Block D: Semiconductor Electronics

EE2301:

Basic Electronic Circuit

Let’s start with diode

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EE2301: Basic Electronic Circuit

Examples of Diode

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EE2301: Basic Electronic Circuit

The Basic Property of a Diode

Let’s have a demo

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How does it work?


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Block C Unit 1 Outline


Semiconductor materials (eg. silicon)

>
Intrinsic and extrinsic semiconductors


How a p
-
n junction works (basis of diodes)


Large signal models

>
Ideal diode model

>
Offset diode model


Finding the operating point


Application of diodes in rectification

EE2301: Basic Electronic Circuit

Semiconductor Electronics
-

Unit 1: Diodes

6

Electrical Materials

Insulators

Conductors

Semi
-
Conductors

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Semiconductor Electronics
-

Unit 1: Diodes

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Semiconductor Applications

Integrated Circuit

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Semiconductor Electronics
-

Unit 1: Diodes

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Semiconductor Applications

TFT (Thin Film Transistor)

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Semiconductor Electronics
-

Unit 1: Diodes

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Intrinsic Semiconductor

Si

Si

Si

Si

Covalent Bonds

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Semiconductor Electronics
-

Unit 1: Diodes

10

Silicon Crystal Lattice

In 3
-
D, this looks like:

Number atoms per m
3
: ~ 10
28

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Semiconductor Electronics
-

Unit 1: Diodes

11

Growing Silicon

We can grow very pure silicon

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Semiconductor Electronics
-

Unit 1: Diodes

12

Conduction

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Semiconductor Electronics
-

Unit 1: Diodes

13

Currents in Semiconductor

Source: http://hyperphysics.phy
-
astr.gsu.edu/HBASE/solids/intrin.html

EE2301: Basic Electronic Circuit

Semiconductor Electronics
-

Unit 1: Diodes

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Carrier Concentration

The number of free electrons available for a
given material is called the
intrinsic

concentration

n
i
.
For example, at room
temperature, silicon has:

n
i

= 1.5 x 10
16

electrons/m
3

1 free electron in about every 10
12

atoms

EE2301: Basic Electronic Circuit

Semiconductor Electronics
-

Unit 1: Diodes

15

Doping: n
-
type

1 Si atom substituted
by 1 P atom

P has 5 valence electrons
(1 electron more)

1 free electron
created

Si

Si

Si

P

Si

Si

-

Electrically neutral

EE2301: Basic Electronic Circuit

Semiconductor Electronics
-

Unit 1: Diodes

16

Doping: p
-
type

Si

Si

Si

B

Si

Si

1 Si atom substituted
by 1 B atom

+

B has 3 valence electrons
(1 electron short)

1 hole created

Electrically neutral

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Semiconductor Electronics
-

Unit 1: Diodes

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p
-
n Junction

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Semiconductor Electronics
-

Unit 1: Diodes

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Diode Physics

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Semiconductor Electronics
-

Unit 1: Diodes

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Diode Physics

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Website: http://www
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g.eng.cam.ac.uk/mmg/teaching/linearcircuits/diode.html

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Biasing and Conventions

v
D
: Voltage of P (anode) relative to N (cathode)

i
D
: Current flowing from anode to cathode

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Diode

Diode begins to conduct a significant amount
of current: Voltage
V
γ

is typically around 0.7V

Diode equation: I
D

= I
0

[exp(eV
D
/kT)
-

1]

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Diode Symbol and Operation

Forward
-
biased
Current (Large)

Reverse
-
biased
Current (~Zero)

+
-

Forward Biased:

Diode conducts

-

+

Reverse Biased:

Little or no current

i
D

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Real diode circuits

+
-

+
V
L
-

+ V
D

-

I
D

V
T

R
T

To find V
L

where V
T

and R
T

are known,

First apply KVL around the loop:

V
T

= V
D

+ R
T
I
D

Then use the diode equation:

I
D

= I
0

[
exp
(
eV
D
/
kT
)
-

1]

At T = 300K,
kT
/e = 25mV

We then need to solve these two simultaneous equations, which is not trivial.
One alternative is to use the graphical method to find the value of I
D

and V
D
.

