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VI Characteristics of Diode

Diode Forward Characteristics... P-N Junction Reverse Bias Char...

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 ashort circuit, like a closed switch, when voltage is applied in the forward direction, and an open circuit, like anopen 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 idealswitch model. When the diode is forward biased it is equivalent to a closed switch in series with a small equivalentvoltage source equal to the barrier potential (0.7 V for Silicon, 0.4 for germanium) with the positive side towards theanode. 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 diodemodel of a diode consists of the barrier potential, the small forward dynamic resistance and the ideal diode. Theresistor approximates the semiconductor resistance under forward bias. This diode model most accuratelyrepresents 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 inforward 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

DIODE REVERSE CHARACTERISTICS

AimAim is to 1. Explain the structure of a P-N junction diode2. Explain the function of a P-N junction diode3. Explain forward and reverse biased characteristicsof a Silicon diode

Related Experiments

When a diode is reverse biased a leakage current flows through the device. This current can be effectively ignoredas long as the reverse breakdown voltage of the diode is not exceeded. At potentials greater than the reversebreakdown voltage, charge is pulled through the p-n junction by the strong electric fields in the device and largereverse current flows. This usually destroys the device. There are special diodes that are designed to operate inbreakdown. 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 pointReverse-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 shouldbe about 60 V.¨Note that maximum current flows after about 50 V in the reverse direction..

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Conclusion

Thus the forward and reverse bias characteristics of the diode have been verified and plotted.