Lecture: 2 (Semiconductor Diodes)
PN Junction Semiconductor Diode
A p-n junction semiconductor diode or p-n diode is two-terminal or two-
electrode semiconductor device, which allows the electric current in only one direction while
blocks the electric current in opposite or reverse direction. If the diode is forward biased, it allows
the electric current flow. On the other hand, if the diode is reverse biased, it blocks the electric
current flow. P-N junction semiconductor diode is also called as p-n junction semiconductor
device.
Diode Characteristics
Important Terms
Diode Equivalent Circuits
DC and AC Resistance Levels
DC resistance or static resistance: The resistance offered by a p-n junction diode when it is
connected to a dc circuit is called dc resistance.
AC resistance or dynamic resistance: The resistance offered by a p-n junction diode when it is
connected to a ac circuit is called ac resistance.
Average AC resistance: If the input signal is sufficient enough to produce a large swing, then
the resistance related to the diode for this region is called as AC average resistance.
***Tangent is a straight line or plane that touches a curve or curved surface at a point, but if
extended does not cross the curve at that point.
Zener Diode
****Write the differences between P-N junction diode and
Zener diode.
Light Emitting Diode (LED)
Light Emitting Diode or LED is nothing but a PN junction diode. When any diode is in forward
biased current flows through the diode. Current is caused in the diode due to flow of free electrons
and holes. During this process of flows of holes and free electrons there will be recombinations
amongst them. That means free electrons will recombines with empty holes. During this
recombinations free electrons in the conduction bands jumps to the holes in the valence band.
During this jump electron emits energy in forms of photon. This energy is equal to the forbidden
energy gap between conduction band and valence band. We know that the energy of photon is E
= hf (f = Frequency of electromagnetic radiation and h = Planks constant). Photon is released in
all electron and hole recombination but light only emit in LED. In normal Si the entire radiation
of electromagnetic wave during recombination is in the form of heat. Because Si and Ge
semiconductor are indirect band semiconductor. In indirect gap semicoductor maximum energy
level of valance band and minimum energy level of conduction band do not occur at the same
momentum of electron. Hence during recombination migration of electron from the conduction
band to valence band the momentum of electron will be changed. The photon regenerated from
the electron will be mostly utilized for the electron momentum. But in indirect gap semiconductor
the maximum of valence band and the minimum of conduction band occur at the same
momentum. Hence there will be no change of momentum of electron during the migration from
conduction band to valence band. So the photon originated due to that migration have not to
provide momentum to electrons. Unlike nomal diodes which are made from Ge or Si the indirect
gap semicnductor materials LED are made from direct gap semiconductor compounded such as
GaAs, GaP, GaAsP, SiC, GaInN.
How does the Light Emitting Diode Work?
The light-emitting diode simply, we know as a diode. When the diode is forward biased, then the
electrons & holes are moving fast across the junction and they are combined constantly, removing
one another out. Soon after the electrons are moving from the n-type to the p-type silicon, it
combines with the holes, then it disappears. Hence it makes the complete atom & more stable and
it gives the little burst of energy in the form of a tiny packet or photon of light.
Application of LED:
Application of Tunnel Diode:
1. Tunnel diode can be used as a switch, amplifier and oscillator.
2. It shows fast frequency response, it is used in high frequency component.
Application of Photo Diode:
1. Photo diode is used as a photo detector.
2. Photo diode in photo sensor, remote control device.
Application of Schottky Diode:
Schottky diode has high switching speed so it is used in digital IC, diode rectifier circuit etc.
Application of Varactor Diode:
It is used in tunning circuit. It is used mainly in radio frequency or RF circuits to provide voltage controlled variable capacitance.
Laser Diode
Application of Laser Diode:
Laser diode is used in laser printing, laser Scanning, fiber optic communications, barcode readers, laser pointers etc.
