ECE-101
ELEMENTS OF ELECTRONICS
ENGG.
1
Solids: Conductors, Insulators
and Semiconductors
• Conductors: mostly metals
• Insulators: mostly nonmetal materials
• Semiconductors: metalloids
2
Solids: Conductors, Insulators
and Semiconductors
Conduction Band: white
Band gap
No gap
Valence Band
in red
Conductor
Insulator
Semiconductor
3
Semiconductors
• Metalloids: semiconducting elements
– low electrical conductivity at room temperature
– Electrical conductivity increases with temp.
• Gap between valence and conduction band
is intermediate in size
4
Semiconductors
• Semiconducting elements form the basis of
solid state electronic devices.
– Metalloids (such as silicon or germanium) are
semiconducting elements whose electrical
conductivity increases as temperature increases.
– A striking property of these elements is that their
conductivities increase markedly when they are
doped with small quantities of other elements.
5
Semiconductors
• Semiconducting elements form the basis of
solid state electronic devices.
– When silicon is doped with phosphorus, it
becomes an n-type semiconductor, in
which electric current is carried by
electrons.
6
Semiconductors
• Semiconducting elements form the basis of
solid state electronic devices.
– When silicon is doped with boron, it
becomes a p-type semiconductor, in
which an electrical current is carried by
positively charged holes
7
Semiconductors
• Semiconducting elements form the basis of
solid state electronic devices.
– Joining a p-type semiconductor to an ntype semiconductor produces a p-n
junction, which can function as a rectifier.
– A rectifier is a device that allows current to
flow in one direction, but not the other.
8
Figure : Effect of doping silicon.
9
Figure :
A p-n junction.
10
The Diode
B
A
Al
SiO
2
p
n
Cross section of pn-junction in an IC process
N-type region
P-type region
doped with donor
impurities
(phosphorus,
arsenic)
doped with
acceptor
impurities (boron)
11
Depletion Region
•Concentration Gradient causes electrons to diffuse from n to p,
and holes to diffuse from p to n
•This produces immobile ions in the vicinity of the boundary
•Region at the junction with the charged ions is called the
depletion region or space-charge region
•Charges create electric field that attracts the minority carriers,
causing them to drift
•Drift counteracts diffusion causing equilibrium ( Idrift = -Idiffusion )
hole diffusion
electron diffusion
p
n
hole drift
electron drift
12
Depletion Region
•Zero bias conditions
hole diffusion
electron diffusion
p
•p more heavily doped
than n (NA > NB)
•Electric field gives rise
to potential difference in
the junction, known as
the built-in potential
(a) Current flow.
n
hole drift
electron drift
Charge
Density

+
x
Distance
-
Electrical
Field
(b) Charge density.

x
(c) Electric field.
V
Potential
-W 1

W2
x
(d) Electrostatic
potential.
13
Forward Bias
hole diffusion
electron diffusion
p
n
hole drift
electron drift
+
-
•Applied potential lowers the potential barrier, Idiffusion > I drift
•Mobile carriers drift through the dep. region into neutral regions
•become excess minority carriers and diffuse towards terminals
•Read about drift and diffusion currents at:
•http://ece-www.colorado.edu/~bart/book/book/chapter2/ch2_10.htm
14
Forward Bias
n p (x)
pn (x)
Lp
pn0
np0
-Wp
p-region
-W1 0
W2
Metal contact to n-region
pn (W2)
minority carrier concentration
Wn
x
n-region
diffusion
Typically avoided in Digital ICs
15
Reverse Bias
hole diffusion
electron diffusion
p
n
hole drift
electron drift
-
+
•Applied potential increases the potential barrier
•Diffusion current is reduced
•Diode works in the reverse bias with a very small drift current
16
pn0
np0
n p0
-Wp
p-region
-W1 0
Wn
W2
Metal contact to n-region
Reverse Bias
x
n-region
diffusion
The Dominant Operation Mode
17
Models for Manual Analysis
+
ID = IS(eV D/T – 1)
VD
ID
+
+
VD
–
(a) Ideal diode model
•Accurate
•Strongly non-linear
•Prevents fast DC bias
calculations
–
VDon
–
(b) First-order diode model
•Conducting diode replaced
by voltage source VDon=0.7V
•Good for first order
approximation
18
Diode Current
VDon  0.7V
VDon  0.7V
Ideal diode equation:
19
Depletion Capacitance

Due to depletion charges
» VD changes space charge
» Forms a capacitor Cj
– Charge modulated by voltage

Ideality factor (m) depends on
junction gradient
20
Secondary Effects: Breakdown

Cannot bear too large reverse biases
» Drift field in depletion region will get extremely large
» Minority carriers caught in this large field will get very energetic
– Energetic carriers can knock atoms and create a new n-p pair
– These carriers will get energetic, too, and so on: thus large currents!
0.1

Two types
» Avalanche breakdown
ID (A)
– Above mechanism
» Zener breakdown
– More complicated
0

–0.1
–25.0
–15.0
–5.0
VD (V)
0
5.0
Can damage diode
21
RECTIFIER
A
rectifier
is
an
electrical
device
that
converts alternating current (AC), which periodically
reverses direction, to direct current (DC), which is in
only one direction, a process known as rectification.
22
TYPES OF RECTIFIERS
Half wave Rectifier
Full wave Rectifier
23
HALF WAVE RECTIFIER
 In half wave rectification, either the positive or
negative half of the AC wave is passed, while the
other half is blocked.
 Because only one half of the input waveform reaches
the output, it is very inefficient if used for power
transfer.
24
HALF WAVE
RECTIFICATION
25
OUTPUT DC VOLTAGE
CALCULATION
 The output DC voltage of a half wave rectifier can
be calculated with the following two ideal equations
26
FULL WAVE RECTIFIER
 A full-wave rectifier converts the whole of the input
waveform to one of constant polarity (positive or negative)
at its output.
 Full-wave rectification converts both polarities of the input
waveform to DC (direct current), and is more efficient.
27
FULL WAVE RECTIFICATION
 In a circuit with a non - center tapped transformer, four
diodes are required instead of the one needed for half-wave
rectification.
 For single-phase AC, if the transformer is center-tapped,
then two diodes back-to-back (i.e. anodes-to-anode or
cathode-to-cathode) can form a full-wave rectifier.
28
FULL WAVE RECTIFIER (Bridge
Rectifier)
29
BRIDGE RECTIFIER
CIRCUIT
30
FULL WAVE RECTIFIER (Centre-tapped
rectifier)
31
FORMULA
The average and root-mean-square output voltages of
an ideal single phase full wave rectifier can be
calculated as:
32