Electronics

The study of electron emission and control by circuit devices.

Vacuum Tubes

• Heated filament called cathode emits electrons in process called thermionic emission
• Other electrodes can control electron emission
• First noticed by Edison, called the Edison effect, 1883
• Led to discovery of electron by Thomson, 1898

• If second electrode (the plate) is made positive, electrons flow and tube conducts
• If plate is negative, electrons are repelled and tube does not conduct
• Tube controls direction of current, called diode
• Triode tube has third electrode called grid
• Acts like adjustable valve
• Used as amplifier, oscillator or switch

Cathode ray tube

• Electrons emitted from cathode are accelerated by positive plate
• Aimed and focused on screen with electric or magnetic fields
• Screen is coated with phosphors that glow when struck by electrons
• TV screens, computer monitors, oscilloscopes

Semiconductors

• Materials with resistivity between good conductors and good insulators
• Silicon, germanium, boron, selenium, some metal compounds
• Can be made into good conductors by adding impurities in process called “doping”
• Adding impurity with an extra electron (P, As, Sb, Bi) to silicon or germanium produces N-type semiconductor with available conduction electrons
• Adding impurity with one less electron (B, Al, Ga) produces P-type semiconductor with electron “holes”, which act like mobile positive charges

Semiconductor Diode

• P-type and N-type material placed together creates P-N junction
• Conduction across junction controlled by connection to external circuit
• If P-type connected to positive potential and N-type to negative, diode conducts
• If connected opposite, diode blocks current: acts like one-way valve

Rectification

• Conversion of AC to DC
• Diode blocks ½ of AC signal creating pulsed DC
• Second diode can invert negative AC pulse helping smooth out pulses
• Capacitors store part of pulse, releasing energy between pulses smoothing out DC

Transistors

• N-P-N or P-N-P sandwich with each layer having connection to circuit
• Center layer can control conduction through outer layers
• Can act as amplifier (boosting the signal), switch or oscillator (producing AC from DC)

Advantages of Transistors

• Much smaller than vacuum tubes
• Run on much lower voltage and current
• Much less heat
• More durable
• Cheaper

Integrated Circuits

• Entire circuit on one chip with many P-N junctions
• Can contain millions of transistors, diodes, capacitors, resistors and connecting wires
• Simplifies circuit construction with only a few external parts