Transcript PN Junction Devices - Physics & Astronomy | SFASU

PN Junction Devices
Electromagnetic Waves

Induction
 A changing magnetic field causes an induced electric
field
 A changing electric field causes an induced magnetic
field

Resonant RLC circuits cause charges to flow
back and forth along the antenna
 The Electric and Magnetic fields are perpendicular
 Electromagnetic waves propagate from the antenna
Electromagnetic Waves
 Travel
at the speed of light
 Predicted by Maxwell’s theory
 Verified as light by experiments of Hertz
 c = lf
 Radio
operates in the MHz range
 l = c/f = 3 X 108 m/s / 106 Hz
 l = 300 m
 Antenna
should be ¼ wavelength for
useful transfer of information
The Electromagnetic Spectrum
Modulation
 Amplitude
Modulation (AM)
A radio wave can be transmitted long
distances.
To get our audio signal to travel long
distances we piggyback it onto a radio
wave.
This process is called MODULATION.
The radio wave is called the CARRIER.
The audio signal is called the
MODULATION.
Amplitude is periodically changed. Information can be sent at a relatively
slow rate, using the high frequency as carrier.
Detecting the variation is done by the speaker. It has a high inductance
so that it does not respond to high frequency, but only to the average.
Frequency Modulation
 Here
the amplitude is constant and the
frequency changes
 Free from static since the sources of static
(lightning, car ignitions, etc.) would show up
as amplitude changes, and FM responds only
to frequency changes
TV Transmission

Video transmitted as AM so that the changing
amplitude can control the changing intensity of
the electron beam
 Beam sweeps the phosphor screen with 525 lines 50
times/sec.
Audio transmitted by FM
 Typical frequencies (channel 6)

 Video Carrier 85 MHz
 Audio 98 MHz
 l = 3.3 m (antenna should be ½ l)
HD TV
 720p50
 1280x720 pixels
• 50 full frames/second (Europe)
• 24, 25, 30, 50, 60 fps in US
 1080i25
 1920x1080 pixels
• 25 full frames/s (Europe)
• 50 or 60 fps (US)
Electrons in Isolated Atoms
 Isolated
atoms have energy levels
 The electrons can only be found in these
energy states
Atoms in Solids
 Atoms
form a lattice structure
The lattice affects the structure of the energy levels of each
atom – we now have joint levels for the entire structure
Band Theory
 Three
bands of energy levels form
 Valence Band – most of the electrons are
here
 Conduction Band – electrons here give the
material electrical conductivity
 Forbidden Band – electrons must jump this
band to get from the valence to the
conduction band
Lattice Bands
Conduction
In order for an electron to become free and
participate in current flow, it must gain enough
energy to jump over the forbidden band
 For semiconductors at room temperature, there
is not enough energy to conduct.
 As temperature increases more electrons have
the energy to jump the forbidden band

 Resistivity decreases
 This is the opposite behavior of conductors
Resistivity
Semiconductor
R
Conductor
T
Semiconductors
 When
an electron becomes free, it creates
a “hole” in the lattice structure
A hole is effectively a positive charge
Electron and Hole Movement
Intrinsic Semiconductor

Elemental or pure semiconductors have equal
numbers of holes and electrons
 Depends on temperature, type, and size.

Compound Semiconductors can be formed from
two (or more) elements (e.g., GaAs)
Extrinsic Semiconductors
A
pure semiconductors where a small
amount of another element is added to
replace atoms in the lattice (doping).
 The aim is to produce an excess of either
electrons (n-type) or holes (p-type)
 Typical doping concentrations are one part in
ten million
 Doping must be uniform throughout the lattice
so that charges do not accumulate
N-Type and P-Type

One valence electron too many (n-type)
 Arsenic, antimony, bismuth, phosphorus

One valence electron too few (p-type)
 Aluminum, indium, gallium, boron
In a dc circuit
The PN Junction Diode
Start with a P and N type material.
Note that there are excess
negatives in the n-type and excess
positives in the p-type
More positive
than rest of N
More negative
than rest of P
Merge the two – some of the negatives
migrate over to the p-type, filling in the
holes. The yellow region is called the
depletion zone.
Biasing the Junction
Apply a voltage as indicated. The free
charge carriers (negative charges in the N
material and positive charges in the P
material) are attracted to the ends of the
crystal. No charge flows across the
junction and the depletion zone grows.
This is called reverse bias.
Switch polarity. Now the negative
charges are driven toward the junction in
the N material and the positive charges
also are driven toward the junction in the
P material. The depletion zone shrinks
and will disappear if the voltage exceeds
a threshold. This is called forward bias.
Diode Circuit Symbols
P material (anode)
N material (cathode)
Reverse Bias
Forward Bias
I-V Curve
Recall Ohm’s Law (V=IR) Put it into slope-intercept form
to get I = V/R. The slope of the graph is 1/R. Large slopes
mean small R.
Types of Diodes
 Rectifier
Diode
 Used in power supplies
 Signal
Diode
 Used in switches, detectors, mixers, etc.
 Zener
Diode
 Voltage regulation – operated reverse bias in
the avalanche region
 Reference
Diode
 Used like zener for voltage regulation
Types of Diodes

