To draw the characteristic curve of a Zener diode and to determine its reverse breakdown voltage

Aim
To draw the characteristic curve of a Zener diode and to determine its reverse breakdown voltage.

Apparatus
A Zener diode (with small reverse breakdown voltage of about 6 volts), [i.e., Vz = 6 V], a ten volt battery, a high resistance rheostat, two 0-10 V voltmeter, one 0-100 mA ammeter, one 20 Ω. resistance, one way key, connecting wires.

Theory
Zener Diode. It is a semiconductor diode, in which the n-type and the p-type sections are heavily doped, i.e., they have more percentage of impurity atoms. This heavy doping results in a low value of reverse breakdown voltage (BVR). This value can be controlled during manufacture.
The reverse breakdown voltage of a Zener diode, is called Zener voltage (Vz). The reverse current that results after the breakdown, is called Zener current (Iz).
to-draw-the-characteristic-curve-of-a-zener-diode-and-to-determine-its-reverse-breakdown-voltage-1
At breakdown, increase of VI increases II by large amount, so that V0 = VI– RI II becomes constant. This constant value of V0 which is the reverse breakdown voltage, is called Zener voltage.
Formula used
to-draw-the-characteristic-curve-of-a-zener-diode-and-to-determine-its-reverse-breakdown-voltage-2

Diagram
to-draw-the-characteristic-curve-of-a-zener-diode-and-to-determine-its-reverse-breakdown-voltage-3

Procedure

  1. Arrange apparatus as shown in circuit diagram.
  2. Make all connections neat, clean and tight.
  3. Note least count and zero error of voltmeters and milli-ammeter. (micro-ammeter)
  4. Bring moving contact of potential divider (rheostat) near negative end and insert the key K. Voltmeters and milli-ammeter will give zero reading.
  5. Move the contact a little towards positive end to apply some reverse bias voltage (VI). Milli-ammeter reading remains zero. Voltmeters give equal readings.
    [i.e.,V0 =VI ... II = 0 (eqn. 2)]
  6. As VI is further increased, II starts flowing. Then V0 becomes less than VINote the values of Vp II and V0.
  7. Go on increasing VI in small steps of 0.5 V. Note corresponding values of II and V0 which will be found to have increased.
  8. As VI is made more and more, II and V0 are found to increase. Values are noted.
  9. At one stage, as VI is increased further, II increases by large amount and V0 does not increase. This is reverse breakdown situation.
  10. As VI is increased further, only II is found to increase, V0 becomes constant. Note values of VI, II and V0.
  11. Increase V1 to a value of 10 V, noting corresponding values.
  12. Record your observations as given ahead.

Observations
to-draw-the-characteristic-curve-of-a-zener-diode-and-to-determine-its-reverse-breakdown-voltage-4
to-draw-the-characteristic-curve-of-a-zener-diode-and-to-determine-its-reverse-breakdown-voltage-5

Calculations
Plot a graph between input voltage VI (column 2) and input current (column 3), taking VI along X-axis and II along Y-axis.
to-draw-the-characteristic-curve-of-a-zener-diode-and-to-determine-its-reverse-breakdown-voltage-6

Result
The reverse’breakdown voltage of given Zener diode is

Precautions

  1. All connection should be neat, clean and tight.
  2. Key should be used in circuit and opened when the circuit is not being used.

Viva Voce

Question. 1. What is reverse current?
Answer. The current due to reverse bias voltage, is called reverse current.

Question. 2. What constitutes the reverse current?
Answer. The reverse current is due to minority carriers.

Question.3. What are minority carriers?
Answer. Free electrons in p-type semiconductor and holes in re-type semiconductor, are called minority carriers. They are present due to breaking of covalent bonds.

Question.4. What is reverse breakdown?
Answer. At certain stage of increased reverse bias voltage, the reverse current increases suddenly. This situation is called reverse breakdown. This phenomenon is called Zener effect.

Question.5. What causes reverse breakdown?
Answer. The rupture of all covalent bonds causes reverse breakdown.

Question. 6. Is the reverse breakdown recoverable?
Answer. Yes. The decrease of reverse bias voltage restores the condition. The broken bonds are reassembled.

Question.7. What is reverse breakdown voltage?
Answer. The reverse bias voltage which causes breakdown, is called reverse breakdown voltage. It is represented by the symbol BVR.

Question. 8. On which factor does the reverse breakdown voltage depend?
Answer. It depends upon the level of doping of re-type and p-type section of the diode.
General purpose diodes have each section lightly doped. They have high value of reverse breakdown voltage.
Zener diodes have each section heavily doped. They have low value of reverse break¬down voltage.

Question.9. What is Zener voltage?
Answer. The reverse breakdown voltage of Zener diodes, is called Zener voltage. It is represented by the symbol Vz.

Question. 10. How does Zener voltage differ for Germanium and Silicon?
Answer. For same order of doping, it is less for Germanium and more for silicon.

Question.11.What is Zener current?
Answer.The reverse current after breakdown, is called Zener current. It is represented by the symbol Iz.

Question.12.How does a Zener diode work as a voltage regulator?
Answer. At breakdown and after, output voltage (V0) becomes constant at value of Zener voltage (Vz) even when input voltage (VI) increases.
Thus, the Zener diode will give same output voltage for all input voltage of higher values. It becomes a voltage stabilizer for voltage equal in value of Zener voltage (Vz). The current drawn does not affect the zener voltage.

Question.13.What is the Knee voltage?
Answer.The forward voltage beyond which the current starts to increase rapidly with voltage is called the cut-in or Knee voltage of the diode.

Question.14.What happens to the potential barrier and depletion layer when a reverse bias is applied to a p-n junction diode?
Answer. Both increases.

Question.15.What is fermi level?
Answer.It is the highest energy level in the conduction band occupied by the electrons at the absolute zero of temperature.

Question.16.What is Zener breakdown?
Answer.Due to small junction width, the junction field is high. Due to this internal high field, there is large production of electron-hole pairs. The corresponding breakdown is called breakdown.

Question.17.What is Zener diode?
Answer. It is specially designed p-n diode whose both sides are heavely doped and work only in the reverse breakdown region.

Question.18.Give one application of a diode.
Answer.Rectifier.

Question.19.What is ideal diode?
Answer.It is a diode which offers zero resistance in forward biasing and infinite resistance in reverse biasing.

Question.20.How the energy gap changes with (i) doping (ii) temperature?
Answer. Decrease with increase in doping and temperature.

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