CHAPTER 3 : SEMICONDUCTOR DEVICES
3.1
DIODE
3.1.1 Structure and schematic symbol




A diode is created by joining N and P-type materials together.
Diode has two terminal electronic components which are anode and cathode.
The N section is called the cathode and the P type called the anode.
The direction of the arrow (anode) in a diode symbol shows the conventional
current direction.
 Figure 3.1 shows the structure and schematic diode symbol.
Anode
Conventional
Current
Direction
P
N
Cathode
Figure 3.1: The Structure And Schematic Diode Symbol.
3.1.2 Biased Voltage
 When voltage is applied to a diode it is called biased voltage.
 Refer Figure 3.2 ,when anode get more positive voltage than cathode or cathode
get more negative voltage than anode ,it is in forward biased condition.
 When anode gets more negative voltage than cathode or cathode gets more
positive voltage than anode, diode is in reverse biased.
A
C
+

A

Diode connected with forward
biased
C
+
Diode connected with reverse
biased
Figure 3.2 :Schematic Diode Circuit
3.1.3 I-V Characteristics For Diode.
Id (mA)
Breakdown
Voltage
Vs
Vd
Barrier
voltage
Is (A)
Figure 3.3 : I-V Characteristics Curve For Diode
 Forward Current(Id) -
Current flow through diode when forward biased .
In unit mA (miliampere).
 Reverse Current(Is)
The lowest current ,leakage current flow through
diode when reverse biased .
In unit μA(microampere).
-
 Knee Voltage
-
Voltage level where the forward current increases.
Knee Voltage = Barrier Voltage
(Si=0.7V , Ge=0.3V)
 Breakdown Voltage -
The applied reverse voltage at which the breakdown
occurs. A large increase in current will damages the
P-N junction diode.
3.1.4 Ideal Diode Concept
 While diode operated, diode characteristics will make analyze process for
electronic circuits become difficulty. The characteristics are :i.
ii.
iii.
Barrier voltage
Forward Current
Reverse Current(Leakage Current)
 In ideal diode concept, we assume diode did not have barrier voltage, no leakage
current, no forward resistance (rd) and no breakdown voltage.
 Figure 3.4 shows diode curve characteristics become ideal when certain part have
been neglected.
Id
Id
Vd
Normal
Vd
rd (neglected)
Vs
Vd
voltage barrier (neglected)
Vs
Is
Normal
Id
Vs
Is
leakage current
(Neglected)
Is
Breakdown voltage
(Neglected)
Figure 3.4: I-V Characteristics When Idealized Diode
 In ideal diode concept, while forward biased, diode acts as close switch (ON)
because no resistance and also no voltage drop.
 While reverse biased, diode act as open switch which is infinity resistance and no
leakage current.
Anode
Cathode
Close switch
Forward biased diode
Anode
Cathode
Open switch
Reverse biased diode
Figure 3.5: Ideal Diode When Forward Biased And Reverse Biased
3.2
Zener Diode
 Zener diode is important in power supply. It can be used to stabilize or regulated
voltage.
 The symbol for Zener diode is shown in Figure 3.6
Cathode
Anode
Figure 3.6 : Zener diode schematic symbol
3.2.1 Characteristic curve for Zener diode
 Figure 3.7 shows the I-V characteristic curve for Zener diode.
Id
Breakdown voltage = Zener
voltage
Vz
Vd
Vs
Is
Figure 3.7 : Diode Zener characteristic curve
 While forward biased, characteristic curve has no different than normal value
 While reversed biased, diode current increase drastically at Breakdown level are
sharper.
 The different between Zener diode and normal diode are Zener diode operate
when reversed biased.
 While forward biased, Zener diode operates as normal diode.
 There are advantages for diode:
i.
ii.
Zener diode is designed to work in reverse bias direction without harming
the diode.
At zener level, reverse voltage across diode ,VR remain constant and the
value is same with zener voltage.
3.2.2 Ideal Zener diode concept
 While Zener diode operation in the Zener region, voltage across diode are same as
Zener voltage (Vd = Vz ). Any changes in the voltage will change the current
value through it.
 Therefore, Zener diode is assumed as battery with Vz value. (Figure 3.8)
Anode
Cathode
-
+
Figure: 3.8
3.3
LED
 A light emitting diode (LED) is a special semiconductor diode designed
specifically to emit light when current flow through it.
 Normally it is used as an indicator lamp either in ‘ON’ or ‘OFF’ condition.
 Figure 3.10 shows the schematic symbols for LED. LED and diode symbols are
similar. The arrow direction that’s come out from P-N shows lights come out
from LED.
Cathod
e
Anode
Figure 3.10 : Schematic symbol for LED
 Same as diode, LED operation is when forward biased. Electron from Ntype material will combine with hole from P-type material. If the
semiconductors are Si and Ge, the result of the recombination is heat.
 When forward biased, the energy of the electrons flowing through the
resistance of the junction is converted directly to light energy.
 Because LED is diode, current will flow only when the LED is connected in
forward biased. The LED must be operated within its specified voltage and
current ratings to prevent irreversible damage.
 Most LED require approximately 1.5 to 2.2V to forward biased them and
safely handle 20 to 30 mA of current.
 Conventional Silicon / Germanium convert energy to heat. Gallium
arsenide diodes convert energy to heat and infrared light. This type of
diode is called an infrared emitting diode (IRED).
 Light emitting diode are used alphabetical and digital displays and as
signaling lamps.
3.4 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 3.11 shows the structure and schematic symbol for NPN and PNP
transistor:
COLLECTOR
N
P
B ASE
N
EMITTER
STRUCTURE
SYMBOL
COLLECTOR
P
N
BASE
P
EMITTER
STRUCTURE
SYMBOL
 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.
3.4.1 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. Refer to the figure 3.12 the 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
E
E
C
N
P
C
P
N
B
N
P
B
Figure 3.12 : Biased Voltage For Transistor
3.4.2 Transistor Operation
E
N
P
- - - - - - - - - -
+
+
+
N
- - - - - - - - - -
C
B
V1
V2
Figure 3.13: Electron Current Flow
 By referring Figure 3.13, since 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.
IC
IE
IB
Figure 3.14 : The electrons flow in transistor
 Arrow in Figure 3.14 above shows the flows of electrons in transistor. Showed
that IC is the largest portion from IE and smallest portion flows as IB. From this we
can obtain:
IE = IC + IB