EE 330 Lecture 35 Parasitic Capacitances in MOS Devices
Exam 2 Wed Oct 24 Exam 3 Friday Nov 16
Review from Last Lecture Cascode Configuration Discuss V CC gm1 gm1 I B VCC V OUT g02 g01 A - β β VXX Q 2 g 0CC g 02 β Q 1 V SS gm1 2VAF AVCC β= 8000100 g01 Vt A 800, 000 VCC This gain is very large and only requires two transistors! What happens to the gain if a transistor-level current source is used for I B?
Review from Last Lecture Cascode Configuration V CC V YY Q 3 V OUT Q 3 V OUT VXX Q 2 Q 2 Q 1 Q 1 V SS
Review from Last Lecture Cascode Configuration Comparisons V DD V CC I B Q 1 V EE V OUT V CC A V -g g m 0 V YY Q 2 Q 1 V EE V OUT A V -gm1 -g g g 2g m1 01 02 01 I B V CC V OUT V YY Q 3 V OUT VXX Q 2 Q 1 V SS A V g g m1 01 β VXX Q 2 Q 1 A V g m1 gm1 g 01 g +g 03 03 Gain limited by output impedance of current scource!! Can we design a better current source? In particular, one with a higher output impedance? V SS
Review from Last Lecture Cascode current sources I X I X V XX Q 2 V XX M 2 I X V YY Q 1 V YY M 1 V SS V SS V SS g 0CC V CC V CC V DD V YY Q 1 I X V YY M 1 All have the same small-signal model V XX Q 2 I X V XX M 2 I X g 02 01 π2 0CC = g 01 +g 02 +g π2 +g m2 g g +g
Review from Last Lecture Cascode Configuration Discuss V CC VXX V ZZ V YY Q 3 Q 4 Q 2 V OUT gm1β A V = g01 2 100 A V = 8000 400,000 2 This gain is very large and is a factor of 2 below that obtained with an ideal current source biasing Q 1 V SS Although the factor of 2 is not desired, the performance of this circuit is still very good This factor of 2 gain reduction is that same as was observed for the CE amplifier when a transistorlevel current source was used
Review from Last Lecture High Gain Amplifier Comparisons ( n-ch MOS) V CC I B V DD V CC V DD V OUT I B V DD M 1 V SS V ZZ M 2 M 1 V OUT V XX M 2 V OUT V ZZ M 3 V OUT V YY V ZZ M 4 M 3 g AV - g m1 01 V SS 1 g A m1 V - 2 g 01 M 1 V SS g AVCC - g g g m1 m2 01 02 M 2 V XX M 1 V SS g AVCC - g m1 01 V OUT M 2 VXX M 1 V SS 1 g g A m1 m2 VCC - 2 g 01 g 02
Review from Last Lecture High Gain Amplifier Comparisons (BJT) V DD I B V CC V CC V OUT V YY I B V OUT Q 1 V OUT Q 1 VXX Q 2 V CC A V V EE -g g m 0 A V V EE g 2g 1 m1 01 A V g g Q 1 V SS m1 01 β VXX V YY Q 3 Q 2 Q 1 V SS V OUT VXX V ZZ V YY V CC Q 3 Q 4 Q 2 V OUT Single-ended high-gain amplifiers inherently difficult to bias (because of the high gain) Biasing becomes practical when used in differential applications These structures are widely used but usually with differential inputs A V g g m1 01 Q 1 V SS gm1β A V = g01 2
Parasitic Capacitors in MOSFET
Parasitic Capacitors in MOSFET C GCH This capacitance was modeled previously and exists when the transistor is operating in triode or saturation But there are others that also affect high-frequency or high-speed operation
Parasitic Capacitors in MOSFET Recall that pn junctions have a depletion region!
