Semiconductor

The conductivity property of a semiconductor lies between the conductors and insulators. It means that the conductivity of semiconductors is not as good as the conductivity of metals, but not as poor as insulators. We can control the conductivity of a semiconductor by introducing impurities into its crystal structure.

The applications of semiconductors include various electronic devices, such as ICs (Integrated Circuits), diodes, and transistors. The common examples of semiconductors include Silicon and Germanium.

Here, we will discuss the following topics:

Semiconductor Materials

History of Semiconductor

Charge Carriers of Semiconductor

Band Gap Energy of Semiconductors

Types of Semiconductor

Doping in Semiconductors

Direct and Indirect Semiconductors

Recombination and carrier generation in semiconductors

Properties of Semiconductor

Advantages and disadvantages of Semiconductors

Applications of Semiconductor

Differences

Semiconductor Materials

The semiconductor materials are generally solid at room temperature. The commonly used solids are found in the powder or polycrystalline form. Some of the available crystalline materials also exist in nature form, such as diamond.

Nowadays, crystals can be developed using the human-made process. A perfect crystal is regular throughout its structure and is rather difficult to achieve. Sometimes, a created crystal ends up with built-in defects. But, it is interesting to know that defective crystals are also useful in the semiconductor materials.

Let's start with the semiconductor materials and their properties.

Among all the types of semiconductor materials, Silicon, Arsenic, Gallium, and Germanium are the most common used materials that are used in making different types of semiconductor devices. Alloy materials are also preferred as the semiconductor materials.

Silicon and Germanium are the basic elemental material used for making various devices. A semiconductor compound GaAs is also a useful material.

Consider the below table that displays the types of semiconductors with the examples.

Where,

Al- Aluminum

As- Arsenic

B- Boron

C- Carbon

Cd- Cadmium

Ga- Gallium

Ge- Germanium

In- Indium

N- Nitrogen

P- Phosphorous

S- Sulfur

Sb- Antimony

Se- Selenium

Si- Silicon

Te- Tellurium

Zn- Zinc

The above names are the full form of the semiconductor materials mentioned in the above table.

Now, we will discuss the properties of the most common materials, i.e., Silicon and Germanium.

Silicon

The atomic number of silicon is 14, and it is represented with the symbol Si. The availability of silicon in Earth's crust is large, which allows its applications for thousand years. In 1906, a radio pioneer and an American engineer named Greenleaf Whittier Pickard invented the first silicon device. The device was named as a silicon radio crystal detector. In 1954, the first silicon junction transistor was fabricated by the American chemist named Morris Tanenbaum at the Bell Labs.

Around the late 1950s, Germanium became the popular semiconductor material than silicon. It was also preferred as a material for transistors. Silicon is scarcely found in its pure form in the Earth's crust.

The first abundant element available in the Earth's crust is oxygen. After it comes to the element silicon, it means that silicon's rank lies in the fiftieth number as per the abundant materials in the Earth's crust.

Germanium

The atomic number of Germanium is 32, and it is represented with the symbol Ge. The performance of the Germanium is better than silicon due to its high carrier mobility. Electron mobility is the semiconductor property that shows movement of an electron in the semiconductor under the applied electric field. The pure form of germanium is a semiconductor.

The appearance of Germanium is similar to that of silicon. The rank of Germanium lies in the fiftieth number as per the abundant materials in the Earth's crust. It means that Germanium rarely appears in high concentrations. At present, the applications of Germanium lie in solar cells, LED, etc.

Both silicon and germanium can react with oxygen to form complex oxides.

History of Semiconductors

The history of semiconductors started around the invention of materials, rectifiers, and transistors' electrical conductivity.

