To plot the forward and reverse bias characteristics of given diodes, determine the cut in voltage of each diode

Semiconductor Diode Characteristic

Aim: To plot the forward and reverse bias characteristics of given diodes, determine the cut in voltage of each diode, the breakdown voltage of the zener diode and to determine forward and reverse resistances of given diodes

Apparatus: DC supply, ammeter, voltmeter, diodes, resistances etc.

Theory: Diode is the device which allows the flow of current only in one direction. A PN junction diode is formed by fusing P and N type semiconductors. The P type material has holes as majority charge carriers while N type has electrons. Near the junction the holes and the electrons combine to form a depletion layer. Due to depletion layer negative ions are produced on P side and positive on N side. This results in the formation of potential barrier which prevents further diffusion of charge carriers.

Biasing is applying external voltage. When P side of the junction diode is connected to positive of the battery and N to the negative, the diode is said to be forward biased. The majority charge carriers are repelled by their respective battery connection and gain sufficient energy to jump over the potential barrier to conduct a large amount of current. On the other hand when P side of the junction diode is connected the negative of battery and N side to the positive of the battery the condition is known reverse bias. The majority charge carriers are attracted by the battery and the depletion layer grows in size. A small current is seen due to the minority charge carriers which soon saturates off. Now if the external voltage is increased   further, at a point known as the breakdown, the current suddenly increases (even in reverse bias mode). This is because of either the zener breakdown (due to the external electric field and a reversible effect) or the avalanche breakdown (thermal effect and irreversible).  Some of the diodes may glow in the forwards bias condition, if they have been designed such that when the electrons and holes recombine they emit in visible region.

Formulae:

V      Δ V
Rdc= ----- ;     Rac= ------
 I      Δ I
                                       
Procedure:

1. Connect the given diodes as shown in the figure for the
    forward and reverse bias conditions.
2. Measure the current in the diode by using suitable ammeter
    for a successive number of the higher applied voltages,  
    without exceeding the meter limits..
3. Sketch the curves of current versus voltage with origin at
    center of the graph.
4. Determine cut-in voltage for each diode and the breakdown
    voltage for the zener diode.
    5.  Evaluate   Rac and Rdc resistances for the diodes.

Observation Tables:

I. Normal Diode (forward bias)

S.No	V (volts)	I (mA)

II. Normal Diode (reverse bias):

S.No	V (volts)	I (mA)

III. Light Emitting  Diode (forward bias)

S.No	V (volts)	I (mA)

IV. Light Emitting  Diode (reverse bias):

S.No	V (volts)	I (mA)

III. Zener Diode (forward bias)

S.No	V (volts)	I (mA)

IV. Zener Diode (reverse biased)

S.No	V (volts)	I (mA)

Diagram:

Results: 

Normal diode :
                  Zener diode
1.  Cut in voltage = ------ 1. Cut in voltage = ------
2.  Rac    = -------         2. Breakdown voltage = ------                                                                    3. R ac = ------
4. R dc   = ------

Precautions and sources of error:

Questions

Q.1   What is a diode?
Q. 2  What is depletion layer and potential barrie ?
Q.3   Explain Zener and avalanche breakdown ?
Q.4   What are the practical applications of this study?