Question

Q.4. (a) What is stimulated emission? Discuss the operation of a LASER diode including a band diagram. (10 pts) (b) Explain quantum mechanical tunneling with an energy band digram. (5) (b) A student was asked to fabricate an ohmic contact. After fabrication, the student found that it was not a contact, but a Schottky diode that she or he fabricated. What mistake could the student have made? Use Figures 3.a and 3,b to explain the mistake. (5 pts) Silicide N Si Ec, Et E, Figure 2.a. One type of metal-semiconductor junction. Figure 2.b. Another type of metal-semiconductor junction. (c) What are the applications of Schottky diode and ohmic contact? (5 pts)

Answer 1

(a)A third type of photon-related electron transitions in an atom is stimulated emission. Suppose an electron is in a higher energy level and a photon comes along with an energy equal to the difference between the electron's energy and a lower energy. What will happen is that the photon will stimulate the electron to fall into the lower energy state, thereby emitting a photon. The emitted photon will have the same energy as the original photon, and viewed as waves we will then have two waves emerging from the atom in phase with the same frequency. Such waves will constructively interfere, leading to a more intense wave.

(b) Quantum mechanical tunneling describes the transition of carriers through a classically forbidden energy state. This can be an electron tunneling from the semiconductor through a dielectric, which represents an energy barrier, to the gate contact of an MOS structure. Even if the energy barrier is higher than the electron energy, there is quantum mechanically a finite probability of this transition. The reason lies in the wavelike behavior of particles on the quantum scale where the wave function describes the probability of finding an electron at a certain position in space. As the wave function penetrates the barrier and can even extend to the other side, quantum mechanics predict a non-zero probability for an electron to be on the other side.

(d) schottky diode are used in

RF mixer and detector diode

Power Rectifier

Solar cell applications

Clamp diode

Use of ohmic contact:

The RC time constant associated with contact resistance can limit the frequency response of devices. The charging and discharging of the leads resistance is a major cause of power dissipation in high clock rate digital electronics. Contact resistance causes power dissipation via Joule heating in low frequency and analog circuits (for example, solar cells) made from less common semiconductors