P-N Junction: Class 12 Physics Notes, Definition, Working Principle, Formation, Types & Examples

The defination of p-n junction as per the NCERT is “A p-n junction is the basic building block of many semiconductor devices like diodes, transistors, etc”. 

Explanation: P-N Junction is a semiconductor device that is used to make the base of electronic components such as diodes, transistors, etc.  

Importance of P-N Junction

As mentioned in the definition, p-n junction is used to make the base of electronic components. Below is the importance. 

Rectifier: P-N Junction can allow the flow of current in one direct (forward bias) and block the current in opposite direction (reverse bias). Due to this property, it is used for converting the AC (Alternate Current) to DC (Direct Current). 

Switching: As mentioned above p-n junction can allow the flow of current in one direction, it can be used as a switch in semiconductors to control the flow of current and blocking in the other end. 

Signal Processing: P-N Junction is also used in signal circuits to display the clip, rectify the signal, and or clamp. 

Form Basis of Transistors: Multiple p-n junctions are used to build the bases of transistors such as BJT and MOSFETs. 

Building Block for Integrated Circuits: Millions or billions of P-N junctions are used to build an integrated circuit (IC). Building Modern microprocessors, memory chips, and digital devices are possible because of p-n junction. 

Other Applications: 

• Solar Cells:  P-N junctions are used to convert sunlight into electricity 

• LED (Light Emitting Diodes) - When p-n junction is under forward bias, led produces light.  

• Photodiodes: Photodiodes are P-N junctions under reverse bias that convert light into electrical current.  

A p-n junctions are classified based on application, structure and operations. Below is the type of p-n junction. 

• Unbiased P-N Junctions 

• Forward biased  

• Reversed biased 

• Step-graded 

• Linearly graded 

• Heterojunction 

• Schottky junction 

Features of P-N Junctions 

Types of p-n junctions	Features
Unbiased	No external voltage
Forward biased	p-side positive, n-side negative, current flow in the same direction
Reversed biased	p-side negative, block current
Step-graded	Sudden doping change
Linearly graded	Slow doping change
Heterojunction	Two different semiconductors with different band gaps
Schottky junction	Metal semiconductor junction

P-N junction working principle is based on the movement of charge carriers from n-type (electron rich) and p-type (hole-rich) semiconductor when brought together. This interaction forms a depletion region. 

Working of P-N Junction: 

Diffusion of Charge Carriers 

• When p and n-type junction are joined, electrons from the n-type and holes from the p-type diffuse across the junction. 

• The electrons and holes are recombined near the junction due to the diffusion process. 

Formation of Depletion Region 

• The diffusion leaves behind fixed ions – negative ions on the p-side and positive ions on the n-side. 

• The immobile ions' charge forms a depletion region, which opposes further diffusion. 

Built-in Potential (Barrier Potential) 

• A potential barrier is built due to the electric field across the depletion region. 

• Due to this barrier, the flow of charge carriers across the junction is opposed. 

Behavior under external voltage: 

Forward Bias 

• In forward bias the positive side of voltage source is connected to positive terminal (anode) and negative terminal (cathode). 

• The external voltage reduces the width of depletion region. This allows the electrons and holes to flow through the junction (p to n side). 

Reverse Bias 

• In reversed bias, the positive side of external voltage source is connected to negative side and negative to positive side. 

• The voltage widens the Depletion region, due to which almost no current flows across the junction. 

• Current flow is blocked in the reverse-bias junction. 

A $p$ -n junction is not the interface between p -type and n - type semiconductor crystals pressed together. It is a single piece of semiconductor crystal having an excess of acceptor impurities into one side and of donor impurities into the other.
(b) Potential Barrier at the Junction: Formation of Depletion Region:

A p-n junction is shown in Fig. The p-type region has (positive) holes as majority charge-carriers, and an equal number of fixed negatively-charged acceptor ions. (The material as a whole is thus neutral). Similarly, the n -type region has (negative) electrons as majority charge-carriers, and an equal number of fixed positively-charged donor ions.

Electrostatic potential energy barrier for holes (potential barrier for holes)
The region on either of the junction which becomes depleted (free) of the mobile charge-carriers is called the 'depletion region'. The width of the depletion region is of the order of ${10}^{-6}\mathrm{m}$ . The potential difference developed across the depletion region is called the 'potential barrier'. It is about 0.3 volt for Ge, p-n junction and about 0.7 volt for silicon $p-n$  junction. It, however, depends upon the dopant concentration in the semiconductor.
The magnitude of the barrier electric field for a silicon junction is ${\mathrm{E}}_{\mathrm{i}}\approx \frac{\mathrm{V}}{\mathrm{d}}\approx \frac{0.7}{{10}^{-6}}=7\times {10}^5{\mathrm{V}\mathrm{m}}^{-1}$
Diffusion & Drift Current : Due to concontration difference hole try to diffuse from p side to n side but due to depletion layer only those hole are able to diffuse from p to n side which have high kinetic energy. Similarly electron of high kinetic energy also diffuse from $n$  to $p$  so diffusion current flow from p to n side.
Due to thermal collision or increase in temperature some valence electron comes in conduction band. If this occurs in depletion region then hole move towards $p$  side & electron move towards n side so a current produce from $n$  to $p$ side. It is called drift current, in steady state both diffusion & drift current are equal & opposite.

In electronic circuits, the semiconductor devices are represented by their symbols. The symbol for the basic device, the p -n junction diode, is shown

 in Fig. The arrow-head represents the $p$ -region and the bar represents the n -region of the diode.
The direction of the arrow is from p to n and indicates the direction of conventional current flow under forward bias. The p -side is called 'anode' and the n -side is called 'cathode'.

P-N junctions are the basic of building many semiconductor devices from simple diodes to complex transistors and optoelectronic devices. Here are the examples of P-N junctions and their key role.

Example of p-n junction

Device	Role of P-N Junction
Solar Cell	Convert light to electricity
LED	Emission of light under forward bias
Photodiode	Detection of light under reverse bias
Diode	One-way current flow
Zener Diode	Voltage regulation
Tunnel Diode	Negative resistance
BJT	Amplification and switching
Varactor Diode	Variable capacitance