Laser and Semiconductors

Population inversion: metastable energy state atoms stay excited longer more excited atoms than those in ground state population inversion achieved

Stimulated emission: when photons of energy equal to E2-E1 passes through medium, either (1): photons absorbed & excited from E1 to E2 or (2): photons de-excited from E2 to E1

Since p.inversion achieved, (2) more likely to occur. Coherent photons are produced; reflected back and forth by mirrors to cause more stimulated emissions and thus laser.

Band theory: At 0K, valence band is completely filled with electrons which are immobile (unable to move in solid)

Higher level is conduction band ; empty, no electrons.

For insulators, thermal excitation insufficient for electron in valence band to acquire 5 eV to jump across band gap,

For semicons, thermal excitation sufficient for electron in valence band to acquire 1 eV to jump across band.

For conductors, bands overlap. Conduction band partially filled with electrons.

Intrinsic Semicon:

Thermal agitation e- in valence band jump band gap equal no. of conduction e- and holes to act as charge carriers form e-hole pairs holes move in dir. of electric field, electrons move opposite net current in same dir. current flows

N-Type Semicon:

4 e- from donor atom covalently bonded to Silicon atom 1 additional e- available to represent a donor level just below conduction band smaller band gap than intrinsic e- are majority carriers, electrical conduction in the presence of electric field

P-Type Semicon:

3 e- from acceptor atom covalently bonded to Silicon atom 1 electron deficiency hole represented by energy level EA just above valence band after thermal agitation, e- in valence band jump band gap to fill EA holes in valence band act as majority charge carriers current flows

P-N junction/depletion region:

The mobile electrons from the n-side and the mobile holes from the p-side flow (diffuse) across the junction and combine e- from n-side fill the holes on p-side leaves the n-side with a positive charge layer (since it has lost electrons), p-side with a negative charge layer (since it has gained electrons) The positive and negative charge layers set up an electric field (or depletion region) in the junction

Electromagnetism

Force acting on charge = mg = BILsinθ = Bqv

Bqv = mv2/r [centripetal force]

Felectricfield = qE

Inserting ferrous core into solenoid

Current pass thru solenoid ext. magnetic field set up magnetic domains of core aligned

in same direction produce a magnetic field that strengthens external magnetic field as

ext. magnetic field is vector sum of magnetic field produced by current in solenoid and by

ferrous core

Velocity selector

FB = FE Bqv = qE V= E/B

Mag.field and e.field arranged perpendicularly to each other. Produce FB and FE respectively in opp. Direction. When FE =FB, FR = 0, electron pass thru undeflected. (only for cases where charged particles have specific speed, v)

Mass spectrometer

Inside dome-like structure:Bqv = mv2/r

In v.selector:v= E/B

Electromagnetic Induction

Flux ∅ = BAcosθ; Flux linkage=NΦ=NBAcosθ

Note: For ‘BAcosθ’, the θ is between B and the normal to the coil or surface. If you use Basinθ(some textbooks use this), then the θ is between B and plane of the area of the surface.

Uniform e field downwards, uniform magnetic field into paper

v.selectorr

Faraday’s Law:

ε=dN ∅ /dt

Lenz’ Law:

Definition: direction of induced emf that drives induced current to flow in a direction to produce an induced magnetic field that opposes the change in magnetic flux linkage

Note: This opposing magnetic effect can be used in damping (refer to Oscillations)

From lenz law eqn,

E = | -d∅ /dt |

ε=¿−Blv∨¿

Magnitude of emf induced across moving conductor -> E = Blv