By: Arif Khan.
Most BSCS students feel too much difficulty while studying Basic Electronic, yes it is hard to understand. But it is very interesting too. In this blog, we will discuss semiconductors.
What is Semiconductor
These materials have got properties that are somewhere in between conductors (These materials have got a huge amount of electrons in the conduction band and therefore they are good conductors of electricity.) and insulators (These materials have got tightly bound electrons in the valence band and therefore electrons cannot flow in them.) so therefore they cannot be classified as good conductors or insulators and therefore they are known as semiconductors.
Atomic Structure of Atom
Before moving forward, let’s understand the structure of an atom. An atom contains a nucleus which is situated in the center and this contains Positively (+) charged Protons and Neutrons with no charge as there are Negatively (-) Charged Electrons that revolve around the nucleus for a neutral atom number of electrons and protons are equal.
Types of Semiconductor
There are two types of semiconductors which is mostly used as mentioned below.
a. Silicon
b. Germanium
Note: Pure semiconductors are called Intrinsic Semiconductors whereas semiconductors with impurities are called Extrinsic Semiconductors.
Characteristics of Germanium and Silicon
| Germanium | Silicon |
| It is unsuitable for certain applications due to high junction leakage currents as it has a relatively narrow energy band gap (0.66eV) | It is comparatively suitable for all applications as junction leakage currents are negligible as the energy band gap is comparatively broader (1.1eV) |
| Germanium devices can be operated up to 100c temperature. | Silicon devices can be operated up to 200c temperature |
| The intrinsic resistivity without any dopant is 47ohm-cm, hence not suitable for high voltage rectifying devices. | The intrinsic resistivity without any dopant is 230,000ohm-cm hence most suitable for high voltage rectifying devices as well as infrared sensing devices. |
| Germanium is costlier as compared to silicon. | Silicon is cheaper as compared to Germanium |
Energy Bands
When we will be discussing the flow of electrons in conductors, semiconductors, and insulators we will be interested in three bands.
Conduction Band
This is the energy band of electron orbital which contains electrons with high energy therefore these electrons have great mobility and therefore constitute the flow of electric current.
Valence Band
This energy band of electron orbital contains electrons with low energy and therefore these electrons will have lesser mobility and therefore will not constitute any current.
Forbidden Band
This is the energy band that separates the conduction band and valence band and is needed to be overcome by electrons in order to move into the conduction band from the valence band.
Energy Band Diagram for Insulator, Semiconductor and Conductor
| Insulator | Semiconductor | Conductor |
| The conduction band and Valence band are greatly separated by the forbidden band gap This is the reason that a huge amount of energy is required in order to make current flow across insulators | The conduction band and valence band is separated but the forbidden band gap for semiconductor is smaller in comparison to that of insulators. The smaller forbidden band gap is the reason that semiconductor exhibits the property of semiconductors and conductors. | The conduction band and valence band is overlapping in conductors and therefore there is no forbidden energy band gap present. This is the reason that conductors have got a large number of mobile electrons and require a small amount of energy to set electrons in motion. |
Covalent Bonds
A covalent bond, also called a molecular bond, is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is known as covalent.
P-type and N-type Semiconductors
There are two types of impurities that can be added to semiconductors one is Tri-Valent impurity such as Boron which can be added to intrinsic semiconductors to make P-type Semiconductors.
Or we can add Penta-Valent impurity such as Phosphorus to make N-type semiconductor
| P Type | N Type | |
| Type of | Extrinsic semiconductor | Extrinsic semiconductor |
| Description | A type of extrinsic semiconductor that carries a positive charge and has an improved conductivity. | A type of extrinsic semiconductor that carries a negative charge and has an improved conductivity. |
| Hole concentration | Larger hole concentration | Less hole concentration |
| Electron concentration | Less electron concentration | Larger electron concentration |
| Charge | The positive charge of the hole | The negative charge of the electron |
| Carriers | Holes are the majority carriers and electrons are the minority carriers | Electrons are the majority carriers and holes are the minority carriers |
| Creation | Created by doping an intrinsic semiconductor with acceptor impurities | Created by doping an intrinsic semiconductor with donor impurities |
| Dopant | A common p-type dopant for silicon is boron. Others include aluminum or gallium. | A common dopant for n-type silicon is phosphorus. Others include antimony and arsenic. |
| Effect of doping | Acceptor impurity creates a hole. | Donor impurities contribute free electrons |
| Contains | Trivalent impurities | Pentavalent impurities |
| Fermi level | Fermi level is below the intrinsic Fermi level and lies closer to the valence band than the conduction band | Fermi level is greater than that of the intrinsic semiconductor and lies closer to the conduction band than the valence band |
Fermi Energy
Fermi energy is often defined as the highest occupied energy level of material at absolute zero temperature. In other words, all electrons in a body occupy energy states at or below that body’s Fermi energy at 0K.
