modern physic notes for o and a level

Understanding Semiconductor diodes

1. Conductors are materials which allow current to flow through them easily. This is because conductors have free electrons which can drift between their atoms.
2. Insulators are materials which do not conduct electrical current.
3. Semiconductor is a material whose resistance is between those of good conductors and those of good insulators.

Doping

1. The conductivity of a semiconductor can be increased by adding a small amount of certain substances (impurities).
2. DOPING is the process of adding a small amount of impurities into the crystalline lattice of semiconductors.

Semiconductors

1. There are 2 types of semiconductors: p-type and n-type.
2. p-type semiconductor:
i. the holes (positive charge) are the majority carriers.
ii. The trivalent atoms are called acceptor atoms because they accept any free electrons to fill the holes.
iii. E.g. boron, indium and gallium.
3. n-type semiconductor:
i. the free electrons (negative charge) are the majority carriers.
ii. The pentavalent atoms are called donor atoms because they supply free electrons.

Semiconductor Diodes

Semiconductor Diodes

1. A semiconductor diode is called the p-n junction diode.
2. it consists of a p-type semiconductor in contact with n-type semiconductor.
3. Regions of P-type is called ANODE.
4. Regions of N-type is called CATHODE.
5. A p-type material meets an n-type material across a bounding region called the depletion layer (p-n junction).
6. In order for current to flow through the diode, the voltage applied across the diode must exceed the junction voltage.
7. Junction voltage is the potential difference that is caused by the movement of the holes and free electron.






FUNCTION OF DIODES


Diode as rectifiers

1. A diode is said to be in a forward-biased arrangement if it only allow the current to flow from the anode to cathode. It is acting as a VALVE
2. A diode can CONVERT alternating current into direct current. This is known as RECTIFICATION. Therefore a diode can act as a RECTIFIER.
3. A RECTIFIER is an electrical device that converts alternating current (AC) to direct current (DC), a process known as rectification. RECTIFIERS have many uses including as components of power supplies and as detectors of radio signals
4. There are TWO ways to convert an alternating current into a direct current.
a. Half-wave rectification
b. Full-wave rectification

Half-wave rectification

1. The current can only flow in the forward direction through the diode.
2. The process of rectification using a diode which ALLOWS CURRENT TO FLOW IN THE HALF-CYCLE is known as half wave rectification

Half Wave Rectification

Full-wave rectification

1. The arrangement of diode in Full-wave rectification is called a bridge rectifier.
2. The process of rectification using four diodes to allow current to flow in a complete cycle and in the same direction is called full-wave rectification.
3. A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient.


Full Wave Rectification


Smoothing


1. The output from a rectifier can be SMOOTHED by connecting a CAPACITOR across the load.
2. During the forward peaks (positive half-cycles), the capacitor is charged up. Energy is stored in the capacitor.
3. In between the forward peaks (negative half-cycles), the capacitor releases its charge (discharge). It discharges partly through the load. The energy stored in the capacitor acts as a reservoir and maintains the potential difference across the load.
4. A capacitor with greater capacitance produces a smoother current. This is because the capacitor can store more charge.
 Transistors

1. Transistor are solid state semiconductor devices that are used to amplify current.
2. These semiconductors are mostly made of silicon and germanium.
3. It has THREE terminals emitter (E), base (B), and collector (C). The emitter emits or sends charge carriers through the thin base layer to be collected by the collector.
4. Transistors can also form parts of the integrated circuits in the microchips. There are 2 types of transistor:
a. n-p-n transistor has thin layer of p-type silicon sandwiched between two layers of n-type silicon.
b. p-n-p transistor has thin layer of n-type silicon sandwiched between two layers of p-type silicon.
5. In a n-p-n transistor the emitter sends negative electrons to the collector.



6. In a p-n-p transistor, the p-type emitter sends positive holes to the collector.


7. In both cases, the arrow on the emitter shows the direction of conventional (i.e.positive charge) current flow.


Functions of terminal in a transistor

Terminal : Function
Emitter (E): Supplies Charge carriers to C.
Collector(C) : Receives charge carriers from E.
Base (B):  Controls the flow of charge carriers from E to C or C to E.

