THE UNIJUNCTION TRANSISTORS
CHAPTER 17 SPECIAL DEVICES
IRISET 70 T3 - FUNDAMENTAL OF ELECTRONICS
CHAPTER 17 SPECIAL DEVICES
17.1 CHARACTERISTICS OF LIGHT
The energy contained in a photon depends on frequency of light. It is expressed as ξ = h.f
ξ = Energy
h = Plank's constant 6.624 X 10-34 Joule-seconds f = frequency
Frequency ( f ) and wave length (λ) of light are related by the relation.
X= C / f
C = velocity of light 3X10 m A 1A0= 1x10-10 metres
Ultra violet , Visible light and Infrared spectrum is shown ill tile Fig.17.1.
Light emitted or falling on a specified area and usually expressed as lumens/square metre or watts/ square metre (refer Fig.17.1).
1 Im/m 2 = 0.1497 X 10-12 W/M2 or watts/square metre.
V I B G Y O R
1000 5000 6000
ULTRA VIOLET VISIBLE LIGHT
7500
7000 A0
INFRA RED
Fig. 17.1 LIGHT SPECTRUM
17.2 Photo resistors
It is a semiconductor device where resistance varies inversely with the intensity of light that falls on the device (photosensitive area). The symbol is shown in Fig 17.2(a) These are constructed with cadmium compounds, such as cadmium sulphide or cadmium selenide. They are more responsive to a particular wavelength of light.
CdS - about 7000 A0 CdSe - about 5500 A0
SPECIAL DEVICES
When no light is incident on the device the cell resistance is maximum and this is called the dark resistance. The resistance decreases with light intensity falling on the device as shown in graph (refer fig 17.2 (b)).
Fig. 17.2 (a) SYMBOL OF PHOTO TRANSISTOR (b) CHARACTERISTICS
17.3 Photo Diodes
Photo diode is a p-n junction device that operates in reverse bias. The symbol of photo diode is as shown in Fig.17.3 (a) circuit. The p-n junction of the device has a small window such that light can strike the junction.
When no light is incident on it, a very small leakage current flow because of reverse bias just like an ordinary diode) as shown in Fig 17.3(b). This current is very small called the dark current. When light is incident, depending on the amount of light energy the reverse current changes (i.e., an increase in the amount of light energy produces an increase in, reverse current).
It can be used as a variable resistance device controlled by light intensity.
Fig. 17.3 (a) SYMBOL (b) CHARACTERISTICS OF PHOTO DIODE
17.4 Light Emitting Diode
In ordinary diode when the electron hole recombination takes place (in forward bias condition) energy is radiated in the form of heat (lower than light frequencies). If light has to be emitted then the frequencies of light have to be emitted. For this the diodes are not just made by silicon or germanium. The light emitting diodes are made using Gallium Arsenide Phosphide (GaAsP) or Gallium Phosphide (GaP) or Gallium Arsenide (GaAs).
The symbol is shown in Fig.17.4. Depending on the type of material used the colour of light (frequencies) is emitted.
SPECIAL DEVICES
IRISET 72 T3 - FUNDAMENTAL OF ELECTRONICS
GaAsP - Red light
GaP - Yellow to green light GaAs - Infra red radiation
They are commonly used in indicator lamps and all the products (displays) Here the light emitted is not monochromatic but a mixture of frequencies of different colours. Size – 4mm X 8mm bar
Fig. 17.4 SYMBOL OF LED 17.5 Laser Diode
Laser-(Light Amplification by stimulated emission of radiation). This light is monochromatic. It has only a single wavelength. Laser diode has a p-n junction formed by two layers of doped GaAs. The wave length of the light can be determined by the length of the junction formed. The LD (laser diode) is shown in Fig. 17.5. In the junction end, one end is highly reflective and other partially reflective end. When junction is forward biased, recombination takes place in the junction and releases photons. These photons try to release more photons. Each photon is identical and has one frequency (monochromatic). LD have a threshold level of current above which the laser action occurs (monochromatic light is emitted).
+
-PN JUNCTION HIGHLY REFLECTIVE
END
N P
PARTIAL REFLECTIVE
END
Fig. 17.5 LASER DIODE 17.6 Solar Cells
In solar cells the light energy is converted directly to electrical energy. A basic solar cell consists of n-type and p-type semiconductor forming p-n junction. The upper area is maximum, exposed to sun with a lead at one end. The lower end is closed with positive contact lead. (as shown in Fig.17.6(b).
Fig. 17.6 (a) SYMBOL OF A SOLAR CELL
SPECIAL DEVICES
S U N L I G H T
C U R R E N T C O L L E C T E D O N T H E G R I D
(M E T A L F I N G E R S
)
Fig. 17.6 (b) CONSTRUCTION OF A SOLAR CELL
M E T A L C O N D U C T O R C O M P L E T E L Y C O V E R I N G B A C K ' P ' R
E G I O N
( B A S E
M A T E R I A )L
D IFFU SED LAYER N - R
E G I O N
~ 3 0 0 µ m
Electrons in semiconductor acquire sufficient energy from sun rays and break away from the parent atoms. This potential difference is created across the junction. Electrons in the semiconductor are pulled across the junction by the force of electric field. If load is connected, these electrons crossing the junction contribute to the load. The symbol is given in Fig 17.6 (b).
A solar cell generates 0.4 V to 0.6 V or less and connects ranging from 4 microamperes to milliampere. So a large number of such diodes (solar cells) are arranged in a array to get higher voltages and currents.
17.7 Photo Transistor
Photo transistor has three leads viz base, emitter and collector. When there is no light and by increasing collector - emitter voltage Vce of the photo transistor, We have dark current (refer figure 17.7(a) and 17.7(b). When light is incident on the phototransistor, by increasing Vce the collector current changes as shown in the characteristic curve (see the graph 17.7(b)). Here intensity of light is A Watts/square metre, B Watts/square metre, and C watts/square metre. In some cases the base-emitter junction is biased suitably depending on the application. The power dissipation should not exceed the rated maximum power dissipation (power = Vce X lc).
SPECIAL DEVICES
Fig.17.7 (a) PHOTO TRANSISTOR (b) CHARACTERISTICS
17.8 OPTO COUPLER
It is a device where light energy is converted to electrical energy. This device enables information (Electrical Signal) to pass from one circuit to another even though the circuits are isolated.
An opto-coupler (typical) which is mounted with a six-pin dual in line package is shown in the Fig 17.8 To identify the pin numbers a dot (spot) is on the top of the package. The package has an LED and a transistor. The LED emits infra-red radiation when biased properly (biased voltage). These radiations fall on the photo transistor and change the collector current (depending on the intensity of radiation). The quality of an opto-coupler depends on :-
SPECIAL DEVICES
Subjective:
1. Write the names of the special devices you know.
2. Draw and explain the working of a PHOTO DIODE.
3. Draw and explain the working of a LED.
4. Draw and explain the working of a LASER DIODE.
5. Draw and explain the working of OPTOCOUPLER.
6. Draw and explain the working of a SOLAR CELL.
SEMI CONDUCTOR MICROWAVE DEVICES - MICROWAVE TRANSISTORS
IRISET 75 T3 - FUNDAMENTAL OF ELECTRONICS