L_MOSFET.pdf

MOSFET’s (metal–oxide–semiconductor field-effect transistor) Current Controlled vs Voltage Controlled Devices  FET ( Field Effect Transistor) Few important advantages of FET over conventional Transistors 1. Unipolar device i. e. operation depends on...

MOSFET’s (metal–oxide–semiconductor field-effect transistor) Current Controlled vs Voltage Controlled Devices  FET ( Field Effect Transistor) Few important advantages of FET over conventional Transistors 1. Unipolar device i. e. operation depends on only one type of charge carriers (h or e) 2. Voltage controlled Device (gate voltage controls drain current) 3. Very high input impedance (109-1012 ) 4. Low Voltage and Low Current Operation is possible (Low- power consumption) 5. Less Noisy as Compared to BJT 6. Very small in size, occupies very small space in Ics.  Types of Field Effect Transistors (The Classification) » JFET n-Channel JFET FET p-Channel JFET MOSFET (IGFET) Enhancement Depletion MOSFET MOSFET n-Channel p-Channel n-Channel p-Channel EMOSFET EMOSFET DMOSFET DMOSFET  MOSFETs MOSFETs have characteristics similar to JFETs and additional characteristics that make them very useful. There are 2 types of MOSFET’s: Depletion mode MOSFET (D-MOSFET) Enhancement Mode MOSFET (E-MOSFET) D-MOSFET Symbols E-MOSFET Symbols  Depletion Mode MOSFET Construction and Working Drain (D) and Source (S) leads connect to the to n-doped regions and these n-doped regions are connected via an n-channel. This n-channel is connected to the Gate (G) via a thin insulating layer of SiO2 The n-doped material lies on a p-doped substrate that may have an additional terminal connection called SS Basics Operation  Basic Operation A D-MOSFET may be biased to operate in two modes: the Depletion mode or the Enhancement mode  p-Channel Depletion Mode MOSFET The p-channel Depletion mode MOSFET is similar to the n-channel except that the voltage polarities and current directions are reversed Enhancement Mode MOSFET’s  Enhancement Mode MOSFET Construction The Drain (D) and Source (S) connect to the to n-doped regions These n-doped regions are not connected via an n-channel without an external voltage The Gate (G) connects to the p-doped substrate via a thin insulating layer of SiO2 The n-doped material lies on a p-doped substrate that may have an additional terminal connection called SS Basics opeartion  Basic Operation The Enhancement mode MOSFET only operates in the enhancement mode. VGS is always positive IDSS = 0 when VGS < VT As VGS increases above VT, ID increases If VGS is kept constant and VDS is increased, then ID saturates (IDSS) The saturation level, VDSsat is reached.  p-Channel Enhancement Mode MOSFETs The p-channel Enhancement mode MOSFET is similar to the n-channel except that the voltage polarities and current directions are reversed.  MCQ Which of the following is NOT a type of transistor? a) NPN b) PNP c) JFET d) SCR  MCQ A JFET has three terminals, namely ………… (A) cathode, anode, grid (B) emitter, base, collector (C) source, gate, drain (D) none of the above  MCQ A MOSFET is a ………… driven device (A) current (B) voltage (C) both current and voltage (D) none of the above  MCQ A MOSFET can be operated with …………….. (A) negative gate voltage only (B) positive gate voltage only (C) positive as well as negative gate voltage (C) none of the above  MCQ The input control parameter of a MOSFET is …………… (A) gate voltage (B) source voltage (c) drain voltage (D) gate current  MCQ The input impedance of a MOSFET is of the order of ……….. (A) 1 Ω (B) a few hundred Ω (C) kΩ (D) several MΩ