Electronics
- lectures for Mechanical Engineering
part 2
Dr. Bogusław Boratyński
Faculty of Microsystems Electronics and Photonics,
Wroclaw University of Technology,
2011
From the course syllabus
Basic literature & figure sources:
G. Rizzoni, Fundamentals of Electrical Engineering, McGraw-Hill
R.F. Pierret, Semiconductor Device Fundamentals, Addison-Wesley Publ.,
B.G. Streetman, Solid State Electronic Devices, Prentice-Hall,
D. Bell, Fundamentals of Electric Circuits, Oxford Univ. Press,
T. Mouthaan, Semiconductor Devices Explained, John Willey&Sons
Additional literature:
W. Marciniak, Przyrządy półprzewodnikowe i układy scalone, WNT,
A. Świt, J. Pułtorak, Przyrządy półprzewodnikowe, WNT,
B.G. Streetman, Przyrządy półprzewodnikowe, WNT
Semiconductor devices
Chapter 3 Electronic devices.
3.1 The p-n junction. Semiconductor diodes.
The p-n junction operation principle.
The Shockley equation – the I-V characteristic.
Ideal and real diodes. Temperature effects.
Bias - operating point. Small signal models.
Breakdown in the junction – Zener diode.
Photodiodes and photovoltaic cells.
Metal-semiconductor contact - the Schottky diode.
Rectifier and voltage regulator circuits.
The p-n diode fabrication
Photolithography process
Source: R.F. Pierret, Semiconductor Device Fundamentals, Addison-Wesley Publishing Comp.
The ideal p-n junction
A real diode and the ideal p-n junction model
- external bias voltage
VA
symbol
A-anode
p-type
In p-type:
majority carriers
- holes
K-cathode
n-type
In n-type:
majority carriers
- electrons
Source: R.F. Pierret, Semiconductor Device Fundamentals, Addison-Wesley Publishing Comp.
The ideal p-n junction electrostatics
Vo - build-in potential,
or diffusion barrier,
or contact potential
in the p-n junction
Vo < Eg /q
Typical values: Vo (Eg)
Ge: 0.4V (0.7eV)
Si: 0.7V (1.1eV)
Source: R.F. Pierret, Semiconductor Device Fundamentals, Addison-Wesley Publishing Comp.
W - the depletion region
(junction region) width
E
Electric field
The ideal p-n junction at equilibrium and under bias
Energy band models under external bias voltage - VA
minority
electrons
Vo
VA <0
actual barrier
Vo + |VA| > Vo
E
majority
electrons
E
VA >0
minority
holes
actual barrier
Vo - VA < Vo
majority
holes
I-V characteristic
Reverse bias:
drift of
minority
carriers
Forward bias:
diffusion of
majority
carriers
Source: R.F. Pierret, Semiconductor Device Fundamentals, Addison-Wesley Publishing Comp.
Energy band models – another view
a p-n junction „formation”
n
p
a p-n junction under bias
n
p
Source: T. Mouthaan, Semiconductor Devices Explained,
John Willey&Sons
The ideal p-n junction under bias
The Shockley equation - current-voltage dependence in the p-n junction
exponential function dependence in forward direction
Io =constant - saturation current ( due to minority carriers flow)
A- junction cross section area
I - V characteristic in lin-lin coordinate system
I - V characteristic in log-lin coordinate system
kT/q = 26 mV
at T=300K
Source: R.F. Pierret, Semiconductor Device Fundamentals, Addison-Wesley Publishing Comp.
The ideal p-n junction under bias
The Shockley equation
Current, I is exponentially dependent
on voltage bias VA (or U)
Typical voltage drop VAknee for real p-n junctions
made of different semiconductors.
kT/q = 26 mV
Example: I0 = 1uA =10-6A, VA = 260mV,
calculated current I = 2.2 10-2 A = 22 mA
For different semiconductors (Eg)
Vaknee
Ge
0.3V
Si
0.6V
GaAs
0.9V
if Eg
> ni
then I0
and VAknee
A real p-n junction under bias
The Shockley equation gives a good proximation of the forward I-V curve for real diodes.
