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 is does current flow when light falls on the photodiode?

Problem 2
(a) Draw an n-channel JFET.

(b) Explain how a JFET works. Explain whether the gate voltage should be positive or negative to operate in the saturation regime.

(c) What is the dominant current mechanism for the source-drain current? (tunneling, drift, diffusion, thermionic emission).

(d) Why is a JFET slower than a MESFET?

Problem 3
(a) A Schottky diode is formed by pressing a metal tip against a silicon wafer. When a light falls on the diode, it works like a solar cell and current flows from the metal tip to the wafer. Explain how you can tell if this is an n-doped or a p-doped wafer from this experiment.

(b) Explain what thermionic emission is.

(c) When is tunneling the dominant current mechanism in a Schottky diode?

(c) Why does the Schottky diode respond faster than the pn diode?

Problem 4
A $pnp$ bipolar transistor is constructed from GaAs, a direct bandgap semiconductor.

(a) What would be the advantages and disadvantages of using GaAs instead of silicon to make a bipolar transistor?

(b) When the emitter-base junction is forward biased, is light emitted like in a light emitting diode? Explain your reasoning.

(c) Draw the minority carrier concentration in a pnp transistor in biased in the forward active regime.

(d) How can you calculate the collector current?