Semiconductor Fundamentals Unit 2 – Diodes and Half-Wave Rectification
Exercise 1 – Diode DC Characteristics
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to test a diode in a typical diode circuit
by using a diode dc characteristic curve. You will verify your results with a multimeter.
DISCUSSION
- Diode characteristic curves graph the current flow versus applied voltage. The right side of
the graph shows the diode operating in the forward bias condition. The left portion of the
graph illustrates the reverse bias condition. - Voltage and current scales on the characteristic curve vary by region, ranging from milliamps
to picoamps or millivolts to volts. - Forward voltages below the barrier voltage will not forward bias the diode. In silicon diodes,
conduction begins when the forward voltage reaches between 0.5 and 0.7V. Germanium
diodes require a forward voltage of about 0.3V. - Once forward voltage produces conduction, small increases in voltage produce large current
increases. This nearly constant voltage is called the forward voltage drop (VF). - Reverse biased diodes will have very small current flows. The current flow is due to the
presence of minority carriers and is referred to as leakage current. - Breakdown voltage is the reverse voltage that causes large current flow through reverse
biased diodes. The current can be large enough to damage the diode. - In general, for calculations, the forward voltage drop of a silicon diode is 0.6V, and 0.3V for
a germanium diode. - Forward resistance is the ratio of the increase in voltage drop divided by the increase in
forward current. - Since forward resistance, or dynamic forward resistance, of a diode is very small, a resistor is
included in the circuit to prevent damage to the diode. - The highest current value that can pass through the diode without causing damage is referred
to as maximum forward current (IF(max)). - Ohmmeters cannot accurately measure the resistance of a diode junction therefore, diode
junctions are tested by checking for continuity only. A forward biased junction will register a
very low resistance indicating continuity. A reverse biased junction, or damaged diode, will
indicate a very high resistance or an overload, indicating a lack of continuity. - Included in this exercise are instructions for testing diodes with the following meters:
LAB-VOLT DIGITAL MULTIMETER
DIGITAL MULTIMETER WITH A DIODE TEST FUNCTION
ANALOG OHMMETER