Parasitic Capacitors in pn junction capacitance MOSFET C Depletion Region For V FB <φ B /2 C JO A C φ B V FB C = CA J0 V 1- φ FB B m
Parasitic Capacitors in pn junction capacitance MOSFET The bottom and the sidewall:
Parasitic Capacitors in pn junction capacitance MOSFET C Depletion Region For a pn junction capacitor C = BOT C A BOT V 1- φ C =C A+C P J BOT SW FB B m C = SW C P SW V 1- φ FB B m
Types of Capacitors in MOSFETs 1. Fixed Capacitors a. Fixed Geometry b. Junction 2. Operating Region Dependent
Parasitic Capacitors in MOSFET Fixed Capacitors Fixed Geometry W C GSO C GDO LD Overlap Capacitors: C GDO, C GSO
Parasitic Capacitance Summary D C GD G B C GS Cutoff Ohmic Saturation C GS CoxWL D CoxWL D CoxWL D C GD CoxWL D CoxWL D CoxWL D L D is a model parameter S
Overlap Capacitance Model Parameters
Types of Capacitors in MOSFETs 1. Fixed Capacitors a. Fixed Geometry b. Junction 2. Operating Region Dependent
Parasitic Capacitors in MOSFET Fixed Capacitors- Junction C BS1 C BD1 Junction Capacitors: C BS1, C BD1
Parasitic Capacitors in MOSFET Fixed Capacitors C GSO C GDO C BS1 C BD1 Overlap Capacitors: C GDO, C GSO Junction Capacitors: C BS1, C BD1
Fixed Parasitic Capacitance Summary C GD D C BD G C GS C BS B C BOT and C SW are model parameters Cutoff Ohmic Saturation C GS CoxWL D CoxWL D CoxWL D C GD CoxWL D CoxWL D CoxWL D C BG C BS C BS1 = C BOT A S +C SW P S C BS1 = C BOT A S +C SW P S C BS1 = C BOT A S +C SW P S C BD C BD1 = C BOT A D +C SW P D C BD1 = C BOT A D +C SW P D C BD1 = C BOT A D +C SW P D S
C BOT and C SW model parameters
Types of Capacitors in MOSFETs 1. Fixed Capacitors a. Fixed Geometry b. Junction 2. Operating Region Dependent
Parasitic Capacitors in MOSFET Operation Region Dependent -- Cutoff C GBCO Cutoff Capacitor: C GBCO
Parasitic Capacitors in MOSFET Operation Region Dependent -- Cutoff C GBCO Note: A depletion region will form under the gate if a positive Gate voltage is applied thus decreasing the capacitance density Cutoff Capacitor: C GBCO
Parasitic Capacitors in MOSFET Operation Region Dependent and Fixed -- Cutoff C GSO C GDO C BS1 C GBCO C BD1 Overlap Capacitors: C GDO, C GSO Junction Capacitors: C BS1, C BD1 Cutoff Capacitor: C GBCO
Parasitic Capacitance Summary C GD D C BD G B C GS C BS S C BG Cutoff Ohmic Saturation C GS C GD C BG C BS C BD CoxWL D CoxWL D CoxWL (or less) C BOT A S +C SW P S C BOT A D +C SW P D
Parasitic Capacitors in MOSFET Operation Region Dependent -- Ohmic C GCH C BCH Note: The Channel is not a node in the lumped device model so can not directly include this distributed capacitance in existing models Note: The distributed channel capacitance is usually lumped and split evenly between the source and drain nodes Ohmic Capacitor: C GCH, C BCH
Parasitic Capacitors in MOSFET Operation Region Dependent and Fixed -- Ohmic C GSO C GCH C GDO C BCH C BS1 C BD1 Overlap Capacitors: C GDO, C GSO Junction Capacitors: C BS1, C BD1 Ohmic Capacitor: C GCH, C BCH
Parasitic Capacitance Summary C GD D C BD G B C GS C BS S C BG Cutoff Ohmic Saturation C GS CoxWL D CoxWL D C GD CoxWL D CoxWL D C BG CoxWL (or less) 0 C BS C BOT A S +C SW P S C BS1 = C BOT A S +C SW P S C BD C BOT A D +C SW P D C BD1 = C BOT A D +C SW P D
Parasitic Capacitors in MOSFET Operation Region Dependent -- Saturation C GCH C BCH Note: Since the channel is an extension of the source when in saturation, the distributed capacitors to the channel are generally lumped to the source node Saturation Capacitors: C GCH, C BCH