The diffusion barrier
(junction built-in potential):
!!! always Vo < Eg /q
Typical values: Vo (Eg)
Ge: 0.3V (Eg =0.7eV)
Si: 0.6V (Eg =1.1eV)
GaAs: 1.0V (Eg =1.4eV)
GaN: 3.0V (Eg =3.3eV)
Vaknee
Ge
Si
GaAs
0.3V
0.6V
0.9V
Source: B.G.Streetman, Solid State
Electronic Devices, Prentice Hall.
GaN
3V
The „knee voltage” value is similar
to the built-in potential value
for a given semiconductor.
Temperature influence on the I-V characteristic
The Shockley equation – temperature dependence
Io =const. If T=const. but, if T then Io  
Temperature coefficients (TC):
Forward voltage - Voltage TC
dV/dT = -2 mV/K @ I=const.
Reverse current - Current TC
(dI/dT)(1/I) = +10%/K @ V=const.
every dT=10K the reverse current doubles
kT/q = 26 mV
only @ T=300K
Application:
Diode as a temperature sensor.
Example (Si diode) at forward bias: dT=70K
at 25 C VA = 620mV @ I = const. (1mA)
at 95 C VA =620mV + 70K · (-2 mV/K) = 620mV - 140mV=
=480mV = 0.48V
dVA = -140mV
Example at reverse bias:
at 25 C Irev = 10nA @ VA = const. (-20V)
at 95 C Irev = 27 x 10nA = 128 x 10nA= 1.28A
A real p-n junction under bias
The Shockley equation + breakdown phenomena at reverse bias
Breakdown – rapid current increase
a typical
Si diode
I-V curve
Source: B.G.Streetman, Solid State Electronic Devices,
Prentice Hall.
Source: R.F. Pierret, Semiconductor Device Fundamentals,
Addison-Wesley Publishing Comp.
A real p-n junction under bias
The Shockley equation + additional Rec. - Gen. currents
Junction breakdown – rapid
current increase
Additional
junction
currents:
- generation
current
- recombination
current
Source: B.G.Streetman, Solid State Electronic Devices,
Prentice Hall.
Source: R.F. Pierret, Semiconductor Device Fundamentals,
Addison-Wesley Publishing Comp.
A real p-n diode
BAV19 diode
I-V measurements
Io = Ig – generation current
at reverse bias
Absolute Maximum Rating from the datasheet:
IF=500mA - dc forward current
VR = 100V - dc reverse voltage
Tj=175C - junction temperature
n –ideality factor
n {1,2}
value dependent on
recombination current
at forward bias
A real p-n diode
Fairchild BAV19, -20, -21 diodes
DC circuit analysis
The load line concept
Finding the operating point - Qpoint
from KVL:
VT= RT iD + vD
and the load line equation is:
iD = -(1/ RT) vD + VT /RT
Operating point is;
iD = 21mA , vD = 1.0 V
Source: G. Rizzoni, Fundamentals of Electrical Engineering,
McGraw-Hill
Small signal equvalent model of a diode
From the Shockley equation:
g = dI/dU = IQ/(kT/q) = IQ/26mV (at 300K)
g-1 = rd - dynamic resistance of the diode
Valid for
low frequency:
f<100kHz
Valid for
high frequency:
f>100kHz
Ctotal - capacitances of a diode
Rseries - parasitic series resistances
rd = g-1
slope=
g2
Rseries
Qp2
slope = g1
Qp1
Values of the model components depend
on the operating point Q (the applied bias),
excluding Rseries = const.
IQ =10mA rd =2.6 Ω
IQ = 1µA rd =26 kΩ
Source: B.G.Streetman, Solid State Electronic Devices,
Prentice Hall.
Source: R.F. Pierret, Semiconductor Device Fundamentals,
Addison-Wesley Publishing Comp.
P-N junction breakdown phenomena
avalanche breakdown
(carrier multiplication)
Zener breakdown (electron tunneling)
original single electron
large voltage bias
small voltage bias
no breakdown
original single electron
1+3 electrons
+3 holes
E
electric field
E
Source: R.F. Pierret, Semiconductor Device Fundamentals,
Addison-Wesley Publishing Comp.
A Zener diode – voltage regulator device
The Zener diode operates at the breakdown at a given voltage Vz
The I-V curve - makes the output voltage constant at Vz
_
Vz = Zener breakdown voltage
Voltage regulator circuit
Izm - max. current value
large ripples
small ripples
Rs
Vz = const.
