Choppers-An Overview

A Chopper is an electronic circuit which controls or reduces a dc supply.  Their function can be compared to an ac transformer.  In an AC transformer, voltage is controlled by changing the turns ratio of the transformer.  In a chopper, voltage is varied by connecting and disconnecting the load from the source many times in a second. 

The chopper is essentially a switching circuit which switches off and on many times.  The output of a chopper is a square wave form while the input is a unidirectional dc waveform. 

Choppers can be used in motor speed controls.  They are increasingly being used in electric automobile technology. 

Choppers are used widely in Electronics in circuits in solar power conversion, speed control of motors in the industry.  They are used to reduce DC voltage to different levels in machines and other electronic equipments

Choppers have high efficiency and can be designed to have very fine control.

Band Theory of Conduction

Electrical conduction in materials occurs due to the free electrons which drift about the atomic lattice.  In an atom, the electrons in the outer most orbit are called the valence electrons.  If the electrons have sufficient energy , they can break free of the atom and flow through the lattice when a voltage is applied.

If the energy levels are graphically represented, we will get a band diagram.

In the Band Diagram, there is the box representing the Conduction band and the box representing the valence band. 

Valence Band

The Valence band is the range of energy levels of the electrons in the outermost orbit of the atom. 

Conduction Band

The conduction band is the range of energy levels all electrons which are involved in conduction. 

In conductors, the valence and the conduction bands overlap.  In  Insulators, the valence and conduction bands are far apart. 

In semiconductors, the distance between the valance and the conduction bands are small.  When external energy in the form of heat or light is applied to the semiconductors, the electrons get excited and jump from the valence to the conduction band. 

The difference between the valence and the conduction band is called the energy gap.

Germanium in Electronics-An Overview

Germanium is used in specific applications such as communication, spectroscopy, etc.  They have largely been replaced with silicon.  Germanium diodes are more expensive compared to silicon.

Germanium diodes have a lower forward bias voltage compared to silicon 0.15 volts.  This enables the use of the Germanium diode at low voltages where silicon cannot be used.

Germanium is also used in photoelectronics application.  Germanium diodes have a smaller band gap  0.66 eV.  This means that the electrons can be excited even by near-infrared radiation. 

Germanium is used in solar cells to capture the energy in near-infrared regions of the light spectrum.  Germanium based sensors are used spectroscopy to detect light radiation at low frequencies.

Silicon Diodes-An Overview

Silicon diodes are diodes in which the P and N materials are made of silicon.  Silicon Diode have a a forward bias voltage of 0.7 volts.  That is, the diode conducts when the voltage across the it in the forward bias is 0.7 volts or greater. 

They are the most widely used diodes in the industry.  Other diodes such as Germanium diodes are used at voltages below 0.7 volts.

The diode can withstand a voltage of 50V or more in the reverse direction.  This is known as the peak inverse Voltage.

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.