chapter 1 2 mayer

8/6/2019 Chapter 1 2 Mayer

1/32

Overview of the two

first chapters

Instructor: Dr.Nguyn V Thng


2/32

Outline

Introduction

Models for MOS transistor

Passive Components in MOS Technology Conclusion


3/32

Introduction

There is significant interaction between circuitand device design

Any analysis is only accurate as theappropriate model is used


4/32

Outline

Introduction

Models for MOS transistor

The influence of device fabrication ondevice characteristics

Conclusion


5/32

Depletion Region of a pn Junction

These junctions all contribute voltage-dependent parasitic capacitances

Important characteristics such as of activedevices depend directly on the properties ofthis region

The width of the depletion region controls thebasic operation of MOS transistor


6/32

Depletion Region of a pn Junction


7/32

The penetration of the depletion layer:

Into the p-type region

Into the n-type region

If either ND or NA is much larger than the other,the depletion region exists almost entirely in thelightly doped region


8/32

Voltage-dependent parasitic

capacitances


9/32

Junction Breakdown

The maximum electric field

There is an large increase in reverse current by the

effect of avalanche breakdown when the reverse

bias approaches the breakdown voltage BV

Zener diodes which operates in the avalancheregion are widely used as voltage references


10/32

Transfer Characteristics of MOS

Devices


11/32

Transfer Characteristics of MOS Devices


12/32

Transfer Characteristics of MOS

Devices In operation, the gate-source voltage modifies the

conductance of the region under the gate, allowing thegate voltage to control the current flowing betweensource and drain

The gate-source voltage VGS required to produce aninversion layer is called the threshold voltage Vt

fSBfT

ox

ss

ox

bfmst

VV

CQ

CQV

JJK

JJ

22

2

0!

!


13/32

NMOS device characteristics.


14/32

The drain current

In active region The effect of channel-length modulation :

Xd is function of VDS in the pinch-off region=>

ID varies with VDS =>

In triode region


15/32

Decomposition of Gate-Source

VoltageVt

VGS Vov

The overdrive

These two components have different

properties


16/32


17/32

Intrinsic Gate-Source and Gate-Drain

Capacitance In the triode region: the channel exists

continuously from source to drain

In the saturation region: the channel pinches offbefore reaching the drain

and


18/32

Output Resistance

The effect of channel-length modulation causes thedrain current to increase when the drain-sourcevoltage is increased


19/32

Basic Small-Signal Model of the MOS

Transistor Vgs =>id =>The total drain current Idincreasing the gate-source voltage in an n-channel

transistor increases the channel conductivity and draincurrent

Compare with p-channel transistor


20/32

BodyTransconductance

The body-source voltage changes the threshold,which changes the drain current

The substrate acts as a second gate => body effect

The ratio gmb/gm is an important quantity in practice

This ratio is the rate of change of threshold voltagewith body bias voltage


21/32

Parasitic Elements in the Small-Signal

Model


22/32

Small-signal MOS transistor equivalent

circuit


23/32

MOS Transistor Frequency Response

The transition frequency fT : the frequency where themagnitude of the short-circuit, common-source currentgain falls to unity

Assume Cgs >>Cgd + Cgd, then

fT increases as the inverse square of the critical devicedimension across which carriers are in transit


24/32

Effective Channel Length

The side or lateral diffusions of the source and thedrain under the gate => Ld

A depletion region exists between the drain region andthe end of the channel => Xd

The Effective Channel Length


25/32

Short-Channel Effects

Short-channel effects become important in MOStransistors at channel lengths of about 1 um or less

Require modifications to the MOS models given

previously

Velocity Saturation from the Horizontal Field

Transconductance and Transition Frequency


26/32

Velocity Saturation from the

Horizontal Field At low field values is proportional to the field, the

velocity at high field values approaches a constant calledthe scattering-limited velocity vscl

The drain current is a linear function of the overdrive(VGS - Vt) when the carrier velocity saturates

=> Model of velocity saturation in an MOSFET by additionof series source resistance to an ideal square-lawdevice.


27/32

Transconductance and Transition

FrequencyWhen the velocity is saturated:

Further decreases in L or increases in (Vgs - Vt) do notchange the transconductance

Reduces the transconductance-to-current ratio for agiven overdrive.

The transition frequency is independent of the overdrive.

The transition frequency is inversely proportional to the

channel length


28/32

Substrate Current Flow in MOS

Transistors

Impact ionization

This phenomenon creates a parasitic resistance fromdrain to substrate and shunts the drain to ac ground.

Model by inclusion of a controlled current generator IDBfrom drain to substrate

Affect to high output impedance of MOS current mirrors(chapter 4)


29/32

Resistors

Diffused Resistors

Polysilicon Resistors

Well Resistors

MOS Devices as Resistor (sheet resistance)


30/32

Capacitors in MOS Technology

Poly-Poly Capacitors

MOS Transistors as Capacitors


31/32

Conclusion

The contents of these two chapters are basicknowledge for Analog IC designer

It is the platform to study the remainingchapters of this book, especially chapter 7


32/32

Thank for your attention !

chapter 1 2 mayer

Documents