Electronic Devices — Semiconductors & the p-n Junction (Medium)

Quick Recap — Semiconductors & the p-n Junction

• Bands: conductors have overlapping bands; insulators a large gap ($>3$ eV); semiconductors a small gap (Si $\approx1.1$ eV, Ge $\approx0.7$ eV). A semiconductor's conductivity rises with temperature.
• Doping: pentavalent donors give n-type (majority electrons); trivalent acceptors give p-type (majority holes). The doped crystal is still electrically neutral.
• p-n junction: diffusion of carriers forms a depletion region with a barrier potential (Si $\approx0.7$ V, Ge $\approx0.3$ V).
• Bias: forward bias lowers the barrier $\Rightarrow$ large current (low resistance); reverse bias widens the depletion region $\Rightarrow$ tiny current (high resistance).

Worked mini-example. In a Si diode circuit with a 5 V supply and a series 430 $\Omega$ resistor, the current is $\dfrac{5-0.7}{430}=10$ mA (0.7 V drops across the diode).