Lab Manual at www.docircuits.com - Voltage / Current characteristics of Silicon Diode

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http://www.docircuits.com/lab-manual/52/vi-characteristics-of-diode
VI Characteristics of Diode
Diode Forward Characteristics...
P-N Junction Reverse Bias Char...
Aim
Aim is to
1. Explain the structure of a P-N junction diode
2. Explain the function of a P-N junction diode
3. Explain forward and reverse biased characteristics
of a Silicon diode
Related Experiments
DIODE
Description
Theory:
The term bias refers to the use of a dc voltage to establish certain operating conditions for an electronic device.
Depending on the magnitude and polarity of the applied voltage the diode is said to be:
Forward Biased, Anode voltage is greater than the Cathode voltage
Reverse Biased, Cathode voltage is greater than the Anode voltage
So, diode is a simple switch that is either closed (conducting) or open (non conducting). Specifically, the diode is a
short circuit, like a closed switch, when voltage is applied in the forward direction, and an open circuit, like an
open switch, when the voltage is applied in the reverse direction.
Let us now take the earlier model one more step. The offset voltage model adds the barrier potential to the ideal
switch model. When the diode is forward biased it is equivalent to a closed switch in series with a small equivalent
voltage source equal to the barrier potential (0.7 V for Silicon, 0.4 for germanium) with the positive side towards the
anode. When the diode is reverse biased, it is equivalent to an open switch just as in the ideal model.
When forward biased, Vg ( 0.7 for Silicon and 0.4 for Germanium ) volts appears across the diode and current flows.
During reverse bias, when the voltage applied across the diode is less than Vg, there will be no current flowing.
Let us now take the earlier model one more step. It is the most accurate of the diode models. The Complete diode
model of a diode consists of the barrier potential, the small forward dynamic resistance and the ideal diode. The
resistor approximates the semiconductor resistance under forward bias. This diode model most accurately
represents the true operating characteristics of the real diode.
¨Static Resistance of a P-N junction diode is the ratio of forward voltage to forward current
¨Dynamic Resistance of a P-N junction diode is the small change in forward voltage to small change in
forward current at a particular operating point.
When a diode is reverse biased a leakage current flows through the device. This current can be effectively ignored
REVERSE
CHARACTERISTICS

2. When a diode is reverse biased a leakage current flows through the device. This current can be effectively ignored
as long as the reverse breakdown voltage of the diode is not exceeded. At potentials greater than the reverse
breakdown voltage, charge is pulled through the p-n junction by the strong electric fields in the device and large
reverse current flows. This usually destroys the device. There are special diodes that are designed to operate in
breakdown. Such diodes are called zener diodes and used as voltage regulators.
¨Take Resistor of 1Kohms
¨Connect to Si Diode.
¨Use Ammeter to measure current and Voltmeter to measure Voltage
¨Setup the circuit as shown
¨One way to measure the VI Characteristics of the diode is the set the DC source at various voltage levels
¨Note the current in the ammeter and voltage in the voltmeter
¨Plot the current against voltage
¨Instead of re-running the experiment at various voltage values from DC source, let us parameterize
¨Click on Simulation Properties – DC Analysis
¨Choose DC Source and Vary the voltage from 0V to 5V to record the voltage and current across the diode
¨Run the experiment
¨On successful run, click on Data Analysis to view the current and the voltage readings
¨Choose to plot against the voltage the current.
¨Edit the properties of the graph to view a closer segment of the plot
¨Select 0.4 V to 1.0 V on the X Axis with interval of 0.01 V
¨On the zoomed graph – measure the
¨Cut off Voltage
¨Static Resistance
¨Dynamic Resistance
¨Hint – to calculate Static and Dynamic Resistances
¨Zoom in the Y axis as well.
¨Dynamic Resistance = Slope of the graph for a particular point
Reverse-bias characteristics
¨Setup the circuit as before – but reverse the diode in the circuit so that it is reversed biased.
¨Go through the same steps as before to record the Current and Voltage Readings but the stop voltage should
be about 60 V.
¨Note that maximum current flows after about 50 V in the reverse direction..
¨
Conclusion
Thus the forward and reverse bias characteristics of the diode have been verified and plotted.