• In 1821, the first person to notice the semiconductor's effect was a German physicist named Thomas Johann Seebeck.
• In 1833, an English Scientist named Michael Faraday noticed that the silver sulfide resistance (dense black solid inorganic compound) decreases with the rise in temperature. The concept was different from that observed in metals.
• The properties of semiconductors were focused on two points called rectification and sensitivity to light for some years.
• In 1878, an American physicist named Edwin Herbert Hall illustrated the diversion of charge carriers under the applied magnetic field. It was termed as Hall Effect.
• The conduction in solids due to the electrons was discovered by the British physicist named J. Thomson. Thus, he is known for his discovery of electrons.
• In 1914, John Koenigsberger categorized the solid materials as insulators (materials that do not conduct electricity), metals, and variable conductors.
• In 1928, the concept of the atomic lattice was introduced by the Swiss-American physicist named Felix Bloch. He published the concept of electron movement following with the atomic lattices.
• In 1930, the impact of impurities on the conductivity of the semiconductors was found by B. Godden.
• The concept of band theory was developed by a British Mathematician named Alan Herries Wilson in the year 1931.
• In 1938, the theory of minority carriers and the p-n junction was introduced by Boris Davydov.
• After the invention of the transistor, the semiconductor industry grew rapidly.
• In 1957, the use of semiconductors exceeded the cost of 100 dollars.

History of devices based on semiconductor

Here, we will discuss the invention of semiconductor devices.

• Various coating over the metals was used for various applications. For example, the selenium and gold coating over the metal was used in the photographic light meters in the year 1930.
• In 1906, an English Engineer named H.J Round observed the emission of light from the Silicon Carbide (semiconductor comprising of silicon and carbon) crystal after the applied electric current or the principle behind LEDs.
• The first semiconductor device was based on Galena's use, a natural mineral form of lead ore. It includes the devices invented by the two physicists named Ferdinand Braun and Jagadish Chandra Bose. The devices were the crystal and radio crystal detector.

Energy bandgap of the Semiconductor

The atoms of solid materials are held together by the forces of repulsion and attraction. It maintains a balance by keeping the atoms at a specific distance known as inter-atomic distance. This distance is not the same for all types of solids.

The energy level diagram of an atom is shown below:

Here, K, L, M, N... is the discrete energy levels that are separated by the number of energy gaps. As shown, the forbidden energy gap between the two energy levels keeps on decreasing as we go upwards. It is the case of an isolated atom.

Now, let's consider the case of solids.

The energy levels of the atoms in a solid are based on quantum mechanics. It clearly explains that the discrete energy levels in solids are closely spaced, which are called bands. The band in solids is separated by a forbidden energy gap (Eg), as shown below:

Here, we will discuss valence and conduction bands in detail.

Conduction Band

The conduction band comprises of the mostly empty energy states. The excited electrons from the valence band jump into the conduction band. The electrons present in the orbits of the conduction band are free to move. The movement of the electrons generates the current. When the electrons cannot jump to the conduction band, the materials lack conductivity.

In a given band, an electron may transfer from one energy state to another.

Valence Band

The valence band consists of the band of electrons that jumps to the conduction band when it gains energy. A bandgap is the energy gap between the valence band and conduction band, as shown below:

Now, let's clearly understand the forbidden energy gap in semiconductors. We will also discuss the energy gap in the case of insulators and metals.

In any solid, the applied voltage generates the magnetic field. The electrons of the solid will experience acceleration.

F = ma

Where,

F is the force generated by the magnetic field

a is the acceleration

As soon as the electrons move in the higher energy states, the electrons' energy starts increasing in the case of empty energy states. For example, if some of the energy states (like 4N) in the valence band are filled, there will be no energy available for electrons' movement in the valence band. In the conduction band, the available empty energy states are greater, but there are no electrons.

Let's consider a case of silicon at 0K.

The applied magnetic field on the silicon at 0K will result in no electrons movement and no current. It means that silicon at 0K acts as an insulator. The increase in temperature will cause the electrons to move due to the increased thermal energy with the temperature rise. Some electron possesses sufficient energy to move, while some electrons possess extra energy that may be greater than the forbidden bandgap energy or Eg. The extra energy allows the electrons to jump to the conduction band. The electrons will jump where there are more empty energy states.

We will now consider the same case of silicon, but at room temperature (around 300K).

A reasonable amount of electrons is present in the conduction band of the Silicon at room temperature. The applied electric field will provide acceleration to the electrons in the conduction band. The electrons will gain energy and move to the available empty energy states in the conduction band. It will acquire velocity and generates a small amount of current. Hence, silicon at room temperature is not an insulator. It starts conducting due to small amount of current.

