6. Semiconductors

Definitions

• Transistors are low impedance, current operated devices
• Thermal Runaway is self destruction of the transistor when the the current passing through a transistor increases, heat is generated
• Depletion layer is an area depleted of mobile electrons (or holes) for carrying current
• Ohmic region is the steep part of the curve and it is similar to resistance

Diagrams

• Forward bias and reverse bias

  

  Image credit: Academics easy

• Field Effect Transistors (source +, drain -)

  

  Image credit: Wikipedia

Circuit Symbols

• Diode

  

• Zener Diode

  

• Variable capacitor diode

  

• Transistors with emitter, base and collector

  

• Field Effect Transistors

  

Circuits

• FET Amplifier

  

  Image credit: Electronics Tutorials

• Common Emitter, Common Base, Common Collector

  

  Image credit: Vintage radio

Graphs

• Diode graph

  

  Image credit: Electronics Tutorials

• FET Graphs

  

  Image credit: Electronics stackexchange

Notes

P-type and N-type

• Germanium has four electrons in its outer orbit
• 4 valence electrons are used in current flow
• Pure Germanium does not conduct electricity
• if a minute quantity of a certain impurity is added, its resistance is greatly reduced

	N-type	P-type
Impurity	Arsenic	Indium
Valency	5	3
Name	N-type: mobility of electrons	P-type: mobility of a missing electron (hole)
Condition	an excess of electrons	an excess of holes

Forward and reverse biased

Forward bias	Reverse bias
(+) terminal of battery is connected to p-type (-) terminal of battery is connected to n-type	(+) terminal of battery is connected to n-type (-) terminal of battery is connected to p-type
depletion layer is very thin	depletion layer is very thick
p-n junction offers low resistance	p-n junction offers high resistance
ideal diode have zero resistance	ideal diode has infinite resistance
current will flow	no current will flow
low impedance	high impedance

Diode

• Current flows when the voltage is positive
• Virtually no current flows when the voltage is negative
• Forward part of the characteristic is not linear
• Small positive voltage is needed for current flow
  • 0.3V for Germanium
  • 0.7V for Silicon (more pd. is needed as there are lesser atom shells to remove the valence electron for current flow)
• Reverse (-V) direction just a few µA flows unless the breakdown voltage is exceeded

Zener diode

• Reverse characteristic can be designed to pass a considerable current
• Pre-designed critical voltage
• Application: voltage stabilizer circuits

Variable capacitance diode

• self capacitance of a diode varies as the voltage is changed
• reverse characteristic can be designed so that this change in capacitance can be used
• Application: electronic tuning circuits

Bipolar Junction Transistor

• PNP (usually Germanium) and NPN (usually Silicon)
• 3 currents at Emitter, Base and Collector are interdependent
• Input and output current are almost the same
• Large difference between the input and output impedances that is the key to transistor amplification because P = I²R
• There is no direct connection between Emitter and Collector

Transistor operation

NPN / PNP	Common base	Common emitter	Common collector*
Current gain	< 1	49	50
Power gain	1000	1000	< 1
Input impedance	Low	Med	Hi
Output impedance	Hi	Med	Lo

* Common collector == emitter follower

For practical transistor biasing, the base should be

• 0.3v more negative than the emitter, for a PNP Germanium transistor
• 0.3v more positive than the emitter, for NPN Germanium transistor
• 0.7v more positive than the emitter, for NPN Silicon transistor
• 0.7v more negative than the emitter, for PNP Silicon transistor

Thermal runway

• Precaution:
  • a resistor in the emitter lead
    • will act in opposition to base bias and reduce current
  • by-pass (decoupling) capacitor is usually connected across the emitter resistor
    • to avoid a reduction of the A by-wanted AC signal

Field Effect Transistor

• Connections are made to each end of the N type channel material
• Small negative Gate voltage:
  • creates a small depletion area
  • current carrying channel is slightly restricted
• medium negative Gate voltage:
  • creates a medium depletion area
  • current carrying channel is further reduced
• higher negative Gate voltage
  • makes the depletion areas meet in the middle
  • completely blocks the channel, cutting off the current flowing from Drain to Source

FET Amplifier

• Source resistor is by-passed with a large capacitor to prevent a reduction in the wanted AC Signal
• The input impedance will be very high; the same value of the gate resistor
• The output impedance will equal the Drain load resistor
• !! Take care to not build up static electricity: don’t wear nylon t-shirt while soldering
• Precaution: It has high impedance

Integrated circuits

• E.g. Transistors, amplifiers, mixers, radio receivers
• Made of Silicon and are connected to the PCB via legs / pins
• Not applicable: Speaker, variable capacitor, coils