Lesson 06 · Semiconductors

Diodes: one-way streets

A diode lets current flow one way only. Look inside a silicon diode at its p side, n side and the depletion region where they meet. Connect a battery forward and the lamp lights; flip it and the current stops. Then read the diode's V-I curve, meet LEDs (diodes that glow) and use a diode to turn AC into one-way pulses.

10 min10 steps5 questionsBeginner

Fig 6 · Interactive

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5 quick questions

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01A 1N4148 diode is reverse biased with 20 V. What happens?

Reverse bias (+ on the n side) widens the depletion region, so only a tiny leak gets through: Vishay gives at most 25 nA at 20 V. Breakdown, where current rushes through, needs at least 100 V for this diode.

02What is the depletion region?

Near the junction, electrons cross and fill holes. That leaves a thin zone with no free holes or electrons, and its leftover charges push back on any more crossing. It acts like a wall until forward bias squeezes it thin.

03About how much voltage does a working silicon diode keep across itself when it conducts?

With its + side on the p side (forward bias), a small silicon diode like the 1N4148 passes about 1 mA at 0.6 V and about 10 mA at 0.72 V (typical, at 25 °C). Below about 0.6 V almost nothing flows.

04A blue LED and a red LED are each run at 20 mA. Which needs more forward voltage?

In Kingbright's 5 mm LED datasheets the blue LED's typical forward voltage is 3.3 V and the red one's is 1.85 V (green is 2.2 V).

05A diode is put in series with an AC supply and a lamp. What does the lamp get?

The diode passes the forward half of each swing and blocks the back half. The result is one-way pulses: half-wave rectified DC. Power supplies often smooth them out afterwards.

Key terms

Glossary

Diode: A part that lets current flow one way only: in at the anode, out at the cathode.
Anode and cathode: The two ends of a diode. Current flows from anode (p side) to cathode (n side); the cathode is marked with a band.
P-N junction: The join between p-type silicon (full of holes) and n-type silicon (full of spare electrons).
Hole: A place in the silicon where an electron is missing. It acts like a moving + charge.
Depletion region: A thin zone at the p-n junction with no free carriers. Its leftover charges block current until it is pushed thin.
Forward bias: Connecting a diode with its p side more positive than its n side, so current can flow.
Reverse bias: Connecting a diode the other way round, so it blocks current, except a tiny leak (until breakdown).
Rectifier: A circuit, often just diodes, that turns AC (which swings both ways) into one-way current.

Fine print

Notes and sources

The board circuit is a teaching model: the 3 V battery, the resistor and the lamp show the idea and are not taken from one datasheet. So is the LED row's 5 V supply (each LED has its own resistor, not drawn).
The V-I curve is the 1N4148's typical forward curve at 25 °C (Vishay datasheet, Fig. 2): about 1 mA at 0.6 V and 10 mA at 0.72 V. Its reverse current, at most 25 nA at 20 V, is too small to draw and is shown as zero; breakdown starts at 100 V or more. The reverse axis is squeezed and broken, so it is not to scale.
LED forward voltages are typical values at 20 mA from Kingbright's 5 mm LED datasheets: red 1.85 V, green 2.2 V, blue 3.3 V. Each part varies a little.
Current is drawn in the conventional direction (from + to -); the electrons in the wires drift the opposite way.
Mains frequency is 50 Hz or 60 Hz depending on the country; the figure slows one cycle to a few seconds.

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