The electrons which have jumped from the valence and to the conduction band are smaller in number. It means that silicon is not a good conductor of electricity. Hence, it Silicon is termed as a Semiconductor.

The Forbidden energy gap of Silicon is 1.1eV.

Let's consider another case for better understanding.

Here, we will discuss the reason for the diamond being insulator at room temperature.

The Forbidden energy gap of the diamond is 5.47eV. At room temperature, the electrons present in the diamond's valence band could not jump to the conduction band. Hence, diamond is considered as an insulator at room temperature, while silicon is a semiconductor.

But, at very high temperatures, a diamond can become a semiconductor.

Band Gap of Conductors

The valence band and the conduction band in the conductors overlap each other. It means that the Bandgap of the conductors is 0 due to the overlapping of these two bands. The conduction band consists of a large number of electrons. It allows the flow of heavy current under the applied electric field.

The bandgap diagram of the conductor is shown below:

Band Gap of Insulators

The materials with a bandgap greater than 5.0eV are considered as Insulators. It is the Bandgap generally at room temperature. The electrons at room temperature do not gain sufficient energy to jump from the valence band to cover the forbidden energy gap and reach the conduction band.

The bandgap diagram of insulators is shown below:

Band Gap of Semiconductors

The materials with a bandgap less than 3.0eV are considered Semiconductors. The valence electrons do not gain enough energy to jump to the conduction band and participate in the current process. Since the Bandgap is less than the insulators, some of the electrons can participate in the current process. It results in a small current flow. Hence, the conductivity of semiconductors lies between the conductors and insulators. The popular semiconductors materials include Silicon (1.1 eV), Germanium (0.7 eV), and GaAs (1.43eV).

The bandgap diagram of semiconductor is shown below:

Bonds in Solids

Bonds in solids can be categorized as Ionic, Covalent, and Metallic bond. The motion of electrons in the ionic and covalent bond is restricted. The electrons in the case of the metallic bond are free to move.

Let's discuss a short description of all three types of binds.

Ionic Bond

The electrons in the outermost orbit of an atom and transferred to the other atom's outermost orbit to create a stable configuration.

Features of Ionic Bonding

• The melting and boiling point of the ionic crystal is high due to the strong bonding.
• Good Insulators

Covalent Bond

In covalent bonds, the neighboring atoms share their valence electrons to form strong covalent bonds. It means no transfer of electrons from one atom to another, like an ionic bond.

The Silicon (Si) crystal has a covalent bond between its atoms. Consider the covalent bond formation in the silicon atom, as shown below:

Features of Covalent Bonding

• Strongly directional
• The melting and boiling point of the covalent crystal is high due to the high binding energies.
• Good insulators as well as fair conductors (semiconductors).

The optical properties are quite similar to that of ionic crystals.

Metallic Bond

The electrons are loosely bound to the atoms in the case of metals. The atoms with one atom in the outermost shell are metallic, while atoms with four electrons in the outermost shell are less metallic. Similarly, more than five atoms (six or seven) in an atom's outermost shell can cause it to lose the metallic properties.

Features of Metallic Bonding

• High thermal and electrical conductivity.
• High absorption coefficients.

Types of Semiconductor

There are two types of semiconductors, intrinsic and extrinsic semiconductor.

Let's discuss this in detail.

Intrinsic Semiconductor

We know that Silicon at room temperature has no electrons in the conduction band. The semiconductor that is free from any crystal defect and impurities are called intrinsic semiconductors. The increase in temperature can result in the electrons to jump from the valence band to the conduction band after gaining thermal energy. The jump of the electrons leaves a hole in the valence band. An EHP (electron hole pair) is thus created in this process.

The increase in temperature can create more holes in the valence band. At a certain temperature the number of holes becomes equal to the number of electrons in the valence band.

Charge carrier concentration in Intrinsic

At 0K, the conduction band of the intrinsic semiconductor consists of no charge carries. The rise in temperature causes the electrons to jump from the valence band to the conduction band.

Extrinsic Semiconductor

The semiconductors with impurities and defects are called extrinsic semiconductors. The process of introducing impurities in the semiconductor is called doping. The doping helps in increasing the conducting properties of a semiconductor. It also provides additional energy levels in the forbidden energy gap.

