To Draw the I-V Characteristic Curve for a P-N Junction Diode in Forward and Reverse Bias
1. Aim
To draw the current-voltage (I-V) characteristic curve for a p-n junction diode in both forward and reverse bias conditions, and to analyze the behavior of the diode under different voltage conditions.
2. Apparatus Used
- Silicon or Germanium p-n junction diode (1N4007 or suitable equivalent)
- DC power supply (0-30V)
- Digital multimeter or ammeter (0-100mA range)
- Digital voltmeter (0-30V range)
- Rheostat/Potentiometer (1kΩ, 2W)
- Resistor (1kΩ, 0.5W) for current limiting
- SPDT switch
- Breadboard and connecting wires
- Graph paper for plotting results
3. Diagram
Fig 1: Circuit diagram for measuring I-V characteristics of a p-n junction diode
4. Theory
A p-n junction diode is a semiconductor device formed by joining p-type (excess holes) and n-type (excess electrons) semiconductor materials. The junction between these two regions creates a depletion region that acts as a barrier to the movement of charge carriers.
Forward Bias: When a positive voltage is applied to the p-side and negative to the n-side, the depletion region narrows. When the applied voltage overcomes the barrier potential (0.7V for silicon, 0.3V for germanium), a large current begins to flow through the diode.
Reverse Bias: When a negative voltage is applied to the p-side and positive to the n-side, the depletion region widens. Only a very small leakage current flows through the diode until the breakdown voltage is reached.
The I-V characteristic curve of a p-n junction diode is non-linear. The current through the diode varies exponentially with the applied voltage as given by the Shockley diode equation:
$$I = I_s \left( e^{\frac{qV}{nkT}} - 1 \right)$$
Where:
- $I$ is the diode current
- $I_s$ is the reverse saturation current
- $V$ is the voltage across the diode
- $q$ is the elementary charge ($1.602 \times 10^{-19}$ coulombs)
- $k$ is the Boltzmann constant ($1.381 \times 10^{-23}$ J/K)
- $T$ is the absolute temperature in Kelvin
- $n$ is the ideality factor (typically between 1 and 2)
5. Formula
Shockley Diode Equation:
$$I = I_s \left( e^{\frac{qV}{nkT}} - 1 \right)$$For Forward Bias (V >> kT/q):
$$I \approx I_s e^{\frac{qV}{nkT}}$$Taking logarithm:
$$\ln(I) = \ln(I_s) + \frac{qV}{nkT}$$For Silicon Diode at room temperature (T = 300K), the equation simplifies to:
$$I \approx I_s e^{\frac{V}{n \times 0.026}}$$For Reverse Bias (V << -kT/q):
$$I \approx -I_s$$6. Procedure
A. Forward Bias Characteristics
- Set up the circuit as shown in the diagram with the switch in the forward bias position (p-side connected to the positive terminal of the power supply).
- Ensure the power supply is set to zero voltage initially.
- Gradually increase the voltage using the rheostat in small steps (0.1V increments up to 1V, then 0.2V increments thereafter).
- For each voltage setting, record the corresponding current reading from the ammeter.
- Continue until you have a sufficient number of readings (at least 10-15 data points).
- Turn off the power supply when done with forward bias measurements.
B. Reverse Bias Characteristics
- Switch the SPDT switch to the reverse bias position (p-side connected to the negative terminal of the power supply).
- Starting from zero, gradually increase the reverse voltage in steps of 1V.
- Record the corresponding reverse current for each voltage setting.
- Be careful not to exceed the reverse breakdown voltage of the diode.
- Take at least 10 readings in the reverse bias region.
- Turn off the power supply and disconnect the circuit when done.
7. Observation Table
Table 1: Forward Bias Readings
| S.No. | Forward Voltage (V) in Volts | Forward Current (I) in mA |
|---|---|---|
| 1 | 0.0 | 0.0 |
| 2 | 0.1 | 0.01 |
| 3 | 0.2 | 0.02 |
| 4 | 0.3 | 0.04 |
| 5 | 0.4 | 0.08 |
| 6 | 0.5 | 0.15 |
| 7 | 0.6 | 0.36 |
| 8 | 0.7 | 2.1 |
| 9 | 0.8 | 10.5 |
| 10 | 0.9 | 27.8 |
| 11 | 1.0 | 45.2 |
Table 2: Reverse Bias Readings
| S.No. | Reverse Voltage (V) in Volts | Reverse Current (I) in μA |
|---|---|---|
| 1 | 0.0 | 0.0 |
| 2 | 1.0 | 0.2 |
| 3 | 2.0 | 0.2 |
| 4 | 3.0 | 0.3 |
| 5 | 4.0 | 0.3 |
| 6 | 5.0 | 0.4 |
| 7 | 10.0 | 0.5 |
| 8 | 15.0 | 0.6 |
| 9 | 20.0 | 0.7 |
8. Calculations
From the observed data, we can calculate various diode parameters:
1. Static Resistance
The static resistance at a particular operating point (V, I) is given by:
$$R_{static} = \frac{V}{I}$$
For example, at V = 0.7V and I = 2.1mA:
$$R_{static} = \frac{0.7V}{2.1 \times 10^{-3}A} = 333.33\Omega$$
2. Dynamic Resistance
The dynamic or AC resistance is given by:
$$r_d = \frac{dV}{dI} \approx \frac{\Delta V}{\Delta I}$$
For example, between V = 0.7V and V = 0.8V:
$$r_d = \frac{0.8V - 0.7V}{10.5mA - 2.1mA} = \frac{0.1V}{8.4 \times 10^{-3}A} = 11.9\Omega$$
3. Ideality Factor
The ideality factor can be calculated from the slope of the ln(I) vs V plot:
$$n = \frac{q}{kT} \times \frac{\Delta V}{\Delta ln(I)}$$
At room temperature (T = 300K):
$$n = \frac{0.026V}{\Delta V} \times \Delta ln(I)$$
4. Knee Voltage
From the forward bias characteristic curve, the knee voltage (where current starts to increase rapidly) is approximately 0.7V for a silicon diode.
