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.

Understanding Semiconductor diodes

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.

Understanding the Nucleus of an Atom

1. Matter is made up of very small particles called atoms.

2. Each atom has a very small and very dense core known as the nucleus.

3. Most of the mass of the atom is contained in the nucleus.

4. The electrons move in orbits around the nucleus.

5. The diameter of the nucleus is about 100 000 times smaller than the diameter of the atom.

6. This means that there are lots of empty space within an atom.

7. The subatomic particles in a nucleus are called nucleons.

8. The two types of nucleons are protons and neutrons.

9. The proton is a positively charged particle. It carries a charge of +e, where e is equal to 1.6 × 10-19 C.

10. The neutron carries no charge. The neutrons has approximately the same mass as the proton.

11. The number of protons in the nucleus of an atom is known as the proton number, Z.

12. The total number of protons and neutrons in the nucleus of an atom is known as nucleon number, Aor mass number.

13. Then number of neutrons, N = A – Z

Nuclide Notation

1. A nuclide is a type of atom with a particular nucleon number. This term is also used for a type of nucleus.

2. The nuclide notation of an atom gives the symbol of the elements, the proton number and the nucleon number of the atom.

Isotopes

1. Isotopes are atoms of the same elements with the same numbers of protons but different number of neutrons.

2. isotopes have the same proton number but different nucleon numbers.

3. All isotopes of an element have the same chemical properties because their electrons are arranged in exactly the same way.

4. Their physical properties such as densities, boiling points and melting points are different.

5. Some elements in nature such as oxygen,carbon, and bromine consist of a mixture of isotopes.

6. Some isotopes of an element are stable while some are unstable. The unstable isotopes or radioisotopes.

7. Radioisotopes will undergo spontaneous decay to emit radioactive rays such as alpha, beta and gamma rays. After radioactive decay, the proton number and nucleon number of the radioisotope may be changed.

Radioactivity

Radioactivity

1. Radioactivity is the spontaneous and random emission of radioactive rays from unstable radioactive materials after which they become more stable.

2. The process is said to be spontaneous because it is not influenced by any physical factors such as temperature, pressure, time, etc.

3. A nucleus is unstable if it is too big. All nuclei with z > 83 or A> 209 are unstable.

4. The emission of radioactive rays is random means that

• Emission occurs at irregular intervals.
• Emission does not occur at the same means.

5. There are three different types of radioactive emissions.

• Alpha particle- a
• Beta particle- B
• Gamma ray-r

6. Table below shows the characteristics of alpha particle, beta particle, and gamma particle.

Characteristic	Alpha particle	Beta particle	Gamma ray
Nature	Positively charged helium nucleus, He	Negatively charged electron, e	Neutral electromagnet ray
In an electric field	Bends to the negative plate	Bends to the positive plate	Does not bend, showing that it is neutral.
In magnetic field	Bends a little showing that it has a big mass. Direction of the bend indicates that it is positively charged.	Bends a lot showing that it has a small mass. Direction of the bend indicates that it is negatively charged.	Does not bend showing that it is neutral.
Ionising power	Strongest	Intermediate	Weakest
Penetrating power	low	Intermediate	High
Stopped by	A thin sheet of paper	A few millimeters of aluminium	A few centimeters of lead or concrete
Range in air	A few centimeters	A few metres	A few hundred metres
Speed	1/20 X the speed of light, c	3%-99% of the speed of light, c	The speed of light,c

Radioactive Detectors

Radioactive Detectors

Geiger-Muller Tube

1. The Geiger-Muller tube is an effective radioactive detector. It can trace alpha particles, beta particles and gamma rays.


2. The outer part of the G-M tube is made of aluminium which acts as the cathode.


3. The middle part of the G-M tube is a metal wire which acts as the anode.


4. The G-M tube is filled with argon gas at low pressure.


5. Initially, the G-M tube must be connected to a high voltage before being used.


6. This high voltage causes some ionization of argon gas.

Cloud Chamber

1. The cloud chamber is made by using a transparent plastic box. The space in it is divided into two parts by a metal.


2. The lower part is filled with solid carbon dioxide. Sponge is used to push the solid carbon dioxide towards the metal plate.


