ECIE 3401 - Laboratory for Electronics I
EXPERIMENT 5 - P-N Junction Diodes

Objectives:

The objectives of this experiment are:

1. To measure the forward and reverse-biased current and voltage characteristics for the P-N Junction Diode.

2. To be able to use the oscilloscope in the X-Y mode to plot the current- voltage curve for the diode.

3. To observe how temperature affects the operational characteristics of a diode.

Parts and Equipment:

Resistors: 1 kW - 2 ea.
Capacitor:  10 µF
Diode: 1N914 (or equivalent) - 2 ea.
Transformer: Step-down transformer, approximately 12V secondary voltage
High impedance digital multi-meter
Oscilloscope
Soldering iron
DC power supply
Breadboard and wire

Textbook Reference:

Hambley, Allan R., "Electronics", 2nd Ed., Prentice Hall, 2000, pp. 131-189.
Serda and Smith, "Microelectronic Circuits", 4th Ed., Oxford University Press, 1998, pp. 122-220.

Material Review:

The diode is the simplest application of the p-n junction, the interface between a semiconductor doped p-type with acceptor atoms and a semiconductor doped n-type with donor atoms. Recall that electrons are the majority carriers in n-type material and holes in p-type material. The simplest junction is a so-called homojunction between two regions of the same semiconductor, each doped a different conductivity type (n or p). Thus, the n-Si/p-Si homojunction is the configuration usually discussed in introductory electronics textbooks. A junction between p and n regions of two different semiconductors is called a heterojunction; for example, n-CdS/p-Cu₂S. Junctions with space charge regions in the semiconductor only can be produced between metals and semiconductors also; these are called metal-semiconductor junctions or Schottky Barriers. In all cases, it is the difference in the Fermi level/energy positions in the two isolated materials and the subsequent equilibration of the Fermi levels in the joined materials (to achieve thermal equilibrium) that cause the transient diffusion of electrons and holes that establishes spaces charge in the narrow depletion region near the junction and the built-in electrical field that produces the potential energy barrier which, in turn, produces the junction/diode behavior.

It can be shown that for the ideal p-n junction diode that

where I_D is the diode current, q the protonic charge (1.602 10^-19 C), n a mechanism factor between 1 and 2 and usually 1 in simple treatments, k Boltzmann's Constant (1.38 10^-23 J/K), and T the Kelvin temperature
(K = ^oC + 273.2). At room temperature (20-25^oC), ( n = 1)kT/q is approximately 25 to 26 mV so often the equation (called the Shockley Equation after William Shockley, one of the inventors of the transistor) is written as

where V_T is approximately 25 mV. This equation does not model the Zener/avalanche breakdown region that occurs under strong reverse bias. The equation predicts that under forward bias (positive V_D), I_D increases quasi-exponentially with V_D:

Thus, ln (I_D)   approximately =  ln (I_s) + V_D/V_T away from the origin in the forward biased region. The Shockley Equation predicts that I_D approaches - I_s as V_D approaches -infinity; i.e., under full reverse bias.

Thus, under reverse bias, the diode current limits at - I_s, usually a small value in the range of nA to µA but increasing with junction area and junction defect concentration (the defects can "short" the energy barrier of the junction).

Note: that V_T = nkT/q is directly proportional to T.  Thus I_D has a strong temperature sensitivity under forward bias. One must also include the temperature sensitivity of I_s which is proportional to the concentration of majority carriers on either side of the junction (I_s = C₁p_n + C₂n_p, where C₁ and C₂ are positive constants). Both concentrations increase with temperature according to

where E_A is an activation energy.

Thus,

Although exp (qV_D/kT) decreases at constant positive V_D with increasing T, normally the increased exp [-E_A/kT] terms in I_s overshadow it to yield an I_D which increases exponentially with increasing T. Under reverse bias,

exp (qV_D/kT) < < 1 and I_D  approximately =  -I_s so the increase in I_s with T causes the reverse bias current ( -I_s) to increase negatively with T.

At sufficiently large reverse voltages, other mechanisms kick in and cause reverse breakdown during which the current increases negatively from the small -I_s value in a nearly vertical manner (on plots of I_D vs. V_D), thus, tending to "clamp" V_D at this voltage value, V_D = -V_Z. By appropriate doping, V_Z can be made fairly small in magnitude (e.g., -5 to -20 V) and such devices are called Zener Diodes. The sharp breakdown is due to two effects: (1) Zener breakdown occurs when the electric field and, hence, force on the bound electrons become sufficiently large that electrons are "ripped" off of their atoms, thus forming electron hole pairs that are free to move. This sudden increase in free conduction band electron density and valance band holes causes the rapid increase in current magnitude. (2) Avalanche breakdown occurs when the free electrons gain sufficient velocity and kinetic energy between collisions with the atoms that upon subsequent collisions with bound electrons, they transfer enough kinetic energy to the bond electrons that they are "knocked" free of the host atoms and become free electrons, again, increasing the free electron and hole concentrations and, hence, current.

