Question

LASER action is found in____________ semiconductor.
A. Direct band gap semiconductor
B. Indirect band gap semiconductor
C. Germanium
D. Silicon

Answer 1

In order to solve this question we need to understand LASER action. So to understand this, we have to understand electron transitions and emissions. So when electron at a lower energy level strike by a photon, it get excited to higher energy level where it stays for very less time and thus it de excites and fall back to lower state by emission of a photon, in this way there would be no laser action, as lasers are high intensity monochromatic beams. So to avoid this type of emission, electrons at higher state after excitation are again hit by a photon, so when it de-excites, it emits two photons which further strikes another electron at higher or metastable state and this way LASER action is performed.

Complete answer:
As explained above, when an electron is excited from a metastable state to a lower state, it emits two photons which again strikes the electron at another higher state. This type of emission is known as stimulus emission.So a released photon must not hit other than an electron, because if it hits another element, say crystal then it would get absorbed and LASER action could not be performed.

So to avoid the same we use direct band gap semiconductor, where there is no hindrance as both electron and hole have same momentum and energy, so it can easily emit photons and emitted photons would again hit the excited electron in the conduction band.

So the correct option is A.

Note: It should be remembered that, we have not used indirect band gap semiconductor because, in indirect band gap, energy and momentum of both electron and hole is not same, also for electron to transit in higher states, it must pass intermediary state, and in this process it would lose its momentum by the expense of release of photons, so LASER action would be hindered.