Characteristics of a point contact diode Lab Manual

Forward Characteristics of Point Contact Diode

1. AIM

To study and plot the forward bias characteristics of a point contact diode and determine its static and dynamic resistance.

2. APPARATUS USED

Point Contact Diode (Germanium diode, e.g., OA79, 1N34A)
DC Power Supply (0-5V, variable)
Voltmeter (0-3V DC range)
Milliammeter (0-50mA range)
Resistor (470Ω, 1W)
Rheostat/Potentiometer (1kΩ)
Connecting wires and breadboard
Graph paper for plotting characteristics

3. DIAGRAM

Characteristics of point contact diode circuit diagram

Fig. 1: Circuit diagram for studying the forward characteristics of point contact diode

4. THEORY

A point contact diode consists of a small metal wire (usually tungsten or phosphor bronze) making point contact with a semiconductor material (typically n-type germanium). The point contact creates a p-n junction at the interface between the metal and semiconductor.

When a diode is forward biased (positive potential to the p-side and negative to the n-side), the barrier potential decreases, allowing current to flow through the diode. As the forward voltage increases, the current increases exponentially according to the diode equation.

Point contact diodes have some distinctive characteristics compared to p-n junction diodes:

Lower forward voltage drop (typically 0.2V for germanium point contact diodes)
Faster switching times due to lower junction capacitance
Higher reverse current leakage
Less mechanical robustness

The forward characteristics of a point contact diode follow the Shockley diode equation:

$I = I_S \left( e^{\frac{qV}{nkT}} - 1 \right)$

Where:

$I$ = Forward current through the diode
$I_S$ = Reverse saturation current
$V$ = Applied voltage across the diode
$q$ = Charge of an electron (1.602 × 10^-19 C)
$k$ = Boltzmann's constant (1.38 × 10^-23 J/K)
$T$ = Absolute temperature in Kelvin
$n$ = Ideality factor (typically between 1 and 2)

At room temperature (around 300K) and for voltage values much greater than the thermal voltage (kT/q ≈ 26mV), the equation can be simplified to:

$I \approx I_S e^{\frac{qV}{nkT}}$

This relationship gives rise to the exponential I-V characteristic curve observed in diodes.

5. FORMULA

1. Static Resistance: The ratio of the DC voltage across the diode to the DC current through the diode at a particular operating point.

$R_{static} = \frac{V}{I}$

2. Dynamic or AC Resistance: The ratio of a small change in voltage to the corresponding change in current at a particular operating point.

$r_{dynamic} = \frac{\Delta V}{\Delta I} = \frac{dV}{dI}$

3. From the theoretical diode equation, the dynamic resistance can also be calculated as:

$r_{dynamic} = \frac{nkT}{qI} \approx \frac{nV_T}{I}$

Where $V_T = \frac{kT}{q} \approx 26$ mV at room temperature.

6. PROCEDURE

Set up the circuit as shown in the diagram (Figure 1). Ensure that the diode is connected with correct polarity for forward bias (anode to positive and cathode to negative).
Before turning on the power supply, set the rheostat/potentiometer to its maximum resistance position to ensure minimum current flows initially.
Turn on the power supply and adjust it to a low voltage (around 0.5V).
Gradually decrease the resistance of the rheostat to increase the current through the diode.
Record the voltmeter reading (V) and the corresponding milliammeter reading (I) for different values of current. Start from 0V and increase in small steps (0.05V increments recommended for the crucial knee region around 0.2-0.3V).
Continue taking readings until the forward voltage reaches about 0.8V or the current reaches the maximum safe value for the diode (typically 50mA for small signal diodes).
Record all readings in the observation table.
Turn off the power supply after completing all measurements.
Plot a graph between the forward voltage (x-axis) and forward current (y-axis).
Calculate the static and dynamic resistance at different points on the curve.

7. OBSERVATION TABLE

Sl. No.	Forward Voltage (V) Volts	Forward Current (I) mA	Static Resistance (R_s = V/I) Ohms	Dynamic Resistance (r_d = ΔV/ΔI) Ohms
1				—
2
3
4
5
6
7
8
9
10

Note: Dynamic resistance can be calculated using consecutive readings as ΔV/ΔI

8. CALCULATIONS

Sample calculation for static resistance:

For a particular reading, if V = 0.3V and I = 10mA (0.01A):

$R_{static} = \frac{V}{I} = \frac{0.3V}{0.01A} = 30\Omega$

Sample calculation for dynamic resistance:

For two consecutive readings:

Reading 1: V₁ = 0.25V, I₁ = 5mA

Reading 2: V₂ = 0.3V, I₂ = 10mA

$r_{dynamic} = \frac{\Delta V}{\Delta I} = \frac{V_2 - V_1}{I_2 - I_1} = \frac{0.3V - 0.25V}{10mA - 5mA} = \frac{0.05V}{5mA} = \frac{0.05V}{0.005A} = 10\Omega$

Theoretical calculation of dynamic resistance:

For a point where I = 10mA and assuming n = 1.2 for a germanium point contact diode at room temperature (300K):

$r_{dynamic} = \frac{n \cdot kT/q}{I} = \frac{1.2 \cdot 26mV}{10mA} = \frac{31.2mV}{10mA} = \frac{0.0312V}{0.01A} = 3.12\Omega$

9. RESULT

1. The forward characteristic (I-V curve) of the point contact diode has been plotted, showing the exponential relationship between voltage and current.

