Semiconductor Electronics: Materials, Devices and Simple Circuits Flashcards
What were the basic building blocks of electronic circuits before the discovery of transistors in 1948?
Vacuum tubes, also known as valves
Describe the structure of a vacuum diode.
A vacuum diode consists of two electrodes: an anode (plate) and a cathode.
How many electrodes does a triode have, and what are they?
A triode has three electrodes: cathode, plate, and grid.
Why is vacuum necessary in the inter-electrode space of a vacuum tube?
Vacuum is required to prevent electrons from losing their energy on collision with air molecules in their path.
What is the direction of electron flow in a vacuum tube, and why are these devices referred to as valves?
Electrons flow only from the cathode to the anode in a vacuum tube, hence they are referred to as valves.
What are some drawbacks of vacuum tube devices compared to semiconductor devices?
Vacuum tube devices are bulky, consume high power, operate at high voltages, and have limited life and low reliability.
What was realized in the 1930s regarding semiconductor materials and their junctions?
It was realized that some solid-state semiconductors and their junctions offer the possibility of controlling the number and direction of flow of charge carriers through them.
How do semiconductor devices differ from vacuum tube devices in terms of the flow of charge carriers?
In semiconductor devices, the supply and flow of charge carriers are within the solid itself, whereas in vacuum tubes, mobile electrons are obtained from a heated cathode and made to flow in an evacuated space.
What are some advantages of semiconductor devices over vacuum tube devices?
Semiconductor devices are small in size, consume low power, operate at low voltages, and have long life and high reliability.
What example is given of a device that works on the principle of vacuum tubes but is being replaced by solid-state electronics?
Cathode Ray Tubes (CRT), used in television and computer monitors, are being replaced by Liquid Crystal Display (LCD) monitors with supporting solid-state electronics.
How are solids classified based on conductivity?
Solids are classified into metals, semiconductors, and insulators based on their relative values of electrical conductivity (s) or resistivity (r).
What are the characteristics of metals in terms of resistivity and conductivity?
Metals possess very low resistivity (or high conductivity), with resistivity values typically ranging from 10^(-2) to 10^(-8) W m and conductivity values from 10^2 to 10^8 S m^(-1).
Describe semiconductors in terms of resistivity and conductivity.
Semiconductors have resistivity or conductivity intermediate to metals and insulators, with resistivity values typically ranging from 10^(-5) to 10^6 W m and conductivity values from 10^5 to 10^(-6) S m^(-1).
How are insulators characterized in terms of resistivity and conductivity?
Insulators have high resistivity (or low conductivity), with resistivity values typically ranging from 10^(11) to 10^(19) W m and conductivity values from 10^(-11) to 10^(-19) S m^(-1).
What types of semiconductors are discussed in the chapter?
The chapter discusses elemental semiconductors (such as Si and Ge) and compound semiconductors, including inorganic (e.g., CdS, GaAs) and organic examples (e.g., anthracene, doped pthalocyanines).
What are the main types of semiconductor devices discussed in the chapter?
Most semiconductor devices discussed are based on elemental semiconductors like Si or Ge and compound inorganic semiconductors. However, some devices using organic semiconductors and semiconducting polymers have been developed since the 1990s.
What are energy bands in solids?
Energy bands in solids are continuous ranges of energy levels formed by the collective behavior of electrons in a crystal lattice, including the valence band (containing valence electrons) and the conduction band (above the valence band).
Describe the concept of the energy band gap.
The energy band gap is the energy difference between the top of the valence band and the bottom of the conduction band. It determines the electrical properties of a material, with larger gaps leading to insulating behavior and smaller gaps allowing for semiconduction or conduction.
How do metals, insulators, and semiconductors differ in terms of their energy band structures?
Metals have overlapping valence and conduction bands or partially filled conduction bands, allowing for easy electron movement and high conductivity. Insulators have a large energy band gap, preventing electron movement and leading to high resistivity. Semiconductors have a small band gap, allowing for some electron movement at room temperature.
What is the lattice structure of Ge and Si?
The lattice structure of Ge and Si is diamond-like, where each atom is surrounded by four nearest neighbors.
How many valence electrons do Si and Ge have?
Si and Ge have four valence electrons each.
What happens to each valence electron of a Si or Ge atom in its crystalline structure?
Each valence electron of a Si or Ge atom tends to share one electron with each of its four nearest neighbor atoms and take a share of one electron from each such neighbor, forming covalent bonds or valence bonds.
What are covalent bonds in the context of Si and Ge atoms?
Covalent bonds are formed when each Si or Ge atom shares one of its four valence electrons with each of its four nearest neighbor atoms and takes a share of one electron from each such neighbor.
What is the significance of the shared electron pairs in covalent bonds?
The shared electron pairs in covalent bonds hold the atoms together strongly, forming a stable structure.
Describe the 2-dimensional representation of the Si or Ge structure and its significance.
The 2-dimensional representation overemphasizes the covalent bond and shows an idealized picture where no bonds are broken. This representation is significant at low temperatures.
How does an increase in temperature affect the covalent bonds in a crystalline lattice?
As the temperature increases, more thermal energy becomes available to the electrons, causing some of them to break away and contribute to conduction.