Electrons at the added level are so close to the valence band and conduction band that they can easily jump to these bands to increase their population. A given amount of doping increases the population of electrons in the conduction band. Such doped semiconductors are termed extrinsic semiconductors.

Ewe can convert the intrinsic semiconductor into an extrinsic semiconductor by applying impurities or doping.

An extrinsic semiconductor is further categorized into P-type and N-type semiconductors. Let's discuss the above types of semiconductors in detail.

P-type

The application of trivalent impurities on the intrinsic semiconductor can result in the formation of the P-type extrinsic semiconductor. Trivalent means the atoms with doping valency of 3. It generally donates excess holes due to its positive charge. Trivalent impurities are often known as acceptor impurities. The acceptor energy level of the P-type is close to the valence band, as shown below:

The examples include Aluminum and Boron. It is called acceptor impurity because the atoms can easily accept electrons from the neighboring atom due to the vacancy. The majority of carriers in a P-type semiconductor are holes. It means that holes are much greater than the number of electrons.

N-type

The application of pentavalent impurities on the intrinsic semiconductor can result in the formation of a N-type extrinsic semiconductor. Pentavalent means the atoms with five electrons in the outermost shell. Pentavalent impurities are often known as donor impurities. It is called a donor because it shares a free electron with the semiconductor. The donor energy level of the N-type is close to the conduction band.

The examples include arsenic and phosphorous. The majority of carriers in a N-type semiconductor are electrons. It means that electrons are much greater than the number of holes.

Charge Carries in the Semiconductor

The charge carriers of a semiconductor are electrons and holes. The electrons possess a negative charge, and holes possess a positive charge. The movement of electrons determines the flow of charge in a semiconductor. But, it does not mean that holes do not participate in the conduction process.

The minority charge carriers mean the smaller number of carriers present in the semiconductor material. The majority carriers signify the greater number of charge carriers. The flow of charge depends on the majority of charge carriers.

Let' discuss the charge carriers in the p-type and n-type semiconductors.

The majority carriers in the p-type semiconductor are holes, while the minority charge carriers are electrons. The majority carriers in the n-type semiconductor are electrons, while the minority charge carriers are holes.

Doping in Semiconductors

The Bandgap of the semiconductor is small as compared to the Bandgap of the conductor. Doping is a process to add impurities to the semiconductor, which increases the semiconductor material's conductivity.

An impurity added to the semiconductor materials is known as Dopant. For example, the addition of pentavalent impurities, such as arsenic, increases the material's conductivity by contributing free electrons. The increase in the number of doping concentration results in higher conductivity. It is due to the presence of extra charge carriers.

Let's first discuss the effect of doping on the materials. It is listed below:

Silicon

Silicon is the essential semiconductor material for the semiconductor industry. Boron and phosphorous are the two important impurities that can be applied on the silicon.

Let's discuss how the trivalent Dopant (boron) and Pentavalent Dopant (phosphorous) contribute to doping with the silicon atom. The diffusion rate of boron and silicon is fast.

Boron

Boron is considered as p-type doping that has three valence electrons. It means that it is missing the fourth valence electron. The silicon atom has four valence electrons. When boron is doped on the silicon, it creates a hole in the silicon lattice. The holes created in the lattice are free to move. Thus, a p-type semiconductor is formed.

Phosphorous

Phosphorous is considered as n-type doping that has fie valence electrons. It means that it has one extra valence electron. The silicon atom has four valence electrons. When phosphorous is doped on the silicon, it forms a covalent bond with the silicon's four electrons. The extra electrons become unbounded to the atoms in the silicon lattice.

Thus, an n-type semiconductor is formed.

Effect of doping on the semiconductors

There are two types of materials on which the doping can be applied. We know that applying doping on the intrinsic materials results in the formation of extrinsic materials. Let's discuss the effect of doping on the types of extrinsic semiconductor materials.

P-type consists of majority carry holes. The p-type doping signifies the doping of trivalent impurities.

N-type consists of a majority of carrier's electrons. The n-type doping signifies the doping of pentavalent impurities.