Varactor Diode
 PN junction exhibits capacitive properties
• Depletion zone acts like a dielectric
• Adjacent material acts like the capacitor plates
 Increasing reverse bias decreases capacitance
– recall
C
o A
d
 Capacitance Effect is destroyed if the forward
bias is great enough to destroy the depletion
zone
Diode Tuning

Varactor used to tune a harmonic circuit, as
before
1
f res 
2 LC

Increasing reverse bias decreases C and
increases fres
Rectification
 Conversion
of ac to dc.
 Many devices (transistors) are unidirectional
current devices
 DC required for proper operation.
Half Wave Rectifier
Full Wave Rectifier
Bridge Rectifier
Operation
Filters
We have now used diodes to produced a pulsed
dc signal.
 Most equipment requires “regulated” dc

 We must remove the “ripple”
 Ripple is departure of waveform from pure dc (flat,
constant voltage level)
• Frequency – so far we have seen pulsed dc at the same
frequency as the input (½ wave) or twice the line frequency
(full wave rectifier)
• Amplitude – a measure of the effectiveness of the filter
Ripple Factor
Vrms ( ripple voltageout)
r
V(averageout)
Low r indicates better filtering
Alternate Definition

Defined also for current
 Iac = effective value of ac harmonic component
 Idc = average of dc component
I rms  I dc2  I ac2
so,
2
I ac  I rms
 I dc2
I ac
r
I dc
For ½-wave rectifier r = 1.21
For full-wave rectifier r = 0.48
2
I
 1
r   rms

I
dc 

High Pass (RC) Filter
 1 

Z  R 2  X C2  R 2  
 2 f C 
I
Vi

Z
Vi
 1 

R  
 2 f C 
2
2
2
High Pass Filter

I is the same through both R and C
Vo  RI 
RVi
 1 

R  
2

f
C


1
2
Vo

Vi


1

1  
 2 f RC 
2
When f is small,
denominator is large and the
voltage ratio is small.
2
When f is large, denominator
is almost one
Typical Results
R = 0.1 MW C = 0.16 mF
Why is it called a high pass filter?
Half Power Point

Note the point at which
Vo
Vi
1
2
 This will make (Vo/Vi )2 = ½
 Recall that one form of power is V2/R, so the
output power is half the input power at this
frequency.
Low Pass (RC) Filter
 1 
2
2
2
Z  R  XC  R  

 C 
I
Vi
Z
2
I
Vo  IZ C  IX C 
C
Low Pass Filter
I
Vo 

C
Vo 
Vi
 1 
 C R 

 C 
Vi
2
2
R C 2  1
Vo
1

2
Vi
1   RC 
Low frequency means denominator is small and Vo / Vi  1
High frequency means denominator is large and Vo / Vi is small
Low Pass Results
Note the half power frequency again.
Half Wave Capacitive Filter

Improving the ripple factor
 During forward bias half-cycle, capacitor is charging
 During the reverse bias half-cycle, the capacitor
discharges through the output resistor
Full Wave Capacitive Filter
 Even
better ripple factor.
L-Section Filter
Full wave
rectifier,
using ground
loop on each
half cycle
• The series inductor opposes changes in current
• The coil stores energy when the current is above average and
releases energy to the circuit when the current falls below average
 Coil chops off (chokes) the peaks of the ac pulses
 L-section delivers more steady current than the capacitive filter alone
 Used for cases of variable loads where good regulation is required
 Output voltage is less than the capacitive filter
P Filter
Full wave
rectifier,
using ground
loop on each
half cycle
Combines the effects of a capacitive and L-section
filter. Regulation is poor when load varies, however.
Precise Voltage Regulation
 If
the load current is varying, but you
require voltage to be constant, R must
vary (V=IR).
The Avalanche Region
 Increasing
reverse bias on the diode
eventually accelerates electrons in the
depletion zone so that they ionize other
atoms.
 The freed electrons ionize other atoms
 Avalanche occurs
 Zener
Diodes are designed so that the
transition potential is very steep
Zener I-V Curve
For a large change of current, voltage remains at VZ
Zener acts like an automatically varying resistor.
Can be obtained with VZ from 2.4 – 200 V.
Zener Regulation Circuit
Since the load is in parallel with the diode, the voltage drop
across RL is always the same as across VR1 and is VZ =
constant Zener voltage
The input voltage V must be greater than VZ.
Zener MUST be operated under load. If not, the zener is still
delivering power (more than usual) and may melt. Recall that
the zener can draw large currents all at the same voltage.
Voltage Multiplication
R protects D from surges
When D is conducting, C charges to Vpeak
During reverse bias half cycle, C discharges through load
at the peak voltage
Vout = Vpeak = 2 Vrms
Half Wave Voltage Doubler
First Half Cycle – CCW – D1 conducting, C1 charged. C2 remains
uncharged because of the low resistance in the forward biased D1
Second Half Cycle – CW – D2 conducting, C2 charged by (source +
C1) – about twice the source voltage
Next Half Cycle – Repeat of first stage with the addition that C2 can
now discharge through RL. Notice that C2 is charged once/cycle,
so output has the same frequency as input.
Clippers and Clampers
 Diodes
can be used in waveform shaping
 Clippers
 Limit or “clip” portions of the signal
 Used for circuit protection or waveform
shaping
 Clampers
 Shift the dc level of the signal
Clippers
Notice that the dc sources V1 and V2 are reverse biasing the diodes in each
branch.
Clockwise half cycle - VS flows to the output until it becomes equal to V1. At that
point diode 1 can conduct, another path is found, and any voltage above V1 goes
through the first diode branch.
Counter Clockwise half cycle - VS flows to the output until it becomes equal to V2.
Excess voltage flows then through diode 2.
Output is “clipped” at V1 and V2.
Special Clipper
 If
V1 = V2 and VS >> V1, then you can
make a good square wave output.
Other Clipper Uses
 Surge
protection
 Noise reduction
Clampers
Vp
0
During the half cycle in which the diode
conducts, the capacitor charges to Vpeak
 If the RC time constant is long compared to the
input frequency, the capacitor cannot discharge
during the other half cycle.
 Vo = Vi + Vpeak

