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06_semiconductor_physics.md

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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

Graphs

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