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Semiconductor Physics  Energy Band Gap Theory It is a theory which explains the quantum state that an electron takes inside metal solid. Every molecule consists of numerous discrete energy levels. This theory explains in detail the way electrons behave inside a...

Semiconductor Physics  Energy Band Gap Theory It is a theory which explains the quantum state that an electron takes inside metal solid. Every molecule consists of numerous discrete energy levels. This theory explains in detail the way electrons behave inside a molecule. In atoms, electrons fill in respective energy orbits which follow Pauli’s exclusion principle. In molecules, two atomic orbitals combine for forming a molecular orbit with two separate energy levels. Further, in solids, 1023 stacked up lines confined in a tiny space will look like a band. Thus, it forms an energy continuum which we refer to as energy bands. With this theory, we get a very useful way of visualizing the difference between conductors, insulators and semiconductors. Energy Band Gap Theory  Energy Band Gap Theory According to Bohr’s theory, every shell of an atom contains a discrete amount of energy at different levels. Energy band theory explains the interaction of electrons between the outermost shell and the innermost shell. Based on the energy band theory, there are three different energy bands: 1. Valence band 2. Forbidden band 3. Conduction band Valence Band The electrons in the outermost shell are known as valence electrons. These valence electrons contain a series of energy levels and form an energy band known as the valence band. The valence band has the highest occupied energy. Conduction Band The valence electrons are not tightly held to the nucleus due to which a few of these valence electrons leave the outermost orbit even at room temperature and become free electrons. The free electrons conduct current in conductors and are therefore known as conduction electrons. The conduction band is one that contains conduction electrons and has the lowest occupied energy levels. Forbidden Band The gap between the valence band and the conduction band is referred to as the forbidden gap. As the name suggests, the forbidden gap doesn’t have any energy and no electrons stay in this band.  Types of electronic materials: conductors, semiconductors, and insulators Conductors- Overlap of the valence band and the conduction band so that at the valence electrons can mov through the material. Insulators- Large forbidden gap between the energies of the valence electrons and the energy at which the electrons can move freely through the material (the conduction band). Semiconductors- Have almost an empty conduction band and almost filled valence band with a very narrow energy gap (of the order of 1 eV) separating the two.  Fermi energy Fermi energy refers to the energy difference between the highest and lowest occupied single-particle states in a quantum system of non-interacting fermions at absolute zero temperature. The value of the Fermi level at absolute zero temperature (−273.15 °C) is known as the Fermi energy. It is also the maximum kinetic energy an electron can attain at 0K. Fermi Level: The highest energy level that an electron can occupy at the absolute zero temperature is known as the Fermi Level. The Fermi level lies between the valence band and conduction band because at absolute zero temperature, the electrons are all in the lowest energy state. Due to the lack of sufficient energy at 0 Kelvin, the Fermi level can be considered as the sea of fermions (or electrons) above which no electrons exist. Types of Semiconductors  Intrinsic semiconductors A Semiconductor which does not have any kind of impurities, behaves as an Insulator at 0 K and behaves as Conductor at higher temperature is known as Intrinsic Semiconductor or Pure Semiconductors. Germanium and Silicon (4th group elements) are the best examples of intrinsic semiconductors and they possess diamond cubic crystalline structure.  Carrier concentration in intrinsic semiconductors When a suitable form of Energy is supplied to a Semiconductor then electrons take transition from Valence band to Conduction band. Hence a free electron in Conduction band and simultaneously free hole in Valence band is formed. This phenomenon is known as Electron - Hole pair generation. In Intrinsic Semiconductor the Number of Conduction electrons will be equal to the Number of Vacant sites or holes in the valence band.  Extrinsic semiconductors The Extrinsic Semiconductors are those in which impurities of large quantity are present. Usually, the impurities can be either 3rd group elements or 5th group elements. Based on the impurities present in the Extrinsic Semiconductors, they are classified into two categories. 1. N-type semiconductors 2. P-type semiconductors  N-type semiconductors When any pentavalent element such as Phosphorous, Arsenic or Antimony is added to the intrinsic Semiconductor, four electrons are involved in covalent bonding with four neighboring pure Semiconductor atoms. The fifth electron is weakly bound to the parent atom. And even for lesser thermal energy it is released Leaving the parent atom positively ionized.  