p-n diode

AIM: a.   To plot forward and reverse characteristics & to determine the cut in voltage of
                 a semiconductor p-n diode.
           b.   To compare the characteristic curves of germanium and silicon diodes.

pn-diode

Semiconductors are the materials having very low conductivity in pure form. At ordinary temperatures say at room temperature; few electrons are available in C.B. along with same number of holes in V.B. (a hole or vacancy is created in V.B. when an electron goes from V.B. to C.B. and so this hole is treated as + charge carrier). The conductivity in pure semiconductors is due to both electron and holes. However the number of free charge carriers is too small to have sufficient current. The conductivity of these materials can be increased by adding impurity in controlled manner. The process of adding impurity to pure or intrinsic semiconductors is called doping and the resulting semiconductor is called doped or extrinsic semiconductor. There can be two types of semiconductors viz; n type and p-type. The n type semiconductor is obtained by doping pure Si or Ge with Vth group elements like P, Sb etc. and it has number of electrons more than number of holes. While p type semiconductor is obtained by doping pure Si or Ge with IIIrd group elements like B, Al etc. and it has number of holes greater than number of electrons. The junction of n type and p type material has revolutionized the concept of a circuit. Due to this; it was possible to design a computer circuit which occupies very small area. These semiconductor devices are very reliable, long lasting and moreover they consume very less power.

APPARATUS: Experimental kit, patch chord

FORMULAE: The forward static resistance is given by

                       R_f = Vf/If

               The forward dynamic resistance is given by

                       r_f = ∆Vf/∆If

           The reverse static resistance is given by

                        R_r = Vr/Ir

INTRODUCTION

Semiconductors are the materials having very low conductivity in pure form. At ordinary temperatures say at room temperature; few electrons are available in C.B. along with same number of holes in V.B. (a hole or vacancy is created in V.B. when an electron goes from V.B. to C.B. and so this hole is treated as + charge carrier). The conductivity in pure semiconductors is due to both electron and holes. However the number of free charge carriers is too small to have sufficient current. The conductivity of these materials can be increased by adding impurity in controlled manner. The process of adding impurity to pure or intrinsic semiconductors is called doping and the resulting semiconductor is called doped or extrinsic semiconductor. There can be two types of semiconductors viz; n type and p-type. The n type semiconductor is obtained by doping pure Si or Ge with V^th group elements like P, Sb etc. and it has number of electrons more than number of holes. While p type semiconductor is obtained by doping pure Si or Ge with III^rd group elements like B, Al etc. and it has number of holes greater than number of electrons. The junction of n type and p type material has revolutionized the concept of a circuit. Due to this; it was possible to design a computer circuit which occupies very small area. These semiconductor devices are very reliable, long lasting and moreover they consume very less power.

THEORY

A p–n junction is formed when single pure crystal is doped with p-type and n- type materials as shown in fig. In p-type, holes are majority carriers and in n-type electrons are majority charge carriers. When junction is just formed; the majority charge carriers diffuses from their respective regions to other region against concentration gradient i.e. electrons diffuse from n-region to p-region and holes diffuse from p-region to n-region and current starts flowing across the junction. When hole diffuses to n -region it leaves –vely charged acceptor ion (dopant or impurity atom) in p region and

when electron diffuses to p region; it leaves +vely charged donor atom in n region. Thus a barrier voltage is developed at the junction due to which further diffusion of charge carriers is stopped. Moreover near the junction; while diffusing; the electrons and holes recombine and a region is formed where no mobile charge carriers are present.This region is called depletion region. 

 The junction can be supplied by an external voltage so as to decrease or increase the width of the depletion region. This is called biasing of a diode.

FORWARD BIASED JUNCTION

If the width of the depletion region reduces after applying the external voltage; the biasing is called forward biasing. The source is connected in such a way that it opposes the barrier potential as shown in fig. The current starts flowing through the junction when the applied voltage exceeds the barrier voltage. The current increases rapidly and attains a saturation value. Thus the forward voltage enhances the diffusion of the majority charge carriers and so the current in forward biased junction is termed as Diffusion current. This current initially increases; when applied voltage is increased because the rate of diffusion increases.

Beyond some value of applied voltage; the current attains saturation value.

This is because; at this voltage; all the charge carriers are

removed from the space charge region  and the rate of diffusion

attains constant  value. Refer the fig.

V-I Characteristics of diode

REVERSE BIASED JUNCTION

 If the width of the depletion region increases then the diode is said to be reverse biased. When this voltage is applied; the majority charge   carriers move away from the junction and width of depletion region increases. However; few minority charge carriers get drifted towards the junction causing small current to flow across the junction. This current is termed as Drift current. This current is very small since the number charge carriers is very small. The number of minority charge carriers depends on the temperature and is independent of applied voltage. Refer fig.

RESISTANCE OF A SEMICONDUCTOR DIODE

In p-n diode the resistances offered in different biasing condition is different. In the forward bias condition,  the resistance is very less of the order of few ohms. The resistance is very high (megaohm) in reverse bias condition.

Static Forward Resistance Rf :

It is the opposition offered by the diode to the dc current through it. At a given operating point it is defined as the ratio of the dc voltage to the dc current.

Dynamic Forward Resistance rf :

It is the opposition offered by the diode to the varying alternating (ac) current. It is defined as the ratio of change in voltage across the diode to the resulting change in current through it. The ac forward resistance is more important as the diodes are generally used with ac voltages.

The Reverse Static Resistance Rr :

It is the ratio of reverse voltage to reverse current at the operating point. The reverse resistance of silicon diode is far larger than that of a germanium diode.

PROCEDURE

1. TO PLOT FORWARD CHARACTERISTICS OF P-N DIODE:

   1. Make the connections as shown in fig for respective diodes (Si, Ge).

   2. Vary the voltage carefully till mA shows deflection. Record voltage.     

   3. Find the voltage required for the current of 1mA.

   4. Now vary the current in steps of 1mA and note down the corresponding value of voltage.

   5. The cut-in voltage, Rf and rf is determined from the forward characteristic curve.

 2. TO PLOT THE REVERSE CHARACTERISTICS:

   1. Make the connections as shown in the fig.

   2. For p-n diode vary the voltage in steps of 0.5V and note down the

        corresponding values of current. Take about fifteen readings. `

  3.   Rr is determined from the reverse characteristic curve.

RESULT:

1. The cut in voltage of the diode is found to be  ----------     

2. The forward dynamic found to be ----------  

3. The forward static resistance found to be ----------  

Answer the following

Q1. In a ckt. you need a diode. You have BJT. Can you it instead?

Q2. Explain using forward characteristics why resistance of diode decreases when bias voltage is increased. 

Q3. Does the diode satisfy Ohm’s law? Explain.

Q4. In a Zener which property of diode material determines the breakdown voltage?
Q5. What do you mean by terms; diffusion and drift currents?
Q6. Why mobility of hole is less than the mobility of an electron?
Q7. Are free electrons normally present in p-type material?
Q8. What is reverse saturation current in a diode? Why is it temp.dependent?
Q9. What is cut in voltages for silicon and germanium? Why cut in voltages for semiconductor

        and zener diodes are different?
Q10. Name five different types of diodes. What is LED?
Q11. Does the diffusion current continue to flow across the junction in no bias condition?
Q12. What are mobile and immobile charge carriers in a p-n junction?
Q13. Why in germanium diode reverse current is more than silicon diode?
Q14. What is rectifier?
Q15. Do the holes physically exist? Why are they termed as positive charge carriers?