PN Connected Diode / PN Junction

PN Connected Diode is created by fusing/bonding two P-type and N-type semiconductor materials.

In the previous lesson, we saw how to make an N-type semiconductor material by adding a silicon atom with a small amount of Antimony, as well as how to make a P-type semiconductor material by adding boron and another silicon atom.

It's fine if we've understood everything so far, but these new additive N-type and P-type semiconductor materials do very little on their own because they are electrically neutral.However, if we combine (or fuse) these two semiconductor materials, they behave in a very different way, merge and produce what is often known as the " PN Connection ".

When N-type semiconductor and P-type semiconductor materials are combined for the first time, a very large density gradient is formed between both sides of the PN connection.The result is that some free electrons from transmitter impurities atoms begin to migrate through this newly created connection to fill holes in P-type material that produces negative ions.

Since electrons are transported along the P-type silicon, N-type silicon PN fusion site, however, they combine in opposite directions to the region where there are positively charged transmitter ions (on the ND negative side) and a large number of free electrons in the receiver-inlet holes just opposite.

As a result, p-type load density is negatively charged receiver ions (NA is filled) throughout the connection and N-type load density is positive along the junction points.This load transfer of electrons and holes throughout the PN connection is known as diffusion.The width of these layers P and N depends on how intensively each side contributes, with receiver density NA and donor density ND respectively.

This process continues back and forth until the number of electrons passing through the connection has an electrical charge large enough to prevent or prevent more load carriers from passing through the connection.Eventually, a state of equilibrium (electrically neutral state) will occur, producing a "potential barrier" zone around the connection area, as the transmitter atoms push through the holes and the receiving atoms push the electrons.

Since no free load carrier can stand in a position where there is a potential obstacle, the zones on both sides of the connection now lack entirely more free carriers than N and P type materials further away from the connection. This area around the PN connection is now called the Depletion Layer, or Depletion Layer.

pn linked diode
PN Connection

The total load on both sides of a PN Connection must be equal and opposite to ensure a neutral charging condition around the connection.If the depletion layer zone has a distance, therefore it is necessary for the Dp positive side, which must penetrate into the silicon with a distance, and a distance Dn : to give a relationship between the two, the negative side is also called this balance to maintain dp * N A = Dn*N D load neutrality.

PN Connected Diode Distance

pn linked diode

Since the N-type material loses electrons and loses P-type holes, the N-type material has become positive according to type P.Then, the presence of impurity ions on both sides of the connection leads to the establishment of an electric field along this region, which is at a positive voltage relative to the N-side P side.Now the problem is that a free charge requires extra energy to overcome the currently existing obstacle in order to cross the junction of the depletion zone.

Created by the diffusion process, this electric field created a "built-in potential difference" with the potential for an open circuit (zero deviation) along the connection:

pn linked diode

Where: E is that zero-sided connection voltage, V T is a thermal voltage of 26mV at room temperature, N D and N A are impurities concentrations and n is intrinsic concentration.

A suitable positive voltage (forward polar) applied between the two ends of the PN connection can provide extra energy to free electrons and holes.The external voltage required to overcome this potential obstacle that currently exists depends very much on the type and actual temperature of the semiconductor material used.

Typically at room temperature, the voltage for silicon along the depletion layer is approximately 0.6 – 0.7 volts and for germanyum it is about 0.3 – 0.35 volts.This potential obstacle will always exist, as seen in the diodes, even if the device is not connected to any external power source.

The importance of this built-in potential throughout the connection is that it opposes the flow of both holes and electrons throughout the connection and is therefore called a potential barrier.In practice, instead of simply merging or merging two separate parts, a PN connection is created in a single material crystal.

The result of this process is that the PN connection has rectifier current-voltage (IV or I–V) properties.Electrical contacts are fused to both sides of the semiconductor so that an electrical connection can be made to an external circuit.The resulting electronic device is generally called PN connection Diode or Signal Diode only.

Then we saw here that a PN connection can be made by combining or distributing different additive semiconductor materials to produce an electronic device called diode, which can be used as the basic semiconductor structure of rectifiers, all kinds of transistors, LEDs, solar cells.

