Prof. Paolo Colantonio a.a. 2011 12

Prof. Paolo Colantonioa.a. 2011‐12

Analogue ElectronicsProf. Paolo Colantonio 2 | 29

Conductors• e.g. copper or aluminum• have a cloud of free electrons (at all temperatures above absolute zero). If an

electric field is applied electrons will flow causing an electric current

Insulators• e.g. polythene• electrons are tightly bound to atoms, so, only a few can break free to conduct

electricity

Semiconductors• e.g. silicon or germanium• at very low temperatures these have the properties of insulators• as the material warms up some electrons break free and can move about, and it

takes on the properties of a conductor – albeit a poor one• however, semiconductors have several properties that make them distinct from

conductors and insulators


• The Silicon is one of the most adopted semiconductor to realize electronic devices, both discrete or integrated

• It is constituted by a tetrahedral cell, with an atom in each vertexes, linked to the other by covalent bounds between the 4 electrons of each atom

• At 0 Kelvin, each electron is linked to its atom and the conductivity is null

• Increasing material temperature, thermal vibration results in some bonds being broken, generating free electrons which move about

• These leave behind holes (positive charge carries) which accept electrons (negative charge carriers) from adjacent atoms and therefore, also move about

• At room temperatures there are few charge carriers, thus pure semiconductors are poor conductors (this is intrinsic conduction)

The atomic structure of

silicon

Temperature


• The addition of small amounts of impurities drastically affects the properties of a semiconductor.

• This process is known as doping.

• Adding Phosphorus P (e.g. pentavalent material) into the crystal lattice of Silicon Si (e.g. tetravalent material), four of P’s valence electrons are tightly bound by the covalent bonding.

• The fifth electron is only weakly bound and is therefore almost free to move within the lattice and contribute to an electric current.

SiSi Si

SiSi Si

SiSi Si

freeelectron

SiSi Si

PSi Si

SiSi Si


• When the added materials form an excess of electrons, they are called donor impurities and produce a n‐type semiconductor

• When the added materials form an excess of holes, they are called acceptor impurities and produce a p‐type semiconductor

• Both n‐type and p‐type materials have much greater conductivity than pure semiconductors: extrinsic conduction

• The dominant charge carriers in a doped semiconductor (e.g. electrons in n‐type material) are called majority charge carriers. The other type are minority charge carriers

• The overall doped material is electrically neutral


• Excepting bulk devices, which exploit basic properties of semiconductors, the large amount of electronic devices are based on the junction of different semiconductor materials with different doping profile, i.e. with different concentration of impurity (e.g., p‐n junction), or by metal and semiconductors (e.g. Schottky junction).

• The modern electronic technology employ a variety of semiconductor• Pure

• e.g. constituted by a single atomic specie, • Composed

• e.g. constituted by different atomic elements like Gallium Arsenide (GaAs), Indium Phosphorus (InP), Gallium Nitride (GaN), Silicon Carbide (SiC), Silicon‐Germanium (SiGe).


• When p‐type and n‐type materials are joined, this forms a pn junction

• The majority charge carriers on each side diffuse across the junction where they combine with (and remove) the charge carriers of the opposite polarity

• Hence, around the junction there are few free charge carriers and we have a depletion layer (also called a space‐charge layer)

• The diffusion of positive charge in one direction and negative charge in the other produces a charge imbalance, resulting in a potential barrier across the junction


• The barrier opposes the flow of majority charge carriers and only a small number have enough energy to surmount it

• This generates a small diffusion current• The barrier encourages the flow of minority carriers and any that come close to it

will be swept over• This generates a small drift current

• For an isolated junction these two currents must balance each other and the net current is zero

Vh

Potential

DistanceIsolated junction

p-type n-type


Forward bias• If the p‐type side is made positive with respect to the n‐type

side the height of the barrier is reduced, thus more majoritycharge carriers have sufficient energy to surmount it

• the diffusion current therefore increases while the driftcurrent remains the same

• there is thus a net current flow across the junction whichincreases with the applied voltage

Reverse bias• if the p‐type side is made negative with respect to the n‐type

side the height of the barrier is increased, thus the number ofmajority charge carriers that have sufficient energy tosurmount it rapidly decreases

• the diffusion current therefore vanishes while the driftcurrent remains the same

• thus the only current is a small leakage current caused by the(approximately constant) drift current