Thus, the simple diode has three major regions (forward bias, reverse bias, and breakdown) and two primary uses. (1) In a sense, the diode acts as a "one-way valve" for current allowing large currents to flow under forward bias and only negligible -I_s values under reverse bias. (2) The huge dI_D/dV_D value at reverse breakdown causes a clamping of V_D to near V_Z Iv the diode has been designed to operate in this region and the total power dissipated is below the power rating of the diode.  If the diode has not been designed to operate in this region, such as rectifier diodes, the power rating will probably be exceeded and the diode will burn out. Thus, diodes can be used in a variety of switching and waveform processing applications.

The average forward bias resistance is defined as I_D/V_D but the instantaneous, dynamic, or "small-signal" resistance is:

which, under forward bias, is approximately

The p-n junction/diode, although simple, is the building block on which most solid state electronic devices are built; for example, the transistor, solar cell, light emitting diode, silicon controlled rectifier, etc. Thus, a firm understanding of this device is prerequisite to a firm understanding of all of modern electronics.

Procedure 1: DC Characteristics

1. Construct the circuit with R = 1 kW.

2. Set V_S at + 15 volts and measure V_D and V_R. Calculate I from Ohms Law, using V_R and R.

3. Set V_S to -15 volts and repeat step 2.

4. Briefly describe what effect the diode has on the D.C. circuit.

Procedure 2: Dynamic Characteristics

1. Construct the circuit above with R = 1 kW. Use a 12.6V center tapped transformer to provide V_S, this will be the two outer terminals on the transformer. Make sure both diodes are inserted in the correct direction. carefully check the circuit before connecting the transformer. Errors in connecting the circuit such as putting both ends of the resister in the same line of holes on the breadboard will cause the fuse or one of the diodes to burn out.

2. With the oscilloscope set for X-Y operation, connect V_D to the X input and V_ID to the Y input. With R = 1kW, the value of V_ID in volts will be equal to the value of I_D in mA.  If the two resistor values are matched and the two diodes have the same characteristics, then the currents in both branches of the circuit will be identical. Therefore plotting the current of the diode D₂ vs. the voltage of diode D₁ will be equivalent to plotting the current and voltage from the same diode.

Set the oscilloscope horizontal voltage scale at 0.1V/div and the vertical scale at 2V/div (this will be 2mA/div for the current flowing through the 1kW resistor). Set the vertical (y-axis or current) zero at the bottom of the screen(-4.00 div) and the horizontal (x-axis or voltage) zero at the left edge of the screen (-5.00 div). The curve you are observing is the forward biases current-voltage curve of the diode.

3. If you switch to voltage time display you can acquire both waveforms with Wavestar and then display the X-Y plot in Wavestar.  make sure that the time per division is set to show at least one full cycle of the waveform or at least all of the positive portion.  Leave the volts per division settings the same as for your X-Y display on the oscilloscope.  After acquiring both waveforms in the voltage time mode, go to view and select XY. Then you will have to select the channel 2 waveform as the Y signal.  If you do not have the display window maximized the Y-axis select box may not show in the window.  Print the current-voltage curve observed from the oscilloscope and label the voltage and current scales with the proper values.  Remember that your origin is now in the lower left hand corner not in the center of the screen.  Mark  the voltage where the curve first visibly leaves the horizontal axis.  Also note and mark the voltages where the curve crosses 8 mA and 16 mA as accurately as possible.  They should be about 6 to 8 divisions from the edge of the screen or between 0.6 and 0.8 volts.

PROCEDURE 3: (Thermal Effects)

1. While observing the curve from the previous procedure, note and monitor the value of the diode voltage at the 16 mA point and perform the following steps.

2. Touch the tip of a hot soldering iron (unplug the iron prior to touching the diode since some iron elements are tied to ground) to the cathode leg of D₁ (as close to the body as possible).  Note and record the maximum change in voltage.  What was the lowest voltage observed at the 16 mA point?  Mark this voltage on the printout also.

3. Use the freeze spray and cool the diode body.  Note any change in voltage.  What was the maximum change at the 16 mA point.  Mark the max change point on your printout.

Procedure 4A: Half-wave Rectification (unfiltered)

NOTE: When using an AC transformer in your circuit, be sure and connect the entire circuit before applying 120 V AC to the primary of the transformer.

