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Explore the World of Semiconductors with this Class 12 Project on Diodes and Transistors


Project on Semiconductor Devices for Class 12 PDF 191




Are you interested in learning more about the fascinating world of semiconductors and how they are used in various electronic devices? Do you want to do a hands-on project that will help you understand the basic concepts of semiconductor physics and their applications? If yes, then this article is for you.




project on semiconductor devices for class 12 pdf 191



In this article, we will explain what semiconductors are and why they are important for modern electronics. We will also discuss how semiconductors are classified into different types based on their electrical properties. Then, we will introduce some common semiconductor devices such as junction diodes, bipolar junction transistors, logic gates and digital circuits. We will also show you how to do a simple project on semiconductor devices for class 12 using some easily available materials. Finally, we will conclude with a summary of the main points and some future scope and challenges in this field.


What are semiconductors and why are they important?




Semiconductors are materials that have electrical conductivity between that of metals and insulators. Metals have very low resistivity (or high conductivity) and allow electric current to flow easily through them. Insulators have very high resistivity (or low conductivity) and do not allow electric current to flow through them. Semiconductors have intermediate resistivity (or conductivity) that can be changed by applying external factors such as light, heat, electric field or magnetic field.


Semiconductors are important because they are the basic building blocks of all electronic devices. By controlling the flow of electrons (or charge carriers) in semiconductors, we can create devices that can amplify, switch, store or process signals. Some examples of electronic devices that use semiconductors are radio, computers, mobile phones, solar cells, LEDs, lasers, sensors and many more.


How are semiconductors classified?




Semiconductors can be classified into two main categories based on their atomic structure: elemental semiconductors and compound semiconductors.



  • Elemental semiconductors are made of single elements from group IV of the periodic table such as silicon (Si), germanium (Ge) or carbon (C).



  • Compound semiconductors are made of two or more elements from groups III-V or II-VI of the periodic table such as gallium arsenide (GaAs), indium phosphide (InP) or cadmium sulfide (CdS).



Elemental semiconductors have a simple crystal structure with four valence electrons in each atom forming covalent bonds with four neighboring atoms. Compound semiconductors have a more complex crystal structure with different numbers of valence electrons in each atom forming covalent bonds with different neighboring atoms.


Basic Concepts of Semiconductor Physics




To understand how semiconductor devices work, we need to know some basic concepts of semiconductor physics such as energy bands, band gap, intrinsic and extrinsic semiconductors, doping, impurities, charge carriers and conductivity.


Energy bands and band gap




In a solid material, the electrons in each atom can occupy different energy levels depending on their distance from the nucleus. The energy levels are discrete and quantized, meaning that the electrons can only have certain values of energy and not any value in between. When many atoms are brought together to form a solid, the energy levels of each atom interact with each other and form continuous bands of energy. The electrons in a solid can only occupy the energy bands and not the gaps between them.


The energy bands are divided into two main types: valence band and conduction band. The valence band is the highest occupied energy band and contains the valence electrons that are involved in bonding with other atoms. The conduction band is the lowest unoccupied energy band and contains the free electrons that can move through the material and conduct electric current. The gap between the valence band and the conduction band is called the band gap. The size of the band gap determines the electrical properties of the material.


In metals, the valence band and the conduction band overlap, meaning that there is no band gap. This allows the electrons to move freely from one band to another and conduct electric current easily. In insulators, the valence band and the conduction band are separated by a large band gap, meaning that there is a large amount of energy required for the electrons to jump from one band to another. This prevents the electrons from moving freely and conducting electric current. In semiconductors, the valence band and the conduction band are separated by a small band gap, meaning that there is a small amount of energy required for the electrons to jump from one band to another. This allows the electrons to move with some difficulty and conduct electric current moderately.


