The PN Junction Diode

A one-way valve for current.

A diode joins P-type and N-type semiconductor. Near the junction carriers combine and leave a depletion region that acts as a barrier. Forward bias shrinks the barrier, and current rises sharply once the voltage passes about 0.7 volts for silicon.

In reverse bias the barrier grows and only a tiny leakage current flows until breakdown. In circuits a diode is nearly always used with a series resistor, because its current would otherwise rise without limit.

• Anode is positive for forward bias; cathode has the stripe.
• Silicon drops about 0.7 V; Schottky diodes drop less.
• Reverse voltage must stay below the rated limit.

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