N-type semiconductors The Intrinsic Semiconductors doped with pentavalent impurities are called N-type Semiconductors. The energy level of fifth electron is called donor level. The donor level is close to the bottom of the conduction band most of the donor level electrons are excited in to the conduction band at room temperature and become the Majority charge carriers. Hence in N-type Semiconductors electrons are Majority carriers and holes are Minority carriers.  P-type semiconductors When a trivalent elements such as Al, Ga or Indium have three electrons in their outer most orbits, added to the intrinsic semiconductor all the three electrons of Indium are engaged in covalent bonding with the three neighboring Si atoms. Indium needs one more electron to complete its bond. this electron maybe supplied by Silicon, there by creating a vacant electron site or hole on the semiconductor atom. Indium accepts one extra electron, the energy level of this impurity atom is called acceptor level and this acceptor level lies just above the valence band. These type of trivalent impurities are called acceptor impurities and the semiconductors doped the acceptor impurities are called P-type semiconductors.  P-type semiconductors Even at relatively low temperatures, these acceptor atoms get ionized taking electrons from valence band and thus giving rise to holes in valence band for conduction. Due to ionization of acceptor atoms only holes and no electrons are created. Thus holes are more in number than electrons and hence holes are majority carriers and electros are minority carriers in P-type semiconductors.  p-n Junction A p–n junction is a combination of two types of semiconductor materials, p-type and n-type, in a single crystal. The "n" (negative) side contains freely-moving electrons, while the "p" (positive) side contains freely-moving electron holes. Connecting the two materials causes creation of a depletion region near the boundary, as the free electrons fill the available holes, which in turn allows electric current to pass through the junction only in one direction. Solar cells and light-emitting diodes (LEDs) are essentially p-n junctions where the semiconductor materials are chosen. The p-n junction can be formed by different methods. They are (i) Grown junction method, (ii) Alloying method and (iii) Diffusion method.  p-n Junction diode p–n junctions represent the simplest case of a semiconductor electronic device; a p-n junction by itself, when connected on both sides to a circuit, is a diode. In PN junction diode, N is at right and P is at left. Majority carriers N region – electrons P region -- holes  Formation of depletion layer NO external connections: The excess electrons in the N region cross the junction and combine with the excess holes in the P region. N region loses its electrons ….. becomes + vly charged 𝑃 region accepts the electrons ….. becomes -vly charged At one point , the migratory action is stopped. An additional electrons from the N region are repelled by the negative charge of the p region. Similarly, An additional holes from the P region are repelled by the net positive charge of the n region. Net result a creation of thin layer of each side of the junction. Which is depleted (emptied) of mobile charge carriers. This is known as Depletion Layer The depletion layer contains no free and mobile charge carriers but only fixed and immobile ions. Its width depends upon the doping level Heavy doped – thin depletion layer Lightly doped – thick depletion layer Formation of depletion layer  Biasing in diodes The process of applying the external voltage to a p-n junction semiconductor diode is called biasing. External voltage to the p-n junction diode is applied in any of the two methods:  forward biasing  reverse biasing.  Forward biasing of PN junction If the p-n junction diode is forward biased, it allows the electric current flow. Under forward biased condition, the p- type semiconductor is connected to the positive terminal of battery whereas; the n-type semiconductor is connected to the negative terminal of the battery.  Reverse biasing of PN junction If the p-n junction diode is reverse biased, it blocks the electric current flow. Under reverse biased condition, the p- type semiconductor is connected to the negative terminal of battery whereas; the n-type semiconductor is connected to the positive terminal of the battery. Biasing of PN junction  VI characteristics of PN Junction The curve drawn between voltage across the junction along X-axis and current through the circuits along the Y-axis. They describe the d.c behavior of the diode.  