2. Mobility was originally defined in Lec. 6 to be the scaling constant between

Question

2. Mobility was originally defined in Lec. 6 to be the scaling constant between drift velocity (Udrift) and electric field (8). Earlier in the same lecture, mobility was defined by the relationship given by Eqs. 2.la and 2.1b. (2.1a) (2.1b) (b) Identify the two key mechanisms that fundmentally limit mobility in bulk semicon- (a) Explain what is meant by the Scattering Time, Tmn, Tmp ductor. (c) Suppose that a semiconductor is submerged in liquid N2 in laboratory experiment cooled to 77 K What is likely to happen to the mobility, and which scattering mechanism defined emulating deep space conditions. In this case, the semiconductor is in Prob. 2b is most impacted? Explain why this is the case. (d) Now suppose this semiconductor is used in a military application with operational temperatures expected to be >600 K. What is likely to happen to mobility? Answer in terms of the scattering mechanisms. e) Suppose that the perfectly period structure of the semiconductor is modified and "defect". some of the semiconductor's atoms are missing: something referred to as a How do you suppose that the mobility would change and why?

Explanation / Answer

A. In semiconductor due to thermal energy the electron are always in random motion, during this there can be collision of electron with electron or with ion or with lattice atoms. Scattering time is defined as the time duration between two collision.

B. Lattice scattering and impurity scattering

impurity scattering is due to the presence of dopants in semiconductor, when the doping is high the number of carriers will also be high. At lower temperatures, the temperature increases the dopants ionisation increases. This tends to create more carriers. This leads to increase in mobility. However at higher temperatures as the carrier number increase they collide with each other more frequent and scattering time decreases which reduces mobility

Lattice scattering

At temperatures greater than 0k vibrating atoms create pressure waves called phonon. As temperature increases phonon increases and electron scattering increasing and mobility decreases.

At low temp impurity scattering is dominant

At high temp lattice scattering is dominant

C) As said in previous question impurity scattering is dominant in low temp at 77k. This reduces the mobility will decrease as at 77k not many donor atoms are ionised. So the carrier concentration is less. Therefore less mobility than 300k temperature.

D. Lattice scattering will be dominant at high temperatures 600k. At this temp the lattice vibration will be really high causing the electrons to collide more frequently. This reduces the scattering time and reduces the mobility compared to 300k temperature

Related Questions

Navigate

• Browse (All)
• Browse 2
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.