1. Build the following circuit using R_L = 1.0 kW.  Connect the oscilloscope to show, V_S, on one channel and, V_L , on the other.  Set the mode switch on the oscilloscope to DC coupling so that the actual value of the output voltage waveforms can be seen.   Set the trigger source to line.  Since the transformer will be plugged into the ac line the internal line connection in the oscilloscope will always be synchronized with your waveforms.  Then adjust the horizontal position control so that the ac waveform starts with a positive zero crossing near the left edge of the display.

2. Adjust the volts/div switch for maximum display on both channels with the zero set to the middle of the screen.  Then print the waveforms showing the correct voltage and time scales.  Identify the input (V_S ) and output (V_L )  waveforms on the graph.

3. Set the zero reference at -3.00 div. and increase the input sensitivity of both channels so the positive part of the waveforms uses the maximum on screen portion of the display.  Print the waveforms and the measurements from Wavestar.  Use the cursor mode to measure the positive peak values of both waveforms.  Also record the delta between the two cursors.  The diode voltage drop will be the difference between the maximum of the input waveform and the maximum of the output waveform.  Note these peak values on your printout of the waveforms.  Record the frequency and period for both waveforms.  Use the multimeter to measure both the DC and the AC_rms voltage across R_L.  Also compare these measurements with the printout of measurements from Wavestar. On the Wavestar measurements, V_DC = cycle mean, and V_{AC rms} = cycle AC rms.  Explain any differences.  The oscilloscope measure mode will also measure the cycle mean or DC voltage and the cycle AC rms voltage.  The cycle rms voltage from Wavestar is the combination of ac and dc and can be calculated by taking the square root of the sum of the square of the dc voltage plus the square of the ac rms voltage.  Compare this value with your meter measurements.

4. Leave the circuit intact for the next procedure.

Procedure 4B: Half-wave Rectification with Capacitor Filter

1. Unplug the power to the transformer and add a 10 µF electrolytic capacitor in parallel with the resistor (R_L), making sure that the capacitor is connected with its negative terminal to ground.  There is usually an arrow on the body of the capacitor which has a small (-) sign printed inside or there will be a (+) symbol at one end and a (-) at the other.

2. After all connections are checked, connect power to the transformer and observe the change in the waveform  across R_L. Continue to use the ac input wave or ac line for the oscilloscope horizontal sweep trigger.

3. Measure the AC and DC voltages across R_L to at least 3 significant figures, using the multimeter.   The zero reference should still be at the -3 div. with the scale set for maximum on screen display.  DC coupling must be used on the oscilloscope to obtain the actual voltage levels with respect to the reference node (gnd.).  Print the waveforms and measurements from Wavestar. Then turn off the display of the input waveform. Print the output waveform by itself.  This can be done in Wavestar by clicking the input waveform off on the display window and printing again without having to acquire again.  Check V_max and V_min of the output waveform. V_max should be the same value as the value measured from procedure 4A.

4. Using the printed waveform from Step 3, draw a horizontal line across the graph at the level of the DC voltage which was previously measured using the multimeter.

5. Switch the oscilloscope to AC coupling and set the zero reference to the center of the screen. Then observe the ripple waveform across R_L by adjusting the volts/div switch for maximum on screen display . Print the ripple waveform and measurements from Wavestar.  The cycle rms voltage from the measure mode of the oscilloscope will now be the same as the cycle ac rms voltage since the dc voltage is removed before the measurements are taken.  Note: the reference is now actually at the level of the average or dc voltage and the maximum point of the waveform is above the dc voltage while the minimum point is below the dc voltage as measured on the screen. Calculate the actual V_max by adding the dc value to the ac coupled V_max and the V_min by adding the dc voltage to the ac coupled V_min.  How do these values compare to the values from the dc coupled measurements?  Also record the delta V, which is the  peak to peak ripple voltage.  For small ripple the average of V_max and V_min will be approximately equal to the DC voltage level. 

QUESTIONS:

1. What is the normal forward operating voltage of a standard silicon diode?

2. Define the following terms:

    a. Forward-biased.
    b. Reverse-biased.
    c. Breakdown voltage.

3. Draw and label the diode voltage-current relationships showing the forward and reverse-biased regions and breakdown voltage regions, and label the approximate voltage (V) in the forward-biased region where the current begins to increase rapidly.

4. Describe the effects of both increasing and decreasing temperature on the operation of a silicon diode. Find the temperature coefficient from the electronics textbook and calculate the actual junction temperature changes for both the heating and the cooling part of the experiment.

5. Describe the output of the half-wave rectifier.  How does it change when the filter capacitor is added to the circuit? 

6. How does the filter capacitor effect the output of the rectifier?  What happens to the D.C. output voltage?  What happens to the a.c. output voltage?

Conclusions:

What can you say about the characteristics of diodes based on your experimental observations?

This page last updated  25 September 2007