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CMOS Transistor and Circuits
Instructed by Shmuel Wimer Eng. School, Bar-Ilan University Credits: David Harris Harvey Mudd College (Some materials copied/taken/adapted from Harris’ lecture notes) Jan 2015 CMOS Transistor
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Outline MOS Capacitor nMOS I-V Characteristics
pMOS I-V Characteristics DC characteristics and transfer function Noise margin Latchup Pass transistors Tristate inverter Jan 2015 CMOS Transistor
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Introduction So far, we have treated transistors as ideal switches
An ON transistor passes a finite amount of current Depends on terminal voltages Derive current-voltage (I-V) relationships Transistor gate, source, drain all have capacitance I = C (DV/Dt) → Dt = (C/I) DV Capacitance and current determine speed Also explore what a “degraded level” really means Jan 2015 CMOS Transistor
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MOS Capacitor Gate and body form MOS capacitor Operating modes
Accumulation Depletion Inversion Jan 2015 CMOS Transistor
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Terminal Voltages Mode of operation depends on Vg, Vd, Vs
Vgs = Vg – Vs Vgd = Vg – Vd Vds = Vd – Vs = Vgs - Vgd Source and drain are symmetric diffusion terminals By convention, source is terminal at lower voltage Hence Vds 0 nMOS body is grounded. First assume source is 0 too. Three regions of operation Cutoff Linear Saturation Jan 2015 CMOS Transistor
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nMOS Cutoff No channel Ids = 0 Jan 2015 CMOS Transistor
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nMOS Linear Channel forms Current flows from d to s e- from s to d
Ids increases with Vds Similar to linear resistor Jan 2015 CMOS Transistor
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nMOS Saturation Channel pinches off Ids independent of Vds
We say current saturates Similar to current source Jan 2015 CMOS Transistor
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I-V Characteristics In Linear region, Ids depends on:
How much charge is in the channel How fast is the charge moving Jan 2015 CMOS Transistor
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Channel Charge MOS structure looks like parallel plate capacitor while operating in inversion Gate – oxide – channel Qchannel = CV C = Cg = eoxWL/tox = CoxWL V = Vgc – Vt = (Vgs – Vds/2) – Vt (Vgc – Vt is the amount of voltage attracting charge to channel beyond the voltage required for inversion) Jan 2015 CMOS Transistor
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Carrier velocity Charge is carried by e-
Carrier velocity v proportional to lateral E-field between source and drain v = mE m called mobility E = Vds/L Time for carrier to cross channel: t = L / v Jan 2015 CMOS Transistor
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nMOS Linear I-V Now we know How much charge Qchannel is in the channel
How much time t each carrier takes to cross Jan 2015 CMOS Transistor
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nMOS Saturation I-V If Vgd < Vt, channel pinches off near drain
When Vds > Vdsat = Vgs – Vt Now drain voltage no longer increases current Jan 2015 CMOS Transistor
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nMOS I-V Summary Shockley 1st order transistor models Jan 2015
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Example We will be using a 0.18 mm process for your project tox = 40 Å
m = 180 cm2/V*s Vt = 0.4 V Plot Ids vs. Vds Vgs = 0, 0.3, 0.6, 0.9, 1.2, 1.5 and 1.8V. Use W/L = 4/2 l Jan 2015 CMOS Transistor
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pMOS I-V All doping and voltages are inverted for pMOS
Mobility mp is determined by holes Typically 2-3x lower than that of electrons mn Thus pMOS must be wider to provide same current In this class, assume mn / mp = 2 Jan 2015 CMOS Transistor
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DC Transfer Characteristics
Objective: Find the variation of output voltage Vout for changes in input voltage Vin. Vtp – Threshold voltage of p-device Vtn – Threshold voltage of n-device Jan 2015 CMOS Transistor
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Recall CMOS device CMOS inverter characteristics is derived by solving for Vinn=Vinp and Idsn=-Idsp Jan 2015 CMOS Transistor
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CMOS inverter is divided into five regions of operation
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I-V Characteristics Make pMOS is wider than nMOS such that bn = bp
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Current vs. Vout, Vin Jan 2015 CMOS Transistor
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Load Line Analysis For a given Vin: Plot Idsn, Idsp vs. Vout
Vout must be where |currents| are equal in Jan 2015 CMOS Transistor
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Load Line Summary Jan 2015 CMOS Transistor
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DC Transfer Curve Transcribe points onto Vin vs. Vout plot Jan 2015
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Operating Regions Revisit transistor operating regions Region nMOS
pMOS A Cutoff Linear B Saturation C D E Jan 2015 CMOS Transistor
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Beta Ratio If bp / bn 1, switching point will move from VDD/2
Called skewed gate Other gates: collapse into equivalent inverter Jan 2015 CMOS Transistor
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DC Transfer function is symmetric for βn=βp
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Noise Margin It determines the allowable noise at the input gate (0/1) so the output (1/0) is not affected Noise margin is closely related to input-output transfer function It is derived by driving two inverters connected in series Jan 2015 CMOS Transistor
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Jan 2015 CMOS Transistor
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Jan 2015 CMOS Transistor
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Impact of skewing transistor size on noise margin
Increasing (decreasing) P / N ratio increases (decreases) the low noise margin and decreases (increases) the high noise margin Jan 2015 CMOS Transistor
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Latchup in CMOS Circuits
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Latchup may be induced by power supply glitches or incident radiation
Parasitic bipolar transistors are formed by substrate and source / drain devices Latchup occurs by establishing a low-resistance paths connecting VDD to VSS Latchup may be induced by power supply glitches or incident radiation If sufficiently large substrate current flows, VBE of NPN device increases, and its collector current grows. This increases the current through RWELL. VBE of PNP device increases, further increasing substrate current. Jan 2015 CMOS Transistor
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Jan 2015 CMOS Transistor
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If bipolar transistors satisfy βPNP x βNPN > 1, latchup may occur.
Operation voltage of CMOS circuits should be below Vlatchup. Remedies of latchup problem: Reduce Rsubstrate by increasing P doping of substrate by process control. Reducing RWELL and resistance of WELL contacts by process control. Layout techniques: separation of P and N devices, guard rings, many WELL contacts (at design). Jan 2015 CMOS Transistor
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Pass Transistors We have assumed source is grounded
What if source > 0? e.g. pass transistor passing VDD Vg = VDD If Vs > VDD-Vt => Vgs < Vt Hence transistor would turn itself off nMOS pass transistors pull no higher than VDD-Vtn Called a degraded “1” Approach degraded value slowly (low Ids) pMOS pass transistors pull no lower than Vtp Jan 2015 CMOS Transistor
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Pass Transistor CKTs As the source can rise to within a threshold voltage of the gate, the output of several transistors in series is no more degraded than that of a single transistor. Jan 2015 CMOS Transistor
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Transmission Gates Single pass transistors produce degraded outputs
Complementary Transmission gates pass both 0 and 1 well Jan 2015 CMOS Transistor
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Transmission gate ON resistance as input voltage sweeps from 0 to 1(VSS to VDD), assuming that output follows closely. Jan 2015 CMOS Transistor
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Tristates Tristate buffer produces Z when not enabled EN A Y Z 1
Z 1 Jan 2015 CMOS Transistor
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Nonrestoring Tristate
Transmission gate acts as tristate buffer Only two transistors But nonrestoring Noise on A is passed on to Y Jan 2015 CMOS Transistor
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Tristate Inverter Tristate inverter produces restored output
Violates conduction complement rule Because we want a Z output Jan 2015 CMOS Transistor
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Multiplexers 2:1 multiplexer chooses between two inputs S D1 D0 Y X 1
X 1 Jan 2015 CMOS Transistor
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Gate-Level Mux Design How many transistors are needed? 20 Jan 2015
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Transmission Gate Mux Nonrestoring mux uses two transmission gates
Only 4 transistors Jan 2015 CMOS Transistor
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Inverting Mux Inverting multiplexer Use compound AOI22
Or pair of tristate inverters Essentially the same thing Noninverting multiplexer adds an inverter Jan 2015 CMOS Transistor
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4:1 Multiplexer 4:1 mux chooses one of 4 inputs using two selects
Two levels of 2:1 muxes Or four tristates Jan 2015 CMOS Transistor
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Sizing for Performance
NMOS and PMOS diffusion + diffusion-gate overlap. Fan-out (input gates) + interconnects. Equivalent gate resistance. Capacitive load of an inverter. S sizing factor. Propagation delay: Inverter delay loaded only by intrinsic. Jan 2015 CMOS Transistor
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Intrinsic cap to gate cap ratio ≈1.
Effective fan-out. The delay of an inverter is only a function of the ratio between its external load cap to its input cap 1 2 N In Out Jan 2015 CMOS Transistor
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It implies that same sizing factor f is used for all stages.
imply It implies that same sizing factor f is used for all stages. The optimal size of an inverter is the geometric mean of its neighbor drives Given and , and the optimal sizing factor is The minimum delay through the chain is Jan 2015 CMOS Transistor
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What should be the optimal N ?
The derivative by N of yields or equivalently having a closed form solution only for γ=0, a case where the intrinsic self load is ignored and only the fan-out is considered. Jan 2015 CMOS Transistor
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