Week 9b OUTLINE The metal oxide field effect transistor MOSFET Structure and operation of the MOSFET MOSFET as a 3 terminal device pn diodes isolate transistors in an IC MOSFET current voltage characteristics The MOSFET as a controlled resistance MOSFET as an amplifier or electronically controlled switch EE42 100 Spring 2006 Week 9b Prof White 1 Modern Field Effect Transistor FET An electric field is applied normal to the surface of the semiconductor by applying a voltage to an overlying gate electrode to modulate the conductance of the semiconductor Modulate drift current flowing between 2 contacts source and drain by varying the voltage on the gate electrode N channel metal oxidesemiconductor field effect transistor NMOSFET EE42 100 Spring 2006 Week 9b Prof White 2 MOSFET NMOS N channel Metal Oxide Semiconductor L channel length W W channel width GATE L Metal heavily doped poly Si s ula n i e d i x o n tor ilicon p type s n DRAIN SOURCE A GATE electrode is placed above electrically insulated from the silicon surface and is used to control the resistance between the SOURCE and DRAIN regions EE42 100 Spring 2006 Week 9b Prof White 3 N channel MOSFET Gate Source IS n IG gate oxide insulator Drain ID n p Without a gate to source voltage applied no current can flow between the source and drain regions Above a certain gate to source voltage threshold voltage VT a conducting layer of mobile electrons is formed at the Si surface beneath the oxide These electrons can carry current between the source and drain EE42 100 Spring 2006 Week 9b Prof White 4 N channel vs P channel MOSFETs NMOS PMOS n poly Si p poly Si n n p p type Si p n type Si For current to flow VGS VT For current to flow VGS VT Enhancement mode VT 0 Enhancement mode VT 0 Depletion mode VT 0 Depletion mode VT 0 Transistor is ON when VG 0V Transistor is ON when VG 0V n denotes very heavily doped n type material p denotes very heavily doped p type material EE42 100 Spring 2006 Week 9b Prof White 5 Why are pn Junctions Important for ICs The basic building block in digital ICs is the MOS metal oxide semiconductor transistor which contains reverse biased diodes pn junctions are important for electrical isolation of transistors located next to each other at the surface of a silicon wafer The junction capacitance of these diodes can limit the performance operating speed of digital circuits EE42 100 Spring 2006 Week 9b Prof White 6 Device Isolation using pn Junctions regions of n type Si a b n n n n n p type Si No current flows if voltages are applied between n type regions because two pn junctions are back to back n region a b n region p region n type regions isolated in p type substrate and vice versa EE42 100 Spring 2006 Week 9b Prof White 7 Transistor A Transistor B n n n n p type Si We can build large circuits consisting of many transistors without worrying about current flow between devices The p n junctions isolate the transistors because there is always at least one reverse biased p n junction in every potential current path EE42 100 Spring 2006 Week 9b Prof White 8 MOSFET Circuit Symbols G NMOS G n poly Si n n S S p type Si PMOS Body G G p poly Si p p S S n type Si EE42 100 Spring 2006 Body Week 9b Prof White 9 Water Model for P channel MOSFET Source Gate Spring Source Gate Drain Drain Flexible membrane Figure 0 1 Schematic symbol and water model for a p channel MOSFET EE42 100 Spring 2006 Week 9b Prof White 10 MOSFET Terminals The voltage applied to the GATE terminal determines whether current can flow between the SOURCE DRAIN terminals For an n channel MOSFET the SOURCE is biased at a lower potential often 0 V than the DRAIN Electrons flow from SOURCE to DRAIN when VG VT For a p channel MOSFET the SOURCE is biased at a higher potential often the supply voltage VDD than the DRAIN Holes flow from SOURCE to DRAIN when VG VT The BODY terminal is usually connected to a fixed potential For an n channel MOSFET the BODY is connected to 0 V For a p channel MOSFET the BODY is connected to VDD EE42 100 Spring 2006 Week 9b Prof White 11 NMOSFET IG vs VGS Characteristic Consider the current IG flowing into G versus VGS IG G S oxide semiconductor VGS IG D VDS The gate is insulated from the semiconductor so there is no significant steady gate current always zero VGS EE42 100 Spring 2006 Week 9b Prof White 12 NMOSFET ID vs VDS Characteristics Next consider ID flowing into D versus VDS as VGS is varied G S VGS VGS VT zero if VGS VT VDS EE42 100 Spring 2006 D oxide semiconductor ID ID VDS Above threshold VGS VT inversion layer of electrons appears so conduction between S and D is possible Below threshold VGS VT no charge no conduction Week 9b Prof White 13 The MOSFET as a Controlled Resistor The MOSFET behaves as a resistor when VDS is low Drain current ID increases linearly with VDS Resistance RDS between SOURCE DRAIN depends on VGS RDS is lowered as VGS increases above VT oxide thickness tox NMOSFET Example ID VGS 2 V VGS 1 V VT VDS IDS 0 if VGS VT EE42 100 Spring 2006 Inversion charge density Qi x Cox VGS VT V x whereWeek Cox 9b Prof ox White tox 14 MOSFET as a Controlled Resistor cont d VDS ID RDS RDS L W L W Rs L W n Qi C V V VDS n ox GS T 2 VDS W I D nCox VGS VT VDS L 2 We can make RDS low by applying a large gate drive VGS VT making W large and or L small EE42 100 Spring 2006 Week 9b Prof White average value of V x 15 ID vs VDS Characteristics The MOSFET ID VDS curve consists of two regions 1 Resistive or Triode Region 0 VDS VGS VT VDS W ID VGS VT L 2 where k n n Cox k n VDS process transconductance parameter 2 Saturation Region VDS VGS VT k n W VGS VT 2 I DSAT 2 L where k n nCox EE42 100 Spring 2006 CUTOFF Week 9b Prof White region VG VT 16 MOSFET regions of operation Cutoff VGS VT ID 0 correction Resistive ID VGS VT VDS Saturation ID VGS VT 2 EE42 100 Spring 2006 Week 9b Prof White 17 MOSFET Uses A MOSFET can be used as A linear amplifier Input voltage applied between gate and source output voltage appears between source and drain or An electronic switch Switches between no current conduction between source and drain and heavy conduction between source and drain as voltage applied between gate and source changes from low to high for NMOSFET EE42 100 Spring 2006 Week 9b Prof White 18 Common Source CS Amplifier …
View Full Document
Unlocking...