• the leakage current is usually negligible (a few nA)

Vh

Potential

DistanceForward bias

V

p-type n-type

+ -V

Vh

Potential

DistanceReverse bias

V

p-type n-type

- +V


• The current flowing through a pn junction can be approximately related to theapplied voltage by the expression

• I is the current through the junction• IS is a constant called the reverse saturation current• q is the electronic charge• V is the applied voltage• k is Boltzmann’s constant,• T is the absolute temperature• is a constant in the range 1 to 2 determined by the junction material (e.g.

approximately 1 for Ge and 1.3 for Si). For most purposes we can assume =1

1kTη

qV

s eIIp-type n-type

+ -V

I


• Thus

1TV

V

s eII

• If V > + 0.1 V, Vs eII 40

• If V < + 0.1 V, sII

• At room temperature (T=300K)• VT26mV 1/VT 40 V−1

11600TVT


• A pn junction is not an ideal diode, but itdoes have a characteristic thatapproximates to such a device(semiconductor diode)

• An ideal diode passes electricity in one direction but not in the other


• Generally have a turn‐on voltage of about 0.5 V• Generally have a conduction voltage of about 0.7 V• Have a breakdown voltage that depends on their construction

• perhaps 75 V for a small‐signal diode• perhaps 400 V for a power device

• have amaximum current that depends on their construction• perhaps 100 mA for a small‐signal diode• perhaps many amps for a power device

AK


• One application of diodes is in rectification

Example: half‐wave rectifier

• In practice, no real diode has ideal characteristics but semiconductor pn junctionsmake good diodes


• Sometimes we represent a diode by an equivalent circuit.• Models have different levels of sophistication

Idealdiode

Idealdiode

VONIdealdiode

VON rON

Idealdiode

VON rON

rOFF

I

V

I

V

I

V

I

VVON

VON VON

Slope=1/rON

Slope=1/rOFF

Slope=1/rON


Conduction Voltage• In a real diode I‐V characteristic it can be noted the

existence of a voltage VON, below of which the currentis negligible (less 1% of diode maximum current).

V

I[mA]

0 0.2 0.6 1.0

500

10

idealcharacteristic

150 °

C

25°C

-55°

C

Logarithmic scale behavior• Increasing the forward voltage V, the voltage drop

across the semiconductor becomes significant andthe device behaves like a resistor

Saturation current• For negative voltage (reverse bias) the current is IS

(generally negligible)

• VON 0.2V for Ge• VON 0.7V for Si

• mA for Si• nA for Ge


Effects of temperature• There is an intrinsic relationship with the operating

temperature T 1qVk

sI I e

T

• For a given current I, the voltage V is inversely proportional to T

• For a silicon diode (similar for germanium), V decreases by about 2 mV per °C

• The diode current is also affected by the reverse saturation current, which

increases with temperature

• IS increases by about 7% per °C

2V mVT C

IS twined for T=10°C


• A reversed‐biased diode has two conducting regions separated by an insulatingdepletion region

• This structure resembles a capacitor• Variations in the reverse‐bias voltage change the width of the depletion layer and

hence the capacitance, namely Transient Capacitance CT

----

----

----

----

----

----

----

++++

++++

++++

++++

++++

++++

++++

++++

p-type n-type

depletion layer

W

A TACW

• A forward‐biased diode shows a capacitive behavior due to the injection of minoritycharges across the junction

• The resulting Diffusion Capacitance CD is greater than CT


• When a varying signal is applied to a diode,the electrical response od diode shows atransient behavior.

• Considering the following situation

RL

iDv(t)i

vD

VF

-VR

0t

t

t

t

IS

p -pn n0

v(t)i

iD

vD

VF/RL

-VR/RL

Forwardbias

Storage ofminority charges

Transitiontime

0VON

-VR


• If a large reverse voltage is applied across a pn junction, a breakdown phenomenoncan be observed, that can be caused by two mechanisms

Zener breakdown• In devices with heavily doped p‐ and n‐type regions the transition from one to the

other is very abrupt and the depletion region is few nanometers thick• This produces a very high field strength across the junction that can pull electrons

from their covalent bonds, resulting in a large reverse current• The current produced by Zener breakdown must be limited by external circuitry to

prevent damage to the diode• The breakdown voltage is largely constant and the Zener breakdown normally occurs

below 5V• The breakdown voltage decreases very slightly with increasing temperature (the

energy required to break the covalent bonds is reduced being increased the energyacquired by each electrons)

