ELECTRONICSSEMICONDUCTOR ELEMENTS

1

QUESTIONS

• What is the Ohm Law formula for calculating voltage across a resistor ?

• Given the current Kirchoffs Law:

σ𝒌 𝒊𝒌 = 𝟎, while

𝒌 = 𝟏, 𝟐, 𝟑, and

𝒊𝟏 = 𝟕𝒎𝑨, 𝒊𝟐 = 𝟑𝒎𝑨,

what is the value of 𝒊𝟑 ? 𝒊𝟑 = −𝟏𝟎𝒎𝑨

• What is the equivalent resistance, between

points A and C, for the given circuit? 10𝛀

• What is the equivalent capacitance?

𝟏. 𝟎𝟎𝟏𝒎𝑭

9

𝒊𝟐𝒊𝟏

𝒊𝟑

𝑨

𝑪

𝑩

𝑨 𝑩𝟐𝒖𝑭 𝟐𝒖𝑭

𝟏𝒎𝑭

𝟑𝛀 6𝛀

𝟖𝛀

𝟐𝛀

𝐔 = 𝐈 ⋅ 𝑹

DIODES

10

SEMICONDUCTOR DIODE

• Diode is an element utilizing pn junction,

placed in a case with connectors

Diode symbol

11

PERFECT DIODE OPERATION

• Perfect diode can be modeled as a directional valve

• With forward polarization

(anode potential higher than cathode`s)

the circuit is closed

• With reverse polarization

(cathode potential higher than anode`s)

the circuit is open

12

ACTUAL DIODE OPERATION

• With forward polarization

• Conduction begins when the voltage reaches knee voltage

• A voltage drop occurs – depending on the particular diode and the current drawn,

thevoltage is usually ~0.7V, it can be always read from the characteristics

• Power dissipated is calculated as P=U*I

• With reverse polarization

• Reverse current of a few µA flows

• Power dissipated is „negligible”

• Exceeding the maximum reverse

voltage can break the diode

13

DIODE CAPACITANCE

• Consider a diode polarized in reverse direction

• An isolator layer occurs between both

p and n parts

• A parasitic capacitance is introduced

• When the polarization is inverted

(voltage goes to positive) the capacitanse

must be first discharged

14

TEMPERATURE DEPENDENCE

• Given a constant current flow, the voltage drop across a conducting diode

drops by 2mV/1degC

• The warmer the diode, the lower the voltage drop, and thus the power

dissipated.

• With rising temperature, the reverse current rises too.

15

DIODE MARKING

• p-semiconductor side is an anode

• n-semiconductor side is a cathode

• The current flows from anode,

to cathode (A-›K)

• On an actual device a stripe

always marks the cathode side

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DIODE PARAMETERS I

• Maximum repetitive reverse voltage VRRM [V]

• Reverse voltage that can be periodically applied to the diode.

• Maximum DC reverse voltage VR or VDC [V]

• Reverse voltage that can be constantly applied to the diode.

• Maximum forward voltage VF [V]

• Forward voltage drop given for the nominal current flowing forward.

• Maximum (average) forward current IF(AV ) [A]

• Maximum forward current. The limit is a result from thermal power dissipation capabilities.

• Maximum peak/surge forward current IFSM [A]

• Peak current that can be applied to diode according to a specified pattern (for example a single pulse for a given duration)

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DIODE PARAMETERS II

• Total power dissipation PD [W]

• The amount of power that can be dissipated from diode to the outside.

• Maximum operating junction temperature TJ [°C]

• Maximum temperaturę, that the junction can operate at, without breaking.

• Thermal resistance R(T) [°C/W]

• Parameter for calculation of the internal temperature at given powerdissipated.

• Reverse (leakage) current IR [µA]

• Reverse current flowing at given negative voltage.

• Typical junction capacitance CJ [pF]

• Typical capacitance, usually expressed in pF.

• Reverse recovery time trr [µs]

• Time for the diode to regain blocking capabilities after voltage polatitychange.

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SCHOTTKY DIODE

• m-s metal semiconductor junction

• Low capacitance

• Short recovery time

• Lower forward voltage drop

• Lower breakdown voltage

• Used in high frequency applications

Schottky diode

symbol19

PN & SCHOTTKY COMPARSION

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DIODES IN PRACTICE

• Example datasheet

• Typical packages

21

DIODE MODELS

Real Ideal Simplified Linearized

According to Shockley’s

equation

No reverse current, no

forward voltageForward voltage

considered

Linear approximation

of the characteristics

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EXAMPLE

• Ideal model

• Simplified model

• Relative error

mAkk

V

RR

VI

VVVV

VVV

R

SS

SRR

DD

03.38.15.1

10

10

0

21

21

21

=+

=+

=

==+

==

mAk

VV

RR

VVI

VVVV

VVVVV

VVV

SS

SRR

DRDRS

DD

61.23.3

4.1104.1

4.1

7.0

21

21

2211

21

=

−=

+

−=

−=+

+++=

==

%1.16%10061.2

03.361.2% =

−=

Assuming ideal model, may lead to large

errors ! 23

DIODE TESTING

• Resistance measurement

• Forward polarization should give a resistance of <1kΩ

• Reverse polarization should give

a resistance of >1MΩ

or out of scale

• Diode testing mode

• Polarized in forward direction should give ~0,7V

• Reverse polarization should

occur as open circuit

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DIODE CHARACTERISCICS EXAMINATION

• Diode and resistor connected inseries

• Function generator producing triangle wave

• Oscilloscope in XY mode

• X channel corresponds to diode voltage

• Y channel corresponds to resitor voltage, thus the current flowing through the

resistor & diode

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ZENER DIODE

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ZENER DIODE

• Utilizes the phenomenon of Zener effect

• Usually works in reverse polarity

• Breakdown voltage is very little dependednt on the current flowing

• Zener voltage ranges between a few Volts up to tens of Volts

Zener Diode symbols

27

ZENER DIODE PARAMETERS

• Zener voltage VZ given for a test current of IZT (mostly 20mA)

• Maximum power dissipated PD(max),

• Maximum current results from max. Power

• Dynamic impedance

• Diode model must consider the breakdown !

