Problem 1
A silicon pn-junction is doped with acceptors to $N_A$ = 1018 1/cm³ on one side and doped with donors $N_D$ = 1017 1/cm³ on the other side. No voltage is applied across the junction.
(a) Sketch the band diagram (conduction band, valence band, Fermi energy) for zero bias.
(b) Indicate $eV_{bi}$ in your drawing.
(c) Draw the charge density. Indicate the values of the maximum and the minimum charge density on the vertical axis.
(d) Light falls on this pn-junction. Which way does the current flow? Explain your reasoning.
Problem 2
Consider an n-channel silicon JFET. The source is grounded and +1 V is applied to the drain.
(a) Plot the gate current as a function of gate voltage for positive and negative gate voltages.
(b) Plot the drain current as a function of gate voltage for positive and negative gate voltages.
(c) What is transconductance? How does the transconductance of a JFET change with drain voltage in the saturation regime?
(d) What is the dominant current mechanism for the source-drain current? (tunneling, drift, diffusion, thermionic emission).
Problem 3
(a) Draw a pnp bipolar transistor showing the emitter, collector, and base contacts.
(b) What are the doping levels of the emitter, the base, and the collector? How does the doping affect the emitter efficiency and the base transport factor?
(c) Explain how the collector current is calculated.
(d) Why would a bipolar transistor be used in a common base configuration?
Problem 4
Make a drawing of a laser diode and describe how it works. What determines the frequency that is emitted? Why is there a threshold current? How is it different from an ordinary light emitting diode?
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 |