Transcript Technical Aspects-5 - Semiconductors

Chelmsford Amateur Radio Society
Advanced Course
Technical Aspects
Part-5 - Semiconductors
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 5: v1.2, 26-Oct-2006
(3) Technical Aspects - Semiconductors
1
Solid State Devices
• Semiconductors form the basis of all modern solid state
devices - diodes, transistors, analogue and digital
integrated circuits etc
• Common Semiconductors are Silicon and Germanium
• Advance Course requires a knowledge of
– Semiconductor theory
– Diodes, including Zeners and Varicaps
– Bipolar & FET Transistors
– Amplifiers - Circuits, Classes, Efficiency
Note: RF Amps inc. Valves covered in Transmitter Course
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 5: v1.2, 26-Oct-2006
(3) Technical Aspects - Semiconductors
2
Origins of Semiconductors
• Silicon and
Germanium are in
Group-IV where each
atom has 4 electrons
in its outer shell
Group IV
Si
Silicon
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Slide Set 5: v1.2, 26-Oct-2006
(3) Technical Aspects - Semiconductors
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Semiconductors
• In pure (intrinsic) Silicon and Germanium all four outer electrons
pair with neighbours in the crystal lattice leaving none free - making
them insulators
• By doping these materials with very small amounts of impurities,
Electron-rich (N-type) or Electron-poor versions (P-type) can be
created
• N-type dopants have one extra electron and come from Group-V
– Phosphorous, Arsenic, Antimony
• P-Type dopants have one less electron and come from Group-III
– Boron, Aluminium, Indium
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(3) Technical Aspects - Semiconductors
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N-type and P-type
• Example shown is
Silicon with n-type
doping by Arsenic
• Arsenic has an extra
electron not used for
pairing up in covalent
bonds, and is free to
move under bias
• In p-type - a positive
‘hole’ is left due to a
shortage which behaves
similar to a real electron
‘Spare’
Electron
As
Si
n-type doping
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Murray Niman G6JYB
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(3) Technical Aspects - Semiconductors
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P-N Junction - The Diode
• A junction between P-type an N-type material will have a charge
across it - a potential barrier
• Near the junction some electrons fill the holes nearby, making the
region devoid of charge, the depletion layer, which is an insulator
• If bias is applied, the depletion layer narrows until electrons will flow
easily from N to P
• If reverse bias is applies the depletion region will widen and stop flow
Junction
N
- -
+
+
P
+
+
Depletion Region
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Advanced Licence Course
Murray Niman G6JYB
Slide Set 5: v1.2, 26-Oct-2006
(3) Technical Aspects - Semiconductors
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Diodes
• Standard Diodes act as rectifiers
• Forward threshold Vf is: Silicon ~0.6v, Germanium ~0.4v
• Reverse Breakdown, Vr usually many volts
+I
Ge
Si
Vr
Vf
-I
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Murray Niman G6JYB
Slide Set 5: v1.2, 26-Oct-2006
(3) Technical Aspects - Semiconductors
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Varicap Diodes
• When Diodes are reverse biased the depletion layer acts as the
insulating layer of a capacitor
• Varicap Diodes also known as Varactors exploit this
• Higher reverse voltages widen the depletion layer, driving the
capacitive plates apart, lowering the capacitor value
• Typical values are of the order of pF
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 5: v1.2, 26-Oct-2006
(3) Technical Aspects - Semiconductors
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Zener Diodes
• In normal diodes, little current flows when reverse biased up to the
point of catastrophic breakdown
• Zener Diodes have a well defined reverse breakdown voltage which
can act as a voltage reference for PSU regulators
• Current in the diode must be limited to avoid excess heat dissipation
+VE
+5.1V
0V
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 5: v1.2, 26-Oct-2006
(3) Technical Aspects - Semiconductors
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Bipolar Transistors
• Ordinary transistors are known as bipolars - two P-N junctions
• Apply ‘bias’ current in the base to control Collector-Emitter current
• Small Signal Gain or ‘Beta’ is the ratio of IC/IB
IC = ß x I B
Collector
Collector
IC
- N-
+
P
+
- - N-
Emitter
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NPN
IE
Base
IB
Emitter
Base
NB: IE = IC + IB
Murray Niman G6JYB
Slide Set 5: v1.2, 26-Oct-2006
(3) Technical Aspects - Semiconductors
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Bipolar Transistors
• Two types - NPN and PNP
Collector
• Base-Emitter similar to Diode characteristic
Base
• ‘Bias’ current in the base controls
Collector-Emitter current
• PNP has negative current in the Base for bias