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Graphical method

Operating point is where
the load line & I
-
V curve
of the diode
intersect

Equation from KVL

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Diode circuit models


Simplify analysis of diode circuits which
can be otherwise difficult


Large
-
signal models: describe device
behavior in the presence of relatively large
voltages & currents

>
Ideal diode model

>
Off
-
set diode mode

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Ideal diode model

In other words, diode is
treated like a switch here

v
D

> 0: Short circuit

v
D

< 0: Open circuit

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Ideal diode model

Circuit containing
ideal diode

Circuit assuming that the
ideal diode conducts

Circuit assuming that
the ideal diode does
not conduct

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Ideal diode example 1

Problems 9.7 and 9.8

Determine whether the diode is conducting or not. Assume diode is ideal

Repeat for V
i

= 12V and V
B

= 15V

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Ideal diode example 1 solution

This slide is meant to be blank

Option 1:

Assume diode is conducting and find the diode current direction

Outcome 1:
If diode current flows from anode to cathode, the assumption is true


Diode is forward biased

Outcome 2:

If diode current flows from cathode to anode, the assumption is false


Diode is reverse biased

Option 2:

Assume diode is not conducting and find the voltage drop across it

Outcome 1:
If voltage drops from cathode to anode, then the assumption is true


Diode
is reverse biased

Outcome 2:

If voltage drops from anode to cathode, then the assumption is false


Diode
is forward biased

EE2301: Basic Electronic Circuit

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Ideal diode example 1 solution

This slide is meant to be blank

Assume diode is conducting

Forward
-
bias diode current (ie anode to cathode)

= (10
-

12) / (5 + 10) =
-
2/15 A


Assumption was wrong


Diode is in reverse bias

Assume diode is not
-
conducting

Reverse
-
bias voltage (ie cathode referenced to anode)

= 12
-

10 = 2V


Assumption was correct


Diode is in reverse bias

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Ideal diode example 1 solution

This slide is meant to be blank

Assume diode is conducting

Forward
-
bias diode current (ie anode to cathode)

= (15
-

12) / (5 + 10) = 1/5 A


Assumption was correct


Diode is in forward bias

Assume diode is not
-
conducting

Reverse
-
bias voltage (ie cathode referenced to anode)

= 12
-

15 =
-
3V


Assumption was incorrect


Diode is in forward bias

EE2301: Basic Electronic Circuit

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Ideal diode example 2

Problem 9.14

Find the range of V
in

for which D
1

is forward
-
biased. Assume diode is ideal

The diode is ON as long as forward bias voltage
is positive

Now, minimum v
in

for
v
D

to be positive = 2V

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Offset diode model

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Offset model example

Problem 9.19

The diode in this circuit requires a minimum current of 1 mA to be above the
knee of its characteristic. Use V
γ

= 0.7V

What should be the value of R to establish 5 mA in the circuit?

With the above value of R, what is the minimum value of E required to maintain
a current above the knee

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Offset model example solution

This slide is meant to be blank

I
D

= (E
-

V
D
)/R

When the diode is conducting, V
D

= V
γ

I
D

= (5
-

0.7)/R

We can observe that as R increases, I
D

will decrease

To maintain a minimum current of 5mA,

R
max

= 4.3/5 = 860
Ω

Minimum E required to keep current above the knee (1mA),

E
min

= (10
-
3

* 860) + 0.7 = 1.56V

EE2301: Basic Electronic Circuit

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Rectification: from AC to DC

Supply is AC

DC required

One common application of diodes is rectification. In rectification, an AC
sinusoidal source is converted to a unidirectional output which is further
filtered and regulated to give a steady DC output.

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Rectifier with regulator diagram

Rectifier

Bi
-
directional
input

Steady DC output

Filter

Regulator

Unidirectional output

We will look at two types of rectifiers and apply the large signal
models in our analysis:

1) Half wave rectifier

2) Full wave rectifier

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Half
-
Wave Rectifier

V
S

~

R
L

On the
positive

cycle


Diode is forward biased


Diode conducts


V
L

will follow V
S

V
S

~

R
L

On the
negative

cycle


Diode is reverse biased


Diode does not conduct


V
L

will remain at zero

V
S

V
L

We can see that the
circuit conducts for
only half a cycle

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Average voltage in a HW Rectifier

1
st

half of
period

2
nd

half
of period

NB: This is equal to the DC term of the Fourier series

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Full
-
Wave Rectifier


Also known as BRIDGE rectifier


Comprises 2 sets of diode pairs


Each pair conducts in turn on each half
-
cycle

V
S

~

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Full
-
Wave Rectifier

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Full
-
Wave Rectifier

Half a period

T/2
–

time

π

–

phase

Repeats for every half a period:

