UNIT - II

UNIT - IIUNIT - II

Hole (majority carriers)

Electron (minority carriers)

Hole (mobile charge)

- Acceptor ions (immobile charge)

-- - -

-- - -

-≈ (p ≈ NA)

Hole (minority carriers)

Electron (majority carriers)

++++

+ + + +

Donor ion(immobile charge)+

Electron (mobile charge)

+≈ (n ≈ ND )

P N----

---- +

+++

+

+++

Space charge region(OR)

Depletion region

JunctionJunction

Ionized acceptorsIonized acceptors Ionized donorsIonized donors

Potential barrier heightPotential barrier height(V(V00))

Potential barrier widthPotential barrier width (W)(W)

The diffusing majority carriers from the two regions recombine near the junction and disappear.

The uncompensated acceptor and donor

ions set up an electric field which halts the further recombination.

The two kinds of majority carriers diffusing across the junction meet each other near the junction and undergo recombination, leaving negative ions on the p-side and positive ions on the n-side of the junction.

This distribution of charges is called space charge region.

P N

CathodeAnode

_+

Evp

From figure the following points to be noted:

Consider that a PN- junction has P-type & N- type materials in close physical contact with each other at the junction.

From figure, the Fermi level EF is closer to the conduction band edge Ecn in the N-type while it is closer to the valence band

edge Evp in the P-type.

The conduction band edge Ecp in the P-type material is higher than the conduction band edge Ecn in the N-type material. Similarly, the valence band edge Evp in the P-type material is higher than the valence band edge Evn in the N-type material.

E1 & E2 indicate the shifts in the Fermi level from the intrinsic conditions in the P & N materials respectively.

Therefore, the total shift in the energy level E0 is given by

E0 = E1 + E2 = Ecp – Ecn = Evp - Evn

The energy E0 (in eV) is the potential energy of the electrons at the PN-junction, which is equal to qV0.

Where,V0 = contact potential (OR) barrier potential ( exists across an open circuited PN- junction)

The diode can be operated in two different ways, as

When positive terminal of the battery is connected to the P-type & negative terminal is to the N-type of the PN-junction diode, known the diode is kept in forward bias.

P N----

---- +

+++

+

+++

Space charge region

Open circuit PN -junction

NP---- +

+++

VF

Forward bias

The applied +ve & -ve potential repels the holes & electrons in P-type & N-type materials. Hence, they can move towards the junction. When the applied potential is more than the internal barrier potential the depletion region & internal potential barrier disappear. Hence, high current flows through the junction. In forward bias the current is due to majority charge carriers (mA).

When negative terminal of the battery is connected to the P-type & positive terminal is to the N-type of the PN-junction diode, known the diode is kept in reverse bias.

P N----

---- +

+++

+

+++

Space charge region

Open circuit PN -junction

----

----

---- +

+++

+

+++

+

+++

P N

Reverse bias

VR

The holes move towards the –ve terminal of the battery & the electrons towards +ve terminal of the battery.

Hence, the potential barrier & width is increased which prevents the flow of charges. Therefore, no current flow across the junction.

But in practice a very small current flows in order of microamperes, due to minority carriers.

The graph is plotting in between the voltage is taking on X-axis & current is on Y-axis, is knownas V-I characteristics of a PN- junction.

These curves are drawn on the basis of diode is connected in forward & reverse bias.

Forward bias

V

I

Knee voltage

(i = mA)

0

Reverse bias

Breakdown voltage

(i = μA)

-V

-I

Reverse bias

Forward bias

V

I

Knee voltage

Breakdown voltage

i = mA

i = μA

-V

-I

The region between knee voltage & breakdown voltage is known as non-ohmic region.

Above the knee & breakdown voltage the current increases.

Breakdown voltage is due to thermally broken covalent bonds.

Diode is conducting in forward bias & non-conducting in reverse bias.

A rectifier is an electronic circuit which converts alternating current to direct current (OR) unidirectional current.

Rectifiers are mainly three types 1.Half wave rectifiers

2.Full wave rectifiers 3.Bridge rectifiers

An electronic circuit which converts alternating voltage (OR) current for half the period of input cycle hence it is named as half-wave rectifier.

The ratio of D.C power output to applied A.C power input is known as rectifier efficiency.

&

Therefore,

since,

An electronic circuit which converts alternating voltage (OR) current into pulsating voltage (OR) current during both half cycle of input is known as full-wave rectifier.

The ratio of D.C power output to applied A.C power input is known as rectifier efficiency.

&

Therefore,

But,

( ) 2πme*KBT

h22 n =

3/2

e(EF – EC ) / KBT

(OR)

Where, ( )

2πme*KBTh2

23/2

= Nc

nc = Nc e(EF – EC ) / KBT

---- (1)

Where, ( )

2πmh*KBTh2

23/2

= Nv

nv = Nv e(EV – EF ) / KBT

---- (2)

nn = Nc e(EFn – Ecn ) / KBT

---- (3)Where,

np = Nc e(EFp – Ecp ) / KBT

---- (4)

Where,

pn = Nv e(Evn – EFn ) / KBT

---- (5)

Where,

pp = Nv e(Evp – EFp ) / KBT

---- (6)Where,

e(EFn – Ecn )

e(EFp – Ecp ) / KBT

nn

np

=

e(Ecp – Ecn ) / KBT nn

np= ---- (7)

Where,

eeVB / KBT nn

np=

(OR)

---- (8)e- eVB / KBT

np nn=

e- eVB / KBT pn

pp=(OR)

---- (9)e- eVB / KBT

pn pp=

np + ∆np = nn e - e(VB -V) / KBT

np + ∆np = nn e- eVB / KBT eV / KBT

e

np + ∆np = np eV / KBT

e

Since, (From eq(8)e- eVB / KBT

np nn=

eV / KBT ∆np = np e - 1 ---- (10)

eV / KBT ie= C1 ∆np = C1 np e - 1 ---- (11)

Where,C = constant (depends on the semiconductor)

eV / KBT ih = C2 ∆pn = C2 pn e - 1 ---- (12)

eV / KBT I = ie + ih = (C1 np+C2 pn) e - 1 -- (13)

- eV / KBT I = ie + ih = (C1 np+C2 pn) e - 1 -- (13)

- eV / KBT e « 1

I = -(C1 np+ C2 pn) = I0 --- (14)

- eV / KBT I = I0 e - 1 -- (15)

- eV / β KBT I = I0 e - 1 -- (16)

LEDs are typically made of compound semiconductors (OR) direct band gap semiconductors like gallium arsenide.

LED is a highly doped diode

substrate

VF

N

POhmicContacts(Al)

Sio2

Photons

+ -

CathodeAnode

+ -

CathodeAnode

P

N

iPhotons

Electron – hole pair

VR

Figure shows thereverse bias of p-i-n diode.

N+

Layer 1

Layer 2

Layer 3

Layer 4

P

i

P+

VRPhotons

Electron – hole pair

Figure shows the reverse bias of avalanche photo diode.

N+ - heavily doped N-region

P+ - heavily doped P-region

P - lightly doped P-region