VIEWS: 31 PAGES: 19 CATEGORY: Engineering POSTED ON: 4/15/2012
Topics To Cover. Device Structure which is Gate, Drain and Source and their physical operation. Then, I-V characteristics of MOSFET. After that, how MOSFET work as inverter and switch.
Analogue Electronics I EAB 2014 Jan 2009 Lecture 7 - MOSFET Introduction 1 Topics To Cover • Device Structure and Physical Operation • V-I Characteristics • MOSFETs as Inverter and Switch • Refer to Chapter 6 Boylestad 2 Basic Structure of MOSFETs Types of MOSFETs Depletion n-MOSFET Enhancement n-MOSFET • Why MOSFETs? Smaller in size and fewer processing steps. 3 Basic Structure of MOSFETs Depletion n-MOSFET Enhancement n-MOSFET S G D S G D SiO2 SiO2 n n n n n p-substrate p-substrate SiO2 is insulator referred to as dielectric – sets up opposing electric field. Responsible for very high input impedance. 4 Basic Structure of MOSFETs Depletion n-MOSFET Enhancement n-MOSFET S G D S G D SiO2 SiO2 n n n n n p-substrate p-substrate No direct electrical connection between gate terminal and channel. M – Metal, O – Oxide, S – Semiconductor. 5 Basic Structure of MOSFETs Depletion n-MOSFET Enhancement n-MOSFET S G D S G D SiO2 SiO2 n n n n n p-substrate p-substrate Enhancement type does not have ‘channel’ connecting drain and source. 6 Physical Operation of MOSFETs Depletion n-MOSFET ID=IS=IDSS S D G + n e e e e n VDD _ p-substrate For example: gate-to-source voltage (VGS) is set to zero and voltage is applied across the drain-to source terminals (VDS) 7 Physical Operation of MOSFETs Depletion n-MOSFET The gate is insulated from the channel. Therefore, the gate current is negligible regardless of the gate- to-source voltage. The channel has finite conductivity and majority carriers (electrons) allows the current to flow from drain to source via the channel when the drain is at a positive potential with respect to the source. This make it as a normally ON device. 8 Physical Operation of MOSFETs Depletion n-MOSFET By controlling the voltage at gate terminal, this enables us to control the current in the device until the gate voltage reaches a value at which the device will turns OFF. Actually, voltage at the gate controls the width of the channel which also determines the concentration of the majority carriers. The more negative the gate-to-source (VGS) voltage is, the smaller is the channel width and the smaller is the drain current 9 Physical Operation of MOSFETs Depletion n-MOSFET The channel width disappears when VGS reaches its pinch-off value, VP and enters the cut-off mode. The decrease in the width of the channel is viewed as the depletion in the majority-charge carriers in the channel. It is for this reason, MOSFET is said to operate in the depletion mode when VGS is less than or equal to zero. When VGS is larger than zero, it is said to be operating in enhancement mode. 10 V-I Characteristics Triode Region Saturation Region 11 Physical Operation of MOSFETs Enhancement n-MOSFET VGS + - IS = I D ID S G D +++ +++ + e e e e e e e n n VDS + + + + + + + _ p-substrate For example: Both (VGS) and (VDS) is set to certain value > 0. 12 Physical Operation of MOSFETs Enhancement n-MOSFET Voltage at gate controls the flow of current between drain and source. When VGS = 0, no current flows between drain and source MOSFETs is said to be in cut-off mode. Transistor will start to activate at its threshold voltage: VT = the value of VGS when the drain current just begins to flow. 13 Physical Operation of MOSFETs Enhancement n-MOSFET When VGS> 0, holes will repel under the gate and attracts electrons from source and drain. This will create a channel connecting both terminals. At VGS= VT the n-channel under the gate will complete the circuit. This channel is also called inversion layer. As VGS is increased slowly above threshold, a deeper channel is induced to allow more current to flow. 14 Physical Operation of MOSFETs Enhancement n-MOSFET For a small value of VDS and a constant VGS, current will flow from drain to source (ID = IS) proportionally to VDS. At this point, the MOSFET acts in triode region and behaves like a voltage- controlled resistor. Further increase in VDS,will create a voltage drop across the channel and cause a taper in the channel depth. When VDS reaches (VGS – VT), the channel pinches off. This leads to saturation so that there is no further increase in current even when VDS continues to increase. 15 Operating Region Enhancement MOSFET NMOS: (VDS and VGS normally positive values) – VGS < VT → Cut off mode, IDS=0 for any VDS – VGS > VT (transistor is turned on) • VDS < VGS – VT → Triode Region • VDS > VGS – VT → Saturation Region PMOS: (VDS and VGS normally negative values) – VGS > VT → cut off mode, IDS = 0 for any VDS – VGS < VT (transistor is turned on) • VDS > VGS – VT → Triode Region • VDS < VGS – VT → Saturation Region 16 Inverter Circuit This circuit is designated as the common source configuration. It is used in digital circuits. When Vin is low, the transistor is off and Vout is high. When Vin is high, the transistor is on and Vout is low. 17 Circuit Model For Switching VGS controls the switch in the model and ron is defined as: 1 ron = K [2(VGS − VT )] 18 Question & Answer ? 19
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