UNIVERSITY OF MASSACHUSETTS DARTMOUTH
COLLEGE OF ENGINEERING
EGR 101 INTRODUCTION TO ENGINEERING THROUGH APPLIED SCIENCE I
PN JUNCTION DIODES AND APPLICATIONS
WHAT IS A PN JUNCTION DIODE?
A PN Junction diode is a 2-terminal semiconductor electronic On/Off switch that allows current
flow in only one direction. The circuit symbol is shown below. The terminals are called the
Anode (A) and the Cathode (K). The state of the diode switch (open or closed) is determined by
the polarity of the voltage across it.
Circuit Symbol and Typical Terminal Identification
Common types of Diode Packages
Figure 1. Some common Diode Packages
CIRCUIT BEHAVIOR OF THE PN JUNCTION DIODE
The behavior of electronic devices, such as the pn junction diode, can be described by their
Volt-Ampere, or (V-I) Characteristic. This is a plot of the current flowing through the device as a
function of the voltage across the device. Since the resulting plot of the diode’s V-I
Characteristic is not a straight line, the diode is referred to as a “non-linear” device. A typical V-I
Characteristic for a pn junction diode is shown here.
Figure 2. A typical PN Junction diode V-I Characteristic
Through experimentation, the current through a diode as a function of the voltage across it can
be mathematically modeled by the following “diode equation”
i  I 0 (e
 v 


 VT 
-1)
From the previous page, the diode equation is
i  I 0 (e
 v 


 VT 
-1)
where
i = the current through the diode in Amperes (A)
v = the voltage across the diode from Anode (+) to Cathode (-) in volts
VT = 25 mv (25 X 10-3 volts) at room temperature (25°C)
I0 = a constant with units of current
η = a constant ranging from 1 to 2 depending upon the diode material
This representation does not account for the reverse breakdown region!
APPLICATION AS A RECTIFIER
Consider the circuit shown below.
XSC1
Ext Trig
+
_
B
A
+
_
+
_
D1
V1
5 Vpk
1kHz
0°
1N4007GP
R1
1kΩ
Figure 3. A PN Junction Diode as a Half-Wave Rectifier
When the applied sinusoidal voltage is on its positive half – cycle, conventional current will want
to flow in the clockwise direction (the direction of the arrow in the diode symbol) and the voltage
polarity across the diode will be + on the Anode and – on the Cathode. This condition is known
as “forward bias”. The diode will act as a short circuit and essentially all of the applied voltage
will appear across the resistor R1. The applied voltage and the voltage across the resistor R1
are shown on the oscilloscope display below. Note that the resistor voltage is the “positive half
– cycle” of the applied voltage, hence the circuit is called a “half – wave rectifier”.
Figure 4. Waveforms for the Half – Wave Rectifier circuit.
If the diode is “turned around”, or installed in the opposite direction as illustrated here, we would
observe the waveforms shown in Figure 6 on the next page.
XSC1
Ext Trig
+
_
B
A
+
_
+
_
D1
V1
5 Vpk
1kHz
0°
1N4007GP
R1
1kΩ
Figure 5. A PN Junction Diode installed in Reverse
Figure 6. A “negative” Half – Wave Rectifier.
In each of the above circuits, the input signal, having an average value equal to zero was
converted into a “unidirectional” signal having a non – zero average value. It can be shown that
the average value of a “half – wave rectified” sinusoid having a maximum value of Vm has an
average value (also known as the DC value) equal to
Vaverage  VDC 
Vm

ANOTHER RECTIFIER APPLICATION
The circuit shown below is a “full – wave” bridge rectifier.
XSC1
Ext T rig
+
_
B
A
+
2
_
+
_
D3
V1
5 Vpk
1kHz
0°
4
1
3
MDA2502
R1
1kΩ
Figure 6. A Full – Wave Bridge Rectifier with the output waveform.
In this circuit, the output waveform consists of both half – cycles of the input voltage. The
average value of the voltage across R1 will be
Vaverage  VDC 
This is twice the value of the half – wave case.
2Vm
