Dopant elements: Group 14 semiconductors
Intrinsic Semiconductor
The two types of extrinsic semiconductor
Intrinsic semiconductor Donor atoms Acceptor atoms Group IV semiconductors Silicon, Germanium Phosphorus, Arsenic Boron, Aluminium Group III-V semiconductors
Aluminum phosphide, Aluminum arsenide, Gallium arsenide Selenium, Tellurium, Silicon, Germanium Beryllium, Zinc, Cadmium, Silicon, Germanium
Intrinsic Semi-conductor:
Conductivity of Semi-conductor increases with increase in temperature.
Intrinsic Semi-conductor has equal number density of electrons and holes i.e. ni = ne = nh
Eg between V.B. and C.B. is of order 1eV. for Si Eg = 1.21eV; for Ge Eg = 0.79 eV
Extrinsic Semi-conductor:
Conductivity increases with the doping
Doping a pure semi-conductor with penta-valent impurity atoms like P, As yields n-type Extrinsic -Semi-conductor.
In n-type Extrinsic semi-conductor, ne > nh ; since each impurity atom contributes one free electron in addition to ‘e-h pairs’ already existing in semiconductor. Since each doped impurity atom (P, As) donates one free electron to the semi-conductor, it is called donor impurity.
The lowest energy level occupied by the donor electron is 0.045eV below the bottom of Conduction Band.
Doping a pure semi-conductor with trivalent impurity atom like B, Al yields a p-type extrinsic semiconductor.
In p-type Extrinsic semi-conductor, nh > ne ; since each impurity atom contributes one free hole in addition to ‘e-h pairs’ already existing in semiconductor. Since each doped impurity atom (B,Al) accepts one free electron from the semi-conductor, it is called acceptor impurity.
This is why diodes conduct in one direction but not the other.
A diode connected as shown below is said to be "forward biased
Diode with d.c. supply.
Figure 5: I–V characteristics of a P-N junction diode (not to scale).
A diode is an electronic component that, in general, will pass current in only one direction
When voltage is applied to a diode and current is flowing through the diode, there will be approximately a .6 volt drop across the diode. In this first diagram, I've included the voltmeter so that you could see how the voltage indicators represent voltage. I'll use the indicators only in the rest of the diagrams. The green rectangular device is a current limiting resistor. It's needed to prevent excessive current flow through the diode when forward voltage is applied.
When the voltage on the cathode is greater than the voltage on the anode, current will not flow through the diode.
Transistor:
A transistor (pnp or npn) has three sections of doped semiconductors with two p-n junctions. It is like two diodes connected back to back.
The three sections are: Emitter, base & collector.
Emitter is highly doped and moderate in size. The emitter is always forward biased w.r.t. base.
Collector: It is large in size but moderately doped. The collector is always reverse biased.
Base: It is lightly doped and thinnest of the three regions. The middle section which forms two pn-junctions between the emitter and collector is
called the base.( the most important factor in the function of a transistor.)
The Transistor as a Switch
In the circuit below we use a rheostat as a variable potential divider to apply a variable voltage across the base and emitter of the transistor to see how this affects the voltage across the collector and emitter.
The Transistor as an AMPLIFIER
Circuit diagram of oscillator
Working of an oscillator
LOGIC GATES
Signal A transfer of information in a certain medium i.e. radio, telephone cable, or fibre optic.
signal time amplitude Analog X(t) continuous continuous Digital X[n] discrete discrete
Applications of Radio waves
Core technology
Kinds of Different radio around: AM and FM broadcast, TV broadcast, Satellite transmission,
GPS signals
Information[voice, image, or data ] is encoded in the sine wave , modulated and transmitted.
Reverse principle at Receiver.
WORKING OF A T.V.
A naughty child at home has swept the screen of your television or computer monitor with a magnet, leaving a strange colour in the swept region, may it be red, blue or green. At this time, you might grieve for your "damaged" television. However, if you only understand the mechanism behind, you can easily "rescue" the screen. As the electron guns eject electrons in different directions, the electrons will reach different fluorescence powders after passing through the metal net (Fig. 1).
we know that the glass of the screen will expand when heated and contract when cooled. If the metal net expands and contracts with a different ratio, the electrons will be on a wrong path when temperature changes. Thus the coefficient of expansion of the metal net and that of the glass must be identical. We found a very suitable alloy to make the metal net - it is called indium steel, which is an alloy of iron, nickel and carbon. This alloy does not expand much when heated, so does glass. However, there is a very big disadvantage of this alloy, that is, it can be magnetized easily. When it is magnetized, electrons in motion will be affected by magnetic force and cannot reach the suitable powders, resulting in strange colours and patterns. Then, what can we do? We can "demagnetize" it, that is, to remove its magnetic property. We can set up a magnetic field with a decreasing strength, that keeps on changing its direction, and this can result in demagnetization. It sounds complex but the method is simple: it is done easily by winding a coil on the screen and let a decreasing alternating current pass through it. But there is an easy way out. Nowadays many computer monitors are equipped with a demagnetizing function that enables a push-button automatic demagnetization. You only have to place a magnetized television in front of a computer monitor and start the demagnetizing function, and the television will be demagnetized !
Encoder
Antenna
Transmitter
Receiver
Decoder
Channel is the physical medium that connects the transmitter and receiver. Channel allocation:
AM: 540~1600KHz
FM: 88~108MHz
TV: 54~72, 76~88, .....MHz Modulation
Q: In telecommunication system what is done to the information before modulation?
example, a telephone encodes speech as a variation in electrical signals.
