MatE/EE 1671 EE/MatE 167 Diode Review. MatE/EE 1672 Topics to be covered Energy Band Diagrams V built-in Ideal diode equation –Ideality Factor –RS Breakdown

MatE/EE 167 1

EE/MatE 167

Diode Review

MatE/EE 167 2

Topics to be covered

• Energy Band Diagrams

• V built-in

• Ideal diode equation– Ideality Factor– RS

• Breakdown

• Capacitance

MatE/EE 167 3

MatE/EE 167 4

Non-equilibrium conditions in a pn junction

• Equilibrium, forward bias, reverse bias

• Carrier injection

• Calculating junction current

• Minority and majority currents

• Diode equation example

MatE/EE 167 5

Equilibrium, forward bias, reverse bias

p n

Reverse B iasV r

E

Forw ard B iasV = V f

p n

E

W

p n

E

E quilibriumV =0

V p

V n

V 0

(V o-V f)

(V o+V r)

MatE/EE 167 6


q(V o-V f)

q(V o+V r)

E cp

E cn

E vp

E vn

E Fn

E Fp

qV o

MatE/EE 167 7


• Equilibrium– The Hole and electron drift and diffusion currents

cancel each other out. No net current.

• Forward bias– The junction potential is lowered by an applied

electric field.

• Reverse bias– The junction potential is increased by an applied

electric field.

MatE/EE 167 8


• Equilibrium– W does not change.

• Forward bias– W is smaller substitute (Vo-V) for Vo in

equation for W.

• Reverse bias– W is larger substitute (Vo+V) for Vo in

equation for W.

MatE/EE 167 9


• Equilibrium• EFp=EFn flat throughout .

• Forward bias• EFp(J) and EFn (J) are separated by q(Vf) (J) .

• Reverse bias• EFp(J) and EFn (J) are separated by q(Vr) (J) .

MatE/EE 167 10


• Equilibrium– No net current .

• Forward bias– Diffusion current is increased because the barrier is lowered and

thus more electrons and hole have enough energy to make it through the barrier. Electrons go from the n-side to the p-side. Holes go from the p-side to the n-side.

– Drift current: small because this depends on the concentration of minority carriers. Thermally generated EHP’s (within a diffusion length of W, are the only carriers that contribute to drift, thus independent of applied bias.

MatE/EE 167 11


• Reverse bias– Diffusion current is decreased because the barrier is higher and

thus less electrons and hole have enough energy to make it through the barrier. Electrons go from the n-side to the p-side. Holes go from the p-side to the n-side.

– Drift current: small because this depends on the concentration of minority carriers. Thermal generated EHP’s (within a diffusion length of W, are the only carriers that contribute to drift, thus independent of applied bias.

MatE/EE 167 12


• Equilibrium: I=I(Diff)-|I(gen)=0|

• Forward bias: I = Io(eqV/kT-1)

• Reverse bias: I=Iop n

V

I

I(gen.)

I(d if f .)

I

V

MatE/EE 167 13

Carrier injection

• Minority carriers dominate

/kTqVd

/kTqVn

p

/kTqVa

/kTqV

pn

oo

oo

e

N

e

nn

e

N

e

pp

nnnnn

ppppp

τDL ,μq

kTD

τDL ,μq

kTD

1/

kTqV

pn

nn

p

p enL

Dp

L

DqAI

MatE/EE 167 14

Calculating junction current

• The mobilities are for electrons in p-type material, and holes in n-type material. From figure 3-23 on page 99:– An electron in p-type Si material (Na=1017cm-3)

would have a mobility of 1000 cm2/V s– A hole in n-type Ge material (Nd=1019cm-3)

would have a mobility of around 100 cm2/V s

MatE/EE 167 15

Calculating junction current

• Minority carrier lifetimes:

n

p

Si 1010-6 s 1010-6 sGe 1010-7 s 1010-7 sGaAs 110-9 s 110-9 sZnSe 110-9 s 110-9 s

MatE/EE 167 16

Minority and majority currents

)()(

)( /

npnn

Lxn

p

pnp

xIIxI

epL

qADxI pn

I

0 xn

i

)(

)(

np

nn

xI

xI

I

p+ n

x0 xn0

MatE/EE 167 17

Reverse bias breakdown

• 5.4 Reverse Breakdown (Streetman)– 5.4.1 Zener Breakdown– 5.4.2 Avalanche Breakdown

MatE/EE 167 18

Reverse bias breakdown

• Under reverse bias a pn junction exhibits a small voltage independent current until a critical voltage is reached Vbr. If the bias voltage exceeds Vbr the current increases dramatically.

• If biased properly with a current limiting diode, you can operate in reverse breakdown mode with out damaging the diode.

MatE/EE 167 19

5.4 Reverse Breakdown• 5.4.1 Zener Breakdown

– This effect applies to heavily doped junctions (p+, n+). This is a low voltage effect.