The p-type dopants are boron, gallium, indium, and Aluminum. Boron doping is popular in CMOS (Complementary Metal Oxide Semiconductors) technology. Aluminum doping is preferred for deep p-diffusions.

The p-type dopants are Antimony, lithium, arsenic, bismuth, and phosphorus. The antimony doping is preferred for its use in buried layers.

Arsenic doping is popular in VLSI (Very Large Scale Integration) circuits.

The other types of dopants are Nitrogen, Xenon, Gold, Platinum, etc. Nitrogen doping is popular for growing defect-free silicon crystals.

Dopant materials for semiconductors

Here, we will discuss the doping materials preferred for different types of semiconductors.

The doping materials preferred for Group IV semiconductors, such as silicon germanium, diamond, etc., are the trivalent or acceptors and donors. The acceptors are the elements of group III. The donors are the elements of group V of the periodic table.

Direct and Indirect Semiconductors

The transition in such types of semiconductors is radiative. It means that the direct and indirect semiconductor depicts an electron's fall from the conduction band to the valence band when it loses energy. The energy that the electron loses while coming from the conduction band to the valence band is given out as radiation corresponding to the bandgap energy (Eg).

Let's discuss direct and indirect semiconductors in detail.

Direct Semiconductors

The semiconductors where the conduction band is situated just above the valence band are called direct semiconductors. The diagram of direct semiconductor is shown below:

In the above diagram, the conduction band energy parabola's energy minima are just above the valence band energy parabola's maxima. It means that the transition in such cases is direct. The direct semiconductors are a good source of optical radiations.

Indirect Semiconductors

The diagram of the indirect semiconductor is shown below:

In this case, the conduction band is shifted to the right side, and its minima have a positive value. Here, an electron transition from the conduction band to the valence band is a two-step process. It first locates a lower energy level (Et). The electron loses energy and comes to the energy level Et. The energy lost in this process is non-radiative and comes out as heat. Under appropriate conditions, the electron now at energy level Et may fall to a valence band release a photon. The indirect semiconductors can be used as a controlled photon radiator.

Mixed Semiconductor

A semiconductor, which is a mixture of both direct and indirect semiconductors called a mixed semiconductor.

The diagram of mixed type semiconductors is shown below:

Recombination and carrier generation in semiconductors

Recombination and carrier generation signifies the elimination and generation of charge carriers. The charge carriers are electrons (negatively charged) and holes (positively charged). These two processes are the basics of different electronic devices, such as LED (Light Emitting Diodes), photodiodes, etc. The transition of an electron between the valence band (lower band or energy state) and the conduction band is based on the generation of electron-hole pair. Let's discuss the recombination and carrier generation process in semiconductors.

Carrier generation

Carrier generation signifies the generation of charge carriers. The transition of the electrons in the case of carrier generation is from the valence band.

The carriers can be generated by the light interacting on the materials or the applied electric field. It generates a pair of free carriers in the semiconductor materials. The electrons across the bandgap can be excited when sufficient energy is applied to it with an external source. It lowers the resistance of the material by generating extra charge carriers.

Recombination

Recombination signifies the elimination of charge carriers. The transition of the electron is reversed as compared to the transition in carrier generation. It means that the transition of the electron is from the conduction band (upper band) to the valence band.

In the recombination process, the jump of electrons (after the electron excitation) from the conduction band to the valence band results in the release of energy in the form of photons. The energy of the released photons can be less or more than the energy that was initially absorbed. The phenomenon of light generation by LEDs is based on such a concept, where the photons are released in the form of light.

The released energy can be in the form of heat or light. When the recombination of charge carriers release phonons instead of photons, the energy is released in the heat.

Properties of Semiconductor

The properties of a semiconductor determine the behavior of the semiconductor materials. Let's discuss the different properties of a Semiconductor.

