Semiconductors

 SEMICONDUCTORS

DEFINITION:

Semiconductors are materials whose physical properties lie in between conductors and insulators i.e., their conductivity is lesser than conductors but greater than insulators.

EXAMPLES AND LOCATION OF SEMICONDUCTORS IN THE PERIODIC TABLE:

Semiconductors are located in the IV-A  and VI-A groups of the periodic table. Carbon C, Silicon Si, Germanium Ge, Stannum St, Plumbum Pb, and Fluorine Fl are the elements of the IV-A group whereas, Tellurium Te and Selenium Se are included in the VI-A group. These are semiconducting elements, however, there is a wide variety of semiconductors compound to this date. 

EXAMPLES OF SEMICONDUCTOR COMPOUNDS:

Some most common semiconductor compounds are:

GaAs (Gallium arsenic), GaN (Gallium nitrate), SiC (Silicon carbide), InP (Indium phosphide), AlGaInP (Aluminium gallium indium phosphide), etc.

MOST COMMON SEMICONDUCTORS:

The most commonly used semiconductors are Si silicon and Ge germanium. 

ENERGY BAND THEORY OF SEMICONDUCTORS:

According to Energy Band Theory at room temperature semiconductors have:

1. Partially filled conduction band

2. Partially filled valence band

3. A narrow forbidden energy gap (of the order of 1eV) between the Conduction and Valence bands.

Limited excitation energy is required to move atoms from the valence band to the conduction band. This energy is usually available at room temperature. Due to this energy, electrons in the valence band jump to the conduction band and so behaves as conductors at room temperature.

Now let us take a quick review of Energy band theory and the other related terms. 

Energy band theory:

Bohr's conventional free electron theory failed to explain the electrical behavior of different materials. According to Bohr, theoretically, certain materials should be conductors but they were insulators practically. Similarly, according to him, certain materials should be insulators but in reality, they were conductors. This puzzle was answered by the Energy band theory. It discusses how conductors, insulators, and semiconductors are classified based on the energy of electrons within the material.

States:

According to Bohr, electrons of an isolated atom are bound to the nucleus and can only have distinct energy levels. However, according to Energy band theory, when a large number of atoms, say N is brought closer to form a solid. Each energy level of the isolated atom is divided into N sub-levels called States under the action of the forces exerted by other atoms in solids. 

Energy band:

These permissible energy states are discrete but so closely spaced that they appear to form a continuous Energy band. 

Forbidden Energy states and gaps:

Between two consecutive permissible energy bands, there is a range of energy states that cannot be occupied by electrons. These are called forbidden energy states and their range is called the Forbidden energy gap. 

Valence band:

The highest occupied band is called the Valence band i.e., the outermost band having the last number of electrons. Electrons of this band are called valence electrons. This band can be either completely filled or partially, but can never be empty.

Conduction band:

The energy band above the valence band is called the conduction band. The electrons of this band are called conductive or free electrons. This band can either be empty or partially filled, but can never be completely filled.

EFFECT OF TEMPERATURE ON SEMICONDUCTORS:

At zero kelvin, there are no electrons in the conduction band, and their valence bands are completely filled. It means at zero kelvin a semiconductor, say, 'Si' or 'Ge' is a perfect insulator. However, with the increase in temperature, the thermally excited electrons jump from the valence band into the conduction band after crossing the narrow forbidden gap by leaving vacancies in the valence band. The conduction increases gradually and the material becomes a semiconductor at room temperature.

TYPES OF SEMICONDUCTORS:

Based on conductivity, there are two categories of semiconductors:

1. Intrinsic Semiconductors

2. Extrinsic semiconductors

INTRINSIC SEMICONDUCTORS:

They are the purest form of semiconductors. Let us take a sample of Silicon as an intrinsic semiconductor. Since it belongs to the IV-A group, it has 4 electrons in its outermost shell. Every silicon atom in the sample forms 4 covalent bonds with 4 other silicon atoms. In this way, all the sample atoms are stable, i.e., each atom has a complete octet and no free electron. Therefore, intrinsic semiconductors are actually insulators.