Intrinsic and extrinsic semiconductors




An intrinsic semiconductor is a pure semiconductor that has no impurities or defects in its crystal structure. In an intrinsic semiconductor, the number of electrons in the conduction band is equal to the number of holes in the valence band. A hole is a vacant position in the valence band that can be occupied by an electron. The electrons and holes are created by thermal excitation, meaning that some electrons gain enough energy from heat to jump from the valence band to the conduction band, leaving behind holes in the valence band.


An extrinsic semiconductor is a semiconductor that has impurities or defects in its crystal structure. The impurities or defects are intentionally added to change the electrical properties of the semiconductor. This process is called doping. The impurities or defects act as dopants that either donate or accept electrons to or from the semiconductor.


There are two types of extrinsic semiconductors: n-type and p-type. An n-type semiconductor is a semiconductor that has more electrons than holes in its crystal structure. This is achieved by doping the semiconductor with donor impurities that have more valence electrons than the semiconductor atoms. For example, doping silicon (Si) with phosphorus (P) creates an n-type semiconductor because phosphorus has five valence electrons while silicon has four. The extra electron from phosphorus becomes free and contributes to the conduction band, while the other four electrons form covalent bonds with silicon atoms.


A p-type semiconductor is a semiconductor that has more holes than electrons in its crystal structure. This is achieved by doping the semiconductor with acceptor impurities that have less valence electrons than the semiconductor atoms. For example, doping silicon (Si) with boron (B) creates a p-type semiconductor because boron has three valence electrons while silicon has four. The missing electron from boron creates a hole that contributes to the valence band, while the other three electrons form covalent bonds with silicon atoms.


Charge carriers and conductivity




The charge carriers in a semiconductor are the electrons and holes that can move through the material and conduct electric current. The movement of charge carriers is influenced by two factors: applied electric field and thermal motion.


When an electric field is applied to a semiconductor, it exerts a force on the charge carriers and causes them to drift in opposite directions. The direction of drift depends on the type and sign of charge carriers. Electrons drift from negative to positive terminal, while holes drift from positive to negative terminal. The drift velocity of charge carriers is proportional to the applied electric field.


Semiconductor Devices and Their Applications




Semiconductor devices are electronic components that use the special electrical characteristics of semiconductors to generate, control, receive, transform, and amplify signals, and convert energy. They are widely used because of their compactness, reliability, power efficiency, and low cost. They have different applications such as power devices, optical sensors, and light emitters, including solid-state lasers. Some common semiconductor devices are junction diodes, bipolar junction transistors, logic gates and digital circuits.


Junction diodes




A junction diode is a device that consists of a p-n junction that allows electric current to flow only in one direction. When the diode is forward biased, meaning that the positive terminal of the battery is connected to the p-type semiconductor and the negative terminal to the n-type semiconductor, the depletion region becomes narrow and the electric field across it decreases. This allows the charge carriers to cross the junction and conduct electric current. When the diode is reverse biased, meaning that the positive terminal of the battery is connected to the n-type semiconductor and the negative terminal to the p-type semiconductor, the depletion region becomes wide and the electric field across it increases. This prevents the charge carriers from crossing the junction and blocks electric current.


Junction diodes have many applications in electronics. They can be used as rectifiers, which convert alternating current (AC) to direct current (DC), as voltage regulators, which maintain a constant output voltage despite variations in input voltage or load current, as switches, which turn on or off a circuit depending on the polarity of the input voltage, and as detectors, which convert an alternating signal into a direct signal that can be measured or amplified.


Bipolar junction transistors




A bipolar junction transistor (BJT) is a device that consists of two p-n junctions connected back-to-back. There are two types of BJTs: npn and pnp. An npn transistor has two n-type semiconductors separated by a thin layer of p-type semiconductor. A pnp transistor has two p-type semiconductors separated by a thin layer of n-type semiconductor. The three regions of a BJT are called emitter, base, and collector. The emitter is the region where charge carriers are injected into the base. The base is the region where a small fraction of charge carriers are controlled by an external voltage or current. The collector is the region where most of the charge carriers are collected by an external voltage or current.


