Semiconductors and doping
A semiconductor such as silicon conducts electricity better than an insulator but worse than a metal. Its conductivity is increased by adding tiny amounts of impurity, a process called doping. Adding a pentavalent impurity gives extra free electrons and makes an n-type semiconductor; adding a trivalent impurity creates "holes" and makes a p-type semiconductor.
The p-n junction diode
Joining a p-type and an n-type together forms a p-n junction diode, which has two terminals: the anode (p-side) and the cathode (n-side).
- Forward bias: anode connected to +, cathode to −. The diode conducts and current flows.
- Reverse bias: anode connected to −, cathode to +. The diode does not conduct; almost no current flows.
Key formula / 关键公式
A diode obeys Ohm's law only when conducting. For the whole circuit while it conducts:
V = IR
and the diode's own voltage drop (about 0.7 V for silicon) must be subtracted from the supply voltage.
Rectification
Because a diode passes current one way only, it is used for rectification — converting alternating current (a.c.) to direct current (d.c.). One diode gives half-wave rectification; four diodes in a bridge give full-wave rectification.
Worked example / 例题
A silicon diode (0.7 V drop) is forward biased in series with a resistor across a 6.0 V supply. If the current is 0.20 A, find R.
Voltage across R = 6.0 − 0.7 = 5.3 V, so R = V/I = 5.3 ÷ 0.20 = 26.5 Ω.
Remember / 记住
- Forward bias conducts; reverse bias blocks.
- n-type has free electrons; p-type has holes.
- A diode is used to rectify a.c. into d.c.