9. Result
Based on the observations and calculations, we can draw the following conclusions:
- The p-n junction diode exhibits nonlinear I-V characteristics.
- In forward bias:
- The diode conducts significantly after overcoming the barrier potential (approximately 0.7V for silicon).
- The current increases exponentially with voltage as predicted by the Shockley equation.
- The static resistance decreases as the voltage increases.
- In reverse bias:
- The diode conducts very little current (in μA range).
- The reverse current is almost constant and independent of the applied reverse voltage (until breakdown).
- The calculated ideality factor of the diode is approximately _____ (to be filled after calculation).
- The knee voltage (barrier potential) of the diode is approximately 0.7V, confirming it is a silicon diode.
Fig 2: I-V Characteristic Curve of P-N Junction Diode showing forward and reverse bias regions
10. Precautions
- Always connect the diode in the correct polarity as per the circuit diagram.
- Use a current limiting resistor to prevent damage to the diode from excessive current.
- Start with zero voltage and increase gradually to avoid sudden surges.
- In forward bias, do not exceed the maximum current rating of the diode.
- In reverse bias, do not exceed the breakdown voltage of the diode.
- Ensure proper connections and tight junctions to minimize contact resistance.
- Use calibrated measuring instruments for accurate readings.
- Allow the circuit to stabilize before taking readings.
- Avoid heating the diode by limiting the time it conducts high current.
- Handle the semiconductor diode carefully to prevent static discharge damage.
- Maintain room temperature during the experiment for consistent results.
11. Sources of Error
- Instrument Errors: Limited precision and accuracy of the voltmeter and ammeter used.
- Temperature Variations: Changes in ambient temperature can affect the diode characteristics.
- Contact Resistance: Poor connections can introduce additional resistance in the circuit.
- Voltage Drop in Ammeter: The internal resistance of the ammeter can affect the voltage across the diode.
- Component Tolerance: Variation in the actual values of resistors from their nominal values.
- Diode Heating: Self-heating of the diode during measurement can alter its characteristics.
- Reading Errors: Parallax errors when reading analog meters.
- Stray Capacitance and Inductance: Can affect measurements, especially at high frequencies.
- Power Supply Fluctuations: Variations in input voltage can affect the measurements.
12. Viva Voice Questions
1. What is a p-n junction diode?
A p-n junction diode is a semiconductor device formed by joining p-type (hole-rich) and n-type (electron-rich) semiconductor materials. It allows current to flow easily in one direction (forward bias) but restricts flow in the opposite direction (reverse bias).
2. Explain the barrier potential in a p-n junction diode.
The barrier potential (also called built-in potential) is the potential difference that forms at the p-n junction due to the diffusion of charge carriers and subsequent formation of a depletion region. It is approximately 0.7V for silicon diodes and 0.3V for germanium diodes at room temperature.
3. Why does a diode conduct heavily after the knee voltage?
When the applied forward voltage exceeds the barrier potential (knee voltage), the depletion region narrows significantly, reducing the resistance to charge carrier flow. This leads to an exponential increase in current as per the Shockley diode equation.
4. What is reverse saturation current?
Reverse saturation current ($I_s$) is the small current that flows when a diode is reverse biased. It is caused by thermally generated minority carriers and is typically in the order of nanoamperes for silicon diodes and microamperes for germanium diodes.
5. How does temperature affect the diode characteristics?
As temperature increases: (1) The barrier potential decreases by approximately 2mV/°C, (2) The reverse saturation current approximately doubles for every 10°C increase, and (3) The forward current at a given voltage increases with temperature.
6. What is the ideality factor in the diode equation?
The ideality factor (n) is a parameter that accounts for the deviation of a real diode from ideal behavior. It typically ranges from 1 (for ideal diodes dominated by diffusion current) to 2 (for diodes where recombination current dominates). It appears in the exponent of the Shockley diode equation.
7. Differentiate between static and dynamic resistance of a diode.
Static resistance is the ratio of DC voltage to DC current at a specific operating point ($R_{static} = V/I$). Dynamic resistance is the rate of change of voltage with respect to current ($r_d = dV/dI$) and represents the small-signal AC resistance of the diode.
8. What happens when a diode reaches its breakdown voltage in reverse bias?
When a diode reaches its breakdown voltage, the reverse current increases rapidly due to either avalanche multiplication or the Zener effect. In avalanche breakdown, carriers gain enough energy to create additional electron-hole pairs through collision. In Zener breakdown, the strong electric field directly generates electron-hole pairs through quantum tunneling.
9. Why is the forward characteristic curve exponential while the reverse characteristic is almost flat?
The forward characteristic is exponential as described by the Shockley equation because the barrier potential decreases with increasing forward voltage, allowing exponentially more carriers to overcome the barrier. In reverse bias, once the depletion region is fully formed, the current is limited to the thermally generated minority carriers (reverse saturation current), which is nearly independent of voltage until breakdown.
10. Compare silicon and germanium diodes in terms of their I-V characteristics.
Silicon diodes have a higher barrier potential (0.7V) compared to germanium diodes (0.3V). Silicon diodes have lower reverse saturation current (nA range) than germanium diodes (μA range). Silicon diodes can operate at higher temperatures and have better stability with temperature changes. Silicon diodes also typically have higher breakdown voltages.