ECE-305: Fall 2016 MOS Capacitance and Introduction to MOSFETs Professor Peter Bermel Electrical and Computer Engineering Purdue University, West Lafayette, IN USA pbermel@purdue.edu 11/4/2016 Pierret, Semiconductor Device Fundamentals (SDF) Chapter 16 (pp. 567-584) Chapter 17 (pp. 611-616) Bermel ECE 305 F16 1
MOS capacitor V G + v S sinwt v S sinwt + - ~ + V G - p-si 2
Simple model for MOSCAP x o K O C ox = K Oe 0 x o W ( f S ) K S C S = K Se 0 W ( f S ) 3 Ground 1 C = 1 + 1 C = C C S ox C = C ox C S C S + C ox C ox C = 1 + C ox C S C ox ( ) 1+ K W f O S K S x o
result x o k ox V G W ( f S ) k Si C ox f S C S C = C ox ( ) 1+ K OW f S K S x o 4
s.s. gate capacitance vs. d.c. gate bias accumulation C C ox depletion C = C ox ( ) 1+ K W f O S K S x o flat band inversion V T V G 5
s.s. gate capacitance vs. d.c. gate bias C flat band accumulation C ox depletion C = C ox ( ) 1+ K OW f S K S x o inversion V T V G 6
capacitance vs. gate voltage C flat band C = C ox accumulation ( ) 1+ K OW f S K S x o C ox depletion inversion V T V G 7
high frequency vs. low frequency C flat band low frequency C = C ox accumulation ( ) 1+ K OW f S K S x o C ox depletion inversion high frequency V T V G 8
high frequency vs. low frequency C low frequency C ox high frequency V T V G 9
high frequency vs. low frequency n + -Si n + -Si MOS capacitor p-si 10
side and top views of a MOSFET Metal Oxide Semiconductor Field Effect Transistor V S = 0 V G V D n-si S L D n-si W L p-type silicon SiO 2 source gate drain side view Bermel ECE 305 F16 top view 11
transistors 12
transistor as a black box terminal 1 There are many kinds of transistors: control black box I 1 terminal 2 terminal 4 MOSFET SOI MOSFET SB FET FinFET MODFET (HEMT) bipolar transistor JFET heterojunction bipolar transistor BTBT FET SpinFET 13
the bulk MOSFET S G D circuit symbol Gate source silicon drain SiO 2 Source Drain Body B (Texas Instruments, ~ 2000) 11/4/2016 Bermel ECE 305 F16 14
the MOSFET as a 2-port device MOSFET circuit symbol common source Drain Gate G D output V DS Source V GS input S current vs. voltage (IV) characteristics ( V ) GS at a fixed V DS transfer ( V G,V S,V ) D ( V ) DS at a fixed V GS output 15
IV characteristics: resistor I + I less resistance I = V R R V more resistance V - I = V R Ohm s Law 11/4/2016 Bermel ECE 305 F16 16
IV characteristics: ideal current source I + I I = I 0 I 0 V - V I = I 0 17
IV characteristics: transistors D G V GS1 S n-channel enhancement mode MOSFET gate voltage controlled resistor linear region V DS gate voltage controlled current source saturation region 18
IV characteristics: real current sources I + I I 0 I = I 0 + V R 0 R 0 I 0 V V - 19
IV characteristics: transistors D G V GS1 S n-channel enhancement mode MOSFET V DS 20
MIOSFET IV: output characteristics D G V GS S n-channel enhancement mode MOSFET subthreshold region V DS linear region saturation region 21
output vs. transfer characteristics output characteristics transfer characteristics low V DS high V DS D I V DS2 > V DS1 G S V DS1 V DSAT V DS V T V GS saturation voltage threshold voltage Bermel ECE 305 F16 22
applications of MOSFETs symbo l switch amplifier D D D G S G S G input signal S output signal 23
n-channel vs. p-channel MOSFET n-mosfet p-mosfet V S = 0 V G > V T V D > 0 V S = 0 V G < V T V D < 0 n-si S channel D n-si p-si S channel D p-si L L p-type silicon n-type silicon side view side view 24
MOSFET device metrics ( ma mm) on-resistance R ON ( W - mm) output resistance: r d ( W - mm) on-current (ma/μm) ( V GS = V DS = V DD ) V GS transconductance V DD V DS g m º D DV GS V DS ( ms mm) 25
MOSFET device metrics (ii) transfer characteristics: ( ma mm) I ON V DS = V DD off-current V DS = 0.05 V V GS V TSAT V TLIN V DD threshold voltage 11/4/2016 26
MOSFET device metrics (iii) log 10 ( ma mm) off-current transfer characteristics: V DS = V DD subthreshold swing: ( mv decade) I ON V DS = 0.05 V DIBL (drain-induced barrier lowering) ( mv V) V GS V T V DD 27
Example: 32 nm N-MOS technology 11/4/2016 Bermel ECE 305 F16 28
summary Given the measured characteristics of a MOSFET, you should be able to determine: 1. on-current: I ON 2. off-current: I OFF 3. subthreshold swing, S 4. drain induced barrier lowering: DIBL 5. threshold voltage: V T (lin) and V T (sat) 6. on resistance: R ON 7. drain saturation voltage: V DSAT 8. output resistance: r o 9. transconductance: g m Our goal is to understand these device metrics. 29
understanding MOSFETs 0 V GS > V T V D n-si n-si p-si x To understand any semiconductor device, we should first draw an Energy Band Diagram. 30