assp e2

What is the total angular momentum J of an Fe^3+ ion (in units of ℏ)? See the figure.  (Note: both 4s electrons are removed, and one 3d.)

• 6
• 2
• 3/2
• 5/2

In an extrinsic semiconductor where the binding energy of electrons on donor levels is 10 meV, roughly at what temperature range are extrinsic electrons excited to the conduction band? (k_B = 1.38e-23 J/K , e=1.6e-19 C)

• 1 K
• 10 K
• 100 K
• 1000 K

The charge accumulation in a planar p-n junction corresponds to a local electric field E and electrostatic potential Φ. The Poisson equation is d^2 Φ / dz^2 = - ρ/ε, where ρ is the signed charge density and ε is the dielectric constant. Assuming box-like ρ(z) profiles near the interface (and zero elsewhere), what is the shape of –e*Φ? 

• constant
• constant outside the centre (ρ≠0) region, and smoothly increases inside by ~E_g
• constant outside the centre (ρ≠0) region, and smoothly decreases inside by ~E_g

What is χ = dM/dH (H-->0)  of a ferromagnet like for T=0, if the magnetic field ( B = µ_0 * H) is aligned parallel to the nonzero magnetization M?

• positive
• negative
• zero
• infinite, in other words the question does not make sense

What condition should be satisfied if we wish to perform ballistic transport experiments in a nanocircuit? (E.g. electron focusing).  Please select the best suited condition. (Note: these are idealized conditions, not perfectly achievable.)

• There should be no inelastic scattering in the sample along the electron trajectory.
• There should be no elastic scattering in the sample along the electron trajectory.
• There should be absolutely no scattering in the sample along the electron trajectory.

What should be the minimum photon energy hf to increase conductivity when we illuminate an intrinsic germanium crystal with a direct band gap E_g=0.8 eV? The conduction and valence band edges are denoted as E_c, E_v, respectively, and the chemical potential is µ.

• µ
• E_g
• E_c-µ
• k_B T

Consider a quantum wire with an elastic scattering center in the middle. When an electron is reflected, what can we say about the occupation number N here (x = L/2) at the initial and final k state?

• ∂N(kinitial>0,t) / ∂t=0
• ∂N(kfinal<0,t) / ∂t=0
• Both of the above
• Neither

A single electron in an otherwise unoccupied band (see picture) is placed in a constant electrostatic field E. Assuming there is no scattering, how does its group velocity evolve in time long after the E field is turned on?

• It increases linearly.
• It oscillates.
• It is nonzero with the same sign as E.
• It is nonzero with an opposite sign as E.

How does the chemical potential µ(T) of an intrinsic (undoped) semiconductor change with increasing temperature T, assuming the effective masses relate as m_e = 2 m_h?

• Constant at the top of the gap (CB min)
• Increases from the middle of the gap
• Decreases from the middle of the gap
• Constant at the bottom of the gap (VB max)

Which quantities equalize when we create a p-n junction? 

• chemical potential µ
• electrochemical potential µ'
• impurity level energies E_d, E_a
• charge carrier densities n,p