Integrate through half a period

NB: This is equal to the DC term of the Fourier series

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Full
-
Wave Rectifier (offset)

V
D
-
on

(only one diode is on)

2V
D
-
on

(two diodes are on)

With offset diodes

With ideal diodes

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Ripple filter

Charging

Discharging

Anti
-
ripple filter is used to smoothen out the rectifier output

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Ripple filter

Approximation: abrupt change in the voltage

From transient analysis: V
M
exp(
-
t/RC)

V
L

Ripple voltage V
r

= V
M

-

V
L min

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Ripple filter example

Problem 9.40

Find the turns ratio of the transformer and the value of C given that:

I
L

= 60mA, V
L

= 5V, V
r

= 5%, V
line

= 170cos(
ω
t) V,
ω

= 377rad/s

Diodes are fabricated from silicon, V
γ

= 0.7V

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Ripple filter example solution

a) TURNS RATIO: To find the turns ratio, we need to find V
S1

and V
S2

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Ripple filter example solution

But V
M

is not equal to V
S1

due to voltage drop across diodes

So we now apply KVL on the secondary coil side:

V
S1

-

V
D

-

V
M

= 0

V
S1

= 5.825 V (V
D

= 0.7V)

Turns ratio, n = V
line

/ V
S1

~ 29

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Ripple filter example solution

b) Value of C: Need to find the RC time constant associated with
the ripple

R
L

= V
L
/I
L

= 83.3
Ω

We know it decays by V
M
exp(
-
t/RC), we now just need to know how long this lasts (t
2
)

V
L
-
min

=
-

V
SO
cos(
ω
t
2
)
-

V
D
-
on

v
so

is negative at this point

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Ripple filter example solution

V
L
-
min

=
-

V
SO
cos(
ω
t
2
)
-

V
D
-
on

2
nd

half of the sinusoid

t
2

= (1/
ω
) cos
-
1
{
-
(V
L
-
min

+ V
D
-
on
)/V
SO
} = 7.533 ms

Decaying exponential:

V
L
-
min

= V
M

exp(
-
t
2
/R
L
C)

EE2301: Basic Electronic Circuit

Examples of Diode

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EE2301: Basic Electronic Circuit

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Electrical Materials

Insulators

Electrons are bound to the nucleus and are therefore
not free to move

With no free electrons, conduction cannot occur

Conductors

Sea of free electrons not bound to the atoms

Ample availability of free electrons allows for
electrical conduction

Semiconductors

Electrons are bound to the nucleus but vacancies are
created due to thermal excitation

Electrical conduction occurs through positive (called
holes) and negative (electrons) charge carriers

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Conduction in Semiconductors

Silicon is the dominant semiconductor material used in the
electronics industry. In a cubic meter of silicon, there are
roughly 10
28

atoms. Among these 10
28
, there will be about

1.5
×
10
16

vacancies at room temperature. This is known as
the intrinsic carrier concentration: n = 1.5
×
10
16
electrons/m
3
.
This corresponds to 1 free electron for every 10
12

atoms.

There will be same number of electrons as holes in intrinsic
silicon since it is overall electrically neutral.

EE2301: Basic Electronic Circuit

Extrinsic semiconductors

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A semiconductor material that has been subjected to the doping process is
called an extrinsic material. Both n
-
type and p
-
type materials are formed by
adding a predetermined number of impurity atoms to a silicon base.

An n
-
type material is created by introducing impurity elements that have five
valence electrons. In an n
-
type material, the electron is called the majority carrier.

An p
-
type material is created by introducing impurity elements that have three
valence electrons. In a p
-
type material, the hole is the majority carrier.

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p
-
n Junction

The pn junction forms the basis of the semiconductor diode

Within the depletion region, no free carriers exist since the holes and electrons at
the interface between the p
-
type and n
-
type recombine.

EE2301: Basic Electronic Circuit

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Response of the depletion region

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+

+
-

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-

+

+

Forward biased:

Voltage on the p
-
type side is higher than the
n
-
type side

Depletion width reduces, lowering barrier for
majority carriers to move across the depletion
region

Large conduction current

57

Reverse biased:

Voltage on the p
-
type side is lower than the n
-
type side

Depletion width increases, increasing the
barrier for majority carriers to move across
the depletion region

Very small leakage current

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Analogy from tides

Depletion region

Depletion region

Forward Biased

Reverse Biased