• High Thermal Conductivity
  The thermal conductivity in a semiconductor can vary depending on the charge carriers' concentration present in the crystal lattice. It also depends on the temperature. The increase in temperature results in the increased thermal conductivity of the semiconductor. The semiconductor with high thermal conductivity is used for improving thermal electronic management.
• Light Emissions
  Some semiconductors produce energy in the form of light when an electric current passes through its circuitry. It means that excited electrons release energy in the form of light compared to heat. Such semiconductors are preferred in applications, such as LEDs, LASER diodes, fluorescent quantum dots, etc.
• Compact size
  The size of semiconductor devices is shrinking by the passing years. Today, semiconductors' applications have reduced the size upto nanometers due to the increasing demand and due to advancements in technology.
• Hetero-junctions
  It is the interface between two differently doped semiconductors. It means hetero-junctions are the interface junctions that occur when two differently doped semiconductors are joined together. It results in the exchange of positive and negative charge carriers between the semiconductors.
• Electrons excitation
  The electrons excitation is a process of transferring an electron to the more excited energy state. Thermal excitation provides energy to the electron to transfer it to a higher energy band. The energy to electrons is generally provided by the lattice vibration in a crystal structure of the semiconductor
• Variable electrical conductivity
  Semiconductors are not good conductors in their natural state. The large gap does not allow all the electrons to move from the valence band to the conduction band. But, different techniques are used to increase the conductivity of semiconductors, such as doping. The outcome of the doping can be a p-type or n-type doped semiconductor. The p-type has a large number of holes, while the n-type has a large number of electrons responsible for the conduction.

Advantages and disadvantages of a semiconductor

Let's discuss the advantages and disadvantages of the semiconductor.

Advantages of Semiconductor

Let's discuss the advantages of semiconductors that made them popular for use in various applications. These are listed below:

• Low Cost
• Long life
• Shock-proof
• Low power consumption
• High reliability
• Good power efficiency
• Compact size
• No warm-up time required
• Semiconductor devices cannot produce current in the absence of any applied voltage

Disadvantages of Semiconductor

The disadvantages of semiconductor are listed below:

• Cannot withstand high power
• High noise level as compared to vacuum tubes
• Difficult operation in high-frequency applications
• Low output power.

Applications of Semiconductor

The use of semiconductors in solids allows the devices to operate at low voltage and less power. Hence, most of the electronic devices are created using semiconductor materials.

Let's discuss the various applications of semiconductors in detail.

Diode

It is the simplest form of electronic device that is based on semiconductors. The diode is a type of device that consists of joined P and N junctions. It allows the current to move in one direction as compared to the other direction easily. It means it blocks one direction so that the current can easily flow through the other. Silicon is the common semiconductor material used as the diode material.

Integrated Circuits (ICs)

The Integrated Circuits or ICs are used in almost every electronic device, such as radio receivers, calculators, sensors, timers, etc. IC comprises a thin silicon chip with electronic items, such as transistors, diodes, etc.

LEDs

LEDs are also created using the semiconductor material. It emits light when current passes through its circuitry. The applications of LEDs include ring lights, flashlights, decorative lights, household bulbs, etc. The recombination of holes (positive charged) and electrons (negative charged) in a semiconductor is responsible for generating lights in LED (Light Emitting Diodes).

Transistors

Transistors are created using semiconductor materials. It is used to switch the current and amplify the signals.

The transistors are used in different devices, such as Integrated circuits, amplifiers, oscillators, switches, etc.

Sensors

The sensors are based on semiconductor materials that detect the movement of particles present in the environment. These are always incorporated with other types of electronic devices. The application of sensors includes optical devices, navigation systems, etc. There are different types of sensors used in different applications, such as motion sensors, temperature sensors, pressure sensors, etc.

Differences

Here, we will discuss the differences between semiconductors, conductors, and insulators.

Semiconductor vs. Conductors

Let's discuss the differences between semiconductors and conductors. It is listed in the below table:

Temperature coefficient

The temperature coefficient can be negative, positive, or zero.

Negative temperature coefficient

It depicts that the temperature rise can result in a decreased value of the resistance.

Positive temperature coefficient

It depicts that the temperature rise can result in the increased value of the resistance. It is due to the smooth flow of electrons present in the conducting materials.

Zero temperature coefficients

The alloy material can have zero temperature coefficients. It means by alloying specific metals; a zero temperature coefficient can be obtained.

Semiconductor vs. Insulator

Let's discuss the differences between semiconductors and insulators. It is listed in the below table: