electronics - polito.it · 2016. 3. 1. · teristic of a mos transistor. compare with corresponding...

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Politecnico di Torino - ICT School

Analog and Telecommunication Electronics

A1 – MOS and Bipolar transistor

» MOS structure and characteristic

» BJT structure and characteristic

» Large signal models

» Small signal model

AY 2015-16

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Lesson A1: MOS and BJT devices

• A refresh lesson!

• MOS and BJT structures and characteristic – Comparison of structure, parameters, models

– Emphasis on similarities

• Operating regions

• Large signal models

• Small signal model

• Difference MOS/BJT

– Some drawings from:» F. Fiori: Introduction to CMOS Analog Circuits.

» F. Bonani: High speed Electron Devices slides

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Structure of N- and P-MOS devices

• N-MOS

• P-MOS

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Symbols fom MOS devices

• Correspondence with BJT: SE; GB; DC

• Current flowing in arrow direction

• N-MOS: (npn BJT)

• P-MOS: (pnp BJT)

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Structure of NPN and PNP devices

• NPN(N-MOS)

• PNP(P-MOS)

– (real devices use mostly planar structures)

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Correspondence MOS/BJT devices

• SE; GB; DC

• Current can flow in arrow direction

• N-MOS npn BJT

• P-MOS: pnp BJT

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MOS output characteristics

• Operating regions– Triode Low VDS

– Saturation Const. Id(“active” region in BJT)

– (cutoff)

VGS

VDS

ID[mA]

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BJT output characteristic

• Operating regions

– Saturation Low VCE

– Active Constant Ic

– (cutoff)

IB

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Switch or amplifier?

• Use as amplifier

– BJT: Active regionMOS: Saturation

• Switch ON– BJT: Saturation

MOS: Resistive

• Switch OFF– Cutoff

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Operation in a circuit

• A LOAD LINEmodels the external circuit

• Operating points must be on load line AND on device I(V) LOAD

LINE

OPERATING POINT

Vcc

Vcc/Rc

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BJT input characteristic and model

• BE junction:– Emitter current (approx.)

– VBE ≈ 0.6 V

• Single model fitsall operating regions

• Strong temperature dependence

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MOS models

• Model depends on operating point

– Low Vgs(sub-threshold):

» Exponential

– Medium Vgs:» Square law

– High Vgs:» Linear

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MOS transcharacteristic

• No unique Id(Vgs) relation

– Weak inversion:exponential

– Strong inversion:quadratic

iD

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MOS Triode region

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MOS as variable resistor

• MOS with low VDS rDS = dVDS/dID depends on VGS

• BJT has fixed dVCE/dIC not good as variable resistor

MOS BJT

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Channel pinch-off

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MOS Channel lenght modulation

• Id depends on Vds

• Model in saturation region must include an equivalent output resistance rO

• Similar to Early effect in BJT

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BJT Early effect

• As VCE increases base region width decreases– In the active region, IC depends on VCE

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BJT Early voltage

• rO in small signal model

• Similar to channel lenghtmodulation in MOS

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MOS large signal model (saturation)

PowerMOS-FET

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BJT Ebers-Moll model

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MOS small-signal model

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MOS gm in triode and saturation regions

• Key parameter: W/L

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BJT small-signal model

• If VBE >> VT

• Linear approximantion(tangent) near an operating point (VBE0, IB0)

• Simplified models– Hibrid

– Transconductance

gm vBE

vBE

B C

E

IC

VBE

VBE0

IC0 VBE0, IB0

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BJT simplified models

1) Simplified model for bias point analysis (active area)

2) Simplified model for small signal analysis, CE configuration.Parameters:hfe iB or gm vBE

gm = IC/VT

hie = VT * hfe/IC

B C

E

IB IB

gm vBE

vBE

B C

E

hOE

hfe iB

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MOS simplified models

1) Bias point analysis with nonlinear model• Define the parameters to be used for small-signal analysis

• Use Id(Vgs) equation (order 2 or better model)

2) Simplified model for small signal analysis (similar to BJT), CS configuration.Parameters: gm , rD

gm = ….(II ord)

rD = dVDS/dID

gm vGS

vGS

B D

ES

G

rD

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MOS parasitic capacitances

• C3 CGS

• C4 CGD

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Summary of MOS and BJT models

• Small signal– BJT, MOS, MOS-FET

– Same linear model (gm or hybrid)

• Large signal: same method, different models– BJT: exponential large signal model (rather simple)

– MOS: lin/exp/quadratic large signal model (complex !)

– Analytic model for BJT

– Analytic or heuristic models for MOS

– Similar effects and countermeasures» Distortion, harmonics, gain compression, …

» Feedback, tuned circuits, ….

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Operating limits: Safe Operating Area

Ohmic region (MOS: a low-value resistor)

Too high current

Too high V x I (power)

Toohigh voltage

Active & SafeOperatingArea (SOA)

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Evaluation of PDMAX

• Bipolar device:– PD = IC VCE; VCE = VCC – IC RC

– Max PD for d(PD)/dIC = 0 VCE = VCC/2

– PDMAX = …

• MOS device – same approach:– PD = ID VDS; VDS = VDE – ID RD

– Max PD for … VDS = VDD/2

– PDMAX = ….

• In both cases PD has parabolic shape– PD = 0 at cutoff (I = 0) and saturation (V ≈ 0)

– Max at the mid-point

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PD vs operating point

• PD has parabolic shape– PD = 0 at cutoff (I = 0)

and saturation (V ≈ 0)

– Max at the mid-point

• Critical parameters must be verified for PDMAX

– VCE = VCC – IC RC

– PDMAX = (ICmax /2) * (VCEmax /2);

• Same shape for MOS– Parameters ID, VDS

IC

PDMAX

PD

ICmax VCE

VCEmaxVCEmax/2

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:

Lesson A1 – final test

• Draw the static Id(Vds) characteristic of a MOS transistor. Compare with corresponding Ic(Vce) characteristic of BJT.

• Describe the difference between n-channel and p-channel devices.

• Describe a small signal model for MOS and BJT and related parameters.

• Which are the limits of small signal models?

• Draw small signal models for MOS and BJT with parasitic capacitances.

• Compare large signal models for MOS and BJT.

• Describe the difference between analytical and heuristic models.

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electronics - polito.it · 2016. 3. 1. · teristic of a mos transistor. compare with corresponding...

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