The fermi energy is the difference in energy, mostly kinetic. In metals, this means that it gives us the velocity of the electrons during conduction. So during the conduction process, only electrons that have an energy that is close to that of the fermi energy can be involved in the process.
This concept of Fermi energy is useful for describing and comparing the behavior of different semiconductors. For example, an n-type semiconductor will have a Fermi energy close to the conduction band, whereas a p-type semiconductor will have a Fermi energy close to the valence band.
Conduction in Semiconductor
n – type Si
When the Semiconductor is N type the donor dopants are able to form a donor level in the band gap near the conduction band, previously where there were no existing states because it is now energetically favorable to do so. This means that the donated electrons will require a much smaller increase in energy to be excited into the conduction band, where the free-flowing electrons can increase conductivity. Therefore, as doping increases, the conductivity of an n-type semiconductor increases as well (more donor states mean more donated free electrons that can be promoted into the conduction band).
p – type Si
When the Semiconductor is P-type, the acceptor dopants are able to form an acceptor level in the band gap near the valence band, previously where there were no existing states, because it is now energetically favorable to do so. This means that the electrons in the valence band can be excited into this acceptor level in the band gap (as opposed to the conduction band) to complete the covalent bond. This process leaves behind free holes that are able to propagate through the valence band, where they can increase the conductivity. Therefore, as doping increases, the conductivity of a p-type semiconductor also increases (more acceptor states mean more free holes that can permeate the valence band).
Doping
Atoms follow a rule called Octet Rule. According to the Octet, rule atoms are stable when there are eight electrons in their valence shell. If not, atoms readily accept or share neighboring atoms to achieve eight electrons in their valence shell. In the silicon lattice, each silicon atom is surrounded by four silicon atoms. Each silicon atom shares one of its electrons in the valence shell with its neighboring silicon atom to satisfy the octet rule. A schematic diagram of an intrinsic semiconductor is shown in the image right.
When we pop in a pentavalent element into the lattice we have doped the silicon lattice with Phosphorous, a pentavalent element. Now pentavalent element has five electrons, so it shares an electron with each of the four neighboring silicon atoms, hence four atoms are tied up with the silicon atoms in the lattice. This leaves an electron extra. This excess electron is free to move and is responsible for conduction. Hence N-type (Negative Type) extrinsic semiconductor (silicon in this case) is made by doping the semiconductor with the pentavalent element.
To create a P-type semiconductor, all we must do is pop a trivalent element into the lattice. A trivalent element has three electrons in its valence shell. It shares three electrons with three neighboring silicon atoms in the lattice, the fourth silicon atom demands an electron but the trivalent atom has no more electrons to share. This creates a void in the lattice which we call it has a hole. Since the electron is deficient, the hole readily accepts an electron, this makes it a P-type (Positive type) extrinsic semiconductor.
Effect on energy Band Diagram of Doping
PN Junction
If a block of P-type semiconductor is placed in contact with a block of N-type semiconductor, the result is of no value. We have two conductive blocks in contact with each other, showing no unique properties. The problem is two separate and distinct crystal bodies. The number of electrons is balanced by the number of protons in both blocks. Thus, neither block has any net charge.
However, a single semiconductor crystal manufactured with P-type material at one end and N-type material have some unique properties. The P-type material has positive majority charge carriers, and holes, which are free to move about the crystal lattice. The N-type material has mobile negative majority carriers and electrons. Near the junction, the N-type material electrons diffuse across the junction, combining with holes in P-type material. The region of the P-type material near the junction takes on a net negative charge because of the electrons attracted. Since electrons departed the N-type region, it takes on a localized positive charge. The thin layer of the crystal lattice between these charges has been depleted of majority carriers, thus, is known as the depletion region. It becomes nonconductive intrinsic semiconductor material. In effect, we have nearly an insulator separating the conductive P and N doped regions.
Depletion Region
When P type and N type semiconductors are formed next to each other by process of doping on the border where they two meet electrons from highly concentrated N region crosses over to electron-deficient P region this forms region where there is electric field directed from N to P thus stopping more holes and electrons to flow in N-Type region and P-type regions respectively and also if there will be any free mobile charge present in this region it will be drifted by this electric field to either side N or P depending on nature of the charge, this will make this region depleted of free charge thus it is called depletion region.
