SEMICONDUCTOR

SEMICONDUCTORS:

            Semiconductor  is a substance which has resitivity in between conductors and insulators.The resitivity of a semiconductor is less than an insulator but more than a conductor.The resistance of a semiconductor is inversely proportional to the temperature . When a suitable impurity is added to a semiconductor , its current conductivity properties change appreciably.

TYPES OF SEMICONDUCTOR:

Semiconductors can be classified into two types :

1.INTRINSIC SEMICONDUCTOR OF PURE OF SEMICONDUCTORS

2.EXTRINSIC SEMICONDUCTOR OR IMPURE OF SEMICONDUCTORS

INTRINSIC SEMICONDUCTOR:

                                       The normal (pure) silicon and Germanium are intrinsic semiconductor .  They  possess all essential conducting characteristics of a semiconductor .The number of electrons present in the outermost orbit of intrinsic semiconductor is four, hence ,they are termed as tetra valent.So, intrinsic semiconductors are tetra valent in nature.

EXTRINSIC SEMICONDUCTOR:

                 The process of  adding impurities to an intrinsic semiconductors is known as Doping.With respect to the type of  impurity added,extrinsic semiconductors are classified into two types.

                                    1.N-TYPE Semiconductors

                                    2.P-TYPE Semiconductors  

N-TYPE SEMICONDUCTORS:

                    When a small amount of pentavalent impurity  (eg: Antimony,Arsenic) is added to a pure semiconductor, it is known as N-type semiconductor.The addition of  pentavalent impurity gives a large number of  free electrons in the semiconductors crystal.The four electrons of intrinsic semiconductor bonds with the four electrons of pentavalent impurity and an extra fifth electron is unbounded and free.The Majority carriers in N-Type are electrons (Negative charges) and Minority carriers are holes (positive charges).N-Type semiconductors are known as Donor impurities because they donate free electrons to the semiconductor crystal.

P-TYPE SEMICONDUCTORS:

                    When a small amount of  Trivalent impurity  (eg: Indium,Gallium) is added to a pure semiconductor, it is known as P-type semiconductor.The addition of  trivalent impurity gives a large number of  holes in the semiconductors crystal.The four electrons of intrinsic semiconductor bonds with the three electrons of trivalent impurity and one hole is created.The Majority carriers in P-Type are  holes (positive charges)and Minority carriers are electrons (negative charges).P-Type semiconductors are known as Acceptor impurities because the holes created can accept the electrons.

P-Njunction:

  A p-type material consists of silicon atoms and trivalent impurity atoms such as boron.

 The boron atom adds a hole when it bonds with the silicon atoms. However, since the number of protons and the number of electrons are equal throughout the material, there is no net charge in the material and so it is neutral. 

  An n-type silicon material consists of silicon atoms and pentavalent atoms such as antimony.

  An impurity atom releases an electron when it bonds with four silicon atoms. Since there is still an equal number of protons and electrons (including the free electrons) throughout the material, there is no net charge in the material and so it is neutral. 

 If a piece of intrinsic silicon is doped so that part is n-type and the other part is p-type, a pn junction forms at the boundary between the two regions and a diode is created.

 The p region has many holes (majority carriers) from the impurity atoms and only a few thermally generated free electrons (minority carriers).

 The n region has many free electrons (majority carriers) from the impurity atoms and only a few thermally generated holes (minority carriers).

The free electrons in the n region are randomly drifting in all

directions.

 At the instant of the pn junction formation,the free electrons near the junction in the n region begin to diffuse across the junction into the p region where they combine with holes near  the junction.

When the pn junction is formed, the n region loses free electrons as they diffuse across the junction.This creates a layer of positive charges (pentavalent ions) near the junction.

 As the electrons move across the junction, the p region loses holes as the electrons and holes combine. This creates a layer of negative charges (trivalent ions) near the junction.

These two layers of positive and negative charges form the depletion region

PN JUNCTION DIODE:

The p region is called the anode and is connected to a conductive  terminal. The n region is called the cathode and is connected to a second conductive terminal.

FORWARD BIASED CONDITION:

An external voltage is applied to cause the diode to conduct current in one direction and block it in the other direction. This process is called biasing.

Forward bias is the condition that allows current through the pn junction.

The external bias voltage is designated as VBIAS. The resistor limits the forward current to a value that will not damage the diode.

The negative side of VBIAS is connected to the n region of the diode and the positive side is connected to the p region.

Another requirement is that the bias voltage, VBIAS, must be greater than the barrier potential.

The electrons are present in the valence band in the p region. The Positive side of the bias-voltage is applied to the anode or p region.

 Since unlike charges attract, the positive side of the bias-voltage source attracts the valence electrons toward the left end of the p region.

The holes in the p region provide the medium or “pathway” for these valence electrons to move through the p region.

 The valence electrons move from one hole to the next toward the left.

 The holes, which are the majority carriers in the p region, effectively (not actually) move to the right toward the junction.

 This effective flow of holes is the hole current.

 

As the electrons flow out of the p region through the external connection (conductor) and to the positive side of the bias-voltage source, they leave holes behind in the p region.

At the same time, these electrons become conduction electrons in the metal conductor.

Electrons diffuse into the depletion region from n region due to the repulsive force of the negative side of battery connected to n region.

As more electrons flow into the depletion region, the number of positive

ions is reduced.

As more holes effectively flow into the depletion region on the other side of the pn junction, the number of negative ions is reduced. This reduction in positive and negative ions during forward bias causes the depletion region to narrow, and current conduction occurs.

REVERSE BIASED CONDITION:

Reverse bias is the condition that essentially prevents current through the

diode.

 

Figure below shows a dc voltage source connected across a diode in the direction to produce reverse bias. This external bias voltage is designated as VBIAS.

 

The positive side of VBIAS is connected to the n region of the diode and the negative side is connected to the p region.

The depletion region is shown much wider than in forward bias or equilibrium.

 The positive side of the bias-voltage source “pulls” the free electrons, which are the majority carriers in the n region, away from the pn junction.

 As the electrons flow toward the positive side of the voltage source, additional positive ions are created. This results in a widening of the depletion region and a depletion of majority carriers.

 In the p region, electrons from the negative side of the voltage source enter as valence electrons and move from hole to hole toward the depletion region where they create additional negative ions.

Reverse bias voltage results in widening of the depletion region and a depletion of majority carriers

and current conduction is blocked.