Problem 1
Consider a pn-junction.

(a) If there is no voltage applied, there is no current. Nevertheless there is an electric field. Provide the formulas for the drift current and the diffusion current and using these formulas write down the equation that states there is not current.

(b) If the junction is reverse biased, what is the expression for the capacitance?

(c) State the relationship between the hole concentration on the p-side, the hole concentration on the n-side, and the built-in voltage \(V_{bi}\).

Solution

Problem 2

Consider an n-channel JFET biased in saturation.

(a) How are the gate-source and gate-drain pn-junctions biased (forward or reverse)?

(b) If a large enough voltage is applied to the drain, breakdown will occur. How can a JFET be modified to increase the breakdown voltage?

(c) If light falls on a JFET biased in saturation, how will the drain current change? Why?

(d) What determines the resistivity of the channel? Provide a formula if you can.

Solution

Problem 3
(a) Draw a cross section of an $n$-channel MESFET showing the source, drain, and gate contacts.

(b) Draw the band diagram (conduction band, valence band, Fermi energy) from the gate to body when no voltage is applied.

(c) What polarity do the voltages have (positive or negative) at the gate and drain when an n-channel MESFET is biased in saturation?

(d) What is thermionic emmission and when does a thermionic emmission current flow in a MESFET?

Solution

Problem 4

(a) Plot the minority carrier concentration in an npn bipolar transistor in forward active mode.

(b) Why is a heterojunction bipolar transistor faster than a normal bipolar transistor?

(c) What is the Early effect and what is punchthrough?

(d) In an npn transistor, where do the holes in the base current recombine?

Solution