EXTRINSIC SEMICONDUCTORS:

To understand Extrinsic semiconductors, let us first see what is Doping?

DOPING:

Doping is a process in which a small number of atoms of material is added to semiconductors in a ratio of 1: 10^6. Doping is essential to increase the conductivity of a material. 

Doped semiconductors are called extrinsic semiconductors. They contain free electrons and can conduct electricity. We will see how it can conduct electricity and where the free electrons come from later. There are two types of extrinsic semiconductors:

TYPES OF EXTRINSIC SEMICONDUCTORS:

N-TYPE SEMICONDUCTORS:

When atoms of the V-A group are doped with intrinsic semiconductors they form N-type semiconductors. Since the impurity is added from the fifth group, therefore, called a pentavalent impurity because it contains 5 electrons in its valence shell.

Example:

Let us take a sample of intrinsic semiconductor silicon and add phosphorus of the V-A group as an impurity. An atom of phosphorus has 5 electrons in its outermost shell and silicon has 4 electrons in its outermost shell. 4 valence electrons of phosphorus will form covalent bonds with 4 neighboring electrons. The fifth electron will not form a bond with other silicon atoms because the octet of phosphorus is completed and will leave the valence shell of phosphorus. In this way addition of phosphorus generates a free electron in the intrinsic semiconductor. The more phosphorus is doped, the more free electrons will be. And in this situation, when we connect a battery to this lattice, free electrons of the lattice will flow toward the positive terminal of the battery resulting in the formation of a current.  

Check the video below for a better understanding:

Facts about N-type semiconductors:

1. It is formed by the doping of pentavalent impurity.

2. In N-type material majority of charge carriers are free electrons.

3. In this case production of current is made possible by the electron donated by phosphorus therefore it is called a donor impurity. 

4. It is called an N-type semiconductor. Where N means negative since the electron is negatively charged. Remember N-type semiconductor does not mean that it has a negative charge.

5. An N-type semiconductor is neutral because it has equal numbers of electrons and protons. Before doping the sample has an equal number of electrons and protons. After doping we get free electrons. One may think that it should be negatively charged. But the point to be noted is that adding a neutral phosphorus atom in a neutral sample will leave the net sample neutral. The addition of pentavalent impurity will free an electron but it is not generating an extra electron. 

P-TYPE SEMICONDUCTORS:

When atoms of the III-A group are doped with intrinsic semiconductors they form P-type semiconductors. Since the impurity is added from the third group, therefore, called a trivalent impurity because it contains 3 electrons in its valence shell.

Example:

Let us take a sample of intrinsic semiconductor silicon and add Aluminum of the V-A group as an impurity. An atom of phosphorus has 3 electrons in its outermost shell and silicon has 4 electrons in its outermost shell. 3 valence electrons of phosphorus will form covalent bonds with 3 neighboring electrons. 1 silicon electron is left without any bond, indicating an electron deficiency. This deficiency of electron is equivalent to a positive charge. It is also known as vacancy of electron or hole. As the presence of an electron creates a negative charge, therefore, the deficiency of an electron is treated as a positive charge. REMEMBER, IT IS NOT AT ALL A POSITIVE CHARGE BUT IT IS EQUIVALENT TO A POSITIVE CHARGE BECAUSE IT CAN BEHAVE AS A POSITIVE CHARGE. This deficiency of an electron creates a hole in the sample. This hole will cause the neighboring electron to move and fill this vacancy. And this process will keep on going, initiating the movement of electrons. In this way the more aluminum is doped in the sample the more holes will be created and result in the flow of electrons. And in this situation when we connect a battery to the lattice, the flow of electrons results in the generation of current.  

Check the animated video for clear understanding👇:

Facts about P-type semiconductors:

1. It is formed by the doping of trivalent impurity.

2. In N-type material majority of charge carriers are holes. It is not because holes are moving. It is incorrect because holes are immobile. Since they can provoke electrons to move therefore called majority charge carriers.

3. In this case production of current is made possible by the electron accepted by an aluminum therefore it is called an acceptor impurity. 

