MOSFET DEVICES (UNIVERSITY)

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Metal-Semiconductor (MS) Junction

Introduction

Schottky Diode

Ohmic Contact



MS Junction

- Metal-semiconductor contacts are the most common form of junction in

integrated circuits

- The great majority of metal-semiconductors are used for ohmic contact

- Ohmic contact has a very low resistance to make the electrons and holes

move freely in and out of the devices

- The other type of metal-semiconductor contact has a rectification

capability and is called Schottky diode

- Schottky diode has a rectification capability, with a large current in forwardbias and very low leakage current in revise bias

- Schottky diodes are often used in digit circuits as fast switches

Introduction



Schottky Diode: Energy Band Diagram

Consider an ideal metal-semiconductor (n-type) junction with metal work

function larger than semiconductor work function.

E0

EFS

Ei

EV

EC

EFM

qqs

qM

metal semiconductor (Si)

…… electron affinityS …… work-function of silicon

M …… work-function of metal

E0 …… vacuum level



Thermal Equilibrium




Forward bias condition (V A positive, semiconductor grounded)


- The Fermi level will be separatedby qV A, EFS will be lifted up and

above EFM.

- Built-in potential decreases and

equals to

- Barrier height remains unchanged,

still equal to

)( Ai V q

B .



Reverse bias condition (V A is negative, semiconductor grounded)


.

- The Fermi level will be separated byqV A, EFM will be lifted up and

above EFS

- Built-in potential decreases and

equals to

- Barrier height remains unchanged,

still equals to

)()( Ai Ai V qV q

B



Thermal Equilibrium

Schottky Diode: Charge Transport

.



Forward bias condition (V A positive, semiconductor grounded)




Reverse bias condition (V A negative, semiconductor grounded)




Overall Current-Voltage Characteristics


Voltage



Electrostatic Analysis

Thermal Equilibrium

- Solving 1-D Poisson equation as in PN junction

++++

+Xd

ND

Q=qNDxd

Q

xd

E

xd

Vbi=

V

qNDxd22Si

xd=[ ]qND

2SiVbi 1/2 solving for xd

qNDxdSi

-

Depletion region

Quasi-neutral region



Electrostatic Analysis

Non-Equilibrium: With applied bias V A



Step 3: Using the following boundary conditions that

Step 4: The current density J can be solved as: (VT=kT/q)

Step 5: From above equation, JS is dependent on V A, a more useful

equation of current-voltage is as follow

n is an empirical factor (called the ideality factor) which ranges from

1.02 to 1.15.

Current-Voltage Characteristics

]1)[exp( T

AS

V

V J J

)exp()(22

T

B

s

D AicnS

V

N V q

kT

N Dq J

where,

]1)[exp(' T

AS

nV

V J J )exp(

22

'

T

B

s

DicnS

V

N q

kT

N Dq J

where,



Ohmic Contacts

Case I: Ohmic contact with a small work function metal

Considering n-type Si with metal

Thermal Equilibrium

0 MSOSMOO

J J J



Ohmic Contacts


Forward Bias

0 SMF MSOSMF MSF SMF F J J J J J J

Note that this JSMF is larger than

JSMF in the Schottky diode

because the ohmic contact,

unlike the Schottky diode whichhas a depletion region, has an

accumulation region and thus

has more electrons to transport

from the semiconductor to metal.



Ohmic Contacts


Reverse Bias

0 MSO MSOSMR MSRSMR R J J J J J J

Note that this JMSO is normally

much larger than the JMSO

in the

Schottky diode



Ohmic Contacts


Current-Voltage Characteristics



Ohmic Contacts

Case II: Ohmic contact with tunneling current mechanism

- Sometimes, it is difficult to form ohmic contact.

(Why)

- In this case, a highly doped layer in

semiconductor is always useful.

- The heavily doped layer increase the junction

barrier height and makes the thickness of the

depletion width extremely narrow, whichincreases the probability of electron tunneling

from the semiconductor to the metal

- This tunneling mechanism thus increases the

current flows through the junction and reducesthe contact resistance.

- For the metal-semiconductor (p-type) junction,

ohmic contact is mainly formed with the

tunneling mechanism.



Comparison: MS versus PN Junction

Performance

Faster in charging/discharging during transient as no accumulation ofminority carrier necessary

No high-level injection problem

Ideality factor close to 1 (ideal case)

Smaller Vbi, smaller turn on voltage and operation range, lowerbreakdown voltage

Applications

Use as rectifiers and other diode circuits that require fast switching

Use as clamping device in BJT to prevent the entering into saturationoperation

Realistic M-S contact and barrier structure (barrier lowering due to mirror

force)

n

metal

cross-section

< qBqVbi

xM

EC

EF

EV

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