VI characteristics of PN Junction Forward bias Reverse bias When it is in forward bias, no current flows until the barrier With reverse bias, potential barrier at the junction increased. voltage (0.3 V for Ge) is overcome.... junction resistance increase...and prevents current flow. Then the curve has a linear rise and the current increases, However, the minority carriers are accelerated by the reverse with the increase in forward voltage like an ordinary voltage resulting a very small current (REVERSE conductor. CURRENT) … in the order of micro amperes. Above 3 V , the majority carriers passing the junction gain When reverse voltage is increased beyond a value, called sufficient energy to knock out the valence electrons and raise breakdown voltage, the reverse current increases sharply and them to the conduction band. the diode shows almost zero resistance. It is known as avalanche breakdown. Therefore, the forward current increases sharply. Reverse voltage above 25 v destroys the junction permanently.  Applications of junction diode As rectifiers to convert AC into DC As an switch in computer circuits. As detectors in radios to detect audio signals. As LED to emit different colors.  Zener diode : Introduction Zener diodes are semiconductor devices that allow current to flow in both directions but specialize in current flowing in reverse. In forward-biased conditions, the Zener Diode works like any normal diode but in the reverse-bias condition, a small leak current flows through the diode. As we keep increasing the reverse voltage it reaches a point where the reverse voltage equals the breakdown voltage. The breakdown voltage is represented as Vz and in this condition the current start flowing in the diode. After the breakdown voltage the current increase drastically until it reaches a stable value. Circuit of Zener: One Zener diode connected with one resistance and battery  V-I Characteristics of Zener diode Forward Characteristics of Zener Diode Forward characteristics of the Zener Diode are similar to the forward characteristics of any normal diode. Reverse Characteristics of Zener Diode In reverse voltage conditions a small amount of current flows through the Zener diode. This current is because of the electrons which are thermally generated in the Zener diode. As we keep increasing the reverse voltage at any particular value of reverse voltage the reverse current increases suddenly at the breakdown point this voltage is called Zener Voltage and is represented as Vz.  Principle of Zener diode  The working principle of Zener diode lies in the cause of breakdown for a diode in reverse biased condition. Normally there are two types of breakdown. 1. Avalanche Breakdown. 2. Zener Breakdown. Avalanche Breakdown The phenomenon of Avalanche breakdown occurs both in the ordinary diode and Zener Diode at high reverse voltage. For a high value of reverse voltage, the free electron in the PN junction diode gains energy and acquires high velocity and these high-velocity electrons collide with other atoms and knock electrons from that atoms. This collision continues and new electrons are available for conducting current thus the current increase rapidly in the diode. This phenomenon of a sudden increase in the current is called the Avalanche breakdown. This phenomenon damages the diode permanently whereas the Zener diode is a specific diode that is made to operate in this reverse voltage area. Zener Breakdown Zener breakdown happens in heavily doped PN junction diodes. In these diodes, if the reverse bias voltages reach closer to Zener Voltage, the electric field gets stronger and is sufficient enough to pull electrons from the valance band. These electrons then gain energy from the electric field and get free from the atom. Thus, for these diodes in the Zener breakdown region, a slight increase in the voltage causes a sudden increase in the current.  Principle of Zener diode Zener Breakdown Avalanche Breakdown This type of breakdown occurs for a reverse bias voltage This type of breakdown occurs at the reverse bias voltage between 2 to 8 V. above 8 V and higher. Even at low voltage, the electric field intensity is strong It occurs for lightly doped diode with large breakdown enough to exert a force. voltage. The valence electrons of the atom such that they are separated As minority charge carriers (electrons) flow across the from the nuclei. device. This type of break down occurs normally for highly doped They tend to collide with the electrons in the covalent bond diode with low breakdown voltage and larger electric field. and cause the covalent bond to disrupt. As temperature increases, the valence electrons gain more As voltage increases, the kinetic energy (velocity) of the energy to disrupt from the covalent bond and the less amount electrons also increases. of external voltage is required. The avalanche breakdown voltage increases with Thus Zener breakdown voltage decreases with temperature. temperature.  Zener diode as voltage regulator Why is Zener Diode used as a regulator? When the reverse voltage is applied to it, the voltage remains constant for a wide range of currents. Due to this feature, it is used as a voltage regulator. The primary objective of the Zener diode as a voltage regulator is to maintain a constant voltage.  