ELECTRONIC DEVICES (7 Marks)

Energy bands in conductors, semiconductors and insulators

(qualitative ideas only ) semiconductor diode I-V characteristics in forward
and reverse bias, diode as arectifier, LV eharaeteristies of L.E.D,phYtdiede,
solarcelt,and zener diode;Zener diede asaveltageregulator.
ELECTRONIC DEVICES
Distinquish Conductors Insulators & Semiconductor on the basis of energy bands
Inside the crystal each electron has a unique position, each electron will have a different energy
level These different energylevels with continuous energy variation form are called energy bands.
The energy band which includesthe energy levels of the valence electrons is called the valence
band. The energy band above the valence band is called the conduction band. With no external
energy, all the valence electrons will reside in the valence band. The gap between the top of the
valence band and bottom of the conduction band is called the energy band gap Eg
Conductors- Here, the conduction and valence bands are overlap. Thus electrons from valence
band can easily move into the conduction band. This situation make a large number of electrons
available for electrical conduction Therefore, the resistance of such materials is low or the
conductivity is high.
Insulators -In these case, a large band gap Eg >3eV exist. There are no electrons in conduction
band and therefore no electrical conduction is possible.
Semiconductor -Here a finite but small band gap <3eV exists. Because of the small band
gap, at room temperature some electrons from valence band can acquire enough energy to
cross the energy gap and enter the conduction band. Hence, the resistance of semiconductors
is not as high as that of the insulators.

Filled witb
C.B.
-Empty few clectoos
Parüally filled
C. B.
E

V.B. Filled V. B.

CONDUCTOR Insulators SEMICONDUCTOR

C, Si and Ge have same lattice structure. Why is C insulator while Si and Ge Intrinsic
semiconductors?
Ans: The 4 bonding electrons of C, Si or Ge lie, respectively, in the second, third and fourth orbit. Hence, energy required
to take out an electron from these atoms will be least for Ge, followed by Si and highest for C.Hence electrons for
conduction in C is negligibly smalil.

-21 -
 MODERN PHYSICS
p-njunction formation : TwO important processes occur during the formation of ap -njunction
diffusion and drift The motion of charge carriers due to the
electric tield is called d
Drift current, is opposite in direction to the diffusion
current.
In the beginning the diffusion current is large but drift
current is small. As the diffusie
process continues, the space charge regions across the junction
extend. Due to concentration
gradient at the junction, holes begin to diffuUse from p- side to n - side and
diffuse from n - side to p- side. This diffusion of majority electrons begin t
gives rise to an electric current from p’n side is charge carriers across the junction
it, the strength of called diffusion current. As a result of
electric field
a crosss the junction increases and thereby dirift
Due to this electric field developed at the current increases
junction, an electron on p-side of the junction
moves to n-side and a hole on n-side of junction
moves to p-side of junction. The motion of
these charge carriers due to electric fieldis
starts, which is opposite in direction to the called drift. As a resut of it, a drift current
the diffusion, current becomes equal to the diffusion current. This process continues until
drift Current. Nhen this stage is
of majority charge
carriers across the junction stops. Now the p-n reached,is the movement
equilibrium state and there is no current across the p-n junction, junction said to be in
Depletion reglon:When an electron diffuses from n
behind an ionized donor (positive charge) on topdue to the concentration gradient it leaves
immoble as it is bonded to the surrounding atoms. n-side which is immobile. This ionized
As the electrons continue to donor is
P. a layer of positive charge on n -side of the junction is developed. diffuse from n to
Similarly when hole diffuses from ptondue to the
gradient, it leaves behind an acceptor (negative
to diffuse from p to n, a layer of
( concentration
charge) which is immobile. As the holes
negative continue
The space-charge region on either charge
on p-side of the junction is
developed.
sides of the Junction which has immobile ions is
as depletion region or layer. known
POTENTIAL BARRIER
The potential tends to prevent the
moverment of
region in the p-n junction called a barrier potentialelectron from then region into the p
The width the depletion region and
potential barrier depends on the nature of the
semiconductor material and doping concentration on the two sides of the p-n junction.
Q: Can we take one slab of the p
type
semiconductor to get p-n junction ?semiconductor and physically join to another n type
Ans No! Any slab ,howsoever flat, will have
spacing and hence continuous contact at roughness
the atomic
much larger than the
inter-atomic crystal
level will not be possible
behave as a discontinuity for the flowing charge .The junction wwil
p-n junction diode under forward blas carriers. P- positive
due tO
When an external voltage V is Dp
semiconductor diode such that p-side is
Mayoruhya
across n-neqatve
positive terminal of the battery and n-side connected to the
to the negative
terminal, it is said to be forward biased. 100
The direction of the applied voltage (V) is 80
opposite to the built-in
potential Vh As a result, the depletion layer 60

