Disadvantages of vacuum tube devices
- bulky
- consume high power
- operate at high voltage
- limited life
- low reliability
Differences between semiconductors and vacuum tubes
- flow of charge carriers are within the solid itself
- no external heating or large evacuated space is required
- small size, consume low power, operate at low voltage, long life, high reliability
Applications of semiconductors
- Liquid Crystal Display (LCD) monitors instead of Cathode Ray Tubes (CRT) in television and computers
- naturally occurring crystal of galena (lead sulphide) was used as detector of radio waves
- pn junction diode in rectifiers, transistors, etc.
Order of conductivity
Metals : 10² - 10⁸
Semiconductors : 10⁵ - 10-6
Insulators : 10-11 - 10-19
Order of conductivity
Metals : 10² - 10⁸
Semiconductors : 10⁵ - 10-6
Insulators : 10-11 - 10-19
Examples of semiconductors
Elemental : Si, Ge
Inorganic : CdS, GaAs
Organic : anthracene
Organic polymers : polyaniline
Define energy band
Energy bands are large numbers of closely spaced energy levels. Different energy levels with continous energy variation form energy bands.
Define valence band and conduction band.
The energy band which includes the energy levels of the valence electrons is called the valence band.
The energy band above the valence band is called conduction band.
Define holes.
Holes are vacancies created by electrons. A hole behaves as an apparent free particle with effective positive charge.
What is doping?
The deliberate addition of a desirable impurity to a pure semiconductor is called doping and the impurity atoms are called dopants.
Name the necessary condition for doping or the characteristic of a suitable dopant.
The sizes of the dopant and the semiconductor atoms should be nearly the same.
Current in a pn junction at equilibrium is
Zero since diffusion current and drift current cancel out each other.
Why does more current flow in forward bias?
In forward bias there is diffusion current while in reverse bias there is drift current only.
Define depletion region.
The space-charge region on either side of a pn junction together is known as depletion region as the electrons and holes taking part in the initial movement across the junction depleted the region if its free charges.
Forward bias involves
- majority carrier diffusion
- minority carrier injection
- reduction in barrier potential
Reverse bias involves
- no diffusion
- minority carrier drift
- increase in barrier potential
Barrier potential or barrier height
It is the potential difference developed across the depletion region of a pn junction due to diffusion of majority charge carriers.