Flow of current in a transistor

1. There are two parts to a transistor circuit: Base circuit and Collector circuit.
2. The current which flows in the base circuit is called the base current, IB while the current which flows in the collector’s circuit is called the collector current, IC.
3. Transistors are composed of three parts – a base, a collector, and an emitter.
4. The base is the gate controller device for the larger electrical supply. The collector is the larger electrical supply, and the emitter is the outlet for that supply. By sending varying levels of current from the base, the amount of current flowing through the gate from the collector may be regulated. In this way, a very small amount of current may be used to control a large amount of current, as in an amplifier.
5. IE = IB + IC

Applications of Transistors

The transistor as an amplifier

1. A transistor can be used to amplify current. This is because a small change in base current causes a large change in collector current.
2. Example is a microphone.
3. Sound waves that are fed into the microphone cause the diaphragm in the microphone to vibrate.
4. The electrical output of the microphone changes according to the sound waves.
5. As a result, the base current is varying because of the small alternating voltage produced by the microphone.
6. A small change in the base current causes a large change in the collector current.
7. The varying collector current flows into the loudspeaker. There, it is changed into the sound waves corresponding to the original sound waves.
8. The frequencies of both waves are equivalent but the amplitude of the sound wave from the loudspeaker is higher than the sound waves fed into the microphone.

Component: Function
Microphone: To change sound signal to electrical signal
Capacitor: To block a steady current from flowing into the transistor and microphone.
Potential divider: To apply a proportion of the total voltage across the emitter-base junction so that the junction is forward-biased.
Transistor: To amplify the input wave form.
Loudspeaker: To change the electrical signal to sound wave.

The transistor as switch

1. In a transistor, no current can flow in the collector circuit unless a current flows in the base circuit. This property allows a transistor to be used as switch.
2. The transistor can be turned on or off by changing the base.
3. There are a few types of switching circuits operated by transistors.

(a) Light-Operated Switch

1. The circuit is designed to light the bulb in a bright environment and to turn it off in the dark.
2. One of the components in the potential divider is a light-dependent resistor (LDR). When it is placed in DARKNESS, its resistance is large. The transistor is switched OFF.
3. When LDR is lighted by bright light, its resistance falls to small value resulting in more supply voltage and raising the base current. The transistor is switched on, collector current flows and bulb lights up.

(b) Heat-operated switch

1. One important component in the circuit of a heat-operated switch is the thermistor.
2. Thermistor is type of resistor that responds to the surrounding temperature. Its resistance increases when the temperature is low and vice versa.
3. When heat is applied to the thermistor, its resistance drops and a greater share of supply voltage is dropped across R. The base current increases followed by a greater increase in the collector current. The bulb will glow and the siren will sound.
4. This particular circuit is suitable as a fire alarm system.

Integrated Circuits (C)

1. An integrated circuit (IC) consists of transistors, resistors, diodes and capacitors combined together in one wafer-thin chip of silicon.
2. This is one wafer-thin chip is called a microchip.
3. The microchip is only a few millimeters square with a thickness of 0.5 mm.

Advantages of an IC:
a. Consumes a small amount of electrical energy.
b. Very little heat is generated.
c. Occupies a small space which reduces the size of circuits.
d. Can be built at low cost. 

Cathode Ray Oscilloscope 

Cathode Ray Oscilloscope (CRO) uses a cathode ray tube to produce visible graphical representations of electrical signals.
CRO

