The P N Junction and the Semiconductor Diode
P N Junction
N and P type semiconductors by themselves are not very useful materials. However when they are brought together in a P N Junction structure interesting things happen.
In the P N junction, electrons in the n material near the p–n interface tend to diffuse into the p region.
As this happens they leave positively charged ions (holes or donors) in the n region. Similarly, holes in the p material near the p–n interface begin to diffuse into the n-type region leaving fixed negatively ions
(acceptors) in the p region. The electric field created by the space charge region opposes the diffusion process for both electrons and holes. This charge migration occurs until equilibrium is achieved. There are two opposing forces: the diffusion process and the electric field generated by the space charge that tends to counteract the diffusion. The charge concentration distribution at equilibrium is shown below with blue and red lines. Also shown are the two counterbalancing phenomena that establish equilibrium. The regions nearby the p–n interfaces lose their neutrality and become charged, forming a space charge region or depletion layer.
The magnitude of the Electric field potential (or voltage) created is determined by the P N junction material. This voltage is commonly referred to as the Barrier Potential.
For Silicon based materials the Barrier Potential is approximately 0.7 V.
For Germanium based materials the Barrier Potential is approximately 0.3 V.

Forward Bias
A P N junction is said to be Forward Biased when a voltage is applied to so that the positive terminal of the battery is connected to the P material.
In the left hand drawing the Depletion Region shrinks, but the battery voltage is not large enough to overcome the Barrier Potential and no current flows in the PN junction. When the battery voltage reaches the Barrier Potential (right hand drawing) the Depletion Region disappears and current now flows in the junction. When current flows the PN junction is said to be Forward Biased.
Reverse Bias
A P N junction is said to be Reverse Biased when a voltage is applied to so that the positive terminal of the battery is connected to the N material. The battery voltage has the effect of widening the Depletion
Region, increases the barrier voltage, the resistance of the depletion region increases with the result that for Reverse Bias no current flows in the P N junction.
Semiconductor Diode

A semiconductor diode (or just a diode) is a real electronic device constructed from a P N junction. The diagram below shows the P N junction, the schematic symbol of a diode, and a physical image of a typical diode. The diode has two terminals called the Anode and the Cathode.
Images of Diodes
Question
Identify the Anode and Cathode of these diodes.
Forward Bias
As with the P N junction, the diode can be connected in Forward Bias. Forward bias means that the voltage connected to the Anode is more positive than the voltage at the Cathode. In forward bias the diode conducts (turns on) and current flows in the circuit.

Question:
For the circuit shown, determine the voltage drop on the diode and the resistor, and the current flowing in the resis
V
D
=
I
R
= I
V
D
R
=
=
What is the purpose of the resistor in this circuit?
Conclusion: The Forward Biased diode behaves like a closed switch.
Reverse Bias
Reverse bias means that the voltage connected to the Anode is more negative than the voltage at the
Cathode. In reverse bias the diode is Off and no current flows in the circuit.
Question:
For the circuit shown, determine the voltage drop on the diode and the resistor, and the current flowing in the resis
V
R
=
I
R
= I
V
D
D
=
=

Conclusion: The Reverse Biased diode behaves like an open switch.
Exercise
Identify whether the diodes show are forward or reverse biased.
View the Silicon Diode Basics Learning Object
Diode Characteristic Curve
Recall that Ohm’s Law can be represented graphically as shown. The linear relation between current I and voltage V indicates that the resistance is a constant value.

The current I versus voltage V graph for a diode is not linear. The graph above shows the characteristics for both Silicon and Germanium diodes. The graph is divided into 2 areas – one for forward bias and one for reverse bias. For forward bias conduction (current flows) starts at about 0.3 V for germanium and
0.7 V for Silicon. In reverse bias ideally the current is zero but the graph indicates that there is a very small leakage current with values of a few µA. If the reverse bias voltage is large enough to exceed the reverse breakdown voltage of the diode then a large current will flow and the diode will be destroyed.
Diode Resistance
From Ohm’s Law we know that R = V/I. If we relate this to the graph we see that the slope of the diode curve is I/V so the conclusion is that the diode resistance R
D
= 1/Slope. There are two points labeled A and B on the graph.
Questions
1.
At point A is the slope of the graph low or high? low or high?
2.
At point B is the slope of the graph low or high? low or high?
At point A is the resistance of the diode
At point B is the resistance of the diode
3.
Calculate the resistance of the diode at points A and B.
Point A: R
D
= 1 V/5 mA = 200 Ω Point B: R
D
= 3 V/50 mA = 60 Ω
Conclusion
The resistance of a diode is not constant but decreases as the current through the diode increases - R
D
α I/I
D
. The diode resistance is often referred to as a dynamic resistance. The diode resistance for reverse bias is, as can be seen, very large.

Diode Data Sheets
Consult the data sheet for the 1N 4XXX series of diodes and find the following information. These are the common diodes used in the lab.
Is the data sheet for one diode or a family of diodes? ____________________
Average Rectified Output Current (Average forward current) = ___________
Peak Repetitive Reverse Voltage (Breakdown voltage) = ______________
Peak Reverse Current (Leakage current) = ________________
Is the Peak Reverse Current (Leakage current) temperature dependent? ____________
Diode Testing
Diodes can be tested using Ohmmeters. When set to Ohms scales the leads of an ohmmeter have a voltage across then that can used to test the diode. Two tests must be performed for a diode. The diagram below shows that the red lead has a positive voltage and is connected to the anode of the diode. This positive voltage forward biases the diode.

In the next diagram the red positive lead of the ohmmeter is connected to the cathode of the diode.
This is reverse bias and the meter indicates a very large (or infinite) resistance.
Both of these tests must be done to check the functionality of the diode.
Using an Analog Meter
The diagrams below indicate testing the diode with an analog meter.
Diode Models
View the Diode Approximations Learning Object