Analog Integrated Circuit Design 2nd Lecture PDF

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MOS transistor – circuit symbols and terminals n-MOS

p-MOS

• basic active component of circuit design • Gate – the control terminal, high impedance • Drain, Source – (usually) symmetrical, interchangeable • don’t forget about the 4th terminal! 38

MOSFET current equation

Poly-Si SiO2







Bulk Silicon

 



   

L

0

 )

Poly-Si Gate

Bulk Silicon SiO2



=

 

Ef



 









Drain

 

y



L

Source0 39

MOS I-V curves Triode (ohmic) region: I D   C OX

(VDS < (VGS-VTH)

W  V DS   V DS  V TH  V GS  L  2 

Saturation region (above threshold / weak inversion): (VDS > VGS -VTH) ID 

 C OX W 2

L

V GS

 V TH

2

K   C OX - technology dependent In saturation region the channel is not fully formed and in a small drift / saturation region carriers are passing at max. speed 40

MOS I-V curves Triode (ohmic) region: I D   C OX

(VDS < (VGS-VTH)

W  V DS   V DS  V TH  V GS  L  2 

Saturation region (above threshold / weak inversion): (VDS > VGS -VTH) ID 

 C OX W 2

L

V GS

 V TH

2

K   C OX - technology dependent Fully featured MOS models are highly complex but can be greatly simplified for hand analysis here 41

MOS I-V curves Weak inversion (subthreshold): (VGS < VTH , VDS > 4 VTH )

ID  KW

V  V GS W exp  TH nV T L 

  

Ef

Strong inversion: (VGS > VTH , VDS > VGS -VTH)  C OX W V GS  V TH ID  2 L

2

Gate

Oxide Substrate

K   C OX - technology dependent • Current in weak inversion drift / diffusion limited • In strong inversion current is limited by drift of free carriers 42

MOS I-V curves log(Ids)

Weak inversion (subthreshold): (VGS < VTH , VDS > 4 VTH )

ID  KW

V  V GS W exp  TH L nV T 

  

Strong inversion

(VGS > VTH , VDS > VGS -VTH)  C OX W V GS  V TH ID  2 L

K   C OX - technology dependent

2

Weak inversion

Strong inversion:

Vgs Vdd

Fully featured MOS models are highly complex but simplified model for hand analysis can give a lot of insights into circuit sizing and optimization 43

MOS operating regions Triode (ohmic) region: • A flat inversion layer forms in the channel – carriers are free to flow from drain to source • Transistor behaves as a resistor at very low Vds Saturation region: • Saturation is entered when pinch-off region appears – Drain cannot “communicate with source – Carriers travel through pinch-off region at saturated speed • In reality: The current increases slightly with Vds – finite output resistance Weak inversion: • The current is due to diffusion rather then drift • Therefore the current gain is exponential near the threshold voltage • Moderate or weak inversion increasingly common in analog • Good VTH matching – bad Ids matching 44

MOS transistor basics D

n-type MOSFET characteristic

Transfer characteristic

G S

• •

Triode region, linear region for small VDS Saturation region

ideal saturation state: dIds / dVds = 0

VTn Output characteristic

saturation drain voltage Vds =Vdsat ≈ VGS –VTH

p-type MOSFET characteristic is similar, but opposite in sign for all voltages 45

MOSFET nonlinear characteristics Triode region 1   I D     V GS  V TnH ( V BS )  V DS   V DS2  2  

0  V DS  V GS  V Tn

Saturation region ID 

 2

 V GS  V TnH ( V BS ) 

2

0  V GS  V TnH  V DS

with the gain factor (or conductance parameter)   K '

  W W W   n C ox    n 0 ox  L L t ox L

Layout parameter! 46

MOSFET transconductance (gm) • Transconductance is one of the key small-signal parameters for transistors in linear circuits • Measures a change of drain current for a given change of VGS • Determines the amplification properties (in saturation): gm 

I D W  V DS  const   C ox V GS  V TH L  V GS



 C ox

W 2ID ID  L V GS  V TH

For MOS strong inversion, saturation:

47

MOSFET transconductance (cont.) • Strong inversion – increases linear with VGS – VTH and as the square root of IDS – Vdsat >> Vt = nkT/q

g mFET 

2 I ds V dsat

• Sub-threshold – gm same equation as for a BJT – exponential current gain – but low current = low speed

48

MOSFET operating regions and small signal characteristics Linear regions of operation The MOSFET is a nonlinear device, but has operating regimes with fairly linear characteristics which are often used for circuit design Linear region In this part of the triode region with VDS 10-18

> 10-18

> 10-18

3 5

2.5 3

2 2

1.5 1.5

5

5

5

5

20

20

20

20

84

Inductors ... even though not used in most analog and digital circuit application (but often in RF circuits!): • RF coils in the order of a few nH can be integrated on-chip • They tyically suffer from a low quality factors due to: – Metal resistance – Eddy currents in the substrate

• For high speed analog designs the inductance of bondwires need to be considered!

85...