Zener diode as voltage regulator A voltage regulator is a device that regulates the voltage level. It essentially steps down the input voltage to the desired level and keeps it at that same level during the supply. The voltage regulator is used for two main reasons, and they are: To regulate the output voltage To keep the output voltage constant at the desired value in spite of variations in the supply voltage. Voltage regulators are used in computers, power generators, alternators to control the output of the plant. The Zener voltage regulator consists of a current limiting resistor RS connected in series with the input voltage VS with the Zener diode connected in parallel with the load RL in this reverse biased condition. The load is connected in parallel with the Zener diode, so the voltage across RL is always the same as the Zener voltage, ( VR = VZ ). When selecting the Zener diode, be sure that its maximum power rating is not exceeded. Imax = Maximum current for Zener diode Imax = Power / Zener voltage.  Rectifier A rectifier is an electrical device that converts the alternating current (AC) , which periodically reverses direction, into the direct current (DC), which flows only in one direction. This process is known as Rectification. Physically, rectifiers take a number of forms, including vacuum tube diodes, mercury-are valves, copper and selenium oxide rectifiers, semiconductor diodes, silicon-controlled rectifiers and other silicon based semiconductor switches. There are two types of rectifiers namely: 1. Half wave rectifier 2. Full wave rectifier Half Wave Rectifier The process of removing one-half of the input signal to establish a dc level is called half- wave rectification. In half wave rectification, the rectifier conducts current during positive half cycle of input ac signal only. Negative half cycle is suppressed. Output frequency of HWR is equal to input frequency. This means when input ac completes one cycle, rectified wave also completes one cycle. fout = fin  Half Wave Rectifier Half Wave Rectifier Circuit A half-wave rectifier is the simplest form of the rectifier and requires only one diode for the construction of a halfwave rectifier circuit. A halfwave rectifier circuit consists of three main components as follows: A diode A transformer A resistive load  Half Wave Rectifier Working of Half Wave Rectifier 1. A high AC voltage is applied to the primary side of the step-down transformer. The obtained secondary low voltage is applied to the diode. 2. The diode is forward biased during the positive half cycle of the AC voltage and reverse biased during the negative half cycle. 3. The final output voltage waveform is as shown in the figure below:  Half Wave Rectifier For the positive half cycle of the AC source voltage, the circuit effectively becomes as shown below in the diagram: When the diode is forward biased, it acts as a closed switch. But, during the negative half cycle of the AC source voltage, the equivalent circuit becomes as shown in the figure below When a diode is reverse biased, it acts as an open switch. Since no current can flow to the load, the output voltage is equal to zero.  Half Wave Rectifier Half Wave Rectifier Waveform The halfwave rectifier waveform before and after rectification is shown below in the figure. A half-wave rectifier is used in soldering iron types of circuits and is also used in mosquito repellent to drive the lead for the fumes. It is used in Pulse generated circuits, for demodulation, in voltage multiplier. Full Wave Rectifier In Full wave rectification current flow through the load in same direction for both half cycle of input ac. This can be achieved with two diodes working alternatively. For one half cycle one diode supplies current to load and for next half cycle another diode works. Centre tap full wave rectifier Circuit has two diodes D1, D2 and a center tap transformer. During positive half cycle, Diode D1 conducts and during negative half cycle, Diode D2 conducts. It can be seen that current through load RL is in the same direction for both the cycles. Full wave bridge rectifier In full wave bridge rectifier, four diodes are arranged in the form of a bridge. The main advantages of this bridge circuit is that it does not requires a special center taped transformer. The input signal is applied across terminals A and B, and the output DC signal is obtained across the load resistor RL connected between terminals C and D. The four diodes are arranged in such a way that only two diodes conduct electricity during each half cycle. D1 and D3 are pairs that conduct electric current during the positive half cycle. Likewise, diodes D2 and D4 conduct electric current during a negative half cycle.  