Forward & Reverse Bias Characteristics of PN Junction Diode

Objective: 

To study and verify the functionality of PN junction diode in forward bias and reverse bias and to

1. Plot Volt-Ampere Characteristics of P-N Diode.
2. Plot Volt-Ampere Characteristics of P-N Diode in XY mode.
3. Find cut-in voltage for P-N Junction diode.
4. Find static and dynamic resistances in both forward and reverse biased conditions.

Components:

S.No.	Name	Quantity
1	Diode (1N4007)	1(One) No.
2	Resistor (1K)	1(One) No.
3	Bread board	1(One) No.

Equipment:

S.No.	Name	Quantity
1	Dual DC Regulated Power supply (0 - 30 V)	1(One) No.
2	Digital Ammeters  ( 0 - 200 mA, 0 - 200 µA)	1(One) No. Each
3	Digital Voltmeter (0 - 20V)	1(One) No.
4	Bread board	1(One) No.
5	Connecting wires (Single Strand)

Specifications:

List of Parameters	SILION DIODE (1N4007)
Maximum Forward Current	1A
Maximum Reverse Current	5.0µA
Maximum Forward Voltage	0.8V
Maximum Reverse Voltage	1000V
Maximum Power Dissipation	30mW
Temperature	-65 to 200° C

Operation:

A PN junction diode is formed  when a single crystal of semiconductor is doped with acceptors impurities (Pentavalent) on one side and donor impurities (Trivalent) on the other side. It has two terminals called electrodes, one each from P-region and N-region. Due to two electrodes it is called (i.e., Di-electrode) Diode.

Biasing of PN junction Diode

Applying external D.C. voltage to any electronic device is called biasing. There is no current in the unbiased PN junction at equilibrium.

Depending upon the polarity of the D.C. voltage externally applied to diode ,the biasing is classified as Forward biasing and Reverse biasing.

Forward bias operation

The P-N junction supports uni-directional current flow. If +ve terminal of the input supply is connected to anode (P-side) and –ve terminal of the input supply is connected the cathode. Then diode is said to be forward biased. In this condition the height of the potential barrier at the junction is lowered by an amount equal to given forward biasing voltage. Both the holes from p-side and electrons from n-side cross the junction simultaneously and constitute a forward current from n-side cross the junction simultaneously and constitute a forward current (injected minority current – due to holes crossing the junction and entering P- side of the diode). Assuming current flowing through the diode to be very large, the diode can be approximated as short- circuited switch.

Reverse bias operation

If negative terminal of the input supply is connected to anode (p-side) and –ve terminal of the input supply is connected to cathode (n-side) then the diode is said to be reverse biased. In this condition an amount equal to reverse biasing voltage increases the height of the potential barrier at the junction. Both the holes on P-side and electrons on N-side tend to move away from the junction there by increasing the depleted region. However the process cannot continue indefinitely, thus a small current called reverse saturation current continues to flow in the diode. This current is negligible; the diode can be approximated as an open circuited switch.

Diode current equation

The volt-ampere characteristics of a diode explained by the following equations:

I = current flowing in the diode, I₀ = reverse saturation current

V = Voltage applied to the diode                                                          

V_T = volt- equivalent of temperature = k T/q = T/ 11,600 = 26mV (@ room temp)

 =1 (for Ge) and 2 (for Si)

It is observed that Ge diodes has smaller cut-in-voltage when compared to Si diode. The reverse saturation current in Ge diode is larger in magnitude when compared to silicon diode.

Circuit Diagram:

Fig. (1) - Forward Bias Condition:

Fig. (2) - Reverse Bias Condition:

Procedure:

Forward Bias Condition:

1. Connect the circuit as shown in figure (1) using PN Junction diode.
2. Initially vary Regulated Power Supply (RPS) voltage V_s in steps of 0.1 V. Once the current starts increasing vary V_s from 1V to 12V in steps of 1V and note down the corresponding readings V_f and I_f.
3. Tabulate different forward currents obtained for different forward voltages.

