ECE - 340 EXPERIMENT IV

DIODE CHARACTERISTICS

Purpose:

In this experiment, you will examine the v-i characteristics of the pn-junction diode and consider its use in some simple circuits.

Parts:

1 - 1N4004 (or 1N4006)

1 - 1N4735

Theory:

A pn-junction diode is fabricated of p-type and n-type semiconductor material. It is a two-terminal device. The terminal connected to the p-type material is called the anode; the terminal connected to the n-type material is called the cathode.

A diode is an inherently non-linear device that conducts current when the voltage across it is of one polarity and does not conduct current when the voltage across it is of the other polarity. Using the notation shown in figure 1 we can write

An analytical expression relatively is given as follows and is graphed in Figure 2.

Where thermal voltage = 26m V at room temperature

n = phenomenological factor = 1 to 2

IS = scaling current

The expression given above is valid only for steady-state conditions. When the diode voltage is switched suddenly from one value to another the current can be considerably different from that indicated above. Switching from no bias or reverse bias to forward bias is a relatively fast process in a pn-junction diode, and the current given by the ideal diode equation above is quickly established. The same can not be said about the opposite switching procedure, however. When a diode is forward biased in has a large amount of excess charge in the region near the pn-junction. When the diode is reverse biased or unbiased it does not. Thus when the diode is switched from forward to reverse bias a large reverse current flows until this charge is removed. The time it takes for this to occur is called the storage time, or ts. After the storage time the current gradually reverts to its steady-state value. The total time after it takes for the diode current and voltage to go to their steady-state values is refereed to as the reverse recovery time, or trr. See Figure 6 for the practical laboratory definitions of ts and trr.

Note: bias means applied voltage.

Procedures:

1. V-I Curve - Point-by-point method.

Set up the circuit shown in figure 3 using a 1N4004. You will want to measure the exact value of Rx using the ohmmeter function of your DVM. Use the following values for : -2 V to -20 V in increments of -2 V, and 0.1 to 2 V in units of 0.1 V. For these values measure and using the DVM. The easiest way to find is to measure and calculate using ohm's law. Note it may be necessary to use different values of Rx to get the desired range of currents. Make sure to choose values of Rx which will allow you to plot the knee in the I-V characteristics at You will not be able to measure the reverse breakdown characteristics of this diode since it occurs at such large negative voltages.

Record the - data. Plot vs. . In your report compare your data with the theory.

2. V-I Curve - Oscilloscope Display

Connect the circuit shown in figure 4. You are going to use the oscilloscope in the X-Y mode, which is different from the usual time-base sweep mode. We will also use the storage feature on the digital oscilloscope. Here we want to measure along the x-axis (channel 1) and (or equivalently ) along the y-axis (channel 2).

Scope Set-up: Option Press

XY Operation Main/Delay

Invert X-axis 1 (button)

Turn to zero. Set volts/div (sensitivity) of channel 1 to 200mV/div. Set volts/div of channel 2 such that each division on the vertical axis represents 1.0mA. Adjust the position knobs so the "dot" on the scope screen is centered. Push the AUTO-STORE button; "STORE" should be visible in the upper left of the scope screen. Increase and note the forward bias characteristic of the diode. Turn off the storage feature by pushing the AUTO-STORE button. Return the "dot" to the scope screen center by turning to zero.

Reverse the leads on and repeat the procedure of the above paragraph to obtain the reverse bias characteristic.

For the forward biased diode, estimate the cut-in voltage and dynamic resistance, .

3. Measurement of Zener breakdown voltage

Use a 1N4735 diode for this part of the experiment. Connect the circuit of figure 4 so that you can measure the reversed biased characteristic using the technique of section 2.

Determine the breakdown voltage, , and estimate the Zener resistance, .

4. Measure of the Storage Time and Reverse Recovery Time.

Set up the circuit of Figure 3, except with the DC power supply replaced by the pulse generator. Use the 1N4004 diode. Set up the pulse generator to produce the waveform shown in Figure 5. Display the diode voltage and current on the oscilloscope. How do they compare with the expected waveforms indicated in Figure 6 and Figure 7? Determine the storage time, , and reverse recovery time, , of the diode.

Note: Channel 1 (X) will provide -Vd instead of Vd