This documents provides an in-depth insight and introduction into diode theory. It goes into detail about the composition of a diode, the low-level materials science of how diodes are fabricated, and it provides worked examples of questions involving circuitry using diodes.
• A PN junction is formed when an N-type material is fused together with a P-type material creating a
semiconductor diode
• When the N-type semiconductor and P-type semiconductor materials are first joined together, a very large
density gradient exists between both sides of the PN junction
• The result is that some of the free electrons from the donor impurity atoms begin to migrate across this newly
formed junction to fill up the holes in the P-type material producing negative ions
• However, because the electrons have moved across the PN junction from the N-type silicon to the P-type
silicon, they leave behind positively charged donor ions (Nd) on the negative side and now the holes from the
acceptor impurity migrate across the junction in the opposite direction into the region where there are large
numbers of free electrons
• As a result, the charge density of the P-type along the junction is filled with negatively charged acceptor ions
(Na), and the charge density of the N-type along the junction becomes positive
• This charge transfer of electrons and holes across the PN junction is known as diffusion
• The width of these P and N layers depends on how heavily each side is doped with acceptor density (Na) and
donor density (Nd) respectively
• This process continues back and forth until the number of electrons which have crossed the junction have a
large enough electrical charge to repel or prevent any more charge carriers from crossing over the junction
• Eventually a state of equilibrium (electrically neutral situation) will occur producing a potential barrier zone
around the area of the junction as the donor atoms repel the holes and the acceptor atoms repel the electrons
• Since no free charge carriers can rest in a position where there is a potential barrier, the regions on either
sides of the junction now become completely depleted of any more free carriers in comparison to the N and P
type materials further away from the junction
• This area around the PN junction is now called the depletion layer
The PN Junction
• As the N type material has lost electrons and
the P type has lost holes, the N type material
has become positive with respect to the P type
• The presence of impurity ions on both sides of
the junction cause an electric field to be
established across this region with the N side
at a positive voltage relative to the P side
• The problem now is that a free charge requires
some extra energy to overcome the barrier that
now exists for it to be able to cross the
depletion layer
• This electric field created by the diffusion
process has created a ‘built-in potential
difference’ across the junction with an open-
circuit (zero bias) potential of:
Zero bias
junction Thermal voltage
voltage (26mV at room temp.) Intrinsic
Impurity
concentration
concentrations
,Overcoming the Potential Barrier
• A suitable positive voltage (forward bias) applied between the two ends of the PN junction can supply the free
electrons and holes with the extra energy needed to overcome the potential barrier
• The external voltage required to overcome this potential barrier that now exists is very much dependent upon
the type of semiconductor material used and its actual temperature
• Typically at room temperature, the voltage across the depletion layer for silicon is about 0.6 - 0.7V and for
germanium about 0.3 - 0.35V
• This potential barrier will always exist even if the device is not connected to any external power source, as
seen in diodes
• The significance of this built-in potential across the junction is that it opposes the flow of holes and electrons
across the junction and is the reason why its called the potential barrier
• In practice, a PN junction is formed within a single crystal of material rather than just simply joining or fusing
together two separate pieces
What’s the Outcome of Forming a PN Junction?
• The result of this process is that the PN junction has rectifying current-voltage (IV or I-V) characteristics
• Electrical contacts are fused onto either side of the semiconductor to enable an electrical connection to be
made to an external circuit
• The resulting electronic device is commonly called a PN junction diode or simply a signal diode
PN Junction Diode
• A PN junction diode is one of the simplest semiconductor devices around
• It has the characteristic of passing current in only one direction
• Unlike a resistor, a diode does not behave linearly with respect the the applied voltage as the diode has an
exponential current-voltage (I-V) relationship
• Because of this relationship, the operation of a diode cannot be described simply using an equation such as
Ohm’s Law
• If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it can supply
free electrons and holes with the extra energy required to cross the junction as the width of the depletion layer
around the PN junction is decreased
• By applying a negative voltage (reverse bias), the free charges are pulled away from the junction resulting in
the depletion layer width increasing
• This has the effect of increasing or decreasing the effective resistance of the junction itself allowing or
blocking the flow of current through the diode’s PN junction
• In the forward bias application, the depletion layer width decreases with a forward voltage
• In the reverse bias application, the depletion layer width increases with a negative voltage
• This is due to the differences in electrical properties on the two sides of the PN junction resulting in physical
changes taking place
• One of the results produces rectification as seen in the PN junction diodes static I-V characteristics
• Rectification is shown by an asymmetrical current flow when the polarity of the bias voltage is altered as
shown:
, Junction Diode Symbol and Static I-V Characteristics
• Before the PN junction diode can be used as a practical device, we first need to bias the junction (that is to
connect a voltage potential across it)
• On the voltage axis above, the reverse bias refers to an external voltage potential which increases the
potential barrier
• An external voltage which decreases the potential barrier is said to act in the forward bias direction
Biasing Conditions
• There are two operating regions and three possible biasing conditions for the standard junction diode:
Zero Bias - No external voltage potential is applied to the PN junction diode
Reverse Bias - The voltage potential connected is negative to the P type material and positive to the N type
material across the diode which has an effect of increasing the PN junction diode’s width
Forward Bias - The voltage potential connected is positive to the P type material and negative to the N type
material across the diode which has the effect of decreasing the PN junction diode’s width
Zero Biased Junction Diode
• When a diode is connected in a zero bias condition, no external potential energy is applied to the PN junction
• However if the diodes terminals are shorted together, a few holes (minority carriers) in the P type material with
enough energy to overcome the potential barrier will move across the junction against the barrier potential
• This is known as the forward current (IF)
• Likewise, holes generated in the N type material (minority carriers) find this situation favourable and move
across the junction in the opposite direction
• This is known as reverse current (IR)
• This transfer of electrons and holes back and forth across the PN junction is known as diffusion
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