0dtdVZ


Avalanche breakdown• Occurs in diodes with more lightly doped materials, where the field strength across

junction is insufficient to pull electrons from their atoms, but is sufficient toaccelerate the electrons within the depletion layer

• The electrons loose energy by colliding with atoms• If they have sufficient energy they can liberate other electrons, leading to an

avalanche effect• Usually occurs at voltages above 5V• The voltage at which avalanche breakdown occurs increase with junction

temperature (the increased temperature increase the thermal vibration, thusincreasing the collision before the electrons reach the sufficient velocity to break thecovalent bonds)

0ZdVdt


Varactor• A reversed‐biased diode has two conducting regions separated by an insulating

depletion region• This structure resembles a capacitor• Variations in the reverse‐bias voltage change the width of the depletion layer and

hence the capacitance• This produces a voltage‐dependent capacitor• The Varactor are diodes constructed to emphasize such capacitive behaviour• They are used in applications such as automatic tuning circuits

Rr

Rs

CT

Rr= reverse bias resistanceRs= semiconductor bulk resistanceCT= reverse bias capacitance

0 5 10 15 20 25Reverse bias [V]

1N916

1N914

25°C

Cap

acita

nce

C [p

F]T

4.0

3.2

2.4

1.6

0.8

0

Symbol

Equivalent circuit


Zener• Uses the relatively constant reverse breakdown voltage to produce a voltage

reference• Breakdown voltage is called the Zener voltage, VZ

iD

vD

VZ

L

L

RVR R

VR

RLV+

R

vDiD

Symbol


Zener• It is used as a voltage stabilizer, since the voltage across the Zener is practically fixed

to VZ value

RLV+

R

vDiD RLV+

R

vDiD

iD

vD

VZ

L

L

RVR R

VR

IncreasingRL

iD

vD

VZ

L

L

RVR R

VR

IncreasingV


Schottky• It is formed by the junction between a layer of metal (e.g. aluminum) and a

semiconductor• Action relies only on majority charge carriers• Much faster in operation than a pn junction diode• Has a low forward voltage drop of about 0.25 V• Used in the design of high‐speed logic gates


Tunnel• High doping levels produce a very thin depletion layer which permits ‘tunnelling’ of

charge carriers• results in a characteristic with a negative resistance region• used in high‐frequency oscillators, where they can be used to ‘cancel out’ resistance

in passive components

Symbol


LED• The Light‐Emitting‐Diode (LED) is a semiconductor diode constructed

in such a way that in the pn junction, when it is forward biased, thecharges injected in the region where they are minoritary (holes inregion n‐type and electrons in region p‐type) will recombine emittinglight.

• Such phenomenon happens in the depletion layer region• A range of semiconductor material can be used to produce infrared

or visible light of various colors• Typical devices use materials such as gallium arsenide (GaAs), gallium

phosphide (GaP) or gallium arsenide phosphide (GaAsP)

SymbolMaterial

Light length(nm)

Color

GaAs 910 Infrared

GaP 560 Green

GaAsP 650 Red

----

----

----

----

----

----

++++

++++

++++

++++

++++

++++

p-type n-type

depletionlayer

Forward biad

Light

Cathode

Anode


Photodiode• They are devices able to transform light in electrical current• Without light the photodiode behaves like a normal diode,

with an I‐V chatacteristic crossing the origin axes• By lighting the pn junction reverse biased, with a proper

incident radiating length (i.e. the energy of the incidentphoton shall be larger than the energy gap), there is anincrease of reverse current due to the increase of freecharges

• The efficiency of the illumination is a function of the lightspot distance from the pn junction

Symbol

I

VIS

IphVph

VB

ndI GdV

Symbol


• If the pn junction of a photodiode is used in the fourth quadrant, being IV negativethen electrical energy is produced

RL

Light I

V

Vph

Iph

RL

Pmax

0.5 0.6phV V With Siliconsilicon

1T

VV

ph SI I I e

Iph short circuit current proportional to the illumination intensity

Vph photovoltaic potential

, ln 1 phph Max T

S

IV V

I

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Prof. Paolo Colantonio a.a. 2011 12