PHYSICAL ELECTRONICS(ECE3540) CHAPTER 9 METAL SEMICONDUCTOR AND - - PowerPoint PPT Presentation

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PHYSICAL ELECTRONICS(ECE3540)

CHAPTER 9 –METAL SEMICONDUCTOR AND

SEMICONDUCTOR HETERO-JUNCTIONS

CHAPTER 9 –METAL SEMICONDUCTOR AND

SEMICONDUCTOR HETERO-JUNCTIONS

Brook Abegaz

Introduction

• Chapter 4: we considered the semiconductor in

equilibrium and determined electron and hole concentrations in the conduction and valence bands, respectively.

• The net flow of

the electrons and holes in a semiconductor generates current. The process by which these charged particles move is called transport.

• Chapter 5: we considered the two basic transport

mechanisms in a semiconductor crystal: drift: the movement of charge due to electric fields, and diffusion: the flow of charge due to density gradients.

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Introduction

• Chapter 6: we discussed the behavior of non-

equilibrium electron and hole concentrations as functions of time and space.

• We developed the ambi-polar transport equation

which describes the behavior of the excess electrons and holes.

• Chapter 7: We considered the situation in which a

p-type and an n-type semiconductor are brought into contact with one another to form a PN junction.

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Introduction

• Chapter 8: We considered the PN junction with

a forward-bias applied voltage and determined the current-voltage characteristics.

• When holes flow from the p region across the

space charge region into the n region, they become excess minority carrier holes and are subject to excess minority carrier diffusion, drift, and recombination.

• When electrons from the n region flow across the

space charge region into the p region, they become excess minority carrier electrons and are subject to these same processes.

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Introduction

• When a sufficiently large reverse-bias voltage is applied

across a PN junction, breakdown can occur, producing a large reverse-bias current in the junction, which can cause heating effects and catastrophic failure of the diode.

• Zener diodes are designed to operate in the breakdown
• region. Breakdown puts limits on the amount of voltage

that can be applied across a PN junction.

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Introduction

• Chapter

9:

we will consider the metal- semiconductor junction and the semiconductor hetero-junction, in which the material on each side

• f the junction is not the same. These junctions can

also produce diodes.

• An Ohmic contact is a low-resistance junction

providing current conduction in both directions. We will examine the conditions that yield metal- semiconductor Ohmic contacts.

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Metal-Semiconductor Junction

• There are two kinds of metal-semiconductor contacts:

 Rectifying Schottky diodes: metal on lightly doped

Silicon.

 Low-resistance Ohmic contacts: metal on heavily doped

Silicon.

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The Schottky Barrier Diode

• Rectifying contacts are mostly made of n-type semiconductors;

for this reason we will concentrate on this type of diode.

• In the ideal energy-band diagram for a particular metal and n-

type semiconductor, the vacuum level is used as a reference.

• The parameter M is the metal work function (in volts), s is the

semiconductor work function, and  is known as the electron affinity.

• Before contact, the Fermi level in the semiconductor was above

that in the metal. In order for the Fermi level to become a constant through the system in thermal equilibrium, electrons from the semiconductor flow into the lower energy states in the metal.

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The Schottky Barrier Diode

• The parameter B0 is the ideal barrier height of the

semiconductor contact, the potential barrier seen by electrons in the metal trying to move into the semiconductor.

• The barrier is known as the Schottky barrier and is given as:
• On the semiconductor side, is the built-in potential barrier.

This barrier, similar to the case of the PN Junction, is the barrier seen by electrons in the conduction band trying to move into the metal Vbi is given as:

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) (     

M B

) (

n B bi

V    

The Schottky Barrier Diode

• Fig. 9.1: Ideal energy-band diagram of a metal-semiconductor junction

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Bn Increases with Increasing Metal Work Function

Theoretically,

Bn= M – Si

 M

 Si

: Work Function

• f metal

: Electron Affinity of Si

qBn Ec Ev Ef E0 q M Si = 4.05 eV Vacuum level, x = 0 x = xn

The Schottky Barrier Diode

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• Schottky barrier height, B ,

is a function of the metal material.

• B is the most important
• parameter. The sum of qBn

and qBp is equal to Eg .