BJTs have three modes of operation: cut-off, active, and saturation. In cut-off mode, both junctions are reverse biased and no current flows through the transistor. In active mode, one junction is forward biased and the other is reverse biased and a small base current controls a large collector current. In saturation mode, both junctions are forward biased and the collector current reaches its maximum value.


BJTs have many applications in electronics. They can be used as amplifiers, which increase the strength of a weak signal, as switches, which turn on or off a circuit depending on the input signal, as oscillators, which generate periodic signals of a desired frequency and amplitude, and as logic gates, which perform basic logical operations such as AND, OR, NOT.


Logic gates and digital circuits




A logic gate is a device that performs a basic logical operation on one or more binary inputs and produces a binary output. A binary input or output can have only two values: 0 or 1, which represent low or high voltage levels respectively. There are seven basic logic gates: AND, OR, NOT, NAND (NOT-AND), NOR (NOT-OR), XOR (exclusive-OR), and XNOR (exclusive-NOR). Each logic gate has a symbol and a truth table that shows its output for every possible combination of inputs.


A digital circuit is a circuit that consists of logic gates connected in a certain way to perform a specific function. Digital circuits can be classified into two types: combinational and sequential. A combinational circuit is a circuit whose output depends only on its present inputs. A sequential circuit is a circuit whose output depends not only on its present inputs but also on its past inputs or outputs.


Logic gates and digital circuits have many applications in electronics. They can be used to design and implement arithmetic and logic units, which perform arithmetic and logical operations on binary numbers, memory units, which store and retrieve binary data, registers, which hold binary data temporarily, counters, which count the number of pulses in a signal, and microprocessors, which are the central processing units of computers.


How to Do a Project on Semiconductor Devices for Class 12




If you want to do a project on semiconductor devices for class 12, you can follow these steps:


Objectives and materials




The objectives of this project are to:



  • Understand the working principle of a junction diode and a BJT.



  • Measure the current-voltage characteristics of a junction diode and a BJT.



  • Build and test a simple rectifier circuit using a junction diode.



  • Build and test a simple amplifier circuit using a BJT.



The materials required for this project are:



  • A junction diode (preferably silicon).



  • A BJT (preferably npn).



  • A multimeter.



  • A breadboard.



  • A variable power supply.



  • Some resistors and capacitors.



  • Some wires and connectors.



Procedure and observations




The procedure and observations for this project are as follows:



  • Connect the junction diode to the multimeter in the resistance mode. Measure the resistance of the diode in both forward and reverse directions. Note down the readings. Observe that the resistance of the diode is low in forward direction and high in reverse direction.



  • Connect the junction diode to the variable power supply in series with a resistor. Vary the input voltage from 0 V to 5 V in steps of 0.5 V. Measure the voltage across the diode and the current through it. Note down the readings. Plot a graph of voltage versus current for the diode. Observe that the current through the diode is negligible until a certain threshold voltage is reached, after which it increases rapidly with increasing voltage. This threshold voltage is called the cut-in voltage or the knee voltage of the diode.



  • Connect the BJT to the multimeter in the resistance mode. Measure the resistance between each pair of terminals: emitter-base, base-collector, and emitter-collector. Note down the readings. Observe that the resistance between emitter-base is low when forward biased and high when reverse biased, while the resistance between base-collector and emitter-collector is high when forward biased and low when reverse biased.



  • Connect the BJT to the variable power supply in series with two resistors: one between base and ground, and one between collector and positive terminal. Vary the input voltage from 0 V to 5 V in steps of 0.5 V. Measure the base current, collector current, and collector-emitter voltage. Note down the readings. Plot a graph of collector current versus collector-emitter voltage for different values of base current. Observe that the collector current is proportional to the base current for a given collector-emitter voltage, while it is almost constant for a given base current when the collector-emitter voltage is above a certain value. This value is called the saturation voltage of the transistor.



the circuit. Observe the input and output waveforms on the oscilloscope or the voltmeter. Note down the peak and average values of the output voltage. Observe that the half-wave rectifier circuit converts the AC input voltage into a pulsating DC output voltage by eliminating the negative half cycle of the input waveform.


