Silicon as a Semiconductor-An Overview

Silicon is the most popular and widely used of the semiconductors.    There are many factors which have made silicon the material of choice in the world of electronics.

Some of the advantages are

1. Silicon is abundant.  Hence, it is also economical.  The extraction process form its ore is cheaper when compared to other materials. 
2. It is strong and easy to handle.
3. It forms a nice stable oxide.
4. Doping is easy.  Both P type materials and N type materials can be formed.
5. It can be easily cut into wafers.
6. It has good mechanical strength. Hence, designing circuits in silicon is easy.
7. Silicon has fewer free electrons in room temperature.  This means that the collector cut-off current in transistor is lower than in other semiconductor materials, such as Germanium.

Depletion Region in the PN junction

When a P type material and a N type material are brought in contact with each other, some of the holes in the P material migrate to the N region and combine with electrons.  Similarly, some of the electrons of  N material migrate to the P region and combine with holes. 

Thus, at the point of contact of the P and N materials, a layer is formed which has no majority charge carriers such as holes or electrons.  This region is called the depletion region as the region has been depleted of its charge carriers. 

The depletion region behaves almost like an insulator.  When a voltage exceeding the barrier potential is applied across the PN junction, current starts to flow.

Barrier Potential in a PN Junction

Barrier Potential in a PN junction refers to the potential required to overcome the barrier at the PN junction.

When a P material and N material are brought in contact in a junction, some of the electrons of the N material near the junction cross over to the P material.  These electrons combine with the holes in the P material.  Similarly, the some of the holes of the P material near the Junction cross over to the N material and combine with the electrons.

The region in the contact area is thus depleted of holes and electrons.  This region is called the Depletion Layer.    The majority charge carriers are absent in this region.  This region almost becomes like an insulator.  Thus, there is no conduction after the depletion layer is formed.

For current to flow through this layer, a specific voltage has to be exceeded.  This is known as the barrier potential.  When an external voltage greater than the barrier potential is applied, the PN junction conducts.  

P type Material and N type Material

P type Materials

The P type material is obtained when a semiconductor is doped with a trivalent impurity such as Aluminium or Boron. P type material is a material which has holes as its majority carriers.  Electrons are the minority Charge Carriers in P type materials.  When a trivalent impurity is added to the crystal lattice of a semiconductor, there is a vacancy for every impurity atom added.  This vacancy is called a hole.

N type Materials

N type materials are made when a semiconductor is doped with a pentavalent impurity.  A pentavalent impurity is one whose atom has five electrons in its outermost orbit (valence electrons).   Examples of pentavalent impurities are Phosphorous, Antimony, Bismuth.  In an N type Material, electrons are the majority charge carriers while holes are the minority charge carriers.

When a semiconductor is doped with a pentavalent impurity for every impurity atom added, there is a free electron.  These electrons are responsible for conduction.

Diffusion Current in the PN junction

When the PN junction is formed, there is movement of the charge carriers across the junction.  The electrons move from the N material across the junction into the P material.  The holes from the P material cross into the N junction.

This movement of charge carriers results in a current across the junction. 

This current is known as diffusion current.  This current occurs in the absence of potential.

Cable Sleeves-An overview

Cable Sleeves are used to enclose a single wire or a group of wires.  Cable sleeves are used to arrange a set of wires going through a panel or to an equipment like a motor.  Cable sleeves are made of different materials from braided metals to rubber and even kevlar. 

Sleeves serve to protect the wires and their insulation from sharp edges.  They can protect wires from UV radiation, moisture, oil and temperature. 

In automobiles, special heat resistant sleeves are used to protect the wires from hot surfaces. 

Teflon sleeves have great cut resistance.  Nylon sleeves have great abrasion resistance. 

Sleeves are available in two broad categories depending on the method of installation. 

The first is the slit sleeve which is cut and the wires are inserted into the sleeve.  The second is the wrap-around, side entry type of sleeve. 

Heat Shrinkable Sleeves

Some sleeves are heat shrinkable.  Heat shrinkable sleeves are made of a polymer which contracts when heat is applied. A stream of hot air from a blower is passed on the sleeve.  This results in the sleeves shrinking and wrapping the wires.  

Electrical Safety Mats

Electrical Safety Mats are important safety equipment.  Safety mats protect personnel from electric shock by providing an insulated surface to work on.  If the worker comes in contact with a live conductor by mistake, he will not get an electric shock as his feet are insulated from the ground by the safety mat.

Safety mats come in different colours.  They are usually made of rubber.  The surface of the mats is ribbed to provide an anti-slip surface to workers. 

Safety mats are also designed to resist aging and ozone. 

Safety mats should also be age and fire resistant.

Safety mats come in different voltage ratings.   They should also be resistant to acids and oils. 

The following is the list of mats and their working voltages

 

Class of Mats	Working Voltage	Colour
0	1000	Red
1	7500	White
2	17000	Yellow
3	26500	Green
4	36000	Orange

 

The most common class of safety mats is the type 0 which is rated for a working voltage of 1000 V.