Semiconductor Material

Introduction To Semiconductor Material and Electronics Engineering:

Electronic devices are widely used in industries for quality and automation.

Electronics is a field of science and engineering, which deals with low power semiconductor devices or electronic devices. With the help of electronics we do Automation, Control, Monitoring, Amplification, Oscillation, Rectification, Clipping, Energy conversion of light to electric, etc

The applications of electronics are

Industrial applications, Communication Engineering, Satellite Communication, Microwave devices and communication and RADAR, Medical Field, Defense field, Control and Instrumentation

Semiconductors:

Semiconductor is defined as a material which exhibits all properties between the properties of insulator and that of the conductors. These materials are normally conductor above room temperature (27 C) and as we decrease temperature the conductivity decreases and material then tends towards insulator.

Semiconductor is basis of electronics field. These are used to design

Several analog and digital Ics,

Computer parts like hard disk, memory, etc

Light to electrical energy and electrical energy to light converters

And Many other applications in different fields like medical, defense, automation fields.

Properties of semiconductor:

1.  It exhibits negative temperature coefficient of resistance.

From this definition it is very clear that if dT is positive, then dR is negative, or resistance decreases on increasing temperature and vice versa. 

So on increasing temperature, resistance decreases and hence conductivity increases and material behaves like a conductor.

and On decreasing temperature, resistance increases and hence conductivity decreases and material behaves like an insulator.

2. Its conductivity lies between conductivity of insulators and conductivity of conductors.

3. At room temperature, it is a semiconductor with reasonable conductivity.

4. Below room temperature, conductivity decreases and it tends to transform into an insulator. At - 273 C (0 K), it is a perfect insulator.

5. Above room temperature, conductivity increases on increasing temperature and it tends to transform into a good conductor.

6. Forbidden energy band gap lies between the gaps in insulator and conductors.

Semiconductors Material:

Normally forth group elements in periodic table are semiconductors.

For Example Si,                                Ge, etc

                    (Silicon – 14 – 2,8,4)  (Germanium – 32 – 2,8,18,4)

Electronic configuration shows that they have four electrons in outer most orbits. We know that atoms in a molecule becomes stable with 8 electronics in outermost orbit. So 4 electrons in outermost orbit of Si or Ge, which is half the maximum possibility in outermost orbit, becomes an important factor which make semiconductors very useful.

Semiconductors Materials other than Forth Group Elements:

Semiconductor material can also be designed using combination of third and fifth group elements.

For example:

GaAs, Ga AlP, GaAlAsP, etc

These semiconductor material more useful because here we get some more forms of semiconductor material like:

Direct type semiconductors

Indirect type semiconductors

Degenerate semiconductors.

                   (We will discuss these topics later.)

Concept of Electron hole pair in Semiconductors:

At room temperature some electrons are ready to conduct in an electronic circuit. As they will get energy, they will move. So in an unbiased semiconductor material electrons are bounded but they are free to move so they are called free electrons.

When an electron leaves its place, a negative charge leaves the place, so a compensatory positive charge at this place becomes present which is due to absence of a charged particle. This charged particle is called ‘Hole’.

Now it is also easy to understand that a hole is always ready to be generated when a free electron is ready to be generated. So electron hole are generated in pair.

Conductivity in Si and Ge elements:

As we know that the forth group element  Si and Ge are having 4 electrons in their outer most orbit. Therefore they are tightly packed structures. Hence their electron affinity is very large means they require a large amount of energy to make an electron free or to break covalent chemical bonding. It emphasize that conduction in Si and Ge is very low. It is obvious that at room temperature, very few electrons and holes are free. So conductivity of pure semiconductor is very low.

Remember: Pure Semiconductor is known as Intrinsic semiconductor.

Impurity Atoms and Doping Process:

Now we have understood that conductivity of pure semiconductor is very low. To increase conductivity of semiconductor, we require to increasing the number of free charge carriers.

To increase free charge carriers, we require adding some extra atoms in the pure semiconductor material. These atoms are known as Impurity atoms.

The process of adding impurity atoms in the semiconductor material, is known as Doping Process.

There are two types of Doping processes

• Acceptor or trivalent impurity doping

• Donor or pentavalent impurity doping

Acceptor impurity atom: III A group elements of periodic table have only 3 valence electrons (Trivalent atoms) and therefore when these atoms are added to the Si or Ge atoms, an electron vacant place is created which behaves as a free hole. Examples are Al, Ga, etc (Element from III A group).

Donor impurity atom: V A group elements of periodic table have 5 valence electrons (pentavalent atoms) and therefore when these atoms are added to the Si or Ge atoms, an extra electron is attached to the element which behaves as a free electron. Examples are Al, Ga, etc (Element from V A group) .

Types of Semiconductor Materials:

There are two types of Semiconductor materials

• Intrinsic semiconductors: Pure semiconductor materials are known as intrinsic semiconductor material. 

In these semiconductor materials        (n = p)

                                         (No of electrons = No of Holes)

• Extrinsic semiconductors: After adding impurity in pure semiconductors using doping process, we obtain an impure semiconductor material, known as Extrinsic Semiconductor Material. 

In these semiconductor materials        (n ≠ p)

                                         (No of electrons ≠ No of Holes)

So we have two cases (Two Types of Extrinsic Material):

(i)No of electrons > No of Holes           (n > p)

 (Negative charges > positive charges)   (n type material)

(ii)No of electrons < No of Holes          (n < p)

(Negative charges < positive charges)    (p type material)

Majority and Minority Charge Carrier:

We know that there are two types of charge carriers in a semiconductor material.

The charge carrier which is larger in concentration is known as majority charge carrier.

The charge carrier which is smaller in concentration is known as minority charge carrier.

Since number of electrons is equal to number of holes in intrinsic semiconductors therefore there is no any majority or minority concept.

In Extrinsic semiconductor, number of electron electrons and number of holes are unequal. So we can say one carrier is majority and another is minority.

Semiconductor Type             (n,p)                   Majority           Minority

N Type                                     n > p                   electrons          holes

P Type                                     n < p                    holes               electrons

Some Questions For Exercise

1. Generally valence electrons in a semiconductor are:

(a) 3          (b) 4              (c) 5             (d) 6

2. The temperature coefficient of resistance in semiconductor is 

(a) zero      (b) positive     (c)  negative    (d) None of these

3. When a pentavalent impurity is added to a pure semiconductor, it becomes

(a) extrinsic semiconductor       (b) intrinsic semiconductor

(c) an insulator                          (d) None of these

4. Ga is added to the pure Ge as an impurity. It becomes

(a) N type       (b) P type        (c) Intrinsic semiconductor       (d) none of These

5. An N type material is formed, select the impurity used:

(a) B,              (b) In,             (c) Se             (d) Sb

Answer:  1 - b,       2 - c,        3 - a,      4 - b,      5 - d

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