40µA
1 mA
mA
Emitter
IB = 60µA
IC
IC
NPN
20µA
E
B
C
PNP
0.5 V
VBE
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Slide Set 5: v1.2, 26-Oct-2006
(3) Technical Aspects - Semiconductors
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Bias Issues
• Bias determines the operating point of a transistor
• Transistors are temperature dependent and have variable gain
• Circuits need to be designed to be relatively independent of this
and give stable operation
• This issue is known as bias stability
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(3) Technical Aspects - Semiconductors
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FETs
d
p-type
• FETs are a semiconductor device similar to a Valve
Depletion
Layer
g
• Operates by a field effect due to Voltage
(as opposed to Current in a Bipolar)
• GDS Terminology refers to Electron flow
s
• n-Channel and p-Channel variants exist
n-Channel
• Insulated Gate FETS give NMOS, PMOS,
and CMOS - which are all static sensitive
Drain
G
Gate
D
G2
S
G1
d
g
Depletion
Layer
about to
pinch off
channel
Source
n-Channel
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p-Channel
Dual Gate
Insulated FET
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s
Slide Set 5: v1.2, 26-Oct-2006
(3) Technical Aspects - Semiconductors
13
Common Emitter
• Three circuit configurations are
possible:-
+V
Common Emitter,
Common Collector and
Common Base
Output
• Common Emitter refers to Emitter
being Common to Input & Output
• A rise in the input voltage turns on
the device harder lowering the
voltage on the collector and output
Input
Low Zin
~1K
Medium Zout
~ 5K
• Thus the circuit inverts, or is said
to give 180° phase change
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Murray Niman G6JYB
Slide Set 5: v1.2, 26-Oct-2006
(3) Technical Aspects - Semiconductors
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Common Collector
• Common Collector is
also popularly called
Emitter Follower
+V
• Output Voltage is similar
to input but can supply
much more current
Input
• So no Voltage gain, but
used for current buffering
• Collector is Common,
as at AC the PSU rails
have zero potential
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High Zin
50k-2M
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Output
Low Zout
10-500 Ohms
Slide Set 5: v1.2, 26-Oct-2006
(3) Technical Aspects - Semiconductors
15
Common Base
• Base is common to
input and output
- thus Common Base
• A positive input voltage will
decrease VBE and reduce IC;
causing the Collector voltage
to rise, so output is in phase
with input
• Mainly used for RF
frequencies
- eg in IF Amplifier chains
+V
High Zout
50k
Input
Low Zin
50 Ohms
Output
Capacitor
ensures no
signal on
the base
• Common base amps amplify
voltage - not current
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 5: v1.2, 26-Oct-2006
(3) Technical Aspects - Semiconductors
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Amplifier Class & Bias
• Class-A, B, AB and C are defined by the
bias and operating region of the
transistor
• Higher Classes aim to reduce wasted
Output Current and improve efficiency
IC
IC
Distorted
Output
Output
VBE
Input signal
normal bias voltage
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VBE
Input signal
low bias voltage
Murray Niman G6JYB
Slide Set 5: v1.2, 26-Oct-2006
(3) Technical Aspects - Semiconductors
17
Amplifier Classes
• Class-A
Biased well on for high fidelity but also results in low efficiency and
high heat dissipation in poweramps
• Class-B
Gives only only half the waveform, so usually used in Push-Pull
configurations. Fairly efficient, but can give crossover distortion
• Class-AB
A variation of above with transistor biased to conduct for more than
half a cycle for better fidelity, but modest dissipation
• Class-C
Nonlinear but efficient - high distortion needs filtering
- Useful for constant amplitudes such as FM and GSM mobile phones
• Other Classes exist but are out of scope: D, E, F, G, H, S etc
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 5: v1.2, 26-Oct-2006
(3) Technical Aspects - Semiconductors
18
Class-B Push-Pull
+15 V
• Class-B only gives half a
sine wave
TR1
• In Push-Pull:TR1 gives positive half,
TR2 give negative half
0.5 Ohms
• Need to keep Bases at
0.7V else crossover
distortion occurs.
Output
bias adjust
0.5 Ohms
• Using diodes to do this
gives a degree of tracking
vs temperature
TR2
Input
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-15 V
Slide Set 5: v1.2, 26-Oct-2006
(3) Technical Aspects - Semiconductors
19
Amplifier Efficiency, 
• Principal efficiency definition, usually expressed as a
percentage.
Collector/Drain/Anode Efficiency:
 = PRFout / PDCin
• For info, other definitions are:
• Power Added Efficiency:
Pae = (PRFout - PRFin) / PDCin
• Overall Efficiency:
Overall = PRFout / (PDCin+PRFin)
– a good criterion (esp for a multistage amp) but not often quoted
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 5: v1.2, 26-Oct-2006
(3) Technical Aspects - Semiconductors
20