  • Connect two junction diodes, two resistors, and a capacitor in a bridge configuration to form a full-wave rectifier circuit. Connect an AC source (such as a transformer or an oscillator) to one pair of opposite terminals of the bridge and an oscilloscope or a voltmeter to another pair of opposite terminals of the bridge. Observe the input and output waveforms on the oscilloscope or the voltmeter. Note down the peak and average values of the output voltage. Observe that the full-wave rectifier circuit converts the AC input voltage into a pulsating DC output voltage by using both half cycles of the input waveform.



  • Connect a BJT, two resistors, and a capacitor in a common-emitter configuration to form a simple amplifier circuit. Connect an AC source (such as an oscillator or a microphone) to the base terminal of the transistor and an oscilloscope or a voltmeter to the collector terminal of the transistor. Observe the input and output waveforms on the oscilloscope or the voltmeter. Note down the peak and average values of the input and output voltages. Observe that the amplifier circuit increases the strength of the input signal by using the base current to control the collector current.



Results and analysis




The results and analysis for this project are as follows:



  • The resistance of a junction diode is low when forward biased and high when reverse biased.



  • The cut-in voltage or knee voltage of a junction diode is about 0.7 V for silicon and 0.3 V for germanium.



  • The current-voltage characteristic of a junction diode is nonlinear and follows the equation: $$I = I_0 (e^V/V_T - 1)$$ where $$I$$ is the diode current, $$I_0$$ is the reverse saturation current, $$V$$ is the diode voltage, and $$V_T$$ is the thermal voltage.



  • The resistance between emitter-base of a BJT is low when forward biased and high when reverse biased, while the resistance between base-collector and emitter-collector is high when forward biased and low when reverse biased.



  • The collector current of a BJT is proportional to the base current for a given collector-emitter voltage, while it is almost constant for a given base current when the collector-emitter voltage is above saturation voltage.



the base current.


  • The voltage gain of a BJT is defined as: $$A_v = \fracV_oV_i = -\beta \fracR_Cr_e$$ where $$A_v$$ is the voltage gain, $$V_o$$ is the output voltage, $$V_i$$ is the input voltage, $$R_C$$ is the collector resistance, and $$r_e$$ is the dynamic emitter resistance.



  • The half-wave rectifier circuit converts the AC input voltage into a pulsating DC output voltage by eliminating the negative half cycle of the input waveform.



  • The peak value of the output voltage of a half-wave rectifier is equal to the peak value of the input voltage minus the cut-in voltage of the diode.



  • The average value of the output voltage of a half-wave rectifier is given by: $$V_DC = \frac1\pi V_m - \frac2\pi V_\gamma$$ where $$V_DC$$ is the average output voltage, $$V_m$$ is the peak input voltage, and $$V_\gamma$$ is the cut-in voltage of the diode.



  • The full-wave rectifier circuit converts the AC input voltage into a pulsating DC output voltage by using both half cycles of the input waveform.



  • The peak value of the output voltage of a full-wave rectifier is equal to the peak value of the input voltage minus twice the cut-in voltage of the diode.



  • The average value of the output voltage of a full-wave rectifier is given by: $$V_DC = \frac2\pi V_m - \frac4\pi V_\gamma$$ where $$V_DC$$ is the average output voltage, $$V_m$$ is the peak input voltage, and $$V_\gamma$$ is the cut-in voltage of the diode.



  • The simple amplifier circuit increases the strength of the input signal by using the base current to control the collector current.



the current gain, and $$R_E$$ is the emitter resistance.


The output impedance of a commo


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