Semiconductor Electronics

Q1.In an intrinsic semiconductor at room temperature:

• AThe number of holes is greater than the number of free electrons
• BThe number of free electrons is greater than the number of holes
• CThe number of free electrons equals the number of holes
• DThere are no free charge carriers

Q2.On the basis of electrical conductivity, which of the following correctly lists materials from highest to lowest resistivity?

• AInsulators > Semiconductors > Metals
• BMetals > Semiconductors > Insulators
• CSemiconductors > Insulators > Metals
• DInsulators > Metals > Semiconductors

Q3.In an intrinsic semiconductor, the conductivity σ is given by σ = ni e (μe + μh), where μe and μh are electron and hole mobilities respectively. If Eg(Si) = 1.1 eV, Eg(Ge) = 0.7 eV, and both have similar mobilities, which has higher intrinsic conductivity at 300 K?

• ASilicon, because it has a larger band gap
• BGermanium, because its smaller band gap leads to higher ni
• CBoth have equal conductivity
• DSilicon, because it has more atoms per unit volume

Q4.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

Q5.The threshold voltage (cut-in voltage) for a silicon diode is approximately:

• A0.2 V
• B0.7 V
• C1.5 V
• D5.0 V