Metal

Depletion layer

Neutral region qBn

Ec Ec Ef Ef Ev Ev

qBp N-Si P-Si

• Fig. 9.2: Energy Band Diagram of Schottky Contact

The Schottky Barrier Diode

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Schottky barrier heights for electrons and holes Bn increases with increasing metal work function

Metal Mg Ti Cr W Mo Pd Au Pt  Bn (V) 0.4 0.5 0.61 0.67 0.68 0.77 0.8 0.9  Bp (V) 0.61 0.5 0.42 0.3 Work Function 3.7 4.3 4.5 4.6 4.6 5.1 5.1 5.7  m (V)

Bn + Bp  Eg

The Schottky Barrier Diode

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• A high density of energy

states in the band gap at the metal-semiconductor interface pins Ef to a narrow range and Bn is

typically 0.4 to 0.9 V

• Question:

What is the typical range of Bp?

qBn Ec Ev Ef E0 qM Si = 4.05 eV Vacuum level, + 

• Fig. 9.3: Fermi Level Pinning

The Schottky Barrier Diode

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Schottky Contacts of Metal Silicide on Si Silicide-Si interfaces are more stable than metal-silicon interfaces. After metal is deposited on Si, an annealing step is applied to form a Silicide-Si contact. The term metal-silicon contact includes and almost always means Silicide-Si contacts. Silicide: A Silicon and metal compound. It is conductive similar to a metal.

Silicide ErSi1.7 HfSi MoSi2 ZrSi2 TiSi2 CoSi2 WSi2 NiSi2 Pd2Si PtSi f Bn (V) 0.28 0.45 0.55 0.55 0.61 0.65 0.67 0.67 0.75 0.87 f Bp (V) 0.55 0.49 0.45 0.45 0.43 0.43 0.35 0.23

• Table. 9.1: Schottky Contacts of Metal Silicide on Si

The Schottky Barrier Diode

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s n d dep s d R bi s n dep d c Bn f c Bn bi

x eN E A W C qN V V x W N N kT q E E q qV                | | ) ( 2 ln ) (

max

Question: How should we plot the CV data to extract bi?

Ev Ef Ec qbi qBn Ev Ec Ef qBn q(bi + V) qV

• Fig. 9.4: Using C-V Data to Determine B

Exercise

• 1. Consider a contact between Tungsten and an n-type

Silicon doped to Nd = 1016 cm-3 at T = 300K. Calculate the theoretical barrier height, built-in potential barrier and maximum electric field in the metal-semiconductor diode for a zero applied bias. Use the metal work function for Tungsten as M = 4.55V and electron affinity for Silicon  = 4.01V.

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) (     

M B

) (

n B bi

V    

) ( 2

d R bi s n dep

qN V V x W    

s n dx

eN E   | |

max

Solution

B0 is the ideal Schottky barrier height. The space charge width at a zero bias is:

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V

M B

54 . 01 . 4 55 . 4 ) (        

V V

n B bi

33 . 206 . 54 . ) (       

cm 10 * 0.207 ) 10 )( 10 * 6 . 1 ( ) 33 . )( 10 * 85 . 8 )( 7 . 11 ( 2 ) ( 2

4

• 16

19 14

    

  d R bi s n dep

qN V V x W 

V x N N e kT

d c n

206 . 10 10 8 . 2 ln 0259 . ln

16 19

                   

cm V x eN E

s n d

/ 10 * 2 . 3 ) 10 * 85 . 8 )( 7 . 11 ( ) 10 * 207 . )( 10 )( 10 * 6 . 1 ( | |

4 14 4 16 19 max

  

  



The Schottky Barrier Diode

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2 2

) ( 2 1 A qN V C

s d bi

   

Using CV Data to Determine B

V 1/C

2



bi

E

v

Ef Ec qbi qBn

• Fig. 9.5: Using C-V Data to Determine C

The Schottky Barrier Diode

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2 / / / / 2 3 2 / ) ( 2 / 3 2 / ) (

A/cm 100 where , 4 2 1 / 2 / 3 2 2

kT q sT kT qV sT kT qV kT q n thx M S n thx n th kT V q n kT V q c

B B B B

e J e J e e T h k qm qnv J m kT v m kT v e h kT m e N n

   

  

      

               

Efn

• q(B  V)

qB qV

Metal N-type Silicon V

Efm Ev Ec

x vthx

4 Constant s ' Richardson

3 2 *

h k qm A

n

 

The Schottky Barrier Diode

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Schottky Diodes

V I Reverse bias Forward bias V = 0 Forward biased Reverse biased

The Schottky Barrier Diode

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) 1 ( ) K A/(cm 100 4

/ / 2 2 3 2 * / 2 *

         

   kT qV sT sT kT qV sT S M M S n kT q

e I I e I I I I h k qm A e KT A I

B





Schottky Diodes

4 Constant s ' Richardson

3 2 *

h k qm A

n

 

The Schottky Barrier Diode

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• IsT of a Schottky diode is 103 to 108 times larger than a PN junction diode,

depending on B . A larger I0 means a smaller forward drop V.

