- P-N Junction Diode
- Diode Function
- PN junction
- Forward bias
- Reverse bias
- • Depletion layer-
- V-I Characteristics of Semiconductor P-N diode
- • Knee voltage
- • Breakdown voltage
- • Doping
- • Reverse Saturation Current
- • The Width of the Depletion Layer
- • Barrier potential/built-in potential
- Application of p-n junction diode
P-N Junction Diode
A diode is a two-terminal polarised electronic semiconductor device. One terminal is Anode(-) and the other is Cathode(+). The cathode is marked with a silver colour or colour band. This is the simplest semiconductor device but this is very important and most useful in electronic circuits.
The main function of the semiconductor diode is the flow of electrons to totally in only one direction across it. It means Diode conducts current in only one direction. so, this is a Unidirectional device. If change the polarity of diode then no current passes through it. So we can say that diode acts as a switch that allows the conduction of current in only one direction. This is the property of an Ideal Diode.
- An ideal diode acts as a short circuit with the same polarity with supply, But in Reverse connection, it acts like Open circuit means no current flows in reverse polarity.
The semiconductor Diode is formed by adding and apply a burning force or crystallization on together of P-type and n-Type materials (p-n junction). Joining of Both materials should of the same base – Ge or Si. The PN-junction is the basic root for semiconductor diodes.
PN junction is made from a single piece of semiconductor of two different properties. One side is made to be P-type material and the other side is made with N-type. Both ends of the PN-junction have different properties. One end has an excess/majority of electrons and the other end has an excess/majority of holes.
The p-type semiconductor is formed by adding trivalent impurities in pure or intrinsic semiconductor and the n-type semiconductor is formed by adding pentavalent impurities in a pure or intrinsic semiconductor.
P-type materials have majority charge carriers of holes and minority charge carriers of electrons. But in N-type materials have majority charge carriers of electrons and minority charge carriers of holes.
Half part of a-Si crystal is doped with a trivalent impurity and a half with pentavalent impurity, we get P-N junction diode. The border where p-type and n-region meet called the junction.
The free electrons move from the negative terminal (cathode) to the positive terminal (anode). But the current flow direction is from the positive terminal to the negative terminal.
If the p-terminal of a diode is connected with positive supply and n-terminal is connected with negative supply then it is called forward bias. If the diode is forward biased, it allows the electric current flow. On the other hand, if the diode is reverse biased, it blocks the electric current flow. In the Reverse bias mode, PN diode works as a short circuit and there is a flow of electric current through it.
When P- terminal of the diode is connected with the Negative terminal of supply (Battery) and the N- terminal is connected with Positive of battery, this connection is called Reverse bias connection. In this connection no Electrons flow and no current flows. Reverse bias mode acts like an Open circuit means there is No Flow of current through it.
• Depletion layer-
Depletion layer created by the initial movement of majority carrier across the junction. Holes concentration on p-type and electron concentration on n-type are very high. Due to the formation of the p-n junction by diffusion electron moves from n-type to p-type and holes move from p-type to n-type. By the combination of electrons and holes at the junction creates ions, and there are presents only ions at the junction. Ions are non- movable.
Therefore when holes pass to electrons through these ions, then +ve ions repel the holes and oppose it passing to electron, And created a resistance wall of very small width. This is called Depletion layer. Width of depletion increases in reverse bias connection. Electric field intensity is created with the depletion layer. and the sign of this electric field is negative because the direction of the electric field is +ve to -ve.
• Depletion layer consists +ve charge and –ve charge on either side of junction.
• Depletion layer opposes only the majority carrier, not minority carrier.
• The depletion layer is also called Space charge region or transition region.
• Depletion layer consists of immobile charged particles.
V-I Characteristics of Semiconductor P-N diode
• Knee voltage
This is the minimum required voltage to start the conduction of current through the diode. This is also known as cut-in voltage. This is the forward voltage at which the diode current starts increasing rapidly.
The knee voltage of si diode is 0.7 voltage and 0.3v of Ge diode.
• Breakdown voltage
This is the minimum amount of voltage of an insulator that makes it electrically conductor.
In Reverse bias connection of pn diode. no, any current flows through the diode, but when we increase the reverse voltage level continuously Then diode get internal damage (breakdown) and start conduction at a fixed level. Breakdown voltage is the Minimum Amount of reverse bias voltage at which diode starts conduction in reverse bias connection. This breakdown characteristics of diode use in Zener Diode which is always used in reverse bias and Limits the circuit voltage.
A material in pure form acts as an insulator. So making it more conductive in nature we need to add some impurity to it. The process of adding impurities in pure (intrinsic material) is called doping to change their electrical properties. Generally, trivalent and pentavalent elements are used to doping to a semiconductor. When a semiconductor is doped with a trivalent impurity(Boron, Aluminium), it becomes P-type semiconductor material. When dope any intrinsic material with Pentavalent impurity (phosphorus, arsenic, bismuth, antimony) then it becomes N-type semiconductor.
• Reverse Saturation Current
Both sides of p-n junction a very small amount of minority charge carrier present. P-type minority charge carrier in the n-type side and n-type minority charge carrier on the p-type side. It also allows to flow a minority charge current. The current which is flowing by the minority carriers is called Reverse current.
When a voltage applied on junction then further external reverse voltage flows due to minority charge carriers and this increases external voltage does not increases reverse current. At a voltage level where the current goes on a fix maximum level and after Only voltage increases and current does not increase. This is called reverse saturation current.
Reverse saturation current remains constant with the increase of voltage, But when voltage will increase continuously then at a Level of voltage when junction will get a breakdown and high reverse current will flow.
Reverse saturation current depends on temperature. If the temperature of junction increases, the minority charge carriers also increases.
• The Width of the Depletion Layer
Practically The value of Depletion width varies from 0.1µm to 0.5µm
A typical value of Depletion width is -0.5µm
By increasing doping concentration Depletion layer width can Reduced
electrons move across the PN junction from the N-type to the P-type, they leave behind positively charged donor ions on the negative side. Holes from the acceptor impurity move 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 through the junction is filled with negatively charged acceptor ions, and the charge density of the N-type along with the junction becomes positive. This charge transfer of electrons and holes across the PN junction is known as diffusion.
• Barrier potential/built-in potential
Simply Voltage form across the depletion layer is called the contact potential. The built-in potential in a semiconductor equals the potential across the depletion region in thermal equilibrium. This is also called built-in potential or barrier voltage/potential or potential hill or diffusion voltage.
Contact potential always appears within the depletion layer It is noted as V0 or Vbi.
Contact potential of any diode can not measure by any instrument.
V0 for Ge diode = 0.1v to 0.5v typically we use V0=0.2v
V0 for Si diode = 0.6v to 0.9v typically V0= 0.7v
Application of p-n junction diode
• As Rectifier to convert AC to DC signal
• Clipper circuit for clipping the signal (changing the wave shape)
• To supply ( to protection from the reverse supply)
• Clamper circuit to restore the dc signal wave
• As Voltage multiplier
• In digital Logic circuit design
• In Demodulation Circuits
Other types of diode