Diodes

Question 1

P type semiconductor

Answer 1

P-type semiconductor has a large number of holes.

Question 2

N-type semiconductor

Answer 2

N-type semiconductor has a large number of free electrons.

Question 3

What happens when p-type and n-type are joined?

Answer 3

When P-type and N-type materials are joined together, a gradient of charge carriers densities is created at the junction. This will cause the electrons to move from N-type material to the P-type material and holes to move from N-type material to the P-type material.

Question 4

Define diffusion

Answer 4

The process of movement of charge carriers from the region of higher concentration to a lower concentration in the absence of an external electric field is called diffusion.

Question 5

Depletion Region

Answer 5

At the junction, N-type material will have positively charged immobile ions and P-type material have negatively charged immobile ions. Thus the regions on either side of p–n interface lose their charge neutrality and become charged. For this reason, it is called space charge region. As the region is devoid or depleted of mobile charge carriers it is also called the depletion region

Question 6

Define barrier potential

Answer 6

The space charge on either side of the junction causes a potential difference across the P-N junction and it is called the barrier potential.

Question 7

Barrier potential of Ge

Answer 7

0.3 V

Question 8

Barrier potential of Si

Answer 8

0.7 V

Question 9

What happens in zero bias?

Answer 9

In the absence of any bias voltage, the net flow of charge carriers in any one direction for a semiconductor diode is zero. This occurs because minority carriers (holes) in the N-type material will encounter a barrier in the depletion region to cross the junction and move to the P-type region. Same is the case for electrons in P-type material. This results in the depletion region with high impedance, and hence no current flows through the diode. The built-in potential varies from 0.3 to 0.7 eV depending upon the type of semiconductor material.

Question 10

What happens in forward bias?

Answer 10

When a negative voltage is applied to the N-type material and a positive voltage is applied to the P-type material, the diode is said to be in a Forward Bias condition. If the external voltage applied is greater than the value of the barrier potential, the carriers start crossing eth junction and hence there will be a forward current. The diode is said to be in the ON condition.

Question 11

What happens in reverse bias?

Answer 11

When a positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material, the diode is said to be in a reverse-biased condition. The positive voltage applied to the N-type semiconductor attracts electrons towards the positive electrode and hence away from the junction. At the same time, the holes in the P-type semiconductor are attracted towards the negative electrode. This results in widening of the depletion layer due to a lack of electrons and holes near the junction and presents a high impedance path for the majority carriers. The height of the potential barrier is increased, which prevents the flow of forward current through the diode. However, the applied potential favours the movement of minority carriers across the junction causing the flow of current in the reverse direction. This current is called reverse saturation current and is represented by I0 or IS.

Question 12

IV characteristic of DIode

Answer 12

I-V characteristics of the practical diode. When the forward-biased voltage is applied to the diode, the current is initially zero and then increases sharply after crossing the cut-in voltage. In this case, the diode behaves like a closed switch. Similarly, in reversed biased condition, the diode behaves like an open switch and very small current flows due to minority charge carriers, which is known as reverse saturation current. In the reverse biased condition, beyond a particular reverse voltage, a sudden rise of current will be observed and this voltage is called the breakdown voltage

Question 13

Define knee point

Answer 13

The point at which this sudden increase in current takes place is called “knee” point.

Question 14

Effect of temperature on reverse current

Answer 14

The reverse saturation current is a temperature dependent parameter. It doubles for every 10o C rise in temperature. Let I01 be the reverse saturation current at temperature T1 and I02 be the reverse saturation current at temperature T2, where T2 > T1.

Question 15

What is the breakdown voltage?

Answer 15

Breakdown voltage is the largest reverse voltage that can be applied without causing an exponential increase in the current in the diode.

Question 16

What happens in Zener breakdown?

Answer 16

n Zener breakdown , the electric field established due to the reverse voltage capapble of getting the electrons out of their covalent bonds and away from their parent atoms as shown in Figure 1.1.13. Electrons are transferred from the valence to the conduction band. In this situation, the current can still be limited by the limited number of free electrons produced by the applied voltage so it is possible to cause Zener breakdown without damaging the semiconductor.

Question 17

What happens in Avalanche breakdown ?

Answer 17

Avalanche breakdown occurs when the applied voltage is so large that electrons achieve kinetic energy sufficiently high and collide with the silicon atoms and knock off more electrons. These electrons are then also accelerated and subsequently collide with other atoms. Each collision produces more electrons which leads to more collisions etc as shown in Figure. 1.1.14. The current in the semiconductor rapidly increases and the material can quickly be destroyed.