Input voltage
Izm ------------
+
Vz
examples
3.3 V
5V
6.3 V
9.1 V
12 V
15 V
24 V
91 V
Output voltage
BZX85C9V1
BZX85B12
tolerance:
Source: B.G.Streetman, Solid State Electronic Devices,
Prentice Hall.
C - 5%
B - 2%
A photodiode structure and operation
Optical absorption in a photodiode
I-V characteristic of an illuminated diode
photogeneration
mechanism
electrons
Reverse biased
photodiode
photons
G - flux of photons
Forward
„self biased”
Solar cell
holes
Source: R.F. Pierret, Semiconductor Device Fundamentals,
Addison-Wesley Publishing Comp.
Photon absorption mechanism
photon
absorption
el. field
profile
depletion region
with an electric field
generated
electron & holes
provide current
-
0
optical absorption constant of the material
photon
absorption
photon
transmission
absorption
edge
Source: T. Mouthaan, Semiconductor Devices Explained,
John Willey&Sons.
Photon absorption
Optical spectrum and the absorption edge λG for various semiconductors
λG [m]=1.24/Eg [eV]
UV
IR
AlGaAs
InGaAs
GaN
Source: R.F. Pierret, Semiconductor Device Fundamentals,
Addison-Wesley Publishing Comp.
Solar cells
Solar cell becomes forward biased due to illumination – no external bias applied
- power source or power converter
I-V characteristics of an illuminated diode
G - flux of photons
- photocurrent
Reverse biased
photodiode
P>0
Forward bias: V>0, but I<0
P= I U < 0 - source of power
P<0
Forward
„self biased”
Solar cell
Source: R.F. Pierret, Semiconductor Device Fundamentals,
Addison-Wesley Publishing Comp.
Solar cells
Solar cell operation
Max. power point
- operating point
Solar spectum: AM1 - outside atm.
AM 1.5 - av. terrestrial P=100mW/cm sq.
slope
1/R load
Efficiency:
Im
Vm
R load
Pin = Psun
FF - fill factor
= 5…..15…..30……40 [%}
 = 5% amorphous sc.
 = 10% polycrystalline sc.
 = 20% single crystal sc.
 = 35% multi-junction cells
 = 40% concentrated sunlight
Source: R.F. Pierret, Semiconductor Device Fundamentals,
Addison-Wesley Publishing Comp.
A metal - semiconductor junction
Type1: Schottky junction
rectifying contact – a diode
Metal
Semiconductor
Type 2: ohmic contact
- small resistance
electron
gas
Metal
Semiconductor
n-type
Schottky
p-n
diode
- saturation current
thermionic current
smaller voltage drop
Source: Source: B.G.Streetman, Solid State Electronic Devices, Prentice Hall.
Source: J. Singh, Semiconductor Devices , John Willey&Sons
Basic rectifier circuits
Half-wave rectifier circuit
Full-wave rectifier circuit
source voltage (f=60Hz, T=1/f=16.7ms)
Large signal diode model
„piecewise linear approximation”
Source: G.Rizzoni, Fundamentals of Electrical Eng., McGraw-Hill
Rectifier circuits + filtering
Bridge rectifier circuit
1k
RL
1k
RL
Source: G.Rizzoni, Fundamentals of Electrical Eng., McGraw-Hill
Rectifier circuits - constant voltage power supply
Bridge rectifier circuit
DC power supply circuit
voltage regulator (Zener diode)
large ripples
small ripples
Rs
Vz = const.
Input voltage
Output voltage
Izm ------------
Source: G.Rizzoni, Fundamentals of Electrical Eng., McGraw-Hill
Electroluminescent diodes
Light Emitting Diode - LED
LED packages for white light emission
Types of monchromatic - to - white light conversion in LED packages
RGB phosphor
3 LED chips
Yellow
phosphor
Blue
M. Rudziński, M. Wesołowski, W. Strupiński, Niebieskie, zielone i białe emitery światła wytwarzane z
półprzewodników AIII-BN, Przegląd Elektrotechniczny, 7, 2014
Diodes and their applications
Different types of diodes - summary
Zener diode - voltage regulators
General purpose diode (rectifying, switching)
Electroluminescent diode, LED – display, indicator, lamps
Varactor diode - tuned circuits
Schottky diode (metal-semiconductor diode) – rectifying,
switching (also in microwave circuits)
Photodiode – photodetector, photovoltaic cell, solar cellsource of power