Reverse Bias Condition:

1. Connect the circuit as shown in figure (2) using PN Junction diode.
2. Vary V_s in the Regulated Power Supply (RPS) gradually in steps of 1V from 0V to 12V and note down the corresponding readings V_r and I_r.
3. Tabulate different reverse currents obtained for different reverse voltages.
4. To get the graph in reverse region (theoretically), remove voltmeter and with reference to the supply voltage note down the reverse current readings in Ammeter because current always selects low reactance path.(Diode have infinite resistance in reverse bias ideally).To get the graph in reverse region (theoretically), replace voltmeter with nano ammeter. Voltmeter has less load resistance when compared to diode. Current conducts in low resistance path.

Diode Characteristics in XY mode:

1. Adjust CRO TIME/DIV knob in X-Y mode.
2. Give the input as triangular voltage waveform from Function Generator.
3. Connect the CRO CH1 across the input and CH2 across resistor.
4. P-N Junction diode characteristics  can be observed.

Observations:

Forward Bias Condition:

RPS Voltage Vs (volts)	Forward Voltage across the diode Vf (volts)	Forward Current through the diode If (mA)

Reverse Bias Condition:

RPS Voltage Vs (volts)	Reverse Voltage across the diode Vr (volts)	Reverse Current through the diode Ir (µA)

Graph:

1. Take a graph sheet and divide it into 4 equal parts. Mark origin at the center of the graph sheet.
2. Now mark +ve X-axis as V_f, -ve X-axis as V_r, +ve Y-axis as I_f and –ve Y-axis as I_r.
3. Mark the readings tabulated for Si forward biased condition in first Quadrant and Si reverse biased condition in third Quadrant.

Fig: V- I Characteristics of PN Junction Diode under Forward & Reverse Bias Conditions

Calculations from Graph:

Cutin Voltage V

Static forward Resistance 

Dynamic Forward Resistance 

Static Reverse Resistance 

Dynamic Reverse Resistance 

Precautions:

1. While doing the experiment do not exceed the readings of the diode. This may lead to damaging of the diode.
2. Connect voltmeter and ammeter in correct polarities as shown in the circuit diagram.
3. Do not switch ON the power supply unless you have checked the circuit connections as per the circuit diagram.

Result: Volt-Ampere Characteristics of P-N Diode are studied.

a) Forward Bias of PN Junction Diode:

1. The Cut in Voltage or Knee Voltage (Vγ) of 1N4007 is _____________Volts.
2. The Dynamic Forward resistance of 1N4007 is  __________________  .
3. The Static Forward resistance of 1N4007 is __________________  .

b) Reverse Bias of PN Junction Diode:

1. The Dynamic Reverse resistance of 1N4007 is __________________  .
2. The Static Reverse resistance of 1N4007 is __________________  .

Outcomes:   Students are able to

1. analyze the characteristics of PN diode
2. calculate the dynamic and static resistance in forward bias and reverse bias.

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Viva Questions:

1.  What are trivalent and penatavalent impurities?

Ans: Doping is the process of adding impurity atoms to intrinsic silicon or germanium to improve the conductivity of the semiconductor. 

Commonly used doping elements

• Trivalent Impurities to make p-Type: Aluminum (Al), Gallium (Ga), Boron(B) and Indium (In).
• Pentavalent Impurities to make n-type: Phosphorus (P), Arsenic (As), Antimony (Sb) and Bismuth (Bi).

2.  How PN junction diode does acts as a switch?

Ans:  Apply voltage in one direction; it acts like an open circuit. Reverse the polarity of the voltage and it acts like a short circuit.

3.  What is diode current equation?

Ans:  

4.  What is the value of V_t at room temperature?

Ans:  25mV

5.  Dynamic resistance expression?

Ans: 

6. What is a semiconductor?

Ans: Semiconductor is a device which conducts partially.

7. What is meant by intrinsic semiconductor?

Ans:Intrinsic semiconductor is pure semiconducor without any significant dopents present.The conductivity of intrinsic semiconductor is Zero at room temperature.

