Problem 1
A photodiode consists of semiconducting layers $n$+ / $n$ / $p$ / $p$+. No bias voltage is applied.

(a) Draw the band diagram (conduction band, valence band, Fermi energy).

(b) Draw the electric field and the charge density. Which way does current flow when light falls on the photodiode?

Problem 2
A MODFET (Modulation Doped Field Effect Transistor) and a HEMT (High Electron Mobility Transistor) are two names for the same device.

(a) How does a MODFET work? What are they used for?

(b) What kind of transport dominates in the drain current: diffusion, drift, thermionic emission, or tunneling?

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) How does the drain current depend on the gate voltage in the saturation regime?

(d) A voltage is applied between the source and the body. What happens if a positive voltage is applied? What happens if a negative voltage is applied?

Problem 4
(a) Describe how a $pnp$ bipolar transistor works and explain the base transport factor and the emitter efficiency.

(b) How would you calculate the electron an hole contributions to the emitter current $I_{En}$ and $I_{Ep}$?

(c) Why does a heterojunction bipolar transistor have more gain than a bipolar transistor?