Features of PN-Linked Diode

When a p-type semiconductor is fused to an n-type semiconductor, a PN-connection diode is formed, creating a potential barrier voltage along the diode connection.

A PN Connection Diode is one of the simplest semiconductor devices and has the ability to pass current in only one direction.However, unlike a resistance, since the diode has an exponential current-voltage (IV) relationship, the diode does not behave linearly according to the applied voltage, and therefore we cannot define its operation using an equation such as the Ohm law.

If a suitable positive voltage (forward polarization) is applied between the two ends of the PN connection, it can provide free electrons and holes with the extra energy they need to pass the connection as the width of the exhaustion layer around the PN connection decreases.

Applying a negative voltage (reverse polarization) causes free loads to be pulled out of the connection, which leads to increased width of the exhaustion layer.This has the effect of increasing or reducing the effective resistance of the connection itself and allows or blocks the flow of the current along the pn-connection of the diodes.

Next, the exhaustion layer expands with an increase in the reverse voltage application and shrinks with an increase in the application of forward voltage.This is due to physical changes that occur due to differences in electrical properties on both sides of the PN connection.One of the results produces straightening, as seen in the static IV (current-voltage) properties of PN connection diodes.The direction is indicated by an asymmetric current flow when the polarity of the pre-recess voltage is changed as shown below.

PN Diode Symbol and Static IV Characteristics

pn linked diode

There are two working zones and three possible "polarization" conditions for the standard PN Connected Diode, and these are:

  • 1. Zero Polarization – External voltage potential is not applied to the PN connection diode.
  • 2. Reverse Polarization – Voltage potential is connected to the P-type material negative, (-and) and positive, (+and) N type material throughout the diode, which has the effect of increasing the width of the PN connection diode.
  • 3. Advanced Polarization – Voltage potential is positive, (+and) connects to type P material and negative, (and) N-type material along the diode, which has the effect of reducing the width of PN connection diodes.

Zero Polar/Prerequisced PN-Connected Diode

When a diode is connected in a zero-polarity condition, external potential energy is not applied to the PN connection.But if the diode terminals short-circuit each other, several holes (majority carriers) in the P-type material with enough energy to overcome the potential barrier will move along the connection against this barrier potential.This is known as "Forward/Direct Current" and is called IF.

Likewise, the holes formed in the N-type material (minority carriers) find this suitable and move in the opposite direction throughout the connection.This is known as "Reverse Current" and is called IR. The back and forth transfer of electrons and holes along the PN connection is known as diffusion, as shown below.

pn linked diode
Zero Polar PN Connected Diode

The potential obstacle that currently exists prevents more majority carriers from spreading along the intersection.However, the potential barrier helps minority carriers (several free electrons in the P-zone and several holes in the N-zone) slide along the connection.

Then, a "Balance" or balance will be established when the majority carriers are equal and both move in opposite directions, so that zero current flows in the net result circuit.When this happens, it is said that the connection is in the state of " Dynamic Balance ".

Minority carriers are constantly produced due to thermal energy, so this state of equilibrium can deteriorate by raising the temperature of the PN connection, which leads to an increase in the formation of minority carriers, thereby causing an increase in leakage current. Since no circuits are connected to the PN connection, an electric current cannot flow.

Reverse Polar PN Connected Diode

When a diode is connected in case of reverse polarization, positive voltage is applied to the N type material and negative voltage is applied to the P-type material.

The positive voltage applied to the N-type material draws the electrons towards the positive electron and away from the port, while the holes at the P-type end are pulled from the port towards the negative electrode.

The clear result is that the depletion layer grows further due to the lack of electrons and holes and offers a high impedance path, being almost an insulator and creating a high potential barrier along the connection, thereby preventing the current from flowing through the semiconductor material.

pn linked diode
Increase in Depletion Layer Due to Reverse Polarization

This represents a high resistance value to the PN connection and practically zero current flows through the connection diode with an increase in pre-voltage.However, a very small reverse leakage current flows from the connection, which can normally be measured in micro amps (μA).

