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Diode Characteristics

BS P-III

Institute of Phsics

Voltage Current Characteristics of diode Objects of the experiment To understand the Voltage-Current characteristics of a janction diode.

Introduction The purpose of this experiment is for you to become familiar with the operating characteristics of the diode. The diode is the physical realization of the PN junction. Depending on which way the diode is biased, current either flows readily or is unable to flow. The circuit symbol for a diode and a diagram showing at which end the band appears on a real diode as well as the equivalent circuits for when the diode is forward- and reverse-biased are given below. Forward and reverse bias are defined by whether VA - VB = V A B is equal to (forward) or less than (reverse) the diode “turn-on” voltage V d . End A is known as the anode and end B as the cathode. So another definition of forward-biased is when the anode is Vd Volts higher in potential than the cathode.

A

Forward biased (VAB> V d)

B

A

B

A

B

r

Vd

Reverse biased (VAB< V d) A B R Figure l-1 The forward-bias resistance r is very small. An “ideal” diode has no forward resistance (i.e., is a short-circuit current path). The reversed-biased diode resistance is very large (many M W ) with the reverse-biased ideal diode operating as an open circuit (i.e., no current flow allowed at all). When the diode is reverse biased, the small current due to minority carriers remain relatively constant, that is independent of the bias voltage up to certain voltage. Beyond this safe level of reverse bias, a phenomenon called "Avalanche Breakdown" takes place when heavy surge of current occurs which may also destroy the diode. The diode must be operated within the safe limit. The Volt-- ampere (V-I)) characteristics of a diode show how currentt (/) in that diode varies with the voltage applies across it experimentally. This . can determined by measuring the current in the diode for successive number of higher applied voltage and plotting a graph of current versus voltage, you will note that very little current follows in the diode for low level of the applied voltage. Thus below ( 0.7 ) forward bias, a silicon diode draws little current. For forward bias voltages equal to or higher than 0.7V , the diode is turn on and permits to currentt to flow. Beyond 0.7V very slight increases in forward biasing voltages result in the increase of current in the diode dramatically. The primary function of the diode is the rectification (process of converting a.c into d.c).

K A

Figure l-2(a) Forward Bias

+

_ V

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Diode Characteristics

BS P-III

V

K

+ +

A

_ V

Figure l-2(b) Reverse Bias

Figure 1-3.Characteristicc curve of a diode

EQUIPMENT •

Oscilloscope

A mili-ameter (range 20mA)

•

Power supply

A bread board A diode (IN4004)

COMPONENTS:

10K

A voltmeter (range 30 v) A few connecting hard wires

resistor

PROCEDURE: 1. Make the circuit as shown in figure 1-2 2. Set the meter indicated by "A" to its '7.5 mA'. Set the meter 'V to read up 4v. Set on the DC supply kit and voltage from zero to 4V and turn the rotary voltage control fully anticlockwise (0V) . 3. Now switch on the power supply and carefully turn the voltage control clock wise whilst watching the ammeter. 4. Set the values of the voltage and observe the values of current. Plot these values on the graph paper. The resulting graph should shows that little current passes until the voltage has risen to 0.6V (for silicon), but the current rises rapidly with further increases in voltage. 5. Change the resistor value, repeat all pervious steps and fill the observation table. 6. Finally plot the V-l characteristics of diode on graph paper.

OBSERVATION TABLE: S.No.

Voltage (VD )

Forword current mA

Voltage (VD )

1

0.2

0.2

2

0.4

0.4

3

0.6

0.6

4

0.5

0.5

5

0.7

0.7

2

Reverse current mA

Diode Characteristics

BS P-III

Diode I-V Characteristics using Oscilloscope The I-V characteristics of a diode can be displayed directly on the oscilloscope using XY mode and the circuit shown in Fig. 6. The voltage across the resistor (which is R (I D)) is used as the Y inp ut and the voltage across the diode vD is used as the X input. Thus a voltage 100 *I D will be the vertical axis, and vD will be the horizontal axis. It must be noted that the voltage units of the vertical scale should be divided by 100 to obtain the diode current in amperes.

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Function generator Fig. 6 – Circuit for I-V Characteristic Measurement On most oscilloscopes the X and Y inputs have a common terminal. If this is the case, vD and vR cannot be displayed directly as described. However, if the common (ground) terminal of the scope inputs are connected to node 2, the vertical input connected to node 3 and horizontal connected to node 1, the desired characteristics can be obtained by inverting the vertical input voltage (a switch on the oscilloscope). The sinusoidal voltage supply will force the diode to go into the forward and reverse biased states. If the frequency of the source is fast enough we will observe a continuous graph of I D vs vD being traced on the scope. 1. 2. 3.

Construct the circuit shown in Fig. 6 Obtain and sketch the I-V characteristics of the diode. the maximum current to approximately 50mA. Plot the I-V characteristics of the silicon diode and the germanium diode

Fig. 7 – Graph Illustrating the Differences between the Si and Ge Diodes

REVIEW QUESTIONS: 1. What do you mean by term Biasing?

2. What is turn on voltage for silicon and germanium at room temperature? 3. What is barrier potential? 4. What is the depletion region?

FINAL CHECK LIST: • Return all equipment and materials to their proper storage area.

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