Semiconductor: conductivity between conductor and insulator (silicon, germanium). Intrinsic: pure. Extrinsic: doped with impurities.
n-type: doped with pentavalent atoms (P, As, Sb), called donors; majority carriers are electrons. p-type: doped with trivalent atoms (B, Al, In, Ga), called acceptors; majority carriers are holes. Both types are electrically neutral overall.
pn junction (diode): n and p joined; a depletion region forms at the junction. Barrier potential: 0.7 V silicon, 0.3 V germanium.
Forward bias: p-side to positive terminal, depletion region narrows, current flows. Reverse bias: depletion region widens, almost no current.
LED: emits light when current passes through it in forward bias.
Transistor: three layers (NPN or PNP), emitter (heavily doped), base (thin, lightly doped), collector (largest). Used as amplifier and switch.
Relay: electrically operated switch. ADC: analogue to digital converter. DAC: digital to analogue converter. Bit: binary digit 0 or 1.
Logic gates: AND, OR, NOT are fundamental. NAND and NOR are universal gates (any gate can be built from them).
Formulas
Ie = Ib + Ic. Current gain β = Ic / Ib (also ΔIc / ΔIb).
Truth tables (A, B to X)
AND (X = A·B): 00 to 0, 01 to 0, 10 to 0, 11 to 1.
OR (X = A+B): 00 to 0, 01 to 1, 10 to 1, 11 to 1.
NOT: 0 to 1, 1 to 0.
NAND: 00 to 1, 01 to 1, 10 to 1, 11 to 0.
NOR: 00 to 1, 01 to 0, 10 to 0, 11 to 0.
Must-know points
Functions of a diode in electronic circuits (give two): rectifier (a.c. to d.c.), protects circuits from reverse polarity, switch, used in LEDs and detectors.
Rectification: a diode conducts only in forward bias, so it passes one half of the a.c. cycle and blocks the other; draw input a.c. and output pulsating d.c.
Diode forward bias (with diagram): battery positive to p-side; free electrons and holes move towards the junction, the depletion region narrows and current flows once the barrier potential is passed.
Transistor action: small base current controls large collector current; as a switch, base current on gives collector current (on), none gives off.
Digital systems are better than analogue: less noise, easy storage and copying, accurate processing. Storage devices (three): hard disk, USB flash drive, memory card, CD/DVD, SSD.
Fax machine (not checked in the book text, from general knowledge): scans the page into light and dark points, converts them to electrical signals, sends them through the telephone line, and a printer at the other end rebuilds the page.
Past-paper questions with model answers
State two functions of a diode in electronic circuits (1.5 + 1.5).
Role of a diode in rectification, with diagram; working of a diode under forward bias, with diagram.
Draw the symbol of an AND gate and its truth table (1+2). Truth tables of AND, OR, NAND, NOR and NOT (Section C).