Schottky & Ohmic Contacts - [PDF Document] (2023)

1

I

VVto

P-n diode I-V

Vto 0.7 V; Iforw up to 100 A, Vrev up to 1000V

The turn-on voltage is relatively high (>0.7 V)

P-n diode performance limitations

2

Switching processes in p-n diodes are relatively slow

Vs

Vd

R

I

When a square wave voltage is applied to a p-n diode, it isforward biased duirng one half-cycle and reverse biased during thenext half-cycle

Using regular p-n diodes, this pulsed current waveform can onlybe obtained with low frequency pulses

Vs

I

t

t

forward

reverse

Under forward bias, the current is

RVVI ds

Under reverse bias, the current is almost equal to zero

3

Vs

Vd

R

I

However, if the pulse frequency is high the reverse currentshows significant increase

High frequency

Vs

I

t

t

I

t

Real p-n diode transient at high frequency

ideal

practical

Switching processes in p-n diodes (cont.)

4

Charge storage and Diode transients

Recall the injected carrier distribution at forward bias

xn-xp

At reverse bias the steady- state minority carrier concentrationis very low.

But not immediatelyafter switching from the forward bias!

xn-xp

Ln Lp

5

Schottky Diodes

Schottky diode has low forward voltage drop and very fastswitching speed.

Schottky diode consists of a metal - semiconductor junction.There is no p-njunction in Schottky diode.

In Schottky diode, there is no minority carrier injection

In 1938, Walter Schottkyformulated a theory predicting theSchottky effect.

metal semiconductor

6

Band diagrams of p-n and Schottky diodes

In Schottky diode, the depletion region occurs only in thesemiconductor region as metal has extremely high electron (hole)concentration.

EC

EV

EF

p n nmetal

EC

EV

EF

7

Schottky Barrier Formation

Work function (): Energy difference between Fermi level andvacuum level. It is aminimum energy needed to remove an electronfrom a solid.

EC

EV

Vaccum level (outside the solid)

Electron Affinity (Xs): Energy difference between the conductionband edge and the vacuum level.

EC

EV

X

Vaccum level (outside the solid)

8

continuedSchottky Barrier Formation

Metal n-type semiconductor before contact

EC

EV

m

Vacuum level (outside the solid)

EFs

metal semiconductor

XsIn metals, the conductance band edge EC and the valence bandEv are the same (both at EF level)

EFm

s

9

continuedSchottky Barrier Formation

After Contact (with n- type material):

EC

m

Vacuum level (outside the solid)

EF

metal semiconductor

Xs

EV

s

Schottky barrier for electrons

10

continuedSchottky Barrier Formation

Before contact (with p-type material):

EC

EV

m

Vacuum level (outside the solid)

EFs

metal semiconductor

Xs

EFm

s

11

continuedSchottky Barrier Formation

m

Vacuum level (outside the solid)

metal semiconductor

EV

EC

EFs

Xss

Schottky barrier for holes

After contact (with p-type material):

12

Schottky diode characteristics

The Schottky barrier height at equilibrium,

ECEF

metal semiconductor

EV

qmqs qs

qbo

smb =

qVbi

The built-in voltage, Vbi

smbiV =The depletion region charge density,

dqN=Note: there is no depletion region in metal

xn

The depletion region width,

02 bin

d

Vx

qN

=

Using energy voltage relationships: m= q m and Xs = q s , we canfind:

13

Schottky diode under bias

ECEF

metal N type

EV

qVbi

xn

Equilibrium

q(Vbi+VR)

metal N type

VR

ECEF

EV

xn

Reverse bias

q(Vbi-VF)

metal N type

VF

ECEF

EV

xn

Forward bias

14

Schottky diode current

Schottky diode has the same type of current - voltage dependenceas a p-n diode:

exp 1SCH SqVI IkT

=

However, important difference is that in Schottky diodes, thecurrent is NOT associated with electron and hole ACCUMULATION(injection, diffusion and recombination) as in p-n diodes.

The current flow mechanism in Schottky diodes is a thermionicemission. The thermionic emission is the process of electrontransfer OVER the Schottky barrier

ECEF

EV

q(Vbi-V)

15

continuedSchottky diode current

The saturation current parameter Is in Schottky diodes dependson the Schottky barrier height:

* 2 exp bsB

qI A T A

k T

=

A* is the Richardsons constant: * 2

*3

4 nqm kAh

=

A is the diode area.

where mn is the electron effective mass, h is the Planckconstant and k is the Boltzmann constant.

16

Microwave Schottky diodes

HSCH-9161 Millimeter Wave GaAs Schottky Diode (Agilent)

17

Ohmic contacts

+-+-

p-type n-type

Any semiconductor device has to be connected to external wiresin order to form an electronic circuit in combination with othercircuit elements. In the case of a p-n diode, for example, contactshave to be provided to both p-type and n-type regions of the devicein order to connect the diode to an external circuit.

18

Ohmic contacts must be as low-resistive as possible, so that thecurrent flowing through a semiconductor device leads to thesmallest parasitic voltage drop.

In good Ohmic contacts, the voltage drop that occurs across thecontact must be low and proportional to the current (so that thecontacts do not introduce any nonlinearities). Since such contactI-Vs follow the Ohm's law, they are usually called ohmiccontacts.

