A diode allows electricity to flow in one direction only and blocks the flow in the
opposite direction. They may be regarded as one-way valves and they are used in
various circuits, usually as a form of protection. There are different types of diode
but their basic functions are the same.
Diode operation: (a) Current flow is permitted; the diode is forward biased. (b) Current
flow is prohibited; the diode is reversed biased.
Diodes are used in many applications like the following:-
Converting AC power from the 60Hz line into DC power for radios,
televisions, telephone answering machines, computers, and many
other electronic devices.
Converting radio frequency signals into audible signals in radios.
Type of diodes:
Properties of Diodes:-
The current-voltage relationship of a diode
diode will operate in one of three regions:
1. Forward bias: When the voltage across the diode is positive the diode
is “on” and current can run through. The voltage should be greater than
the forward voltage (VF) in order for the current to be anything
2. Reverse bias: This is the “off” mode of the diode, where the voltage is
less than VF but greater than -VBR. In this mode current flow is (mostly)
blocked, and the diode is off. A very small amount of current (on the
order of nA) – called reverse saturation current – is able to flow in
reverse through the diode.
3. Breakdown: When the voltage applied across the diode is very large
and negative, lots of current will be able to flow in the reverse direction,
from cathode to anode.
Regions of Operation:-
bipolar junction transistor (BJT) has three operating regions:
1. Cut off ( for NPN BJT)
2. Active region ( for NPN)
3. Saturated ( for NPN)
In active region, for silicon BJT, and for Germanium BJT.
In saturated region, .
MOSFET (Metal-Oxide-Semiconductor Field-Effect-Transistor)
A three terminal semicondcuting device
Current through two terminals (Drain and Source) can be controlled by voltage in
third terminal (Gate)
One can make an amplifier with MOSFET
MOSFET is also used as switch in modern micro processors
MOSFET Regions of Operation :-
There are three regions of operation in the MOSFET:
1– When VGS <VT, no conductive channel is present and ID = 0, the
2– If VGS <VT and VDS <VDS,sat, the device is in the triode region of
operation. Increasing VDS increases the lateral field in the channel,
and hence the current. Increasing VGS increases the transverse field
and hence the inversion layer density, which also increases the
3– If VGS <VT and VDS >VDS,sat, the device is in the saturation region
of operation. Since the drain end channel density has become
small, the current is much less dependent on VDS , but is still
dependent on VGS, since increased VGS still increases the inversion
Cut-off region (VGS≤VT)
Linear (or Triode) region (VGS>VT and VDS≤VGS−VT)
Saturation region (VGS>VT and VDS≥VGS−VT)
JFET (Field-effect transistor)