01Chapter semiconductor physics 4-2.ppt

01Chapter semiconductor physics 4-2.ppt

1. EE 3110 Microelectronics I Suketu Naik
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Course Outline
1. Chapter 1: Signals and Amplifiers
2. Chapter 3: Semiconductors
3. Chapter 4: Diodes
4. Chapter 5: MOS Field Effect Transistors (MOSFET)
5. Chapter 6: Bipolar Junction Transistors (BJT)
6. Chapter 2 (optional): Operational Amplifiers

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Chapter 4:
Diodes
Part II

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4.5 Rectifier Circuits
Figure 4.20: Block diagram of a dc power supply
The primary application of diode is the rectifier –
Electrical device which converts alternating current
(AC) to direct current (DC)
One important application of rectifier is dc power
supply.

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step #1: increase / decrease rms magnitude of
AC wave via power transformer
step #2: convert full-wave AC to half-wave DC
(still time-varying and periodic)
step #3: employ low-pass filter to reduce wave
amplitude by > 90%
step #4: employ voltage regulator to eliminate
ripple
step #5: supply dc load
.

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4.5.1 The Half-Wave Rectifier
Half-wave rectifier –
utilizes only alternate
half-cycles of the input
sinusoid
Constant voltage
drop model is
employed.

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4.5.1 The Half-Wave Rectifier
Small inputs?
Regardless of the
model employed,
one should note that
the rectifier will not
operate properly
when input voltage
is small (< 1V)
Those cases require
a precision rectifier
(diode with op
amps).

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4.5.2 Full-Wave Rectifier
Center-tapping of the transformer, allowing “reversal”
of certain currents…

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When instantaneous source voltage is positive, D
1
conducts while D
2
blocks…
4.5.2. Full-Wave Rectifier

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when instantaneous source voltage is negative, D
2

conducts while D
1
blocks
4.5.2 Full-Wave Rectifier

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Center-tapped Transformer

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An alternative implementation of the full-wave rectifier
is bridge rectifier
Does not require center-tapped transformer
Four diodes instead of two
4.5.3 Bridge Rectifier

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when instantaneous source voltage is positive,
D
1 and D
2 conduct while D
3 and D
4 block
4.5.3 Bridge Rectifier

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when instantaneous source voltage is negative,
D
3
and D
4
conduct while D
1
and D
2
block
4.5.3 Bridge Rectifier

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4.5.4. The Rectifier with a Filter Capacitor
 Why is this example unrealistic?
 Because for any practical application,
the converter would supply a load
(which in turn provides a path for capacitor discharging)

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4.5.4. The Rectifier with a Filter Capacitor

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








output voltage for state #1
output voltage for state #2
O I
t
RC
O peak
v t v t
v t V e
4.5.4. The Rectifier with a Filter Capacitor

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4.5.4. The Rectifier with a Filter Capacitor
Precision rectifier – is a device which facilitates
rectification of low-voltage input waveforms
How?

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4.6: Limiting and Clamping Circuits
Q: What is a limiter or
clamping circuit?
A: One which limits
voltage output.

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single limiters
employ one diode
double limiters
employ two
diodes of
opposite polarity
linear range may
be controlled via
string of diodes
and dc sources
zener diodes may
be used to
implement soft
limiting

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soft vs. hard limiterQ: How are limiter
circuits applied?
A: Signal processing,
used to prevent
breakdown of
transistors within
various devices.
Why use soft?
4.6: Soft vs Hard limiter

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4.6.2 The Clamped Capacitor or DC Restorer
Q: What is a DC restorer?
A: Circuit which provides the dc
component of an AC without
DC value.
Q: Why is this ability important?
A:
1) Average value of the output is
effective way to measure duty
cycle
2) Duty cycle is modulated to
carry digital data (PWM): use
DC restorer followed by RC low
pass filter

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Q: What is a voltage
doubler?
A: One which
multiplies the
amplitude of a wave or
signal by two.
How?
4.6.3 The Voltage Doubler
dc restorerpeak rectifier

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4.7 Special Diodes
Schottky-Barrier Diode or
Schottky Diode
metal and moderately doped
semiconductor junction
current flows from metal to
semiconductor
current is conducted by
majority carriers: can switch it
on and off faster than p-n
junction
forward voltage drop is lower
than p-n junction
(0.3-0.5 V)

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4.7 Special Diodes
Varactors
reverse-biased p-n junction
junction capacitance is a
function of reverse bias voltage
how?
voltage variable capacitor
tuning of receivers, Phase
locked loops
Anode Cathode

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Photodiodes
reverse-biased p-n junction
expose to light: covalent bonds
break, electron-hole pairs are created
free electrons sweep to n side and
holes to p side
reverse current is created
photocurrent is proportational to
intensity of incident light
convert light into electric signal
applications: CD-ROM, fiber-optic
what happens when you don't reverse
bias the photodiode and expose it to
light?
4.7 Special Diodes
Anode Cathode
P
o
...P
2
= light levels

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4.7 Special Diodes
LEDs
convert forward current into light
forward bias region: when minority
carriers diffuse into p and n sides,
they recombine with majority
carriers, e.g. electrons with holes.
recombination: emission of light
special semiconductor material:
direct band-gap
known spectra of light when
electrons leave orbit
emitted light is proportional to
number of recombinations which is
proportional to the forward current
Anode Cathode