Full wave bridge rectifier Working When an AC signal is applied across the bridge rectifier, terminal A becomes positive during the positive half cycle while terminal B becomes negative. This results in diodes D1 and D3 becoming forward biased while D2 and D4 becoming reverse biased. The current flow during the positive half-cycle is shown in the Fig. 1: Fig. 1 During the negative half-cycle, terminal B becomes positive while terminal A becomes negative. This causes diodes D2 and D4 to become forward biased and diode D1 and D3 to be reverse biased. The current flow during the negative half cycle is shown in the Fig. 2: Fig. 2  Full wave bridge rectifier It is observed that the current flow across load resistor RL is the same during the positive and negative half-cycles. The output DC signal polarity may be either completely positive or negative. In our case, it is completely positive. If the diodes’ direction is reversed, we get a complete negative DC voltage. Thus, a bridge rectifier allows electric current during both positive and negative half cycles of the input AC signal. The output waveforms of the bridge rectifier are shown in the below figure. Output frequency of full Wave Rectifier Output frequency of FWR is equal to double of input frequency. This means when input ac completes one cycle, rectified wave completes two cycle. fout = 2fin Full wave bridge rectifier Full wave bridge rectifier  Transistors : Introduction Transistors is an electronic device made of three layers of semiconductor material that can act as an insulator and a conductor. The three layered transistor is also know as the bipolar junction transistor. Definition: Transistor is a three terminal device which allows current to flow from high resistance region to low resistance region. A transistor is a type of semiconductor device that can be used to conduct and insulate electric current or voltage. A transistor basically acts as a switch and an amplifier. In simple words, we can say that a transistor is a miniature device that is used to control or regulate the flow of electronic signals.  Transistors : Introduction Parts of a Transistor A typical transistor is composed of three layers of semiconductor materials or, more specifically, terminals which help to make a connection to an external circuit and carry the current. A voltage or current that is applied to any one pair of the terminals of a transistor controls the current through the other pair of terminals. There are three terminals for a transistor. They are listed below: Base: This is used to activate the transistor. Collector: It is the positive lead of the transistor. Emitter: It is the negative lead of the transistor.  Transistors Bipolar Junction Transistor (BJT) The three terminals of BJT are the base, emitter and collector. A very small current flowing between the base and emitter can control a larger flow of current between the collector and emitter terminal. Furthermore, there are two types of BJT, and they include: P-N-P Transistor: It is a type of BJT where one n-type material is introduced or placed between two p-type materials. In such a configuration, the device will control the flow of current. PNP transistor consists of 2 crystal diodes which are connected in series. The right side and left side of the diodes are known as the collector-base diode and emitter-base diode, respectively. N-P-N Transistor: In this transistor, we will find one p-type material that is present between two n-type materials. N-P-N transistor is basically used to amplify weak signals to strong signals. In an NPN transistor, the electrons move from the emitter to the collector region, resulting in the formation of current in the transistor. This transistor is widely used in the circuit. Transistors  Transistor Configurations There are three types of configuration, which are common base (CB), common collector (CC) and common emitter (CE). In common base (CB) In common collector (CC) In common emitter (CE) configuration, the base terminal configuration, the collector configuration, the emitter terminal of the transistor is common terminals are common between is common between the input and between input and output the input and output terminals. the output terminals. terminals.  Numerical Q1. Given the intrinsic carrier concentration of silicon at 300 K Q2. For an intrinsic semiconductor, the intrinsic carrier is 1.5 × 1010 cm−3, calculate the number of charge carriers per concentration (ni) at 300 K is 1.5 × 1010 cm−3. Calculate the cubic meter. number of free electrons and holes. A) 1.5 × 1016 m−3 A) 1.5 × 1010 cm−3, 1.5 × 1010 cm−3 B) 1.5 × 1020 m−3 B) 3 × 1010 cm−3, 3 × 1010 cm−3 C) 1.5 × 1014 m−3 C) 1.5 × 1020 cm−3, 1.5 × 1020 cm−3 D) 1.5 × 1018 m−3 D) 0, 0 Answer: B) 1.5 × 1016 m−3 Answer: A) 1.5 × 1010 cm−3, 1.5 × 1010 cm−3 Explanation: To convert cm−3 to m−3, multiply by 106. Explanation: In an intrinsic semiconductor, the number of free Therefore, 1.5 × 1010 cm−3 = 1.5 × 1010 × 106 = 1.5 × 1016 m−3. electrons (n) and holes (p) are equal to the intrinsic carrier concentration (ni).  Q4. A Zener diode has a breakdown voltage of 5.6 V and is used in a voltage regulator circuit. If the input voltage is 12 V and the Q3. A silicon diode has a forward voltage drop of 0.7 V when a series resistance is 100 Ω, what is the current through the Zener current of 10 mA is passing through it. What is the resistance of diode when the load current is 20 mA? the diode in this condition? A) 20 mA A) 70 Ω B) 16 mA B) 7 Ω C) 36 mA C) 0.07 Ω D) 56 mA D) 0.7 Ω Answer: B) 16 mA Answer: A) 70 Ω Explanation: The total current through the series resistor is: Explanation: The resistance R is given by 𝑅 = 𝑉/𝐼. 12 V − 5.6 V 0.7 V 𝐼total = = 64 mA 100Ω 𝑅= = 70Ω 10 × 10 A The current through the Zener diode: 𝐼 = 𝐼total − 𝐼load = 64 mA − 20 mA = 44 mA