Chapter summary

The whole chapter in one list.

• In metals the charge carriers are negative electrons; in semiconductors, electrons and holes (like positive charges).
• Every broken bond frees an electron and makes a hole, so a semiconductor has equal numbers of free electrons and holes. With a potential difference across it, holes move to the negative side, electrons to the positive, and current flows from positive to negative.
• Doping an intrinsic semiconductor with a group V element makes n-type; with a group III element, p-type.
• With the p side positive the depletion layer shrinks and, once the voltage exceeds the barrier, current flows: forward bias. With the p side negative the layer widens and no current flows: reverse bias.
• Diodes rectify an alternating current or voltage.
• The potential barrier of a p-n junction is about 0.7 V for silicon and about 0.3 V for germanium.
• In a pnp transistor the current flows from emitter through base to collector (arrow in); in an npn transistor from collector through base to emitter (arrow out). Carriers always go from emitter to collector.
• A transistor can be a DC current amplifier, a signal (AC) amplifier and a switch.

The textbook's summary gives 0.2 V for germanium; its body text, twice, gives 0.3 V, which is used here.