Ohmic contacts to semiconductors are often made using Schottkycontacts

1xp

kTqV

IS

1xp

kTqV

IS

p-n junction

Ohmic contact

Ohmic contacts

19

Rectifying Schottky contactsn-type semiconductor

metal semiconductorn-type m> s

Rectifying Schottky contact creates an electron depletion regionat the metal-semiconductor interface

20

p-type m< s

p-type semiconductor

metal semiconductor

Rectifying Schottky contacts

Rectifying Schottky contact creates a hole depletion region atthe metal-semiconductor interface

21

Schottky contacts(Rectifying contacts)

Ohmic Contacts (Non-rectifying contacts)

Criteria: n-type m> s p-type m< s

Criteria: n-type m< s p-type m> s

Non - rectifying Schottky contacts

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Ohmic Contact to n-type semiconductor

Majority carriers are electrons;there is no potential barrierfor electrons in both forward or reverse directions:

Non - rectifying Schottky contacts

m< s

Non-rectifying Schottky contact creates an electron accumulationregion at the metal-semiconductor interface. The electronconcentration in the contact region is higher than that in thebulk. The resistance of the contact region is low.

23

Ohmic Contact to p-type semiconductor

Majority carriers are holes; there is no potential barrier forholes in both forward or reverse directions:

Non - rectifying Schottky contacts

m> s

Non-rectifying Schottky contact creates a hole accumulationregion at the metal-semiconductor interface. The hole concentrationin the contact region is higher than that in the bulk. Theresistance of the contact region is low.

24

Ohmic Contact under biasOhmic contact to

n-type semiconductor ECEF

EV

metal N type

V

Positive bias at metal

ECEF

EV

metal N type

V

Negative bias at metal

ECEF

EVNo barrier, so almost no contact voltage drop

The voltage is evenly distributed in the bulk

Electron reservoir at the interface

25

continuedOhmic Contact under biasOhmic contact to

p-type semiconductor ECEF

EV

metal N type

V

Positive bias at metal

metal N type

V

Negative bias at metal

ECEF

EV

ECEF

EV

Hole reservoir at the interface

26

Tunneling Schottky contacts

Metal - n-type contact example

Issue:Not for all semiconductors, it is possible to find themetal with m > sIf the condition m > s is not met, theSchottky contact creates a depletion region at the MetalSemiconductor interface.Solution: heavily doped semiconductor

Schottky contact to a heavily doped semiconductor creates atunneling contact with very low effective resistance.

EC

EV

EF

W

Depletion region width = W

EC

EV

EF 1~D

WN

- +-+

Low-doped material large W

Highly-doped material small W

27

Tunneling Schottky contacts for high voltage devices:

only sub-contact regions are heavily doped

n-type material; ND and dn are chosen to provide the requiredoperating voltage

p+ -type material (heavily doped)

Bottom metal contact

Top metal contact

dn

dp

n+ sub-contact layer

28

Sub-contact doping by annealingDuring high-temperatureannealing, metal atoms diffuse into semiconductor and create donorimpurities. The contact material needs to be properly chosen tocreate donor (acceptor in p-materials) type of impurities.

n-type material; ND and dn are chosen to provide the requiredoperating voltage

p+ -type material (heavily doped)

Bottom metal contact

Top metal contact

dn

dp

n+ annealed region

29

The contact resistanceA quantitative measure of the contactquality is the specific contact

resistance, c, which is the contact resistance per unit contactarea.

sandwich type devices

also called vertical geometry devices

The contact resistance of each contact in a sandwich-typestructure(VERTICAL structure):

RCV=CV/A, where A is the contact area. CV is specific contactresistance for vertical structures: [CV] = cm2

Typical current densities in sandwich type devices can be ashigh as 104 A/cm2. Hence, the specific contact resistance of 10-5cm2 is needed to maintain a voltage drop on the order of 0.1 V.

30

Contact resistance of planar structures

In planar structures, contact resistance is inverselyproportional to the contact width W but no longer proportional tothe total contact area. The current density is larger near thecontact edge. The contact resistance of planar structures istypically given by the contact resistance per unit width, Rc1.Thelateral contact resistance RC and unit-width contact resistance RC1are related as:

1cC

RRW

=

Planar,or lateral geometry

device structureactive layer

substrate (device holder)

Current

W

31

Sheet (per square) resistance of thin films L

The resistance R of a thin semiconductor film between the twocontacts,LR

tW=

For thin films, commonly used thin film characteristic is socalled resistance per square or sheet resistance:

sqR t

=

tW

sqLR R

W= When L = W, R = Rsq

32

Transmission Line Model (TLM) method to determining contactresistance

L=1m 2m 3m

W

Resistance Rn,n+1 between two adjacent contacts in the TLMpattern,

WL

RR2R 1n,nsqc1n,n+

+ +=

Where Ln,n+1 is the distance between the contacts number n andn+1, Rsq is the resistance of the semiconductor film persquare,

t

33

Transmission Line Model (TLM) plot

From the Y axis intercept we can find the value of RC.From theslope of R (L) plot we can find the film resistance per square:Re

sist

ance

()

Distance between contact pads L (m)

2Rcsq

LR RW

=

R

L