• A Schottky diode is the preferred rectifier in low voltage, high current

applications.

kT q sT kT qV sT

B

e AKT I e I I

/ 2 /

) 1 (

 

  

I V

Schottky B

I V

PN junction Schottky diode B diode

Applications of Schottky Diodes

Exercise

• 2. Consider a Tungsten-Silicon diode with a barrier

height of BN = 0.67V and JsT = 6*10-5 A/cm2. Calculate the effective Richardson constant.

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4 Constant s ' Richardson

3 2 *

h k qm A

n

 

kT q Bn

e T A

/ 2 * sT

J

 



Solution

• 1. Using the relation for the reverse saturation current

density:

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kT q Bn

e T A

/ 2 * sT

J

 



2 2 / 2 *

114 cm K A e T J A

kT q sT

Bn

 

 

The Schottky Barrier Diode

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AC DC AC AC DC

utility power

110V/220V PN Junction rectifier

Hi-voltage MOSFET

inverter

100kHz Hi-voltage

Transformer

Schottky rectifier

Lo-voltage

50A 1V feedback to modulate the pulse width to keep Vout = 1V

• Fig. 9.6: Switching Power Supply

Applications of Schottky Barrier Diode

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• Synchronous Rectifier:

For an even lower forward drop, replace the diode with a wide-W MOSFET which is not bound by the tradeoff between diode V and leakage current.

• There is no minority carrier injection at the

Schottky junction. Therefore, Schottky diodes can

• perate at higher frequencies than PN junction

diodes.

Comparison of Schottky Barrier Diode and the PN Junction Diode

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• The ideal current-voltage relationship of the Schottky barrier

diode are of the same form as the PN Junction Diode, there is

• nly a magnitude difference in the reverse-saturation current

densities and the switching characteristics.

• The current in a PN Junction is determined by the diffusion of

minority carriers while the current in a Schottky barrier diode is determined by thermionic emission of majority carriers over a potential barrier.

• The effective turn-on voltage of the Schottky diode is less than the

PN Junction diode.

• The Schottky diode is a high-frequency device than the PN

Junction diode, therefore can be used in fast-switching application in pico-second time.

Metal-Semiconductor Ohmic Contacts

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• Ohmic

Contacts: metal-to-semiconductor contacts providing conduction in both directions, and having the current throught the

• hmic contact as a linear function of applied voltage.
• Ohmic contacts can be classified as ideal (non-rectifying barrier) or

tunnel barrier.

• A) Ideal (Non-rectifying) Barrier
• Check the Energy band Diagram (the case where m < s)

Metal-Semiconductor Ohmic Contacts

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• Now, if a positive voltage is applied into the metal, there is no

barrier to electrons flowing from the semiconductor into the metal.

• If a positive voltage is applied to the semiconductor, the effective

barrier height for electrons flowing from the metal into the semiconductor will be Bn = n which is fairly small for a moderately doped semiconductor therefore electrons can easily flow from the metal into the semiconductor.

Metal-Semiconductor Ohmic Contacts

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• B) Tunneling Barrier
• In a metal to semiconductor contact, the space charge width is

inversely proportional to the square root of the semiconductor doping.

• As the doping concentration in the semiconductor increases, the

probability of tunneling through the barrier increases.

Semiconductor Heterojunctions

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• Semiconductor

Heterojunctions are formed between two semiconductor materials with different bandgap energies.

• Such a junction is useful because it can create a potential well at

the interface where electrons are confined to in the direction perpendicular to the interface but are free to move in the other direction.

• In order to have a useful heterojunction, the latice constants of the

two materials must be well-matched.

• Examples of Heterojunctions: GaAs - AlGaAs

Exercise

• 3. Consider Silicon at T = 300K doped at Nd = 7*1018cm-3.

Assume a rectifying Schottky barrier with Bn = 0.67V. Consider the density of energy states for Silicon NcSi = 2.8*1019cm-3. Calculate the space charge width for the Schottky rectifying diode.

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2

2 1

      

d bi s n

qN V x 

Solution

• 1. Using the relation for the reverse saturation current

density:

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kT q Bn

e T A

/ 2 * sT

J

 



2 2 / 2 *

114 cm K A e T J A

kT q sT

Bn

 

 

Picture Credits

• Semiconductor Physics and Devices, Donald Neaman, 4th

Edition, McGraw Hill Publications.

• Modern

Semiconductor Devices for Integrated Circuits, Prof. Chenming Calvin Hu, UC Berkeley (Free e-Book Download)

• http://www.eecs.berkeley.edu/~hu/Book-Chapters-and-Lecture-Slides-download.html

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