Problem 1
A region of a silicon wafer is doped with donors at a concentration of 3 ×10¹⁷ cm^-3 and with acceptors at a concentration of 2 ×10¹⁶ cm^-3. Assuming that all of the dopants are ionized, what is the density of electrons in the conduction band and the density of holes in the valence band at 300 K?

For silicon, $n_i = $ 1.5 ×10¹⁰ cm^-3 at 300 K.

Solution

Problem 2

(a) Draw a cross section of an n-channel JFET showing the source, gate, and insulating substrate. (Make a large drawing that is about as wide as the paper).

(b) The JFET is biased in saturation. Draw the depleted region and put + or - signs in regions where there is positive or negative charge.

(c) Add arrows to the drawing of the JFET to indicate the direction of the electric field at each point.

(d) Where are there tunnel contacts in a JFET?

Problem 3
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) If the transistor is unbiased, which depletion region is wider? Why?

(b) For an unbiased transistor, is the maximum electric field larger in the emitter-base junction or in the base-collector junction? Why?

(c) Plot the minority carrier concentration in forward active mode.

(d) Calculate the equilibrium electron concentration in the collector. (n_i = 1.5 × 10¹⁰ cm^-3)

Problem 4
Describe how a semiconductor laser diode works. What determines the frequency that is emitted? Why is there a threshold current? How is it different from an ordinary light emitting diode?