Z

D

ZMV

PI

(max)=

==

= 28

mA2

mV56

Z

ZZ

I

VZ

VZ

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VOLTAGE STABILIZER

• Zener diodes are mostly used as a source of reference voltage

• The circuit given stabilises the output voltage, regardless of the input voltage,

provided:

• When V>VZ the diode operates in breakdown area:

• then V0≈VZ

• Diode current I =𝑉−𝑉𝑧

𝑅

• Diode power P = 𝐼𝑉𝑧

• Resistor limits the current and

prevents the diode from overheating!

V= V0 +RI

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GRAHICAL ANALYSIS

• Operating point moves when changing the source voltage

• Output voltage depends on the steepness of the characteristics,

thus it depends on dynamic impedance

30

REFERENCE VOLTAGE SOURCE

• Simple Zener diode circuit stabilizes output voltage

• Input voltage must by higher than the Zener voltage

• Any load applied must be considered in current flow calculation

• Power is dissipated in the resistor and

in the diode or the load

(dependent on the load)

• The circuit has very low efficiency

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SIMPLE APPLICATIONS

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CUTTING SIGNAL I

• Output waveform can be affected using diodes

• Diode applied in series:

• Diode cuts one of the half-waves (depending on the direction)

• The other half wave is affected by the diode voltage drop

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CUTTING SIGNAL II

• Diode in paralell

• Problem – circuit is shorted for

forward polarity

• Resistor added in series

• Diode voltage drop must be considered

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LIMITING SIGNAL I

• Limiting voltage to a certain level with a diode and a voltage source

• Limiting both half waves (positive and negative)

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LIMITING SIGNAL II

• The same application utilizing Zener diodes

• Single Zener diode will limit the voltage to forward voltage drop i one direction

and to zener voltage (VZ) in another direction

• Zener diodes can be connected in series, thus giving voltage cutting at the level of

Vz+0,7V

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LIMITING CIRCUITS

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VOLTAGE SPIKES PROTECTION

• Voltage spikes protection:

• Must withstand big energies for short periods of time

• Must react instantly

• Zener diodes and both Transil diodes (or TVS) may be used

• Transil (TVS - transient voltage supressor) is a specialized diode designed for

suppresing high voltage spikes

• Example protection circuit:

Transil symbol

38

VARISTORS

• Varistor is non-linear resistor

• Characteristics given in the graph

• Compared to transil, varistor:

• Has longer time of reaction

• Can withstand bigger energies

Varistor symbol

39

VOLTAGE SPIKES PROTECTION

• Example 1 – redirecting the high voltage spike to

power supply (it has a relatively big capacitance,

so can absorb the energy)

(-0,7V ÷ +5,7V) utilizing standard diodes

• Example 2 – electro-static discharge

protection

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AMPLITUDE DETECTION

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• Amplitude detection

• No synchronization necessary

AMPLITUDE DETECTION

• Demodulation of a signal (AM radio signal)

• Proper selection of components RLC1 allows to filter the signal and find its corresponding

„envelope”

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VOLTAGE DOUBLER I

• Single pulse voltage doubler

• Negative half-wave – C1 charges to Vs

• Positove half-wave – C2 charges to VS + VC1

• The resulting voltage is

the input voltage doubled

• Voltage drops must be

considered

43

VOLTAGE DOUBLER II

• Double pulse voltage doubler

• Negative half-wave – C1 charges to VS

• Positive half-wave – C1 charges to VS

• C3 voltage is sum of VC1 + VC2

• The resulting voltage is

the input voltage doubled

• Voltage drops must be

considered

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VOLTAGE MULTIPLIER

• Voltage multipliers:

• x3 (on the right)

• x4 (below)

45

OPTO-ELECTRONICS

46

OPTICAL APPLICATIONS

• Optical radiation detection (photodiode)

• Optical radiation emission (LED)

• Electrical power generation (solar cells)

47

PHOTODIODE

• Works in reverse polarity

• Current proportional to light flux

Photodiode symbol

48

LED I

• LED – light-emitting diode

• Color (wavelength) depends on the

semiconductor type used

• Radiation intensity is proportional

to the current flowing

49

LED symbol

LED II

• pn-junction is enclosed in a package

(shorter lead is cathode)

• Dual color LED`s are twosiodes

enclosed in a single package

50

SELECTING LED RESISTOR

• Diodes are usually designed for the current of 5-20mA

• Resistor selection, given the source voltage, diode voltage drop

and diode nominal current

( )

( )

1.8...2.0

20

8

8 1.8310

20

F

F

out pk

out pk F

S

F

V V

I mA

V V

V V V VR

I mA

=

=

=

− −= = =

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LED APPLICATIONS

• Simple applications:

• Diode as an indicator

• 7-segment display:

• Lighting etc.

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SUMMARY

Forward direction

Reverse direction

• Anode potential higher

than cathode

• Voltage drop of ~0.7V

• Current limited by power

dissipation

• Cathode potential higher

than anode

• Reverse current of ~µA

• Breakdown voltage

• p = anode

• n = cathode,

marked by a strip on the

package

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THANK YOU

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