2. The knee voltage (threshold voltage) of the germanium point contact diode was found to be approximately _____ volts.

3. The static resistance values were calculated at different operating points and found to decrease with increasing forward current, ranging from _____ Ω to _____ Ω.

4. The dynamic resistance values were calculated and found to be consistently lower than the static resistance values, ranging from _____ Ω to _____ Ω.

5. The experimental values of resistance were found to be in reasonable agreement with the theoretically expected values, with discrepancies attributed to temperature variations and component tolerances.

10. PRECAUTIONS

Always connect the diode with the correct polarity in the circuit. Reverse connection can damage the diode in the presence of high voltage.
Start with minimum voltage and gradually increase it to avoid sudden current surges through the diode.
Do not exceed the maximum current rating of the diode. For most point contact diodes, this is typically around 50mA.
Use appropriate range of voltmeter and milliammeter to get accurate readings.
Ensure all connections are tight and secure before starting the experiment.
Handle the point contact diode carefully as they are mechanically fragile compared to junction diodes.
Allow some time between consecutive readings to prevent heating of the diode, which can affect its characteristics.
Turn off the power supply when changing connections.
Take more readings around the knee voltage region (0.2-0.3V for germanium diodes) for better characterization.
Keep the circuit away from heat sources as diode characteristics are temperature-dependent.

11. VIVA VOICE QUESTIONS

1. What is a point contact diode and how does it differ from a p-n junction diode?

A point contact diode consists of a metal wire making point contact with a semiconductor surface. It differs from p-n junction diodes in having lower forward voltage drop (0.2V for germanium compared to 0.7V for silicon junction diodes), faster switching speeds, higher reverse leakage current, and lower mechanical robustness.

2. Why is germanium often used in point contact diodes instead of silicon?

Germanium has a lower bandgap (0.67eV) compared to silicon (1.1eV), resulting in a lower forward voltage drop (about 0.2-0.3V compared to 0.6-0.7V for silicon). This makes germanium diodes more suitable for low-voltage applications and signal detection.

3. What is the knee voltage or threshold voltage of a diode? What is its approximate value for a germanium point contact diode?

The knee voltage or threshold voltage is the minimum forward voltage required for significant current flow through the diode. For a germanium point contact diode, it is approximately 0.2-0.3V.

4. Explain the difference between static resistance and dynamic resistance of a diode.

Static resistance is the ratio of DC voltage to DC current at an operating point (V/I), whereas dynamic resistance is the ratio of small change in voltage to the corresponding change in current (ΔV/ΔI or dV/dI) at that point. Dynamic resistance represents the slope of the I-V curve at a given point and is typically lower than static resistance in the forward bias region.

5. Why does the forward resistance of a diode decrease with increasing current?

As the forward current increases, more charge carriers cross the junction, effectively reducing the barrier potential. The exponential relationship between current and voltage means that large increases in current correspond to relatively small increases in voltage, resulting in a decreasing resistance as per Ohm's law (R = V/I).

6. What are the main applications of point contact diodes?

Point contact diodes are primarily used in high-frequency applications such as RF signal detection, AM radio demodulation, radar systems, microwave receivers, and as mixer diodes in communication circuits due to their low junction capacitance and fast switching speeds.

7. How does temperature affect the characteristics of a point contact diode?

Increased temperature reduces the bandgap of the semiconductor, resulting in increased reverse saturation current and reduced forward voltage drop. This means the diode conducts more easily at higher temperatures. The dynamic resistance also decreases with increasing temperature according to the relation r_d = nkT/qI.

8. Why is the I-V characteristic of a diode non-linear?

The I-V characteristic is non-linear because the current through a diode follows an exponential relationship with voltage (I ∝ e^V), as described by the Shockley diode equation. This is due to the carrier diffusion mechanism across the junction barrier and the statistical nature of carrier movement.

9. What is the ideality factor 'n' in the diode equation, and what does it represent?

The ideality factor 'n' is a measure of how closely the diode follows the ideal diode equation. It typically ranges from 1 to 2. A value of 1 represents an ideal diode where diffusion current dominates, while values closer to 2 indicate that recombination current in the depletion region is significant. For point contact diodes, the value is typically between 1.2 and 1.5.

10. How can you determine the knee voltage from the forward characteristic curve?

The knee voltage can be determined by identifying the point on the I-V curve where the current begins to increase rapidly with small increases in voltage. Graphically, it is the voltage at which the curve begins to bend sharply upward from the x-axis. More precisely, it can be found by extrapolating the linear portion of the curve to the voltage axis.

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FORWARD CHARACTERISTICS OF POINT CONTACT DIODE