The document discusses capacitance in PN junctions. It describes how a depletion layer forms at the PN junction interface when there is no external voltage. When the junction is reverse biased, the depletion layer widens and acts as a dielectric between the P and N regions, forming transition/space charge capacitance. When forward biased, minority carriers are injected, forming diffusion capacitance which is much larger than transition capacitance. Formulas for calculating both capacitances in step-graded and linearly-graded junctions are provided.

1. TRANSITION AND DIFFUSION
CAPACITANCE
SUBMITTED BYJITENDER SINGH
02620703111
IT (3rd sem.)

2. CONTENTS
1.
2.
3.
4.
5.
PN JUNCTION
DEPLETION LAYER
CAPACITANCE
TRANSITION CAPACITANCE
DIFFUSION CAPACITANCE

3. PN JUNCTION
Above figure represents two blocks of semiconductor materials
,one P type and the other N type .
The pn junction is produced by placing a layer of p type
semiconductor next to the layer of n type semiconductor. The
interface separating the n and p regions is referred to as the
metallurgical junction.

4. STEP GRADED JUNCTION
A junction is said to be step graded if there is an abrupt change from acceptor ion concentration on
the P-side to donor ion concentration on the N-side.Such a junction is get formed in alloyed or
fused junction. Usually the acceptor density Na and the donor density Nd are kept unequal.
LINEARLY GRADED JUNCTION
A junction is said to be linearly graded if the charge density varies gradually with the distance in
transition region. Such a junction gets formed in a growth junction diode.

5. DEPLETION LAYER
Metallurgical Junction
Na
-
P
-
-
Nd
-
-
-
-
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n
Space Charge
Region
ionized
acceptors
Ionized
donors
E-Field
+
h+ drift
_
+
=
h+ diffusion
e- diffusion
_
= e- drift

6. DEPLETION LAYER
Figure represents the net positively and negatively charged
regions for a semiconductor P-N junction where there are no
external connection, no applied voltage , no field etc. The net
positive and negative regions induce an electric field near the
junction in the direction from the n-region to the P-region. Since
the electrons and the holes both pushed out of the space charge
region of the electric field, the region is depleted of any mobile
charge is called the depletion region.

7. WIDTH OF
DEPLETION LAYER IN
FORWARD BIAS
when the P-N junction is Forward
Biased ,the width of depletion
region is reduced and the barrier
potential is also reduced .

8. WIDTH OF
DEPLETION LAYER IN
REVERSE BIAS
When the P-n junction is reverse biased ,
the depletion region is widened and the
barrier potential is also increased.

9. CAPACITANCE
Capacitance is the property to hold charge by a capacitor.
In case of a parrallel plate capacitor the formula for the
capacitance is given by
C=ɛA/D=Q/V
A = cross section area of capacitor plates,D=separation b/w
capacitor plates, ɛ=absolute permittivity of the medium b/w the
capacitor plates, Q=charge stored on capacitor
plates,V=potential applied

10. TRANSITION
CAPACITANCE
When a p-N junction is reverse biased the
depletion region acts like an insulator or
dielectric material while the P and N type
regions on either side have a low resistance and
act as the plates. Thus P-N junction may be
considerd as a parrallel plate capacitor.The
jumction capacitance is termed as space charge
capacitance or transition capacitance and is
denoted by CT.
As mentioned earlier, a reverse bias causes
majority carriers to move away from the
junction, thereby uncovering more immobile
charges. So the thickness W of the depletion
layer increases with the increase in reverse bias
voltage . This increase in uncovered charge with
applied voltage may be considered a capacitative
effect . The incremental capacitance may be
defined asCT = dQ/dV where dQ is
increase in charge with increase in voltage, dV.

11. FORMULA FOR TRANSITION CAPACITANCE IN
CASE OF STEP JUNCTION
The transition capacitance in this case is given asCT = ɛA/W
ɛ = absolute permittivity of the semiconductor medium
A = cross section area of P-N junction
W = width of depletion layer and is given as-

12. FORMULA FOR TRANSITION
CAPACITANCE IN CASE OF
GRADED JUNCTION
•
In this case we assume
•
So in this case -
NA = ND

13. DIFFUSION
CAPACITANCE
When a P-N junction is forward biased ,a capacitance which is
much larger than the transition capacitance , comes into play .
This type of capacitance is called the Diffusion Capacitance and
is denoted by CD.

14. FORMULA FOR DIFFUSION CAPACITANCE
C = dQ/dV
The formula for CD is given by
where dQ represents the change in
D
the number of minority carriers stored outside the depletion region when a change in voltage across
diode ,dV, applied .
If ɽ is mean lifetime of charge carriers, then a flow of charge Q yields a diode current I is given as
I = Q/ ɽ
In case of forward bias current is given by-
I = I0 ( e V/ɳV T - 1)
Q = ɽ I0 (e V/ɳV T - 1) = ɽ I0 e V/ɳV T
So diffusion capacitance
CD = dQ
dV
= d( ɽ I0 e V/ɳV T )
dV
= ɽ ( I + I0 )
ɳV
So
CD
as e V/ɳV T >> 1
T
= ɽ I0
ɳV
T
here
I >> I0

15. GRAPH FOR
TRANSITION AND
DIFFUSION
CAPACITANCE