8. What is the order of energy gap in a pure semiconductor?

Ans: The energy gap in pure semiconductor is 1eV.

9. What is an extrinsic semiconductor?

Ans: The intrinsic semiconductors are added with other materials to improve its conductivity which are called extrinsic semiconductors.

10. What is a doped semiconductor?

Ans:Extrinsic semiconductor is called doped semiconductor.

11. What is doping?

Ans: The process of adding other material to the crystal of intrinsic semiconductors to improve its conductivity is called doping.

12. What are two different types of impurities?

Ans: 1) Pentavalent atoms  2) Trivalent atoms

13. To which group does a (i) p-type, (ii) n type impurity belong?

Ans: (i) p-type impurities are-Gallium, Indium & Boron- Group -3 elements

         (ii) n-type impurities are- Arsenic,Bismath & Phosphorous- Group-5 elements

14. What are the charge carriers in a pure semiconductor?

Ans: Electrons and holes are charge carriers in pure semiconductor.

15. What are the charge carriers in n-type semiconductor?

Ans:Electrons are majority charge carriers in n-type semiconductor.

16. What is the effect of temperature on conductivity of a semiconductor?

Ans:The conductivity of an intrinsic semiconductor increases with increase in temperature.The conductivity of an extrinsic semiconductors decreases with the increase in temperature, the number of majority carriers is nearly constant, but mobility decreases.

17. What is junction diode?

Ans:The p-n junction forms a popular semiconductor device called p-n junction diode with two terminals one from p-type material and other from n-type material.

18. What is meant by forward bias?

Ans:Forward bias means connecting p-region to +ve terminal and n-region to -ve terminal of the battery, where diode in ON condition.

19. What is meant by reverse bias?

Ans:Reverse bias means connecting p-region to -ve terminal and n-region to +ve terminal of the battery, where diode in OFF condition.

20. What is knee voltage?

Ans:The minimum voltage required to turn on a device is called Knee voltage or cut in voltage.Which is represented by V .

21. What is reverse breakdown?

When a PN junction is reversed biased it allows very small current to flow through it. ... If reverse bias is made too high, the current through PN junction increases abruptly and the voltage at which this phenomenon occurs is called breakdown voltage

22. What are the semiconductor materials in use?

Ans:Silicon, Germanium, Gallium arsenide.

23. Why is Silicon used popularly compared to Germanium?

Ans: Silion is less temprature sensitive compared to Germanium and easily available from sand.

24. How many valence electrons are there in each atom of a semiconductor?

Ans: 4 - valence electrons are there in each atom of a semiconductor. 

25. What are the p type doping materials and n type doping materials?

Ans:(i) p-type impurities are-Gallium, Indium & Boron

        (ii) n-type impurities are- Arsenic,Bismath & Phosphorous

26. How many valence electrons are there in P type doping materials and in n type doping materials?

Ans: 3-valence electrons in p-type material and 5-valence electrons in n-type material.

27. Draw the ideal, practical and piecewise linear characteristics of a PN junction diode.

Ans: 

28. What is the static resistance of a diode? 

Ans: It is the ratio of voltage to current at a point in V-I characteristics of the diode.

29. What is the dynamic resistance of a diode?

Ans:It is the ratio of change in voltage to change in current from V-I characteristics of the diode.

30. How PN junction diode does act as a switch?

Ans:When PN junction diode in forward bias it conducts and allow the current through it like ON switch, when PN junction diode in reverse bias it does not conducts like OFF switch.

31. What is cut-in voltage? What are its values for Si and Ge diodes?

Ans:The minimum voltage required to turn on a device is called cut in voltage .for Si diode (1N4007)=0.5V and For Ge diode=0.2V

32. Write the Diode current Equation.

Ans:

I = current flowing in the diode, I₀ = reverse saturation current

V = Voltage applied to the diode                                                          

V_T = volt- equivalent of temperature = k T/q = T/ 11,600 = 26mV (@ room temp)

 =1 (for Ge) and 2 (for Si)