4. It is called a P-type semiconductor. Where P means positive since the hole is equivalent to a positive charge which causes current flow. Remember P-type semiconductor does not mean that it has a positive charge.

5. A P-type semiconductor is also neutral just like N-type semiconductors.

SEMICONDUCTORS AS PN-JUNCTION DIODE:

If we join  P and N-type material, the boundary so formed is called a PN-junction.

Soon after the formation of this junction electrons in the N-region that are mobile particles start diffusing towards P-region rapidly and start filling up the holes. The atoms in N-region that donate electrons will form positive ions and those in P-region that accept electrons will form negative ions. 

After some time a +ve charged layer at the N-type and a -ve charged layer at the P-type region will form. This is known as a potential barrier. Barrier means blockage. PN-junction is known as a potential barrier because initially, electrons form ions briskly but soon after the formation of a negative layer at the P-region, it starts repelling the incoming electrons from N-region. This means that further transfer of electrons is not possible. The same is the case for holes at P-region. If they wish to move towards N-region, they cannot, because the positive layer at N-region will repel them. But we do not consider it because holes are immobile. The potential barrier is also known as the depletion region. The word Depletion means deficiency of something. This region is deficient in moving electrons since electrons cannot cross this region or barrier while moving therefore, the region is said to be depleted from free charge carriers and hence called a depletion region. It is also known as Diode. Di means double and ode means electrode therefore diode means double electrodes. Double electrodes are referred to as P and N-region where the P-region is called +ve and the N-region is called a -ve electrode.

This PN junction diode is further used for rectification and many other purposes.

Value of potential barrier or depletion region:

The value of the potential barrier depends upon the semiconductor material used in the formation of P-type and N-type materials. If the material used is Si (silicon), the value of the potential barrier will be 0.7 volts and in the case of germanium, the barrier value reduces to 0.3 volts. 

Let us see the graphical representation of the potential barrier formed by both Silicon and Germanium.

graphical representation of potential barrier formed by Silicon

The horizontal line of the graph is representing the zero (0) value of the potential barrier. When P and N-type materials are joined together the potential start increasing and attains a maximum of 0.7 volts. This means that the barrier so formed is stopping the electrons by performing 0.7 J work on it. This indicates that if electrons of the N-region get energy greater than 0.7 joules then they will cross the barrier. 

graphical representation of potential barrier formed by Germanium

Similarly, in the case of germanium, the maximum value of potential attained is 0.3 volts and likewise, electrons of the N-region will cross the barrier if they get energy greater than 0.3 joules.

SEMICONDUCTORS AND ELECTRONIC CURRENT:

In conductors, there is a bulk quantity of free electrons therefore whenever a battery or power source is connected to a conductor a huge amount of current flows through the conductor. It is not easy to control this current at a micro level. This current is known as an Electric current. However, in the case of semiconductors, there is one free electron in thousands of atoms. This will not create a bulk amount of current which can be controlled at a micro level. And this current is known as Electronic current which is produced by semiconductors. Hence we can say that the current formed in metals is called electric current. It has its own advantages and uses. But if we consider digital devices such as our computers and smartphones, we need a current that can be controlled with complete accuracy and precision and here electronic current is used. This is the reason we use semiconductors when forming computer chips and other digital and electronic devices.

USES OF SEMICONDUCTORS:

There are millions and billions of uses of semiconductor materials in the present-day world. A few of them are listed below:

1. Semiconductors are used in making electronic and digital devices.

2. They can be used for the formation of a diode. The diode can be used as a rectifier.

3. It can be used as a transistor.

4. Photodiodes that can convert light energy into electrical energy can also be formed using semiconductors.

WORLD WITHOUT SEMICONDUCTORS:

The necessity and importance of semiconductors in the present-day world cannot be ignored. It is present almost everywhere around us. Semiconductors are used in our mobiles, computers, smart watches, toothbrushes, toys, alarms, and much more. They are present in modern automobiles as well. Microwave ovens, heaters, and refrigerators also operate using semiconductors. The use of semiconductors is increasing from time to time. Hence, we can say that transistors are quite tiny but incredibly useful.