Semiconductor Electronics

Valance Band

1. In solids, the valence band is the highest range of electron energies in which electrons are normally present at absolute zero temperature.

Conduction Band

1. The conduction band is the band of orbitals that are high in energy and are generally empty. In reference to conductivity in semiconductors, it is the band that accepts the electrons from the valence band.
2. 

Energy Gap

1. In graphs of the electronic band structure of solids, the band gap generally refers to theenergy difference (in electron volts) between the top of the valence band and the bottom of the conduction band in insulators and semiconductors.
2. 

3. Classification Of Solids On The Basis Of Energy Bands

4. Conductors
   the substances which allow the passage of electrical current through them are known as conductors. The valence band and conduction band over lap on each other and they have large number of free electrons. Conductivity decreases with rise in temperature. The temperature coefficient of resistance is positive. Examples for conductors are all metals, human body etc.
   
   Insulators
   The materials in which forbidden energy gap is very large are insulators. In this there is no free charge carriers. With the rise in temperature, electrical conductivity increases slightly. These substances do not allow electric current through tem. The temperature coefficient of resistance is negative. Examples for insulators are rubber, diamond etc. 
   
   Semi conductors
   The materials whose conductivity is in between that of conductors and insulators is known as semi conductors. The valence band and conduction band are separated by very small forbidden gap and when temperature was increased, electrons are transferred from valence bond to conduction band. Conductivity increases with increase in temperature. The temperature coefficient of resistance is negative. Examples for semi conductors are silicon, germanium etc. 
   
   Intrinsic semi conductors
   Semi conductors in the purest form are called intrinsic semi conductors. In this the number of electrons in the conduction band is equal to the number of holes in the valence band. The Fermi energy level lies in the middle of the forbidden gap. At OK intrinsic semiconductors behaves as an insulator. 
   
   Extrinsic semi conductors
   The semi conductors with the presence of impurities are called extrinsic semi conductors. The process of adding impurities to pure semi conductors for increasing of conductivity is in two types. 
   
   P-type semi conductors
   When a trivalent impurity is added to the pure semi conductor it is called p-type semi conductor. In this, the number of holes in the valence band is greater than electrons in conduction band. The Fermi energy level lies nearer to the valence band. The impurity atom requires one electron, hence, it is acceptor impurity. 
   
   N-type Semi conductor
   When a penta valent impurity is added to the pure semi conductor, it is called n-type semi conductor. In this number of electrons in the conduction band is greater than number of holes in the valence band. The majority charge carriers are electrons. The Fermi energy level lies nearer to the conduction band. The penta valent impurity atom gives an electron, hence it is donor impurity.

PN Junction

In its basic form a semiconductor diode is formed from a piece of silicon by making one end P type and the other end N type. This means that both ends have different characteristics. One end has an excess of electrons whilst the other has an excess of holes. Where the two areas meet the electrons fill the holes and there are no free holes or electrons. This means that there are no available charge carries in this region. In view of the fact that this area is depleted of charge carriers it is known as the depletion region.

The semiconductor diode PN junction with no bias applied

Even though the depletion region is very thin, often only few thousandths of a millimetre, current cannot flow in the normal way. Different effects are noticed dependent upon the way in which the voltage is applied to the junction. If the voltage is applied such that the P type area becomes positive and the N type becomes negative, holes are attracted towards the negative voltage and are assisted to jump across the depletion layer. Similarly electrons move towards the positive voltage and jump the depletion layer. Even though the holes and electrons are moving in opposite directions, they carry opposite charges and as a result they represent a current flow in the same direction.

The semiconductor diode PN junction with forward bias

If the voltage is applied to the semiconductor diode in the opposite sense no current flows. The reason for this is that the holes are attracted towards the negative potential that is applied to the P type region. Similarly the electrons are attracted towards the positive potential which is applied to the N type region. In other words the holes and electrons are attracted away from the junction itself and the depletion region increases in width. Accordingly no current flows.

The semiconductor diode PN junction with reverse bias

PN Junction Diode

The effect of adding this additional energy source results in the free electrons being able to cross the depletion region from one side to the other. The behaviour of the PN junction with regards to the potential barrier’s width produces an asymmetrical conducting two terminal device, better known as the PN Junction Diode.

PN Junction Formation

1. P-n junctions are formed by joining n-type and p-type semiconductormaterials, as shown below. Since the n-type region has a high electron concentration and the p-type a high hole concentration, electrons diffuse from the n-type side to the p-type side.

Semiconductor Diode

When a p-type semiconductor material is suitably joined to n-type 

semiconductor the contact surface is called a p-n junction. The p-n junction is 

also called as semiconductor diode.

                                               Note                                             

• A diode is an electrical component acting as a one-way valve for current.
• When voltage is applied across a diode in such a way that the diode allows current, the diode is said to be forward-biased.
• When voltage is applied across a diode in such a way that the diode prohibits current, the diode is said to be reverse-biased.
• The voltage dropped across a conducting, forward-biased diode is called the forward voltage. Forward voltage for a diode varies only slightly for changes in forward current and temperature, and is fixed by the chemical composition of the P-N junction.
• Silicon diodes have a forward voltage of approximately 0.7 volts.
• Germanium diodes have a forward voltage of approximately 0.3 volts.
• The maximum reverse-bias voltage that a diode can withstand without “breaking down” is called the Peak Inverse Voltage, or PIV rating.

                                                                                                                                                                   

 Forward biasing

• When external voltage applied to the junction is in such a direction that it cancels the potential barrier, thus permitting current flow is called forward biasing.

• To apply forward bias, connect +ve terminal of the battery to p-type and –ve terminal to n-type as shown in fig.2.1 below.


• The applied forward potential establishes the electric field which acts against the field due to potential barrier. Therefore the resultant field is weakened and the barier height is reduced at the junction as shown in fig. 2.1.

• Since the potential barrier voltage is very small, a small forward voltage is sufficient to completely eliminate the barrier. Once the potential barrier is eliminated by the forward voltage, junction resistance becomes almost zero and a low resistance path is established for the entire circuit. Therefore current flows in the circuit. This is called forward current.

Reverse biasing

• When the external voltage applied to the junction is in such a direction the potential barrier is increased it is called reverse biasing.

• To apply reverse bias, connect –ve terminal of the battery to p-type and +ve terminal to n-type as shown in figure below.

• The applied reverse voltage establishes an electric field which acts in the same direction as the field due to potential barrier. Therefore the resultant field at the junction is strengthened and the barrier height is increased as shown in fig.

• The increased potential barrier prevents the flow of charge carriers across the junction. Thus a high resistance path is established for the entire circuit and hence current does not flow.

I-V Characteristics

We can now plot the current density-versus-voltage (J-V) curve for a PN junction, taking into account the special features of reverse and direct bias that we have discussed before (fig. EC8). This curve shows the existence of an “offset” (or “threshold” ) voltage Voff in the case where the ohmic resistance of the semiconductors is weak.

Cut in Voltage

1. There is a definite forward voltage at which the diode starts to conduct significantly. This is called the knee voltage or cut-in voltage and is equal to the barrier potential of the p-n junction.

Breakdown Voltage

The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive. Thebreakdown voltage of a diode is the minimum reverse voltage to make the diode conduct in reverse

Reverse Saturation Current

When the diode is reverse biased, there is a very little flow of current due to minority carriers. This current is of the order of nano-ampere to micro-ampere. This current does not change significantly with bias voltage. The current is called reverse saturation current and it remains almost constant with the change in reverse bias voltage