Transducer Encoding may involve the transformation of energy from one form into another – for example, sound energy must be transformed into electrical energy for radio transmission. Devices that do this are called transducers – for example, a microphone in this case.
Working Principle of a Radio
Q: What are the different modes of communication? Explain them giving an example for each. 3
A: 2 basic modes of communication: point-to-point and broadcast.
In point-to-point communication mode, communication takes place over a link between a signal transmitter and a receiver. Example: telephony.
In broadcast there will be large number of receivers corresponding to a single transmitter. Example: Radio or TV.
Wireless communication. One desirable feature of radio transmission is that it should be carried without wires i.e., radiated into space. At audio frequencies, radiation is not practicable because the efficiency of radiation is poor. However, efficient radiation of electrical energy is possible at high frequencies ( > 20 kHz).
For this reason, modulation is always done in communication systems.
Composition of Communication Systems
Communication systems can be broken down in to several components, each with its own specific function. The table below explains these key components and what they do.
Encoder converts information to a form that can be sent. Modulator varies the wave carrying the signal. Decoder translates information to a form that can be understood. Storage where information is held. Transmitter sends signals from one place to another. Receiver picks up the signal that has been sent to it. Transducer changes signal from one form to another. Amplifier increases the amplitude of a signal.
The diagram below demonstrates a system by example of speech.
Transmitting and Receiving
A common method of transmission is fibre optic cables, they are simply wires that carry light. They are used for internet transmission and there are huge networks under the oceans.
The signal along these is digital. A digital signal is either 'on' or 'off' compared to an analogue signal which has a variety of different values.
Signal quality can be reduced in transmission by attenuation where the signal becomes weaker with distance from losing energy. Or noise where the amplitude is distorted.
Because of attenuation the signal must be amplified by repeaters, but in an analogue signal this means the noise is also amplified and hence the resultant signal received is of poorer quality.
However because of the simplicity of a digital signal it can undergo regeneration to it's original form. It is for this reason that digital signals are seen as better than analogue.
Storage: Computer Disks
As explained above, digital information is either 'on' or 'off' and so could be represented as either 1 or 0, this forms the basis of binary code that is used as the basis for computing.
A CD has a series of bumps on it that represent 1 or 0, these are read using light. See the diagram below.
The diode laser emits a small beam of concentrated light (laser beam) on to the disk surface. This is then reflected back and can be detected by the photodiode by a change in the intensity of the light reflected. This is why if there is a scratch on your CD surface it could interfer with the quality
Basic elements of Modulation and Detection
MODEM = MOdulator-and-DEModulator= also called dataset
A modem acts as a modulator in transmitting mode and as a demodulator in receiving mode.
Microwave communication is possible only if the position of satellite becomes stationary with respect to the position of earth. So, these types of satellites are known as geostationary satellites.
What is Microwave Propagation?
Electromagnetic waves having frequency greater than that of the television signals are known as Microwaves. These waves have very less wavelength nearly few millimeters. Due to their low wavelength, these waves bend properly from the corners of the objects coming in their way. These are basically used to generate beam signals in fixed directions.
LASER
Spontaneous: 1. Happening or arising without apparent external cause; self-generated.
2. Arising from a natural inclination or impulse and not from external incitement or constraint.
PROPERTIES OF LASER LIGHT
MONOCHROMATICITY
DIRECTIONALITY
Conventional source
COHERENCE
This light is said to be "incoherent," meaning that the light beam has no internal order
Coherent light waves
Viewing laser speckle [ruin or impair the eye]
Spontaneous emission
STIMULATED EMISSION OF LIGHT
The coherent light of the laser is produced by a "stimulated-emission" process (Figure 10). In this case, the excited atom is stimulated by an outside influence to emit its energy (photon) in a particular way.
Fig. 10 The stimulating agent is a photon whose energy (E3–E2) is exactly equal to the energy difference between the present energy state of the atom, E3 and some lower energy state, E2. This photon stimulates the atom to make a downward transition and emit, in phase, a photon identical to the stimulating photon. The emitted photon has the same energy, same wavelength, and same direction of travel as the stimulating photon; and the two are exactly in phase. Thus, stimulated emission produces light that is monochromatic, directional, and coherent. This light appears as the output beam of the laser.
Fig. 11 Absorption of light
Fig. 12 Elements of a laser
Population inversion is key to producing laser.
Types of Lasers:
GAS LASERS: Hellium Neon;
SOLID CRYSTALLINE AND GLASS LASERS: Neodymium-YAG Laser
Yttrium aluminum garnet (YAG)
SEMICONDUCTOR LASERS GaAs
Laser Applications
Medical applications Welding and Cutting Surveying Garment industry Laser nuclear fusion Communication Laser printing CDs and optical discs Spectroscopy Heat treatment Barcode scanners Laser cooling DVD Spiral
The key to DVD technology is the increased data-storage capacity, and this is enhanced even further by the use of double-sided and double-layered discs. This is one of the factors that allows the technology to far outstrip the old Video CD format, as well as VHS tape.
CD-DVD Comparison
Digital versalite disc DVD
41 Semiconductor Devices/Electronics and Basics of LASERS -By: Md. Abdul Mumeed 009667433412