and the barrier height is width decreases 40

forward, bias is (Vo -V) reduced. The effective barrier under 100 80 60 40 0
The motion of the majority
gives rise to current. The charged carriers on
total diode forward either side 20
Sum of hole diffusion current is
to electron current and conventional current due
30

diffusion. The magnitude of this current is usually in mA.

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 MODERNI
PHYSICS
diode under reverse bias due to mineuty caven
junction
N Whenaneexternal voltage Vis applied across $a semiconductor diode such that
p"sideis n- side to thebpositive
connected to the negative terminal of the battery andpatential an
biased.
minal, it is said to be reverse
the built-in potential Va. Asa
edirection of the applied voltage (V) is same as the direction of The effectve barrier
increases.
eut the depletion layer width increases and the barrier height is
(V +V),
under reverse bias is
rise to current. The
he motion of the minority charged carriers on either side gives
uA.
magnitude of this cCurrent is usually in
resistance of the circuit shown in
Determine the current through the
fn. la) and (b) when similar diodes D, and D, are connected.

Ans : (a) 0.1 A (b) zero

(a)
200 2V (b) 202
?
following diagram, is the junction diode forward biased or reverse biased
o In the
[CBSE 2017]
+5V.
reverse bias?
Which of the folowing statements is not true for p-n junction diod under2024 (s)]
0. independent of the applied voltage. [CBSE
(a) The current is almost n-side.
(b Holes flow from p-side to region increases.(Vo+V)
(c) Electric field in the depletion
connected to +ve terminal and p-side to -ve terminal of
the battery.
(d) n-side of the junction is
junction diode as a half wave rectifier.
Q. Explain p - n
InputAC. of rec tifier
Transformer

Primary secondary oUTTU vOLTA

(b)

P
ta)
negative and P, is positive. Due to mutual
P, is p-njynction diode is forward
cycle of the inputA.C.,
During the negative half positive, S, becomes negative. The
induction, S, becomesThus, we get output across-load resistance. P, is negative. Due to
and
biased and it conducts.
half cycle of the input A.C., P, is positive
Thep-njunction.diode is reverse
During the positive
becomes
positive.
negative and S, is across load resistance. The process is
mutual induction, S, thus no output
not conducts and
biased and it does process is called half
That is why the
repeated.
converts half of the A.C. into D.C.
Thus this circuit rectifier.
wave rectification.
diode as a full wave is positive. Due
to
junction negative and P,
Q. Explain p
n
of the inputA.C., P, is The p-njunction diode
cycle negative.
half
During the negative becomespositive, S, becomes Hence the diode D, conducts.
S, reverse biased.
mutual induction, and the diode D, is D,.
forward biased resistancedue to diode Due to
D, is output
across-load
is positiveand P, is negative.
Thus, weget the inputA.C., P,
half cycleof
Duringthe positive
3
 deing ataing impunti
MODERN PHYSICS
mutual induction, S, becomes negative and S, is positive. The p-njunction diode D
forward biased and the diode D, is reverse biased. Hence the diode D, conducts. Th
we get output across-load resistance due to diode D,. The prOcess is repeated. Thus th
circuit converts complete A
C. into D.C. That is why the process is called fuli wave
rectification.

Centre Tap
P Transforrner Input AC. of ee tufier
Dlode lD)

Centre S
| Tap
Due to Due to Dur to bue ta
Diode 2(D) K Output

(a
put frequency 50 Hz What ls
Q. In a half wave rectification, what is the output frequencyi
innutfrogunew (Ans 50Hz, 100Hz).
the out put of a full wave rectifier forthe same
-tuivan a. suppose a pure Si crystal has 5x of102"atomandn. It is doped by 1ppm concentration of
e that n, = 1.5 x 10lm-. Is
boron. Calculate the concentration holes 5x1D8xi8
p-type the doped crystal n-type or p-type? nh : DEVICES nesxo2)}sot6 j3
ELECTRONIC