Transistor PDF

Transistor 1  Transistor Transistor is a three terminal electronic device Transistor Amplifies the weak input signal Transistor is used as switch and amplifier with various electronic devices. Transistors consume less power hence they have grea...

Transistor 1  Transistor Transistor is a three terminal electronic device Transistor Amplifies the weak input signal Transistor is used as switch and amplifier with various electronic devices. Transistors consume less power hence they have great efficiency. 2 1.Transistor tree Transistor Classifications  Types of Transistor Bipolar Junction Transistors (BJT) – Three layer sandwich of doped semiconductor materials. Field Effect Transistor (FET) – Two layers of semiconductor material. Junction Field Effect Transistor (JFET) Metal Oxide Semiconductor Field Effect Transistors (MOFET) Phototransistors – Light detectors that possess internal gain 4  Two basic types of transistors are the bipolar junction transistor (BJT) and the field-effect transistor (FET). The BJT is used in two broad areas- as a linear amplifier to amplify an electrical signal and as an electronic switch.  The term bipolar reflects the fact that there are two types of carriers, holes and electrons which form the currents in the transistor. If only one carrier is employed (electron or hole), it is considered a unipolar device like field effect transistor (FET).  The transistor is constructed with three doped semiconductor regions separated by two pn junctions. The three regions are called Emitter (E), Base (B), and Collector (C). layers of transistor A transistor consists of three layers of semiconductors: Base Emitter Collector The Bipolar Junction Transistor (BJT) is the simplest type of transistor 9  One type consists of two n regions separated by a p region (npn), and the other type consists of two p regions separated by n region (pnp). The term bipolar refers to the use of both holes and electrons as current carriers in the transistor structure.  The pn junction joining the base region and the emitter region is called the base-emitter junction. The pn junction joining the base region and the collector region is called the base-collector junction BJT are classified into two types: NPN Transistor PNP Transistor 12 The schematic symbols for the npn and pnp bipolar junction transistors. The base-emitter (BE) junction is forwarded-biased and the base- collector (BC) junction is reverse-biased.  Transistor Currents The arrow on the emitter of the transistor symbols points is the direction of convention current. These diagrams show that the emitter current (IE) is the sum of the collector current (IC) and the base current (IB), IE = IC + IB  DC Beta (𝛽 DC) and DC Alpha (𝛼 DC) The dc current gain of a transistor. is the dc beta (𝛽DC) 𝛽𝐷𝐶 = 𝐼𝐶 / 𝐼𝐵 The dc alpha (𝛼 DC) is The ratio of the dc collector (IC) to the dc emitter current (IE) Example 1: Determine the dc current gain 𝛽DC and the emitter current IE for a transistor where IB = 50 μA and IC = 3.65 mA. Solution:  VBB forward-biases the base-emitter junction and VCC reverse-biases the base-collector junction. When the base-emitter junction is forward-biased, it is like a forward-biased diode and has a nominal forward voltage drop of VBE ≅ 0.7 V. Example 2: Determine IB, IC, IE, VBE, VCE, and VCB in the circuit. Assume 𝛽DC = 150.  Working of Transistor Transistor works as a switch or amplifier Base part of transistor is gate controller device for large amount of electricity The collector supplies large amount of electricity The emitter gives us the output. The current flowing from collector is regulated by changing levels of current from the base.  NPN Transistor In NPN transistors, the middle layer i.e. p-type material is sandwiched between two outer layers i.e. two n-type materials. Hence, these transistors are known as NPN transistors When emitter supplies electrons, at the same time base pulls electrons from emitter. This movement of electrons creates a flow of electricity through the transistor  PNP Transistor In PNP transistors, the base is formed by n-type material and the outer two layers i.e. emitter and collector are made up of p-type material. Hence, these transistors are known as PNP transistors. When emitter supplies holes, at the same time base pulls holes from emitter. This movement of holes creates a flow of electricity through the transistor  Three possible ways to connect Bipolar Transistor within an electronic circuit with one terminal being common to both the input and output. 1. Common Base Configuration - has Voltage Gain but no Current Gain. 2. Common Emitter Configuration - has both Current and Voltage Gain. 3. Common Collector Configuration - has Current Gain but no Voltage Gain. 1- The Common Base (CB) Configuration The voltage gain Vout IC × R L Av = = Vin IE × R in The Common Emitter (CE) Configuration  In the Common Emitter or grounded emitter configuration, the input signal is applied between the base, while the output is taken from between the collector and the emitter. the input impedance is LOW as it is connected to a forward-biased PN-junction, the output impedance is HIGH as it is taken from a reverse-biased PN- junction.  IC IC Alpha, 𝛼 = and Beta, 𝛽 = IE IB IC = 𝛼. IE = 𝛽. IB IE = IC + IB 𝛽 𝛼= 𝛽+1 𝛼 𝛽= 1−𝛼  The Common Collector (CC) Configuration current gain This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit. high input impedance, in the region of hundreds of thousands of Ohms while having a relatively low output impedance.  IE = IC + I B IE IC + IB Ai = = IB IB IC Ai = +1 IB Ai = 𝛽 + 1 Characteristic Common Base Common Emitter Common Collector Input Impedance Low Medium High Output Impedance Very High High Low Phase Angle 0o 180o 0o Voltage Gain High Medium Low Current Gain Low Medium High Power Gain Low Very High Medium Operating Modes of Transistors Depending on the biasing conditions like forward or reverse, transistors have three major modes of operation namely cutoff, active and saturation regions. 