1.Why can thermal generation in semiconductors cannot be used for meaningful cur
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Question
1.Why can thermal generation in semiconductors cannot be used for meaningful current conduction?
2.What is basic purpose of doping? Group III elements are acceptor or donor dopants?
3.What is difference between acceptor and donor dopants? Which of following are acceptor dopants; Boron (B) and Phosphorous (P)?
4.What is difference between drift current and diffusion current?
5.Mobility of electrons (µn=1350 cm2/V.s) is 2.5 times higher than the mobility of holes (µp=480 cm2/V.s) in Si? What is intuitive reason?
6.Mobility of carriers (electrons and holes) is higher (µn=1350 cm2/V.s & µp=480 cm2/V.s) in intrinsic silicon compared to the doped silicon (µn=1110 cm2/V.s & µp= 400 cm2/V.s)? What is intuitive reason?
7.What is the relationship between diffusion constant (D) and mobility (µ) and what are its units? {Note: this is the most fundamental relation called Einstein relation and resulting value is called thermal voltage VT = kT/q = 25.9 mV}
8.What is difference between bound charges and free charges? Draw the diagram showing the state of pn junction with open-circuit terminals before formation of depletion layer? Show the free and bound charges!
9.Draw the diagram showing the state of pn junction with open-circuit terminals after formation of depletion layer? Show the free and bound charges! Also show the potential distribution.
11. In actual PN junction, why the width of depletion layer is not same on N and P side?
10. When the width of depletion layer is different in P and N regions, why the charge stored on each side is same?
12.Why diffusion current or saturation current (Is) depends upon the temperature and not on the junction voltage (Vo)?
13.The the junction voltage (Vo) is about 0.7V in silicon diodes? Can we measure this voltage by voltmeter at the terminal of the diodes? If not why?
14. In forward bias diode the major component of current is diffusion current. This diffusion current consists of two parts. What are those two parts and are they majority carrier or minority carrier?
Explanation / Answer
1.Why can thermal generation in semiconductors cannot be used for meaningful current conduction?
Because at room temprature conductivity offered through thermal generation in intrinsic semiconductors is very small to be considered for any meaningfull applications.
2.What is basic purpose of doping? Group III elements are acceptor or donor dopants?
Doping is the intentional introduction of impurities into an intrinsic semiconductor for the purpose of modulating its electrical properties. In general, doping leads to increased conductivity due to the higher concentration of carriers in the semiconductor. Group II elemnts are acceptor dopants. Doping with Group III elements, which are missing the fourth valence electron, creates "broken bonds" (holes) in the silicon lattice that are free to move. The result is an electrically conductive p-type semiconductor.
3.What is difference between acceptor and donor dopants? Which of following are acceptor dopants; Boron (B) and Phosphorous (P)?
Acceptor atoms have "broken bonds" (holes) in silicon lattice, which results in free movement of holes due to acceptance of valance band electrons from the silicon. Donor atoms have unbounded electrons in silicon lattice, these donated electrons constitute for increased conductivity of the semiconductor. Boron:- Acceptor. Phosphorus:- Donor.
4.What is difference between drift current and diffusion current?
Diffusion is the process of particles distributing themselves from regions of high concentration to regions of low concentration. Drifting is the process of movement of charge carriers under the presence of an electric field. The difference between drift current and diffusion current is that drift current depends on the electric field applied: if there's no electric field, there's no drift current whereas diffusion does not need external forces to act upon a group of particles.
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