20-03-2012, 04:17 PM
Junction Field Effect Transistor
chap4-FET LUX.ppt (Size: 4.22 MB / Downloads: 66)
Introduction (FET)
Field-effect transistor (FET) are important devices such as BJTs
Also used as amplifier and logic switches
Types of FET:
MOSFET (metal-oxide-semiconductor field-effect transistor)
Depletion-mode MOSFET
JFET (junction field-effect transistor)
Junction FETs (JFETs)
JFETs consists of a piece of high-resistivity semiconductor material (usually Si) which constitutes a channel for the majority carrier flow.
Conducting semiconductor channel between two ohmic contacts – source & drain
N-channel JFET
Major structure is n-type material (channel) between embedded p-type material to form 2 p-n junction.
In the normal operation of an n-channel device, the Drain (D) is positive with respect to the Source (S). Current flows into the Drain (D), through the channel, and out of the Source (S)
Because the resistance of the channel depends on the gate-to-source voltage (VGS), the drain current (ID) is controlled by that voltage
JFET Characteristic Curve
To start, suppose VGS=0
Then, when VDS is increased, ID increases. Therefore, ID is proportional to VDS for small values of VDS
For larger value of VDS, as VDS increases, the depletion layer become wider, causing the resistance of channel increases.
After the pinch-off voltage (Vp) is reached, the ID becomes nearly constant (called as ID maximum, IDSS-Drain to Source current with Gate Shorted)