– Barrier is thin due to high abrupt doping

– When the reverse bias voltage is large enough, electrons can tunnel to the p-side, and holes can tunnel to the n-side (section 2.4.4)

– Reverse bias of a p+/n+ junction• leads to large electric field (106V/cm)

• leads to covalent electrons being “ripped away”

MatE/EE 167 20

5.4 Reverse Breakdown

• 5.4.1 Zener Breakdown

E cp

E cn

E vp

E vn

E Fn

E Fp

qV o

d

e -

I

V

MatE/EE 167 21

5.4 Reverse Breakdown

• 5.4.2 Avalanche Breakdown– Lightly doped junctions, tunneling can not

occur• W increases with reverse bias.

– Impact ionization• A carrier can be accelerated by a high electric field

with enough kinetic energy to knock an electron out of the lattices covalent bond and make an EHP. One carrier can cause many carriers to be created.

• To design Vbr, use figure 5-22 on page 190.

MatE/EE 167 22

Reverse Breakdown

• 5.4.2 Avalanche Breakdown

E

W

p n

e -

h+ e -

e -

MatE/EE 167 23

Metal Semiconductor junctions:

5.7 Metal-semiconductor junctions5.7.1 Schottky barriers

5.7.2 Rectifying contacts

5.7.3 Ohmic contacts

5.7.4 Typical Schottky barriers

MatE/EE 167 24

Schottky barriers

• Diode like behavior can be mimicked by applying clean metal to a clean semiconductor.– Easy to do and faster switching times can be realized.

• n-type– Semiconductor bands bend up causing a more positive

region near the interface, which attracts electrons from the metal to the interface interface.

• p-type– Semiconductor bands bend down causing a more

negative region near the interface, which attracts holes from the metal to the interface.

MatE/EE 167 25

Schottky barriersn-type

E cE Fs

qm qsq

M etal

Semiconductor

E FM

E v

m > s

M etal

E cE FM

E v

qbqm

E Fs

q(m sqV o

W

p-type

E c

E Fs

qm qsq

M etal

Semiconductor

E FM

E v

m < s

M etal

E c

E FM

E v

E Fs

q(s mqV o

W

MatE/EE 167 26

Rectifying contacts

• Apply a forward bias to the Metal of the M/S(n) diode and the contact potential is reduced by Vo-V

– Allows electrons to diffuse into metal.• Apply a forward bias to the Semiconductor of the

M/S(p) diode and the contact potential is reduced by Vo-V

– Allows holes to diffuse into metal.

MatE/EE 167 27

Rectifying contacts

• Apply a reverse bias to the Metal of the M/S(n) diode and the contact potential is increased by Vo+Vr.– Electrons have to overcome a voltage

independent barrier to diffuse into metal.• Apply a reverse bias to the Semiconductor of the

M/S(p) diode and the contact potential is reduced by Vo+Vr.– Holes have to overcome a voltage independent

barrier to diffuse into metal.

MatE/EE 167 28

Rectifying contacts

• Current flows primarily by majority carriers is both cases.

• Very little charge storage occurs, which leads to fast switching speeds.

MatE/EE 167 29

Ohmic contacts

• Metal/semiconductor ohmic contacts– linear near the origin, non-rectifying

• Two methods of fabrication– Choose a metal with a workfunction that aligns the

fermi levels with majority carriers. (Al for p-type Si, Au for n-type Si

– Dope the semiconductor heavily so that W is very thin so that tunneling occurs (Al on p+ or n+ Si)

– Heavy doping all ways improves ohmic behavior.

MatE/EE 167 30

Ohmic contactsp-type

E c

E Fs

qm qsq

M etal

SemiconductorE FM

E v

m > s

M etal

E c

E FM

E v

E Fs

q(m sqV o

W

MatE/EE 167 31

Ohmic contactsn-type

E cE Fs

qm qsq

M etal

Semiconductor

E FM

E v

m < s

M etal

E cE FM

E v

qbqm

E Fsq(s mqV o

W

MatE/EE 167 32

Real Schottky barriers

• In Si, there is a thin oxide in between the metal and semiconductor.

• Surface states arise from the crystal ending– This can pin the fermi level to midgap in GaAs

• If a metal semiconductor junction is alloyed the interface is blurred between metal/metal-semiconductor/semiconductor.

• Contact design is very dependant on your process.

MatE/EE 167 33

Equations

2ioo npn

ppp

nnn

DL

DL

q

kTD

1enL

Dp

L

DqAI qV/nkT

pn

nn

p

p

2i

da

n

NNln

q

kTVo

2

1

2

1

)(

2

2)(

2

da

da

ooj

da

da

da

dao

NN

NN

VV

qA

VVd

dQC

WNN

NNqAQ

NN

NN

q

VVW

MatE/EE 167 34

Breakdown

Documents

MatE/EE 1671 EE/MatE 167 Diode Review. MatE/EE 1672 Topics to be covered Energy Band Diagrams V built-in Ideal diode equation –Ideality Factor –RS Breakdown