Clamper Features
 Output
has the negative peaks “clamped”
to zero.
 Could have the positive peaks clamped by
just reversing the diode.
 Clamping
here is always at zero,
independent of the amplitude of the input
 Capacitor always charges to Vpeak
 Time constant is long
Clamping at any voltage
 Insert
a dc source
 Capacitor now charges to Vpeak + V
Vp
V
Solar Cells
(Photovoltaics)
Incoming photons collide with electrons in the valence
band of the p-type material.
These electrons are promoted to the conduction band,
increasing the number of minority carriers.
This increases the reverse bias. Watch the Animation
Solar Cell I-V Curve
When curve crosses
the horizontal axis (I =
0), we note the open
circuit voltage, V0C
When curve crosses the
vertical axis (V = 0),
we note the short
circuit current, ISC
ISC and V0C vs. Light level
V0C is logarithmic
ISC is linear
Spectral Responses
Si is more sensitive in the IR. Se matches the human eye better.
Thermistors
 Semiconductors
generally have a negative
temperature coefficient – as the
temperature increases, the resistance
decreases
 This occurs because more electrons can be
promoted to the conduction band at higher
temperatures. More electron flow means
lower resistance.
 Can develop circuits that respond to
temperature changes.
Typical Temperature Response
LEDs

When forward biased, electrons from
the N-type material may recombine
with holes in the P-type material.
 System energy is decreased
 Excess energy emitted as light
 Indium gallium nitride (InGaN)
semiconductors have been used to
make colored LEDs
• Stop lights
 Progress toward white LEDs is
promising
7-segment displays
Since the diode provides light, no external or
internal source of light is needed to see the
display.
If the diodes are visible when not lit, the result
can be hard to read
Photodiode
With no light we have a small
“dark current” (Idark) due to
thermal electron-hole pairs in
the depletion zone.
With light the reverse bias
current increases (Idet)
Typical:
Dark Vr = 3 V, I = 25 mA
R = Vr/I = 120 kW
For Intensity 25000 lumens/m2
Vr = 3 V, I = 375 mA
R = 8 kW
So we can use it as a variable
resistor controlled by light
Photoresistor
Current vs Intensity
Response is very linear
so photodiodes are used
to measure light intensity
Spectral Response
Typically, more
sensitive to longer
wavelengths.
Laser Diodes
Used in forward bias.
Electrons move into depletion zone
and recombine with holes,
producing light (like an LED).
P material
N material
Highly
Reflective
Mirror
Partially
Reflective
Mirror
More electron-hole recombinations
can be stimulated by this photon,
producing more photons at the
same wavelength.
The mirrors reflect the photons
back and forth through the
depletion zone, stimulating more
photon at each pass.
Eventually, the beam passes out of
the right hand mirror.
Laser Diode Application
 Used
as CD and DVD detector
Laser Diode
Photodiode
Also used as bar code readers, laser pointers, fiber optics, etc.
Optocouplers
 LED-photodiode
pairs
 Used in circuits where surges are a problem
 Isolates one part of the circuit from another
• Only connection is light
LCD
 Liquid
Crystal Display
 State between solid and liquid
 Requires only a little heat to change material
into a liquid
• Used as thermometers or mood rings
 Electric currents are used to orient the
crystals in predictable ways
• The liquid crystals polarize the light (either internal
light source or external) to give light and dark
areas on the display