Problem 1
(a) Describe how a solar cell works.
(b) The depletion region of a solar cell has a certain thickness in the dark. What determines this thickness? What happens to the depletion width when light falls on the solar cell?
(c) If a semiconductor has an indirect bandgap, what consequence does this have for a solar cell?
Problem 2
(a) Draw an n-channel JFET.
(b) Explain how a JFET works.
(c) Where are there tunnel contacts in this device? What is the purpose of tunnel contacts? Draw the band diagram (valence band, conduction band, Fermi energy) of a tunnel contact.
(d) Why is a JFET slower than a MESFET?
Problem 3
(a) Draw an $p$-channel MOSFET showing the source, drain, gate, and body contacts.
(b) How should this MOSFET be biased so that it is in the saturation regime?
(c) A voltage is applied between the source and the body that reverse biases the source-body junction. Explain how this shifts the threshold voltage. Which direction is the threshold voltage shifted and how far can it be shifted?
(d) Explain what latch-up is in CMOS circuits.
Problem 4
In a silicon $pnp$ bipolar transistor, the emitter is doped to 1019 cm-3, the base is doped to 1014 cm-3, and the collector is doped to 1013 cm-3.
(a) Why is the transistor doped this way?
(b) What is the equilibrium concentration of minority carriers in the collector?
(c) How can you calculate the collector current?
Quantity | Symbol | Value | Units | |
| electron charge | e | 1.60217733 × 10-19 | C | |
| speed of light | c | 2.99792458 × 108 | m/s | |
| Planck's constant | h | 6.6260755 × 10-34 | J s | |
| reduced Planck's constant | $\hbar$ | 1.05457266 × 10-34 | J s | |
| Boltzmann's constant | kB | 1.380658 × 10-23 | J/K | |
| electron mass | me | 9.1093897 × 10-31 | kg | |
| Stefan-Boltzmann constant | σ | 5.67051 × 10-8 | W m-2 K-4 | |
| Bohr radius | a0 | 0.529177249 × 10-10 | m | |
| atomic mass constant | mu | 1.6605402 × 10-27 | kg | |
| permeability of vacuum | μ0 | 4π × 10-7 | N A-2 | |
| permittivity of vacuum | ε0 | 8.854187817 × 10-12 | F m-1 | |
| Avogado's constant | NA | 6.0221367 × 1023 | mol-1 |