Diodes and Transistors
Objectives of this lab:
In the first part of this lab, you will become familiar with different types of diodes, their
properties and characteristics, and understand their behavior in the context of solid state
physics. You will do this by observing the I-V characteristics of these devices as they are
coupled in series with a resistor.
In the second part, you will modify your setup to observe the I-V characteristics of a
bipolar junction transistor (BJT) and a field-effect transistor (FET) as you vary the base
current or gate voltage.
Method
A current-voltage characteristic can be obtained with the
voltage divider illustrated to the right, and an XYconnected oscilloscope. The voltage over the resistor
gives the diode current. The oscilloscope you will use
has one pole grounded, so ground must be placed
between the diode and variable resistor to obtain a
voltage across the component. This means that the
function generator must be floating, not grounded.
This can be done with the equipment pictured. The
function generator in the photograph is an LMF2 but the
IEC F34 also works. The oscilloscope is showing the
characteristic for a normal red LED, with current on the
Y axis, and voltage on the X axis.
Most of the scopes give a stable picture in the frequency
range 100-1000Hz. If you go too high you run into
problems such as inductive reactance, which show up as
hysteresis effects. Choose a Helipot (1k) as a variable
resistor, and mount the components on an experiment
board, since many different diodes will be studied.
Getting Started
Begin with both inputs grounded (GND) and adjust the settings so that the trace is in the
center of the screen. Practice by placing a known resistor in place of the diode. Choose
the same resistance on the variable resistor, and you will get a 45° line on the screen
when the gains of both channels are the same.
Part 1: Diodes
Now you are ready to test the diodes. Please keep in mind that diodes do not like strong
currents, and the point of the decade resistor is to limit the current. Currents should be
5-10 mA, which can be measured most easily with a current meter in series with the
diode.
Reports: Make accurate sketches of the I-V characteristic curves, with important features
labeled, plus comments and explanations.
Study the following types of diodes:
1. Ge- and Si- diodes. Measure and note the offset voltage (E0), and compare with
the band gap energies Eg for Ge and Si.
2. Red LED (the classic red LED with wavelength :~660 nm). Study the relationship
between the offset voltage, and where the diode begins to light. Choose a voltage
where the diode is lit, and change the frequency to ~1 Hz. At which points on the
I-V curve does the light from the LED turn on and off? Go back to the original
frequency.
3. LEDs with different colors. Note the relationship betwen the offset voltage and
the LED wavelength.
4. Zener and avalanche diodes. These diodes are normally used with reverse bias, so
we are interested in studying the point when the voltage is negative and
breakdown curves. It is possible that the Farnell function generator may not be
able to deliver sufficient negative voltage. Connect an extra pair of wires, and
place the diode slightly away from the other connections. Warm the Zener and
avalance diodes, and study how the breakdown voltage changes.
5. Schottky diode. Note the offset voltage.
6. Tunnel diode. Take an extra look at how the curve is drawn. You should see a
marked hysteresis effect. Is this diode made with Ge or Si?
7. Solar cell. This doesn’t like to have current forced through it, so only apply a
small voltage (a few V). Study how the I-V characteristics change when the solar
cell is covered or illuminated. If you have problems with hysteresis, reduce the
frequency.
Part 2: Transistors
For this part, you are provided with a bipolar junction transistor (BJT) and a field-effect
transistor (FET). Connect the emitter and collector (for the (BJT) or the source and drain
(for the FET) across the X-input of the oscilloscope, as you did for the diodes. Examine
the IV characteristics for an unconnected base (gate).
Now, use a DC voltage source in series with a decade resistor to provide an adjustable
base current or gate voltage to the third lead of the transistor. For each transistor, measure
and draw the I-V characteristic curves for different values of IB/VG. Estimate the
amplification factor β for each device.
Additional measurements
If time permits, you can try these ideas:
1. Connect a diode to a high-speed square wave generator and a fast oscilloscope.
Observe the current response when the diode switches quickly between forward
and reverse bias.
2. Use the BJT to construct a simple inverting log amplifier. Draw a schematic of
the circuit, and measure the gain.