Chap.1 Physics and Modelling of MOSFETs 반도체 연구실 신입생 세미나 박 장 표 2009 년 1 월 8 일

Contents Basic MOSFET Characteristics Current – Voltage Characteristics p-Channel MOSFETs Geometric Scaling Theory Small – Device Effects Small Device Model 2

1.1 Basic MOSFET Characteristics The MOS Threshold Voltage Body Bias 3

Basic MOSFET Characteristics MOSFET used as a Switch I D determine by V GS & V DS ( also V SB affects lesser degree ) 4

Basic MOSFET Characteristics W, L are important dimension for electrical characteristics Aspect ratio : W / L 5

Basic MOSFET Characteristics  V GS V T : active mode  ID depends on the voltages applied The MOS Threshold Voltage : used to enhance the conduction between the drain and source 6

Basic MOSFET Characteristics  MOS system : altering the charge distribution at the surface  7

Basic MOSFET Characteristics For small values of V G  Create depletion region referred to as bulk charge  The surface charge is made up entirely of bulk charge  Bulk charge consists of ionized acceptor atom, it is immobile 8

For V G > V T  initiates thin electron inversion layer when V G = V T  Basic MOSFET Characteristics 9

The MOS Threshold Voltage Basic MOSFET Characteristics 10

Basic MOSFET Characteristics Body Bias 11

1.2 Current – Voltage Characteristics Square-Law Model Bulk-Charge Model 12

Current – Voltage Characteristics Cutoff when V GS < V T 13

Current – Voltage Characteristics Active when V GS > V T 14

Current – Voltage Characteristics Square-Law Model 15

Current – Voltage Characteristics Channel Length Modulation 16

Current – Voltage Characteristics 17

Current – Voltage Characteristics 18

Current – Voltage Characteristics Bulk-Charge Model 19

1.3 p-Channel MOSFETs 20

p-Channel MOSFETs 21

p-Channel MOSFETs 22

 Cutoff ( V SGp < l V Tp l )  Active (V SGp > l V Tp l ) p-Channel MOSFETs 23

1.4 MOSFET Modelling Drain-Source Resistance MOSFET Capacitances Junction Leakage Currents 24

MOSFET Modelling 25

MOSFET Modelling Drain-Source Resistance 26

MOSFET Modelling MOSFET Capacitances 27

MOSFET Modelling MOS-Based Capacitances 28

MOSFET Modelling 29

Depletion Capacitance MOSFET Modelling 30

Depletion Capacitance in Drain & Source region MOSFET Modelling 31

 Zero-bias source/drain bulk capacitance MOSFET Modelling 32

 C av using a simpler LTI element General model for voltage-dependent depletion capacitance m : grading coefficient, such that m<1 MOSFET Modelling 33

Device Capacitance Model  Use the LTI average of the depletion capacitance MOSFET Modelling 34

Junction Leakage Currents MOSFET Modelling 35

 Drain / Source are always at a voltage greater than or equal to 0v  Bulk is will always exhibit leakage flows regardless of the state of the conduction of the transistor MOSFET Modelling 36

General doping profile ( m : grading coefficient ) MOSFET Modelling 37

1.5 Geometric Scaling Theory Full-Voltage Scaling Constant-Voltage Scaling Second-Order Scaling Effects 38

Geometric Scaling Theory 39

Geometric Scaling Theory 40

Geometric Scaling Theory Full Voltage Scaling 41

Constant-Voltage Scaling Geometric Scaling Theory 42

Second-Order Scaling Effects Geometric Scaling Theory  First-Order Scaling Effects deals with MOSFET dimensions, doping level, voltages, and currents  Second-Order Scaling Effects for example of by increased impurity scattering  Second-Order Scaling Effects for example of in V T In the flat band voltage as is scaled 43

1.7 Small-Device Effects Threshold Voltage Modifications Mobility Variations Hot Electrons 44

Small-Device Effect Threshold Voltage Modifications Basic threshold voltage Charge – voltage relation by area  Gate voltage does not support all of the bulk char with an area of WL 45

Short-Channel Effect Small-Device Effect 46

Small-Device Effect Using Pythagorean theorem 47

Small-Device Effect 48

Narrow Width Effect Small-Device Effect total area of region 49

Small-Device Effect Since the area for Another approach : empirical factor When W 50

Mobility Variations Small-Device Effect  Ignore the V GS induces the field effect will alter the local electric field 51

Small-Device Effect [Exam] L=0.5um, V DS =2V, estimate the Channel electric field  Electron temperature For low electric fields : cold electron region curve goes nonlinear : warm electron region reaches the : hot electron region by Particle kinetic energy to the thermal energy 52

Small-Device Effect Hot Electrons  Particularly important : L < 1 um  Highly energetic particles can leave the silicon and enter the gate oxide  leading to instability of the threshold voltage  Long-term reliability problems may result  May induce leakage gate currents and excessive substrate currents The LDD MOSFET  Particularly important : L < 1 um  Highly energetic particles can leave the silicon and enter the gate oxide  leading to instability of the threshold voltage  Long-term reliability problems may result  May induce leakage gate currents and excessive substrate currents Maximum value of the built-in electric field 53

1.7 Small Device Model 54

Small Device Model Critical electric field Field-dependent velocity 55

Small Device Model Non-saturated current Saturation current for V DS > V Sat 56