1-Active Mode In this mode, the transistor is generally used as a current amplifier. current flows between emitter and collector and the amount of current flow is proportional to the base current. two junctions are differently biased that means emitter-base junction is forward biased whereas collector-base junction is reverse biased. Characteristic-Curve-of-BJT Cutoff Mode In this mode, both collector base junction and emitter base junction are reverse biased. BJT in this mode is switched OFF and is essentially an open circuit. Cutoff Region is primarily used in switching and digital logic circuits. Saturation Mode In this mode of operation, both the emitter-base and collector-base junctions are forward biased. Current flows freely from collector to emitter with almost zero resistance. In this mode, the transistor is fully switched ON and is essentially a close circuit. Saturation Region is also primarily used in switching and digital logic circuits.  Types of FET They are JFET and MOSFET.  have three terminals (like BJTs). The three terminals are: Gate (G), Drain (D) and Source (S). field-effect transistor is a unipolar device constructed with no pn junction in the main current-carrying path require only the majority charge carriers to operate Field Effect Transistor are classified into Junction Field Effect transistors (JFET) and Metal Oxide Semiconductor Field Effect Transistors (MOSFET).  JFET A junction field effect transistor is a three terminal semiconductor device in which current conduction is by one type of carrier i.e., electrons or holes. An FET is a three-terminal unipolar semiconductor device. It is a voltage controlled device  JFET Constructional details A JFET consists of a p-type or n-type silicon bar containing two pn junctions at the sides The bar forms the conducting channel for the charge carriers If the bar is of n-type, it is called n-channel JFET If the bar is of p-type, it is called a p-channel JFET The two pn junctions forming diodes are connected internally and a common terminal called gate is taken out Other terminals are source and drain taken out from the bar  JFET JFET polarities The voltage between the gate and source is such that the gate is reverse biased. The drain and source terminals are interchangeable i.e., either end can be used as source and the other end as drain. Schematic Symbol of JFET  MOSFET A field effect transistor (FET) that can be operated in the enhancement- mode is called a MOSFET. It stands for Metal Oxide Semiconductor Field Effect Transistor. A MOSFET is an important semiconductor device and can be used in any of the circuits covered for JFET. However, a MOSFET has several advantages over JFET including high input impedance and low cost of production. high input impedance and low output impedance  MOSFET Types of MOSFETs: Depletion-type MOSFET or D-MOSFET: The D-MOSFET can be operated in both the depletion-mode and the enhancement-mode. For this reason, a D-MOSFET is sometimes called depletion/enhancement MOSFET. Enhancement-type MOSFET or E-MOSFET: The E-MOSFET can be operated only in enhancement-mode. The manner in which a MOSFET is constructed determines whether it is D-MOSFET or EMOSFET. Depletion Mode The depletion mode MOSFETs are generally known as ‘Switched ON’ devices, because these transistors are generally closed when there is no bias voltage at the gate terminal. If the applied gate voltage more negative, then the channel width is very less and MOSFET may enter into the cutoff region.  MOSFET D-MOSFET: It is similar to n-channel JFET except with the following modifications/remarks : (i) The n-channel D-MOSFET is a piece of n-type material with a p-type region (called substrate) on the right and an insulated gate on the left as shown in Fig. The free electrons flowing from source to drain must pass through the narrow channel between the gate and the p-type region (i.e. substrate). 54 (ii) A thin layer of metal oxide (usually silicon dioxide, SiO2) is deposited over a small portion of the channel. A metallic gate is deposited over the oxide layer. As SiO2 is an insulator, therefore, gate is insulated from the channel. Note that the arrangement forms a capacitor. One plate of this capacitor is the gate and the other plate is the channel with SiO2 as the dielectric. (iii) It is a usual practice to connect the substrate to the source (S) internally so that a MOSFET has three terminals source (S), gate (G) and drain (D). (iv) Since the gate is insulated from the channel, we can apply either negative or positive voltage to the gate. Therefore, D-MOSFET can be operated in both depletion-mode and enhancement-mode.  The depletion type MOSFET transistor is equivalent to a “normally closed” switch. The depletion type of transistors requires gate – source voltage (VGS) to switch OFF the device. The n-channel MOSFET operates in the depletion mode when a negative gate-to-source voltage is applied. The depletion mode MOSFET transistors are generally ON at zero gate-source voltage (VGS). Enhancement Mode The Enhancement mode MOSFET is commonly used type of transistor. This type of MOSFET is equivalent to normally-open switch because it does not conduct when the gate voltage is zero. Enhancement Mode With a positive gate voltage,  the device is turned off when no voltage is applied to the gate. if the bias voltage is zero or negative (-VGS) then the transistor may switch OFF If the positive voltage (+VGS) is applied to the N-channel gate terminal, then the channel conducts  MOSFET E-MOSFET Its gate construction is similar to that of D-MOSFET except the E-MOSFET has no channel between source and drain unlike the D-MOSFET. 62  The substrate extends completely to the SiO2 layer so that no channel exists. The E-MOSFET requires a proper gate voltage to form a channel (called induced channel). It is reminded that E-MOSFET can be operated only in enhancement mode. N Channel Enhancement MOSFET Symbol V-I-Characteristics-of-Enhancement-Mode-MOSFET Current-Flow-in-Enhancement-Mode-MOSFET Applications MOSFETs are used in digital integrated circuits, such as microprocessors. Used in calculators. Used in memories and in logic CMOS gates. Used as analog switches. Used as amplifiers.  Use of transistors in a computer – I Transistor is used in memory chips as a switch Number of transistors in computer helps to store large amount of binary numbers to represent ordinary numbers and alphabets Transistors used in microprocessors and printer logic board Switching transistor is used in SMPS Thin film transistor is used in monitor. 68