Diodes
A diode is the simplest semiconductor device: a one-way valve for electrical current. Just as a check valve in a plumbing system allows water to flow in one direction and blocks it in the other, a diode allows current to flow from anode to cathode when the voltage is applied correctly, and blocks it when the voltage is reversed. This simple ability makes the diode one of the most versatile components in electronics — used for rectification, signal detection, voltage clamping, protection, and much more.
- The PN junction: why a diode is a one-way valve
- Forward voltage and the diode characteristic
- Reverse breakdown
- Rectification: turning AC into DC
- Common diode types
- Diodes in ham radio
The PN Junction: Why a Diode Is a One-Way Valve
A diode is made by joining two types of semiconductor material. The P-type material has an excess of positive charge carriers called holes. The N-type material has an excess of negative charge carriers called electrons. At the boundary where these two materials meet — the PN junction — electrons from the N side and holes from the P side diffuse across and recombine, creating a thin region called the depletion layer where no free charge carriers remain.
The depletion layer acts as a barrier. When you apply voltage with the positive terminal to the P side (anode) and the negative terminal to the N side (cathode), you are pushing the charge carriers back toward the junction. Once the applied voltage exceeds the built-in barrier voltage, the depletion layer collapses and current flows freely. This is forward bias.
When you reverse the polarity — positive to the cathode, negative to the anode — you pull the charge carriers away from the junction, widening the depletion layer. No current can flow. This is reverse bias.
The PN junction. Holes in the P region and electrons in the N region recombine at the boundary to form the depletion layer. Forward bias collapses this barrier; reverse bias widens it.
View LargerForward Voltage and the Diode Characteristic
A diode does not behave like a resistor. It has a threshold voltage — called the forward voltage drop (Vf) — below which almost no current flows. Once the applied voltage exceeds Vf, current increases very steeply with even small increases in voltage.
For practical purposes, a forward-biased silicon diode drops approximately 0.6–0.7 V across it regardless of the current. This voltage drop is nearly constant over a wide range of currents, which is why it appears in circuit calculations as a fixed voltage source rather than a resistance.
| Diode material | Typical Vf | Notes |
|---|---|---|
| Silicon (general purpose) | 0.6 – 0.7 V | Most common; 1N4001, 1N4148 |
| Germanium | 0.2 – 0.3 V | Older type; used in vintage receivers for low-level detection |
| Schottky | 0.15 – 0.4 V | Very fast switching; low forward drop; used in RF detectors |
| LED | 1.8 – 3.5 V | Forward drop depends on color; emits light when conducting |
⚖ Experiment: Measure Diode Forward Voltage
Directly measure the forward voltage drop of a silicon diode using a simple circuit and a multimeter. This confirms the 0.6–0.7 V characteristic discussed above.
- 9 V battery
- 1N4001 or 1N4148 diode
- 470 Ω resistor
- Multimeter
- Breadboard and wires
- Connect the 9 V battery in series with the 470 Ω resistor and then the diode (anode toward the positive battery terminal, cathode toward the negative terminal).
- Set your multimeter to DC volts (10 V range).
- Measure the voltage across the resistor — it should be about 8.3 V, confirming current is flowing.
- Now measure the voltage directly across the diode (anode to cathode).
- Reverse the diode in the circuit (cathode toward positive). Measure again.
Forward biased: the diode drops approximately 0.6–0.7 V and the resistor sees about 8.3 V. Reverse biased: the diode drops nearly the full 9 V (it is blocking), and almost no voltage appears across the resistor. This is the one-way valve in action.
Reverse Breakdown
If the reverse voltage applied to a diode is increased beyond the diode's peak inverse voltage (PIV) or reverse voltage rating, the junction breaks down and current flows in the reverse direction — often destructively. For general-purpose rectifier diodes, you must always ensure the PIV is well above the peak AC voltage it will experience. The 1N4001 is rated at 50 V PIV; the 1N4007 at 1000 V PIV.
One special type of diode — the Zener diode — is designed to break down at a precise voltage and operate stably in that region. That is the subject of the next lesson.
Rectification: Turning AC Into DC
The most widespread use of diodes is rectification — converting AC into pulsating DC. A single diode passes only one half of the AC cycle (half-wave rectification). Four diodes arranged in a diamond pattern form a bridge rectifier that conducts on both halves of the AC cycle (full-wave bridge rectification), producing a much smoother pulsating DC output that is easier to filter.
A bridge rectifier uses four diodes to conduct on both halves of the AC cycle. The current path through the bridge reverses each half-cycle, but always flows in the same direction through the load.
View LargerThe output voltage of a bridge rectifier is approximately Vpeak minus two diode drops (0.6 V each), because current must pass through two diodes in series. For a 12 V RMS supply: Vpeak = 12 × √2 ≈ 17 V, minus 1.4 V = approximately 15.6 V DC before filtering.
Diodes in Ham Radio
Diodes appear throughout ham radio equipment:
- Power supply rectifiers: Bridge rectifiers convert line voltage AC to DC in every linear power supply and linear amplifier.
- Flyback (freewheeling) diodes: Placed across relay coils and motor windings to absorb the voltage spike when the coil is switched off, protecting the driver transistor.
- RF signal detection: In envelope detectors for AM demodulation, where the diode rectifies the RF carrier to extract the audio envelope.
- Voltage clamping: Diodes prevent circuit nodes from swinging outside safe voltage limits, protecting sensitive inputs.
- T/R switching: PIN diodes are used as RF switches in antenna relay-less transmit/receive switching (covered in the next lesson on special diodes).
- SWR protection: Back-to-back zener diodes protect receiver inputs from transmit power breakthrough.
Frequently Asked Questions
Which terminal of a diode is the anode and which is the cathode?
The anode is the positive terminal — the P-type side — and the cathode is the negative terminal — the N-type side. On a physical diode, the cathode is usually marked with a stripe or band at one end. In a schematic symbol, the triangle points in the direction of conventional current flow (anode to cathode), and the bar on the arrowhead represents the cathode.
Why does a diode have a voltage drop?
The voltage drop is the energy needed to overcome the built-in potential barrier at the PN junction. Once the applied voltage exceeds this threshold, charge carriers have enough energy to cross the junction. The barrier voltage is a fixed property of the semiconductor material — about 0.7 V for silicon. This drop appears as heat dissipated in the diode.
What is PIV and why does it matter when choosing a rectifier diode?
PIV (Peak Inverse Voltage) is the maximum reverse voltage a diode can withstand without breaking down destructively. In a power supply, the diodes in the bridge rectifier each see the full peak AC voltage when reverse biased. You must choose diodes with a PIV rating comfortably above the peak supply voltage — typically at least 2× the RMS voltage. Using a 1N4007 (1000 V PIV) on a 240 V utility supply (peak 340 V) provides a safe margin.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.