3. The upper part is filled with molecules of alcohol vapour released from the felt which is initially soaked in alcohol.


4. When the alcohol vapour diffuses downwards, it will become colder. Thus, a supersaturated condition will be produced in the space in the lower part of the chamber.


5. When the radioactive rays enter the upper part, the ionization of air will occur. Saturated alcohol vapour will move above the ions. Droplets of liquid alcohol on the ions will cause the formation of misty tracks.


6. Steps to ensure clear tracks:




• The transparent Perspex cover is rubbed with a soft cloth to produce charges which will remove all ions in the chamber before any radioactive rays enter.


• The cloud chamber must be placed horizontally to ensure smooth flow of particles in it.


• If light is used, it must shine on the area superated with vapour and not on the black base of the chamber in order to avoid heating it.


7. Normally, the tracks produced are not uniform. This shows that the radioactive rays are produced randomly.


8. There are three types of tracks as shown in Table below.

Types of radioactive rays	Explanation
Tracks of alpha particles	The alpha tracks are thick and straight. This shows that alpha particles have the strongest ionizing power and the biggest mass.
Tracks of beta particles	The beta tracks are thin and curvy. This shows that beta particles have low ionizing power and small mass.
Tracks of gamma ray	Their tracks are short, curvy and spiky from the middle. It shows that it has the lowest ionizing power.

9. The number of radioactive tracks produced will decrease after a while. This is because after some time, the condensation of alcohol vapour on the radioactive source will block the emission of radioactive rays.

Spark counter

The wire gauze and thin wire are connected to a voltage of more than 2000 V.

The voltage is increased slowly until sparks are produced in between.

The sparks are formed due to ionisation of the air.

The voltage is then decreased until no sparks are formed.

The radioactive source is brought close to the wire gauze.

The radioactive rays will ionize the air molecules between the wire gauze and thin wire. Positively charged ions will be attracted to the negatively charged gauze and the negatively charged ions will be attracted to the positively charged thin ions.

Secondary ionization will occur due to the collision between the ions and the air molecules.

Therefore, sparks are formed.

The number of sparks measured the intensity of radioactive rays from its source randomly.

The spark counter can only trace alpha particles which have high ionizing power.

Electroscope

When charged plate of the electroscope is exposed to the source of alpha particles, the gold leaf will collapse slowly.

This is due to the ions and electron are produced by the alpha particles which will neutralize the charge in the electroscope.

The rate of collapse of the gold leaf indicates the strength of the radioactive source.

Photographic Plate

All types of radioactive rays will darken the photo film. The effect is like sunlight acting on it.

The ionization effect by the radioactive rays will decompose silver bromide crystals on the film.

Films which are exposed to sunlight will show white lines representing radioactive tracks.

Films are kept in the badges worn by workers as a tracer device of radioactive rays.

The main disadvantage of using a film as a radioactive tracer is that it needs to be processed in order to prove the presence of radioactive rays.

Half Life

Half-life

Concept of Half-life

The reactivity or activity of a radioactive material is the rate of decay of the material.

The rate of decay is the same as the number of atoms which decay or are emmited every second.

The rate of decay of a radioactive materials depends on the number of atoms that have not yet undergone decay. Thus, the reactivity of a radioactive material will decrease with time.

The half-life of a radioactive element is the time taken for half the number of atoms in a sample of radioactive atoms to decay.

Decay curve.

The half-life of the same radioactive element is the same but the half-lives of different radioactive elements are different.

The value of half-life is not influenced by factors such as temperatures, pressure and etc.

Usage of Half-life

Half-life in Archeology

Carbon-14 has a half-life of 5600 years.

Humus, animals and plants absorb carbon-14 through carbon dioxide gas in the atmosphere. A small amount in CO2 exists as carbon-14.

Living animals and vegetable have a constant amount of Carbon-14 because the c-14 decayed will always replaced.

However or dead beings the amount of C-14 in it will decrease because new C-14 will not be absorbed causing its reactivity to decrease.

When an antique or human skill are found, their age can be determined by

Measuring the reactivity of C-14 in it.

Determine the ratio of decay carbon-14 against intact carbon-14.