N-Channel JFETs (negative current flow) or acceptor impurities for P-Channel (positive current flow).

 

[phyzguy]

What about JFET? There is no such insulating layer between the gate and the channel.. Then?

There is a reverse-biased diode between the gate and the channel in a JFET. So the only current that flows into the gate is the diode reverse leakage current, which is very small. The gate current is not as low as a MOSFET, but it is much lower than the base current in a BJT.