CHARGE DENSITIES

3. R.M.K COLLEGE OF ENGINEERING
AND TECHNOLOGY
DEPARTMENT OF ECE
EC8252-ELECTRONIC DEVICES
SECOND SEMESTER-I YEAR- (2020-2024 BATCH)
Mrs. P.Sivalakshmi AP/ECE
SESSION:6
DATE: 22.04.2021

4. Contents:
Charge densities in the Intrinsic & Extrinsic semiconductor
PN Diode-Zero bias, Forward Bias & Reverse bias of a diode.
Energy Distribution of Electrons
Carrier Concentration in Intrinsic semiconductor
Calculation of Fermi Energy levels
Potential barrier- In built potential.
Space Charge Width(w)

5. Charge densities in the Intrinsic semiconductor
 The number of atoms per cubic centimeter is 5x1022 and the electron density is
ni = 1.5x1010/cm3
 This means that 1.5x1010 atoms have each released one electron and consequently
1.5x1010 hole have been created.
 So the hole density (pi) is the same as the electron density (ni) in intrinsic semiconductor.
Pi = ni

6. Charge densities in the Extrinsic semiconductor
Let ND= Concentration of donor atom (charges per cm3)
NA = Concentration of Acceptor atom (charges per cm3)
The total number of electrons per unit volume is given by acceptor atom concentration NA and electron concentration ‘n’.
i. e: Negative charge Density = NA + n
The total number of hole per unit volume is given by donor atom concentration ND and hole concentration ‘P’
i. e: Positive Charge Density = ND + p
Under equilibrium condition or neutral
Positive charge density = Negative charge density
ND + p = NA + n
For ‘n’ type semiconductor; NA = 0
n= ND + p = ND ; since n>>p
For ‘p’ type semiconductor; ND = 0
p= NA + n = NA ; since p>>n

7. N – Type Material
As per above equations;
nn = ND ; pn = NA
As per the Mass Action Law:
(n) (p) =ni
2
(nn) (pn) =ni
2
pn = ni
2/ND
As per above equations;
np = ND ; pp = NA
As per the Mass Action Law:
(n) (p) =ni
2
(np) (pp) =ni
2
np = ni
2/NA
P – Type Material

8. Barrier Formation in P-N Junction Diode
• The holes from p-side diffuses to the n-side while
the free electrons from n-side diffuses to the p-
side.
• This movement occurs because of charge density
gradient.
• This leaves the negative acceptor ions on the p-
side and positive donor ions on the n-side
uncovered in the vicinity of the junction.
• Thus there is negative charge on p-side and
positive on n-side.
• This sets up a potential difference across the
junction and hence an internal Electric field
directed from n-side to p-side..

9. • Equilibrium is established when the field
becomes large enough to stop further diffusion
of the majority charge carriers.
• The region which becomes depleted (free) of
the mobile charge carriers is called the
depletion region.
• The potential barrier across the depletion
region is called the potential barrier.
• Width of depletion region depends upon the
doping level.
• The higher the doping level, thinner will be
the depletion region.

10. P-N Junction at
Zero Bias