Oscilloscope


The graphs produced consist of a horizontal axis which is normally a function of time, and a vertical axis which is a function of the input voltage.
Many physical quantities can be converted into a corresponding electric voltage. The oscilloscope is a useful tool in many physics experiments.
The componentd in a cathode ray tube consists of a vacuum glass tube with an electron gun, a deflection system for deflecting the electron beam, and a flourescent coated screen.
Electron gun
In a cathode ray tube, a beam of electrons is produced by heating the filament with a small voltage supply. the power supply can be AC or DC. The electron beam emerging from the electron gun passes between two pairs of deflection plates, i.e. X and Y - plates mounted horizontally and vertically.
Deflection system
CRO has a fluorescent screen. When the screen is struck by a beam of electrons, wave forms will be traced out on the screen. The kinetic energy of the electrons is changed to light energy.
There is a bright spot on the screen when the beam strikes. By changing the vertical gain on the Y-plates, the beam is deflected vertically. The beam can be moved up and down and if it moves fast enough, the dots will appear as a line.
When an AC supply is connected to the Y-plates, the electron beam will move vertically. The amount of vertical movement can be amplified by increasing gain control. The vertical movement of the electron depends on the vertical gain control and it can be adjusted, using the VOLTS/DIV control. The control is adjusted so that the resulting display is neither too small nor too large, but it fits the screen.
The horizontal deflection plates or X-plates produced a left to right movement. The movement is produced by a circuit called the time base inside the oscilloscope. The time base produces a saw tooth wave form. During the rising phase ( the rising line) of the voltage, the spot is driven at a uniform rate from left to right across the screen. During the falling phase, (the straight vertical line downwards) the electron beam returns rapidly from right to left.
Sawtooth wave form
http://www.hasdeu.bz.edu.ro/softuri/fizica/mariana/Mecanica/Waves_4/transp11.7.gif
The spot is moving in a very short duration and will not appear on the screen. The time base has a changing voltage across the X-plates so that the spot moves from left to right across the screen again. The speed at which the spot sweeps across the screen horizontally can be controlled by altering the frequency of the time base.

Properties of cathode rays

The properties of cathode rays can be studied using apparatus such as the maltese cross tube and cathode ray deflection tube.
Maltese cross tube
The maltese cross tube has a glass bulb and a hot cathode and an anode enclosed in it. The anode has a hole in the centre so that electrons can pass through it anc shoot across the vacuum. In the middle of the bulb is a second anode in the shape of a Maltese cross. At the end of the tube, there is a fluorescent screen.


The streams of electrons which leave the cathode and shoot across the vacuum are called cathode rays. The edges of the shadow of the Maltese cross on the screen are sharp. This is because the electrons are travelling in straight lines. Invisible cathode rays travelling across the tube, cast a shadow of the cross on the screen. When electrons strike the screen, the fluorescent screen will glow and light is emitted.
The beam of electrons can be moved by a magnetic field.
Deflection tube

The properties of a beam of electrons in an electric field can be investigated using a deflection tube as shown above. The cathode is connected to about 6 V AC power supply (AC: Alternating current). The electron gun produces a narrow beam of electrons.
The vertical screen is coated with fluorescent material which will glow when electrons strike it. It can show the path of the beam. There are two horizontal metal plates one above the other. When a voltage is applied accross the metal plates the electron beam will be deflected towards the positive plate.

Thermionic Emission

Electrons are attracted to the nucleus of an atom. There is a strong attractive force between the electrons and the positive charges of the nucleus. In order to move these electrons farther from the atom, energy is needed. This can be done by heating a metal using electric current. We can look at a vacuum diode and see how electrons move between the cathode and anode.Thermionic emission
A vacuum diode consists of a glass bulb containing two electrodes. One electrode is called the anode and the other is the cathode. The cathode is made up of tungsten filament. The cathode can be heated by a small current connected to the filament. This filament when heated will release electrons from its surface. These electrons can be attracted to the anode when there is a high ptential difference applied between the anode and the cathode.


The filament is connected to a 6 volt external battery (usually). When it is heated, a large number of electrons are free to move. As a result, a cloud of electrons is found outside the metal surface of the filament. Many of these electrons are held back by the attractive force of the atomic nucleus. Some of the electrons gained enough energy and escape from its surface. This effect is called thermionic emission.
Thus, Thermionic emission can be defined as the escape of high energy electrons from the surface of a tungsten filament.
Thermionic emission can be used to produce a continuous flow of electrons in a cathode ray tube. When the cathode is connected to the anode by an extra high tension (EHT) voltage supply, a narrow beam of fast electrons will move to the anode. The beam of electrons moving from the cathode to the anode is called cathode rays.
Thus, Cathode ray is the beam of electrons moving from the cathode to the anode in a cathode ray tube.