SEMICONDUCTOR DEVICES

statementdizzyeyedSemiconductor

Nov 1, 2013 (3 years and 11 months ago)

98 views

SEMICONDUCTOR
DEVICES

Diodes as a semiconductor devices

Symbol and Structure

Diodes is made by joining p
-
types and n
-
types semiconductor materials.

Diodes are undirectional devices that
allow current to flow through them only
in one direction.

Biasing Diode


There are two operating regions and three possible "biasing"
conditions for the standard
Junction Diode

and these are:



Zero Bias
-

No external voltage potential is applied to the PN
-
junction.



Reverse Bias
-

The voltage potential is connected negative, (
-
ve) to
the P
-
type material and positive, (+ve) to the N
-
type material across
the diode which has the effect of
Increasing

the

PN
-
junction width.



Forward Bias
-

The voltage potential is connected positive, (+ve) to
the P
-
type material and negative, (
-
ve) to the N
-
type material across
the diode which has the effect of
Decreasing

the

PN
-
junction width.


Zero Biased

The potential barrier exists
discourages the diffusion of any
more majority carriers across the
junction.

The potential barrier helps minority
carriers (few free electrons in the P
-
region and few holes in the N
-
region) to drift across the junction.

Then an "Equilibrium" or balance
will be established when the
majority carriers are equal and both
moving in opposite directions, so
that the net result is zero current
flowing in the circuit. When this
occurs the junction is said to be in a
state of "
Dynamic Equilibrium
".

Reverse Biased

N material is connected to the
positive terminal of the voltage
source, V and the P material is
connected to the negative terminal of
the voltage source,V.The effect is that
charge carriers in both sections are
pulled away from the junction.Free
electrons on the N side are attracted
away from the junction because of the
attraction of the positive terminal of
the voltage source,V.Likewise,hole in
the P side are attracted away from
the junction because of the attraction
of the negative terminal of the voltage
source,V.Its will increases the
depletion region.

Forward

Biased

N material is connected to the
negative terminal of the voltage
source, V and the P material is
connected to the positive terminal of
the voltage source,V.The voltage
source must be large enough to
overcome the internal barrier potential,
VB. The voltage source repels free
electrons in the N side across the
depletion zone and into the P side.
Once on the P side, the free electron
falls into a hole. The electron will then
travel from hole to hole as it is
attracted to the positive terminal of the
voltage source, V.

Its will decreases the depletion region.

I
-
V Characteristic Curve for silicon Diode

Zener Diode

Zener Diode symbol and I
-
V Characteristics



Zener Diode

is used in its



i. reverse bias or reverse breakdown mode





Diodes anode connects to the negative supply.



From the I
-
V characteristics its show that the zener diode has a region in


its reverse bias characteristics of almost a constant negative voltage


regardless of the value of the current flowing through the diode and


remains nearly constant even with large changes in current as long as


the zener diodes current remains between the breakdown current IZ(min)


and the maximum current rating IZ(max).





This ability to control itself can be used to great effect to regulate or


stabilise a voltage source against supply or load variations.Zener diode


will continue to regulate the voltage until the diodes current falls below


the minimum IZ(min) value in the reverse breakdown region.

Light Emitting Diodes

LED are the most visible type of diode, that emit a fairly narrow bandwidth of
either visible light at different coloured wavelengths, invisible infra
-
red light for
remote controls or laser type light when a forward current is passed through
them.

LED are semiconductor devices that convert electrical energy into light energy

Light emitting diodes are available in a wide range of colours with the most
common being RED(GaAsP), AMBER(GaAsP,

YELLOW

(GaAsPN) and
GREEN (AlGaP)

Biased Voltage



Biased voltage applied to transistor to operate successfully.

There are two type of biased voltage supplied to the transistor as a signal for it
operations. Two main condition in biasing a transistor are:


i.

E
-
B(emitter
-
base) junction must be forward biased

ii.

C
-
B (Collector

base )junction must be reverse biased

N

P

N

P

N

P

E

B

C

E

B

C

Bipolar Junction Transistor (BJT)


Like diode, BJTs are formed by P and N region and as we are already the point
at which P and N region join is known as a junction.

The bipolar junction transistor is a three terminal current operated
semiconductor device.

The terminals are called the emitter, base and collector. The emitter and

collector is made up of the same type of semiconductor material.

Figure above shows the structure and schematic symbol for NPN and PNP
transistor:

The arrow at the transistor symbol shows the direction of conventional current
when it is operates.

Either NPN or PNP, the base is very thin .Contains less majority current carrier.

Emitter functions as charge provider or majority current carrier in transistor.

Collector has to collect charge for circuit operates.

Base is the junction which is control the current flow.

Transistor Operation

N

P

N

E

B

C

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

+
+
+

I
E

I
B

I
C

E
-
B junction has been forward biased by V1, E
-
B junction will flow the
current because of electrons at emitter (N
-
type) will move by negative
potential supply V1. These electrons tried to go to positive potential V1
through base (P
-
type). But because of base only a thin layer and have small
amount of hole, not all electrons can flows. Only little electrons current can
flow at the base known as base current, IB.

The others electrons will collect at base layer in
large amount. Because of collector layer has
been connected to positive terminal of V2, the
positive potential will attracted that electrons flow
through collector as current collector, IC.

IE = IC + IB