At a final point, if the reverse polarization voltage applied to the diode is raised to a high enough value in Vr, it will cause the PN connection of the diode to overheat and malfunction due to the avalanche effect around the connection.This can cause the diode to short-circuit and cause maximum circuit current flow, which is indicated as a downward slope in the reverse static characteristic curve below.

Inverse Characteristic Curve of PN-Linked Diode

pn linked diode

Sometimes this avalanche effect has practical applications in voltage stabilized circuits where a series of limitation resistance with diode is used to limit this reverse breakage current to a predetermined maximum value, thereby producing a constant voltage output along the diode.Such diodes are often known as Zener Diodes and will be discussed in a later course.

Advanced Polar PN-Connected Diode

In case of advanced polarization, when a diode is connected, negative voltage is applied to the N type material and positive voltage is applied to the P type material.If this external voltage is greater than the value of the potential barrier, 0.7 volts for silicon and 0.3 volts for the germanium, potential obstacles will be overcome and the current will begin to flow.

This is because the negative voltage pushes the electrons towards the junction point, giving them energy to merge with holes pushed in the opposite direction by diagonal and positive voltage towards the merger.This results in a characteristic curve of zero current flowing up to this voltage point, called "elbow" in static curves, and then a high current flows through the diode with little increase in external voltage, as shown below.

Advanced Characteristic Curve for PN-Linked Diode

pn linked diode

Applying the forward polarity voltage to the connection diode causes the exhaustion layer to be very thin and narrow, which represents the low impedance path along the connection, thereby allowing high currents to flow.The point at which this sudden increase in current occurs is shown as the "elbow" point in the static IV characteristic curve above.

pn linked diode
Decrease in Depletion Layer Due to Forward Deviation

This represents the low resistance path through the PN connection, which allows very large currents to flow through the diode with only a small increase in pre-voltage.The real potential difference throughout the connection or diode is kept constant at about 0.3v for germanyum and about 0.7v for silicon connection diodes with the effect of the depletion layer.

Since the diode can transmit "infinite" current above this elbow point, it effectively becomes a short circuit, so resistances are used serially with the diode to limit the flow of current.Exceeding the maximum forward current specification causes the device to consume more heat-shaped power than is designed, causing the device to malfunction very quickly.


The connection area of the PN Connection Diode has the following important features:

  • Semiconductors contain two types of load carriers, "Holes" and "Electrons".
  • While electrons are negatively charged, holes are positively charged.
  • A semiconductor can be contributed by donor impurities such as Antimony (N-type additives), so that it contains moving loads, which are mainly electrons.
  • A semiconductor can be contributed by receiving impurities such as Boron (P-type additive), so that it contains moving loads, which are mainly holes.
  • The connection zone itself does not have a load carrier and is known as a depletion zone.
  • The connection (depletion) zone has a physical thickness that changes with the applied voltage.
  • When a diode is zero-polar, no external energy sources are applied, and a natural Potential Barrier is developed along a depletion layer, which is about 0.5 to 0.7v for silicon diodes and about 0.3 volts for germanium diodes.
  • When a connection diode is forward-polarized, the thickness of the depletion zone decreases, and the diode acts as a short circuit that allows the full-circuit current to flow.
  • When a connection diode is inverted, the thickness of the depletion zone increases, and the diode acts as an open circuit that blocks any current flow (only a very small leakage current will flow).

Above we also found that the diode is a nonlinear device with two terminals that depend on polarity depending on the polarity of the voltage applied to the IV characteristic, VD diode or forward polarity , V D > 0 or Reverse Tilt , V D  < 0 .In both cases, we can model these current-voltage properties for both an ideal diode and a real silicon diode as shown:

PN Linked Diode Ideal and Actual Characteristics

pn linked diode

In the next lesson on diodes, we will look at the small signal diode, sometimes called switching diode, which is used in general electronic circuits.As the name suggests, the signal diode is designed for low voltage or high frequency signal applications such as radio or digital switching circuits.

Signal diodes such as 1N4148 pass very small electrical currents, unlike high-current main pointing diodes, where silicon diodes are usually used.In addition, in the next lesson, we will examine the characteristic curve and parameters of the Signal Diode static current-voltage.