S08 Semiconductor Devices PDF

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Lecture on semiconductors...

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Electronics Engineering Fundamentals Bachelor’s Degree in Aerospace Engineering Semiconductor devices

Electronics Technology Department

Outline • Semiconductorfundamentals • Semiconductordevices • Diode • TransistorMOSFET

ElectronicsEngineeringFundamentals

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Semiconductor fundamentals • Solidswithcrystallinestructurespresentspecialproperties • Siliconisoneofthemostusedsemiconductormaterials T=0ºK

ElectronicsEngineeringFundamentals

How do electrons move along the lattice?

T > 0ºK

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Semiconductor fundamentals • Energylevels  Bandstructure.Availableenergylevelsfor crystallinesolidsthatarenotforsingleatoms • Valenceband • Conductionband • Bandgap Semiconductors have band-gaps around 1 eV Conductor

Insulator

Semiconductor

E

E

E

CB CB

CB Eo VB

Eo

GAP

Eo

VB VB

ElectronicsEngineeringFundamentals

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Semiconductor fundamentals • Conductors:  Conduct electricity  They have free electrons not linked to atoms  Allows the formation of metallic joints. E.g.: Copper • Insulators:  Do not conduct electricity  Do not have free electrons  Share their electrons by covalence joints. • Semiconductors:  Half way between conductors and insulators  They conduct when they have free charges (negative or positive charge carriers)  Intrinsic or Extrinsic semiconductors

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Semiconductor fundamentals • Extrinsicsemiconductors:Addingimpuritiestoincreasethe carriers TYPE n: Substitution of atoms in the crystal with atoms with one electron more in the valence band.

TYPE p: Substitution of atoms in the crystal with atoms with one electron less in the valence band.

Ie: Phosfor (P). DONOR Atom

Ie: Boron (B). ACCEPTOR Atom

e- : Majority carriers h+ : Minority carriers

ElectronicsEngineeringFundamentals

T=0ºK

h+ : Majority carriers e- : Minority carriers

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Semiconductor devices • Electronics is based on using semiconductor devices • These devices present different behaviours depending on their operating conditions. • Advanced manufacturing process for integrated devices. • Diodes • Transistors

ElectronicsEngineeringFundamentals

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Diode

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Diode • PNjunction

+

P

Free charge: h+

+ -+ - + -+ -+- + -

– –+ –+ –+ –+ –+ –+ N

-

+

Fixed charge

Free charge: e-

Depletion region (no free majority carriers due to recombination)

E +

Volts

At equilibrium  No overall current

-

Potential barrier ElectronicsEngineeringFundamentals

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Diode Forward bias

• PNjunction P

+ -+ - + -+ -+- + -

P

N

-

+

Fixed charge

–+ –+ –+ –+ –+ –+

N

+++ – – – + + + +I – – – – +++ – – – D

+-

Free charge

Net current flow, ID

E

+ +

Volts

-

Volts Potential barrier

ElectronicsEngineeringFundamentals

-

Potential barrier 10

Diode • PNjunction

Reversebias

P

+ -+ - + -+ -+- + -

P

N

-

+

Fixed charge

–+ –+ –+ –+ –+ –+

+ - + +

N

+-

– –+ – Free charge

NO current flow

E

Volts

+

-

+

Volts Potential barrier

ElectronicsEngineeringFundamentals

-

Potential barrier 11

Diode • Twoterminalsemiconductor • Allowthecurrentflowinonedirection:Anode

Cathode

Metallic Contacts

p

STRUCTURE

SYMBOL

id

n

Cathode

Anode

+

vd

-

PACKAGING Anode

ElectronicsEngineeringFundamentals

Cathode

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Diode - model • Idealmodel 1ª Approximation: Ideal Diode

id

Equivalent Circuit

A <

Reversebias ON

C

ID

Cathode

Anode

vd=0

+

id>0

Si VD > 0 => ON

vD

-

vd OFF Equivalent Circuit

A

C

id=0

SHORTCIRCUIT

Forward bias Si VD < 0 => OFF

OPEN CIRCUIT

vd 0 => ON

+ V -

ON

vd=V id>0 V

OFF Equivalent Circuit

A

vd DATASHEET

Constant DC voltage drop (V)

Si VD < 0 => OFF OPEN CIRCUIT

C

id=0 vd Vt

VDS

VGS

N

If VGS >>> Vt

IS S

IG = 0

ID = IS

ElectronicsEngineeringFundamentals

NMOS 22

MOSFET - model • EquivalentCircuitModel:n‐channelMOSFET Saturation region

𝐼  𝑉  𝑉



Cutoff region

𝐼  0 BiasPoint=OperatingPoint  VGS,VDS,ID ElectronicsEngineeringFundamentals

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MOSFET - applications • Amplifiers Input signal vgs Small Signal (AC)

vds

Output signal

Bias (DC)

ElectronicsEngineeringFundamentals

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MOSFET - applications • Amplifiers vi+ vi-

VCC

vo

+ _

𝑇𝐿082 VEE

ElectronicsEngineeringFundamentals

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MOSFET - applications • Switch:Fixingcurrent

Lamp / Motor

ElectronicsEngineeringFundamentals

Forsituations whereVindoes notprovide currentenough foryourload

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MOSFET - applications • Switch:Digitalcircuits • Saturation • Cut‐off

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MOSFET – Bias point (operating point) 1. Assumethetransistorisworkinginsaturation(ID constant) 2. Applysaturationequations 𝐼  𝑉  𝑉 VGS≥Vt



IG= 0 NMOS

3. Usecircuitanalysistechniques(Kirchhoff’sLaws) 4. Determinecurrentsandvoltages 5. Checktransistorisactuallyworkinginsaturation VDS≥VGS +Vt

ElectronicsEngineeringFundamentals

NMOS

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MOSFET – Example VGG =2V,RG =2.5MΩ ,VDD=5V,RD =2kΩ k=0.5mA/V2 Vt=1V

RD RG +

VGG

IG + VGS -

ID + VDS -

+ -

VDD

-

ElectronicsEngineeringFundamentals

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