Lecture 12

Semiconducting junctions

The PN-Junction

One of the simplest bipolar devices, important for the understanding of more complex devices (bipolar = both electrons and holes contribute to device characteristics).

Semiconductor devices: Inhomogeneous semiconductors

All solid-state electronic and opto-electronic devices are based on doped semiconductors.

In many devices the doping and hence the carrier concentrations are non-homogeneous.

In the following section we will consider the p-n junction which is an important part of many semiconductor devices and which illustrated a number of key effects

+ +

+

++

-

-

-

--

A

Diode

Nonlinear I-V characteristics

V

I

Forward bias

Reverse bias

np

+ +

+

+

-

-

-

--

+

+

+

-

--

AA

++

++

-

-

-

-

-

+

-

+

The p-n semiconductor junction: p-type / n-type semiconductor interface

We will consider the p-n interface to be abrupt. This is a good approximation.

n-type ND donor atoms per m3

p-type NA acceptor atoms per m3

Consider temperatures ~300K Almost all donor and acceptor atoms are ionised.

impurity atoms m-3

NA

ND

x x = 0

p-type n-type

ND (x) = ND (x>0) = 0 (x<0)NA (x) = NA (x>0) = 0 (x<0)

impurity atoms m-3

NA

ND

x x = 0

p-type n-type

impurity atoms m-3

NA

ND

x x = 0

p-type n-type

ND (x) = ND (x>0) = 0 (x<0)NA (x) = NA (x<0) = 0 (x>0)

p-n interface at x=0.

Electron andhole transfer

Consider bringing into contact p-type and n-type semiconductors.

n-type semiconductor: Chemical potential, (Fermi level) below bottom of conduction band

p-type semiconductor: Chemical potential, above top of valence band.

Electrons diffuse from n-type into p-type filling empty valence states.

n-type semiconductorp-type semiconductor

EC

EV

EC

EV

EC

EV

EC

EV

Electrons

Holes

EC

EV

EC

EV

e0

p-type semiconductor n-type semiconductor

Electrons

Holes

EC

EV

EC

EV

e0

p-type semiconductor n-type semiconductor

EC

EV

EC

EV

e0

p-type semiconductor n-type semiconductor

Electrons

Holes

Electrons diffuse from n-type into p-type filling empty valence band states.

The p-type becomes negatively charged with respect to the n-type material.

Electron energy levels in the p-type rise with respect to the n-type material.

A large electric field is produced close to the interface.

Dynamic equilibrium results with the chemical potential (Fermi level) constant throughout the device.

Note: Absence of electrons and hole close to interface -- depletion region

Band Bending

Junction

0dx

dEF

At equilibrium the Fermi level gradient equals zero!

p-n junction

I

IV characteristics :

The principle working of a pn-junction

P-doped

N-doped

Negatively charged

electrons + positively charged

immobile donors

Positively charged holes +

negatively charged immobile acceptors

+-

No electrons or holes, only charged donors/acceptors (DEPLETION LAYER)

electrons

holes

P-doped

N-doped

The principle working of a pn-junction

+- electrons

holes

No Voltage

P-doped N-doped

+ -+- electrons

holes

Forward bias

current

- ++- electrons

holes

Reverse bias

“no” current

“No” current(Leakage current)

Large current

Current

Voltage

Circuit symbol:

I

IV characteristics :

Ec

Ev

Ec

Ec

e-e-Drift (thermally exc.) Diffusion (E-field)

diffusiondrift jj

driftdiffusion jj

driftdiffusion jj

No bias

Forward bias

Reversebias

V

V

V0

0

j

I-V CharacteristicsHole current:

• diffusion Ipd = C1Npexp (-eVbi/(kT))

• drift Ipu = CNpn = Ipd = C1Npexp (-eVbi/(kT))

• at forward bias IpF = C1 Np exp (-e(Vbi- V) /(kT))

• Ip = IpF - Ipu = C1Np exp (-e(Vbi- V) /(kT)) – C1Np exp (-eVbi/(kT)) =

C1Npexp [-eVbi/(kT)][exp(eV/(kT)-1] =Ipd [exp(eV/(kT))-1]

Electron current:

In = Ind [exp(eV/(kT))-1 with Ind = C2Nn exp (-eVbi/(kT))

I = Io [exp(eV/(kT)-1]

Io = Ind + Ipd = (C1 Np + C2Nn) exp (-eVbi/(kT))

Rectifier

Ac transfers into dc

a) b)

I

t

Based on the photovoltaic effect-solar cell-photodetectors

photodiode

Avalanche diode

• Powielanie lawinowe (Vprzebicia>6Eg/e)

p

n

-elektrony

uzyskują energię

--

+

aby kreować pary elektron-dziuraprzez zderzenie nieelastyczne

Wykład VI

Zener diode

Light is absorbed if ; EHP are created; electric field separates carriers

• Short-circuit (U = 0)

-

EC

EV

EC

EV

F

0

hf

Isc = q Nph(Eg)

ID (A)

VD (V)

Isc

photodiodeghf E

photodiode• Open circuit

EC

EV

EC

EV

qVbi

qVOC

ID (A)

VD (V)

Voc

Id = Io [exp(eVoc /kT)-1]

Isc – Id = 0

This current balances photogenerated current, Isc

ln( 1) lnsc scoc

o o

I IkT kTV

q I q I

Solar cell

Transfers solar energy into electric energy

P = I x U=I2 x R= U2/R

LED

Ge Si GaAs

Semiconductor laser

0FC FVE E