unbiased diode, forward biased , reverse biased diode,breakdown,energy hills
DESCRIPTION
Presentation of Unbiased diode, Forward biased , reverse biased diode,breakdown,energy hillsTRANSCRIPT
GOVERNMENT ENGINEERING COLLEGE - BHARUCH (014)
SUB : ELECTRONICS DEVICES & CIRCUITS (2131006)
E.C Div A
Topics :The Unbiased Diode, Forward Bias, Reverse Bias, Breakdown, Energy Levels, The energy Hill, The Barrier Potential and Temperature , Reverse-Biased Diode
Created by:
Meghwal Dinesh S.130140111041
Mistry Keyur R. 130140111043
Nakrani Pratik R. 130140111046
The Unbiased Diode
What is Diode :- A semiconductor device with two terminals, typically allowing the flow of
current in one direction only.
What is Unbiased Diode :- when we do not apply any source of energy or electricity than a
diode is said to be unbiased diode.
The Unbiased Diode FORMATION OF P-N JUNCTION
p type semiconductor
n type semiconductor
AnodeCathode
Junction
P side is called as N type is called as
HolesElectrones
MajorityMinority
Minority
Majority
Diffusion is same
If p region is connected to the positive terminal of external dc source & n region is connected to negative terminal of dc source is said to be forward biasing.
Forward bias
IfIf
Forward biasing of a diode Symbolic Representation
Operation of a Forward biased Diode
--
--
-
+
+
++
+
+
+
-
-
p n
R
Current limiting resister
V
Due to negative terminal of external souce connected to n-region so electrons are pushed towards p-side
Due to Positive terminal of external souce connected to p-region so holes are pushed towards n-side
The holes Converting into the negative ions into neutral atomes
The Electrones Converting into the Positive ions into neutral atomes
Due to this The width of deplation region will reduce
Due to reduction in the depletion region width the barrier also reduced.
Hence a large number of electrons & holes can cross the junction under the influence of externally connected DC voltage
As we can say that the flow of electrons is current so that the current produce in this position said to be forward Current
If p region is connected to the negative terminal of external dc source & n region is connected to Positive terminal of dc source is said to be reverse biasing.
Reverse bias
Reverse biasing of a diode Symbolic Representation
Ir Ir
Operation of a Reverse biased Diode
-----
+
+
++
+
+
+
-
-
p n
RCurrent limiting resister
VHoles in P region are attracted towards negative terminal of supply
Electrones in n region are attracted towards positive terminal of supply.
Due to this The width of deplation region will incresases
Due to increases in the depletion region width the barrier also increases.
Minority carrier in p-region attracted towards positive of supply.
So that few electrons are flow so the current is also tiny that current states at this position said to be reverse saturation current due to minority carrier
-----
+++++
The process of widening is not take place at longer time due to there is no steady flow of current from n-side to p-side
BreakdownThe reverse saturation current flowing in reverse biased diode is dependent only on temperature & independent on reverse applied voltage externally.The breakdown in reverse biased diode can take place due to following effects.
Avalache effect
Zener effect
BREAKDOWN
Due to large voltage velocity
of minority carriers will
increase to great exent
Therefore Kinetic energy also
increses
While traveling minority carriers will collide with stable atomes & import some of
kinetic energy To the valance
electrons present in co-valent band
Due to this additional energy these electones will break the
covalent bonds &jump into the
conduction band to free for
conduction.
Now this free electones will be
accelerated &they knockout
some more valance
electrons by means
collisions.
THIS “CHAIN REACTION” IS
CALLED as “AVALANCHE
EFFECT”
Breakdown due to Avalache effect
Due to heavy doping of p & n side
of the diode the deplation region is narrow in reverse biased condition.
Therefore electrical field which is the voltage per unit distance is very
intense across the deplation region
This intense electric field can pull some of valence electrons by breaking the covalent bonds these electrons
than become free electrons.
The large number of such electrons can constitute through diode this is called as the breakdown
due to zener effect
Break down due to Zener effect
Each electrons orbit has an energy leval associated with it. The electrons in the inner orbits are more closely bound to the nucleus to the nucleus and posses less energy.
As we move towards the valance shell, the bliding force between nucleus & electrons reduces and the electrons reduces & elctrones posses higher energy.
Energy Levals
Shell 1 Lowest energy
Energy leval increses as we move away from the nucleus
Valance orbit has highest energy leval
Energy Bands
First band
Formed by cluster of energy leval in first shell
Concept of forbidden gap:
• Forbidden gap is energy gap that separates the conduction band & Valence bands.• For any material the forbidden gap may be large, small , or even nonexistent. The
materials are classified as conductor , insulator, semiconductors based on relative widths of forbidden gap.
Energy Hill
A Kp-region n-region
Energy
Conduction band
Valence band
The Valence & Conduction bands are slightly lower energy levals
This is Due to difference in the atomic characteristics of the pentavalent(n-type) & Trivalent(p-tpye) impurity atomesMajority
carriers(Holes)
Minority carriers(electrons)
Minority carriers(Holes)
Majority carriers(electrons)
Inernal temperature of p-n junction is known as the junction temperature
Whereas the air surrounding the device is called as ambient temprerature
Barrier Potential & Temperature
For conducting Diode
Junction Temp. > Ambient Temp.
Barrier Potential 1junction temperature
So that Due to Incresed Temp. barrier potential decreases more electrons & hole pair produced . These will diffuse into deplation region to narrow it down.
The barrier potential of a silicon diode decreases at 2 mV per degree Celsius rise in junction temperature.-2mV/
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