Parasitic Capacitors in MOSFET Operation Region Dependent and Fixed --Saturation C GSO C BS1 C GCH C BCH C GDO C BD1 Overlap Capacitors: C GDO, C GSO Junction Capacitors: C BS1, C BD1 Saturation Capacitors: C GCH, C BCH 2/3 C OX WL is often attributed to C GCH to account for LD and saturation This approximation is reasonable for minimum-length devices but not so good for longer devices
Parasitic Capacitance C GD Summary D C BD G B C GS C BS S C BG Cutoff Ohmic Saturation C GS CoxWL D CoxWL D + 0.5C OX WL CoxWL D +(2/3)C OX WL C GD CoxWL D CoxWL D + 0.5C OX WL CoxWL D C BG CoxWL (or less) 0 0 C BS C BOT A S +C SW P S C BOT A S +C SW P S +0.5WLC BOTCH C BOT A S +C SW P S +(2/3)WLC BOTCH C BD C BOT A D +C SW P D C BOT A D +C SW P D +0.5WLC BOTCH C BOT A D +C SW P D
Parasitic Capacitance Summary C GD D C BD G B C GS C BS S C BG Cutoff Ohmic Saturation C GS CoxWL D CoxWL D + 0.5C OX WL CoxWL D +(2/3)C OX WL C GD CoxWL D CoxWL D + 0.5C OX WL CoxWL D C BG CoxWL (or less) 0 0 C BS C BOT A S +C SW P S C BOT A S +C SW P S +0.5WLC BOTCH C BOT A S +C SW P S +(2/3)WLC BOTCH C BD C BOT A D +C SW P D C BOT A D +C SW P D +0.5WLC BOTCH C BOT A D +C SW P D
Recall: Small-signal and simplified dc equivalent elements Element ss equivalent Simplified dc equivalent C Large Capacitors C Small C L Large Inductors L Small L Diodes Simplified MOS transistors (MOSFET (enhancement or depletion), JFET) Simplified Simplified Have not yet considered situations where the small capacitor is relevant in small-signal analysis
Amplifiers with Small Capacitors Recall: V DD If capacitors are large R 1 R 3 C 3 V OUT V OUT C 1 Q 1 R 2 R 4 C 2 R L R 1 //R 2 Q 1 R 3 R L A g R R // V m1 3 L
Amplifiers with Small Capacitors What if C 1 and C 2 large but C 3 not large?: V DD R 1 R 3 C 3 V OUT C 3 V OUT C 1 Q 1 R 2 R 4 R L R 1 //R 2 Q 1 R 3 R L C 2 C 3 V out R 1 //R 2 V be g π g m V be R 3 R L
Amplifiers with Small Capacitors What if C 1 and C 2 large but C 3 not large?: V 1 C 3 V out R 1 //R 2 V be g π1 g m1 V be R 3 R L From KCL: VOUT sc3 GL V1 sc3 V1 sc3 G3 gm 1VIN VOUT sc3 Solving: VOUT sc3g m1 V sc3 G G3 G3G Equivalently: V g sc R R IN L L OUT m1 3 3 L VIN sc3rl R3 1 Serves as a first-order high-pass filter A V ω
Amplifiers with Small Capacitors Consider parasitic C DB V DD V out R D V gs g m1 V gs R D C DB V out M 1 C DB By KCL: V in V sc G g 1 V OUT DB D m IN -V SS Causes gain to decrease at high frequencies Solving: V V Equivalently: V g sc G OUT m1 IN DB D g R sc R 1 OUT m1 D V IN DB D A V ω
Amplifiers with Small Capacitors Consider parasitic C GS, C GD, and C DB V DD C GD V out R D V gs C GS g m1 V gs R D C DB V out V in C GD C GS M 1 -V SS C DB By KCL: V s C C G g V sc V OUT DB GD D m1 IN GD IN Causes gain to decrease at high frequencies Has one LHP pole and one RHP zero Solving: V g 1 sc s C C G OUT m GD V Equivalently: V IN DB GD D OUT V R g 1 sc s C C R D m GD IN DB GD D 1 A V 1 ω
Amplifiers with Small Capacitors Consider parasitic C GS, C GD, and C DB V DD R D V out V in C GD C GS M 1 -V SS C DB Device parasitics problematic at high frequencies C DB, C GD and C GS effects can be significant Value of parasitic capacitances strongly dependent upon layout Device parasitics usually not a problem at audio frequencies Causes gain to decrease at high frequencies: has one high frequency LHP pole and one high frequency RHP zero.
End of Lecture 35