Biasing a PN Junction
Under no voltage or unbiased condition, the p-n junction diode does not allow the electric current. If the external forward voltage applied on the p-n junction diode is increased from zero to 0.1 volts, the depletion region slightly decreases. Hence, a very small electric current flows in the p-n junction diode. However, this small electric current in the p-n junction diode is considered negligible. Hence, this cannot be used for any practical applications.
If the voltage applied on the p-n junction diode is further increased, then more free electrons and holes are generated in the p-n junction diode. This large number of free electrons and holes further reduces the width of the depletion region. Hence, the electric current in the p-n junction diode increases. Thus, the depletion region of a p-n junction diode decreases with an increase in voltage. In other words, the electric current in the p-n junction diode increases with the increase in voltage.
When the external voltage is applied to the p-n junction diode in such a way that, negative terminal is connected to the p-type semiconductor and positive terminal is connected to the n-type semiconductor, holes from the p-side are attracted towards the negative terminal whereas free electrons from the n-side are attracted towards the positive terminal. In reverse biased p-n junction diode, the free electrons begin their journey at the negative terminal whereas holes begin their journey at the positive terminal. Free electrons, which begin their journey at the negative terminal, find large number of holes at the p-type semiconductor and fill them with electrons. The atom, which gains an extra electron, becomes a charged atom or negative ion or motionless charge. These negative ions at p-n junction (p-side) oppose the flow of free electrons from n-side.
PN Junction Current Voltage Characteristics
Reverse Break Down Voltage:
The amount of reverse bias that will cause a p-n junction (diode) to break down and conduct in the reverse direction. Do not be confused into thinking that this “breakdown” means that the diode has been damaged. It simply refers to the voltage required to overcome the natural tendency of a p-n junction to not conduct in a reverse direction. This is also known as Avalanche Break Down
Zener Break Down Voltage:
Also known as Zener Effect, is like an electrical breakdown condition in a Zener diode due to the reverse bias voltage exceeding the breakdown voltage resulting in the tunneling of electrons from the valence to the conduction band leading to a large number of free minority carriers that suddenly increases the reverse current.
| Basis For Comparison | Avalanche Breakdown | Zener Breakdown |
| Definition | The avalanche breakdown is a phenomenon of increasing the free electrons or electric current in semiconductor and insulating material by applying a higher voltage. | The process in which the electrons are moving across the barrier from the valence band of the p-type material to the conduction band of the lightly filled n-material is known as the Zener breakdown. |
| Depletion Region | Thick | Thin |
| Junction | Destroy | Not Destroy |
| Electric Field | Weak | Strong |
| Produces | Pairs of electron and hole. | Electrons. |
| Doping | Low | Heavy |
| Reverse potential | High | Low |
| Temperature Coefficient | Positive | Negative |
| Ionization | Because of collision | Because of Electric Field |
| Breakdown Voltage | Directly proportional to temperature. | Inversely proportional to temperature. |
| After Breakdown | Voltage varies. | Voltage remains constant |
Diod
A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals
A diode is operated under two conditions
Forward Bias
Reverse Bias v
It is viewed like a switch in a circuit which allows the flow of current when connected in forwarding bias that is how it acts like a close switch and when it is connected in reverse bias it stops the flow of current thus acting as an open switch.
Testing the Diod with a Multimeter Using Ohms
We first take the ohmmeter and place the positive probe on the anode of the diode (the black part of the diode_ and the negative probe on the cathode of the diode (the silver strip), as shown above. In this setup, the diode should read a moderately low resistance, maybe a few tens of thousands or low hundreds of thousands of ohms. For example, you may read 230KΩ.
Open Diode
If the diode reads high resistance in both directions, this is a sign that the diode is open. A diode should not measure very high resistance in the forward biased direction. The diode should be replaced in the circuit.
Shorted Diode
If the diode reads low resistances in both directions, this is a sign that the diode is shorted. A diode should not measure low resistance in the reverse biased direction. The diode should be replaced in the circuit.
Testing the Diod with a Multimeter Using Volts
Because diodes drop a specific voltage across their terminals with their threshold voltage being exceeded, we can use these properties to see if a diode is reading a healthy and correct voltage across its terminals.
Open Diode
If you reading a very high voltage across the diode, such as the voltage you are supplying it, the diode is open and, thus, defective, and should be replaced.
Shorted Diode
If you reading no to very little voltage across the diode, then the diode is shorted and should be replaced. So there you have it, these are 2 strong tests you can do to test whether a diode is good or not.
Your are always welcome to write comments and suggestions.
