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Section 4.3: Semiconductors/Transistors
8. Solid State and Thermionic Devices
Thermionic Devices
  • Thermionic devices (valve devices) consist of vacuum tubes with a filament which acts as a cathode and at least one other electrode. Diodes have 2 electrodes, triodes have 3 electrodes, and pentodes have 5 electrodes.
  • Thermionic devices utilise thermionic emission which is the spontaneous emission of electrons from solids when heated to high temperatures. The filament in the vacuum is heated by an electric current which causes it to release electrons. These electrons are accelerated towards an anode because of the high potential energy.
  • Diodes are suitable for rectification (the conversion of AC current into DC current), because electrons will only flow if the negative terminal is attached to the cathode. The difference between diodes and triodes is that a triode has a third electrode (grid) which can control the amount of current. Triodes act as amplifiers of current.
  • Thermionic devices require a separate heating circuit to heat the cathode to a sufficient temperature to allow for thermionic emission. These devices also require a near vacuum to allow the electrons to flow between the electrodes.

Solid State Devices
  • Modern appliances use solid state devices, such as transistors and integrated circuits.
  • Solid state devices are made from semiconductors, in particular using both p-type and n-type semiconductors.
  • The interface between a p-type semiconductor and an n-type semiconductor acts a diode. Electrons move across the junction from the n-type semiconductor to the p-type semiconductor, which results in the neutralising of available holes in the p-type semiconductor material.
  • The zone adjacent to the interface called the depletion zone exerts a force to prevent any more electrons moving into that region. This means that the electron flow is confined to one direction only which is from the negative terminal to the n-type semiconductor then into the p-type semiconductor to the positive terminal (conventional current flows in the reverse direction).
  • If the device is connected in the opposite way, then it acts as a large resistor preventing most of the current from flowing through the device.

Comparison between Thermionic Devices and Solid State Devices
  • Solid state devices have the following advantages over thermionic devices:
  • Solid state devices are smaller than thermionic devices, which has allowed for the smaller more complex devices. They are also less fragile then thermionic devices, which have relatively short lifetimes.
  • Solid state devices consume less electrical energy than thermionic devices and produce less heat. This is because solid state devices can operate at room temperature and at normal air pressure.
  • Solid state devices can operate much faster than thermionic devices and do not require a ‘start up’ time to become operational.

Comparison between Triodes and Transistors
  • Triodes required high voltages, while its solid-state equivalent, the transistor can operate off about 0.6 Volts.
  • Both use a small external circuit in order to modulate the current in the main circuit.
  • The triode valve involves electrons travelling through a vacuum are modulated using the grid, while in transistors, the electrons travel in semiconductors, with the flow of electrons between the two components affecting the flow of electrons between the other two components, by altering the conductivity of the middle semiconductor.

Practicals and Research
2. The Invention of the Transistor
  • In the same way that solid-state diodes replaced thermionic diodes, the invention of the transistor replaced the thermionic devices used for amplification such as the thermionic triode.
  • A transistor is a semiconductor device that changes the size or direction of current because of small changes in the potential difference across the device.

There are two types of transistors, both of which achieve the same purpose:

  • An npn-transistor, which consists of a thin p-type semiconductor called the base between two n-type semiconductors, called the emitter and collector. In this type of transistor, current flows from the collector to the emitter. (The electron flow is the opposite direction). The npn-transistor is powered on when sufficient current is supplied to the base.
  • A pnp-transistor, which consists of a thin p-type semiconductor called the base between two n-type semiconductors, called the emitter and collector. In this type of transistor, current flows from the emitter to the collector. (The electron flow is the opposite direction). The pnp-transistor is powered on when there is no current supplied to the base.
3. Transistors Impact on Society
Integrated Circuits
  • The development of transistors enabled the eventually development of integrated circuits. These silicon chips contained many small transistors and resistors, and replace the need for multiple transistors.
  • These enable the miniaturisation of electronic devices, and hence the development of electronic devices.
Microprocessors
  • By 1971, microprocessors had been developed. Microprocessors contain a control circuit, arithmetic circuit, a logic circuit and a CPU all on a single chip.
  • Many devices use microprocessors, such as cars, toys, game consoles, digital watches, personal computers, cameras, DVD players and microwaves. Most of these are hidden from view and contain programs etched in the silicon that cannot be altered. Microprocessors are designed for a particular purpose.
 
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