Semiconductor Electronics

Q1.Why do LED (Light Emitting Diode) semiconductors need direct band gap materials (like GaAs), while Si and Ge (indirect band gap) are not suitable for LEDs?

• ASi and Ge do not allow electron-hole recombination
• BIn direct band gap materials, electron-hole recombination releases energy as photons directly; in indirect band gap (Si, Ge), energy is released as heat (phonons) making photon emission very inefficient
• CGaAs has a larger band gap than Si and Ge
• DSi and Ge cannot be doped with both p-type and n-type impurities simultaneously

Q2.In an n-type semiconductor, the donor energy level ED is located:

• AAt the middle of the band gap
• BSlightly below the bottom of the conduction band (EC)
• CSlightly above the top of the valence band (EV)
• DIn the valence band

Q3.Why cannot a p-n junction be made simply by pressing a p-type slab against an n-type slab?

• AThe two materials would repel each other electromagnetically
• BThe surface roughness (~μm scale) is much larger than the inter-atomic spacing (~2-3 Å), preventing atomic-level contact required for junction formation
• CDiffusion of carriers would be prevented by air between the slabs
• DThe different work functions of p and n materials prevent junction formation

Q4.The energy band gap for diamond (C) is about 5.4 eV, for silicon it is 1.1 eV, and for germanium it is 0.7 eV. Which statement correctly explains why carbon (diamond) is an insulator while Si and Ge are semiconductors?

• ACarbon atoms are heavier than Si or Ge atoms
• BThe very large band gap of diamond (5.4 eV) prevents thermal excitation of electrons at room temperature, while Si and Ge have small enough band gaps for some electrons to be thermally excited
• CCarbon does not form covalent bonds in diamond
• DSi and Ge have fewer valence electrons than carbon

Q5.In a p-n junction under forward bias, the process of 'minority carrier injection' refers to:

• AMajority carriers crossing from one side to the other, becoming minority carriers on the other side
• BExternal injection of electrons through a metal contact
• CThermal generation of carriers near the junction
• DBreakdown of covalent bonds at the junction