Pn Junction – Class 12 | Chapter – 14 | Physics Short Notes Series PDF for NEET & JEE
Pn Junction: A pn junction is a type of junction between a p-type and an n-type semiconductor material, forming a region where the properties of the semiconductor change abruptly. The interface between the p-type and n-type materials is called the depletion region, which is a region that contains no mobile charge carriers, such as holes or electrons.
When a pn junction is formed, electrons from the n-type material diffuse across the junction into the p-type material, where they recombine with holes. At the same time, holes from the p-type material diffuse across the junction into the n-type material, where they recombine with electrons. This creates a region near the junction that is depleted of mobile carriers and has a fixed electric field, which acts to repel further diffusion of carriers across the junction.
The depletion region acts as a barrier to the flow of current in the reverse bias direction, meaning that when a negative voltage is applied to the p-type material and a positive voltage is applied to the n-type material, the depletion region widens and the junction becomes more resistant to current flow. However, when a positive voltage is applied to the p-type material and a negative voltage is applied to the n-type material, the depletion region narrows and the junction becomes less resistant to current flow.
The pn junction has several important properties that make it useful in a wide range of electronic devices. One of the most important properties is its rectification behavior, meaning that it allows current to flow in only one direction. This property is used in diodes, which are electronic components that allow current to flow in one direction while blocking it in the opposite direction. Another important property of the p-n junction is its ability to emit light when current flows through it, which is used in light-emitting diodes (LEDs) and laser diodes.
Formation of Pn Junction
A pn junction is formed by bringing together a p-type and an n-type semiconductor material. The process of forming a p-n junction typically involves two main steps: doping and diffusion.
Doping is the process of intentionally adding impurities to a semiconductor material to change its electrical properties. In the case of a p-n junction, the p-type material is doped with a trivalent impurity such as boron, which has one less valence electron than the silicon atoms in the semiconductor lattice. This creates a “hole” in the lattice where an electron is missing, resulting in a surplus of positively charged holes in the material.
The n-type material is doped with a pentavalent impurity such as phosphorus, which has one more valence electron than the silicon atoms in the semiconductor lattice. This creates a surplus of negatively charged electrons in the material.
Once the p-type and n-type materials have been doped, they are brought together to form a p-n junction. At the interface between the p-type and n-type materials, the surplus electrons from the n-type material diffuse into the p-type material, where they recombine with the holes. Similarly, the surplus holes from the p-type material diffuse into the n-type material, where they recombine with the electrons. This diffusion process creates a region near the junction that is depleted of mobile charge carriers, creating a fixed electric field that acts to repel further diffusion of carriers across the junction.
The resulting p-n junction has rectifying behavior, meaning that it allows current to flow in only one direction. When a positive voltage is applied to the p-type material and a negative voltage is applied to the n-type material, the junction becomes forward-biased, and current flows easily through the junction. However, when a negative voltage is applied to the p-type material and a positive voltage is applied to the n-type material, the junction becomes reverse-biased, and current is blocked by the depletion region.
Formulas of Pn Junction
There are several formulas that describe the behavior of a pn junction. Here are a few of the most important ones:
- Depletion region width: The width of the depletion region, also known as the space charge region, is given by the following formula:W = √((2εsVbi)/((1/Na)+(1/Nd)))where:
- W is the width of the depletion region
- εs is the permittivity of the semiconductor material
- Vbi is the built-in potential of the p-n junction
- Na and Nd are the acceptor and donor impurity concentrations in the p-type and n-type materials, respectively.
- Built-in potential: The built-in potential of a pn junction, which is the potential difference across the junction in thermal equilibrium, is given by the following formula: Vbi = (kB*T/q)ln(NaNd/ni2) Where:
- kB is the Boltzmann constant
- T is the temperature in kelvins
- q is the elementary charge
- Na and Nd are the acceptor and donor impurity concentrations in the p-type and n-type materials, respectively
- ni is the intrinsic carrier concentration of the semiconductor material.
- Current-voltage relationship: The current-voltage relationship of a pn junction can be described by the Shockley diode equation, which relates the current through the junction to the applied voltage: I = Is*(exp(qV/(kBT))-1), Where:
- I is the current through the junction
- Is is the reverse saturation current
- V is the applied voltage across the junction
- kB is the Boltzmann constant
- T is the temperature in kelvins
- q is the elementary charge.
These formulas are important for understanding the behavior of p-n junctions and designing electronic devices that use them.
Applications of Pn Junction
Pn junctions are used in a wide range of electronic devices, including:
- Diodes: Pn junction diodes are one of the most basic electronic components, used to control the flow of electrical current in a circuit. They are widely used in rectifiers, voltage regulators, and signal detectors.
- Solar cells: Solar cells use p-n junctions to convert sunlight into electrical energy. When photons from the sun strike the junction, they can create electron-hole pairs, which can then be separated by the electric field of the junction and collected as electrical current.
- Transistors: Bipolar junction transistors (BJTs) use two p-n junctions to control the flow of current through the device. By applying a small current or voltage to one junction, the larger current or voltage flowing through the other junction can be controlled.
- Light-emitting diodes (LEDs): LEDs are p-n junctions that emit light when a current is applied. The color of the light emitted depends on the materials used to make the junction.
- Integrated circuits: Pn junctions are used in the fabrication of integrated circuits, which are the building blocks of modern electronics. By creating complex networks of p-n junctions on a single chip, a wide range of electronic functions can be achieved in a small and efficient package.
- Thermoelectric generators: Pn junctions can be used in thermoelectric generators, which convert temperature differences into electrical energy. When one side of a p-n junction is heated and the other is cooled, a voltage is generated across the junction.
Characteristics of Pn Junction
The characteristics of a pn junction are determined by a variety of factors, including the doping concentration of the p-type and n-type regions, the temperature, and the applied voltage. Here are some of the key characteristics of a pn junction:
- Forward bias: When a voltage is applied across the p-n junction in the forward bias direction (positive to the p-type material and negative to the n-type material), the depletion region becomes narrower and the current through the junction increases rapidly. The voltage at which significant current begins to flow is called the forward voltage, and it typically ranges from about 0.3 to 0.7 volts depending on the materials used.
- Reverse bias: When a voltage is applied across the p-n junction in the reverse bias direction (positive to the n-type material and negative to the p-type material), the depletion region widens and the current through the junction decreases. A small amount of current can still flow due to minority carrier diffusion, but it is much smaller than the current in the forward bias direction.
- Breakdown voltage: If the reverse voltage is increased beyond a certain point, called the breakdown voltage, the electric field across the depletion region becomes strong enough to ionize the semiconductor material, creating a large amount of current flow. This can damage or destroy the junction if the current is not limited.
- Capacitance: P-n junctions have a capacitance that depends on the width of the depletion region. When a voltage is applied to the junction, the depletion region width changes, which changes the capacitance.
- Junction resistance: P-n junctions have a small resistance when forward-biased and a much larger resistance when reverse-biased. The resistance can be modeled as a diode with a voltage-dependent current.
- Thermal effects: The behavior of p-n junctions can change with temperature. The built-in potential decreases with increasing temperature, while the reverse saturation current increases. This can affect the performance of devices that use p-n junctions.
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