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 is forward biased duirng one half-cycle and reverse biased during the next half-cycle

Using regular p-n diodes, this pulsed current waveform can only be 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 current shows 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 concentration is 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 fast switching 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 the Schottky effect.

metal semiconductor

6

Band diagrams of p-n and Schottky diodes

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

EC

EV

EF

p n nmetal

EC

EV

EF

7

Schottky Barrier Formation

Work function (Φ): Energy difference between Fermi level and vacuum level. It is aminimum energy needed to remove an electron from a solid.

EC

EV

ΦΦ

Vaccum level (outside the solid)

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

EC

EV

X

Vaccum level (outside the solid)

8

…continued…Schottky Barrier Formation

Metal – n-type semiconductor before contact

EC

EV

Φm

Vacuum level (outside the solid)

EFs

metal semiconductor

Xs

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

EFm

Φs

9

…continued…Schottky Barrier Formation

After Contact (with n- type material):

EC

Φm

Vacuum level (outside the solid)

EF

metal semiconductor

Xs

EV

Φs

Schottky barrier for electrons

10

…continued…Schottky Barrier Formation

Before contact (with p-type material):

EC

EV

Φm

Vacuum level (outside the solid)

EFs

metal semiconductor

Xs

EFm

Φs

11

…continued…Schottky Barrier Formation

Φm

Vacuum level (outside the solid)

metal semiconductor

EV

EC

EFs

XsΦs

Schottky barrier for holes

After contact (with p-type material):

12

Schottky diode characteristics

The Schottky barrier height at equilibrium,

EC

EF

metal semiconductor

EV

qφm

qχs qφs

qφbo

smb χ−φ=φ

qVbi

The built-in voltage, Vbi

smbiV φ−φ=

The depletion region charge density,

dqN=ρNote: there is no depletion region in metal

xn

The depletion region width,

02 bin

d

Vx

qNε ε

=

Using energy – voltage relationships: Φm= q φm and Xs = q χs , we can find:

13

Schottky diode under bias

EC

EF

metal N type

EV

qVbi

xn

Equilibrium

q(Vbi+VR)

metal N type

VR

EC

EF

EV

xn

Reverse bias

q(Vbi-VF)

metal N type

VF

EC

EF

EV

xn

Forward bias

14

Schottky diode current

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

exp 1SCH SqVI IkT

⎡ ⎤⎛ ⎞= −⎜ ⎟⎢ ⎥⎝ ⎠⎣ ⎦

However, important difference is that in Schottky diodes, the current 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 thermionic emission. The thermionic emission is the process of electron transfer OVER the Schottky barrier

EC

EF

EV

q(Vbi-V)

15

…continued…Schottky diode current

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

* 2 exp bs

B

qI A T A

k Tφ⎛ ⎞

= − ×⎜ ⎟⎝ ⎠

A* is the Richardson’s constant: * 2

*3

4 nqm kAh

π=

A is the diode area.

where mn is the electron effective mass, h is the Planck constant 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 wires in order to form an electronic circuit in combination with other circuit elements. In the case of a p-n diode, for example, contacts have to be provided to both p-type and n-type regions of the device in order to connect the diode to an external circuit.

18

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

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

Ohmic contacts to semiconductors are often made using Schottky contacts

⎟⎠⎞

⎜⎝⎛ −1xp

kTqV

IS

⎟⎠⎞

⎜⎝⎛ −1xp

kTqV

IS

p-n junction

Ohmic contact

Ohmic contacts

19

Rectifying Schottky contactsn-type semiconductor

metal semiconductorn-type Φm> Φs

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

20

p-type Φm< Φs

p-type semiconductor

metal semiconductor

Rectifying Schottky contacts

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

21

Schottky contacts(Rectifying contacts)

Ohmic Contacts (Non-rectifying contacts)

Criteria:• n-type Φm> Φs• p-type Φm< Φs

Criteria:• n-type Φm< Φs• p-type Φm> Φs

Non - rectifying Schottky contacts

22

Ohmic Contact to n-type semiconductor

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

Non - rectifying Schottky contacts

Φm< Φs

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

23

Ohmic Contact to p-type semiconductor

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

Non - rectifying Schottky contacts

Φm> Φs

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

24

Ohmic Contact under biasOhmic contact to

n-type semiconductor EC

EF

EV

metal N type

V

Positive bias at metal

EC

EF

EV

metal N type

V

Negative bias at metal

EC

EF

EVNo barrier, so almost no contact voltage drop

The voltage is evenly distributed in the bulk

Electron reservoir at the interface

25

…continued…Ohmic Contact under biasOhmic contact to

p-type semiconductor EC

EF

EV

metal N type

V

Positive bias at metal

metal N type

V

Negative bias at metal

EC

EF

EV

EC

EF

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 the metal with Φm > Φs

If the condition Φm > Φs is not met, the Schottky contact creates a depletion region at the Metal – Semiconductor interface.Solution: heavily doped semiconductor

Schottky contact to a heavily doped semiconductor creates a tunneling 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 required operating 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-temperature annealing, metal atoms diffuse into semiconductor and create donor impurities. The contact material needs to be properly chosen to create donor (acceptor in p-materials) type of impurities.

n-type material; ND and dn are chosen to provide the required operating 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 contact quality is the specific contact

resistance, ρc, which is the contact resistance per unit contact area.

sandwich type devices

also called “vertical geometry” devices

The contact resistance of each contact in a sandwich-type structure(“VERTICAL” structure):

RCV=ρCV/A, where A is the contact area. ρCV is specific contact resistance for vertical structures: [ρCV] = Ω×cm2

Typical current densities in sandwich type devices can be as high as 104 A/cm2. Hence, the specific contact resistance of 10-5 Ω×cm2 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 inversely proportional to the contact width W but no longer proportional to the total contact area. The current density is larger near the contact edge. The contact resistance of planar structures is typically given by the contact resistance per unit width, Rc1.The lateral contact resistance RC and unit-width contact resistance RC1 are 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 two contacts,LR

tWρ=

For thin films, commonly used thin film characteristic is so called “resistance per square” or “sheet resistance”:

sqRtρ

=

tW

sqLR R

W= When L = W, R = Rsq

32

Transmission Line Model (TLM) method to determining contact resistance

L=1μm 2μm 3μm

W

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

WL

RR2R 1n,nsqc1n,n

++ +=

Where Ln,n+1 is the distance between the contacts number n and n+1, Rsq is the resistance of the semiconductor film “per square”,

t

33

Transmission Line Model (TLM) plot

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

esis

tanc

e (Ω

)

Distance between contact pads L (μm)

2Rcsq

LR RWΔ

Δ =

ΔR

ΔL