A diode is a semiconductor component that conducts mainly in one direction and blocks in the opposite direction. This asymmetry allows signal rectification, input protection, and voltage spike control.
The diode is the simplest semiconductor, but it’s the fundamental piece that allows us to direct electron traffic. Without diodes, we couldn’t convert the alternating current from our outlets into the direct current needed by our devices.
We can imagine a diode as a check valve in plumbing: it allows water to flow freely in one direction but completely blocks the flow if the water tries to go back the other way.
How a Diode Works
A diode is composed of the junction of two semiconductor materials (P-N junction). It has two terminals:
- Anode (A): the positive terminal.
- Cathode (K): the negative terminal (usually marked with a gray or black band on the component body).
If we connect the positive side of the battery to the anode and the negative side to the cathode, the diode behaves (almost) like a closed switch. Current flows.
However, there’s a “toll.” To open the gate, the diode charges a small amount of energy. In standard silicon diodes, this voltage drop is approximately 0.7 V.
If you have 5 V and pass through a diode, you will have 4.3 V on the other side. Keep that in mind!
If we apply positive voltage to the cathode relative to the anode, the diode becomes reverse-biased. A small leakage current flows until we approach its breakdown voltage.
No diode can block an infinite voltage. If we exceed its reverse limit, it can enter breakdown and be damaged, unless it is designed to operate in a controlled manner in that region.
Types of Diodes
| Type | Forward Behavior | Reverse Behavior | Primary Use |
|---|---|---|---|
| Rectifier | Conducts (0.7 V drop) | Blocks | Rectification, protection, flyback |
| Zener | Conducts (0.7 V drop) | Maintains fixed voltage (Vz) | Voltage reference, stabilization |
| Schottky | Conducts (0.3 V drop) | Blocks | Fast switching, efficiency |
Rectifier Diodes
When we talk about “normal” diodes, the undisputed king is the 1N400X series (with the 1N4007 being the most famous).
It is a robust, cheap, and ubiquitous component.
- Maximum current: 1 ampere.
- Maximum reverse voltage: 1000 volts.
These are the values for the 1N4007. For other diodes, consult their datasheet.
Main Uses
- Rectification: converting AC (which goes back and forth) into DC (which only goes one way). If we put a diode on an alternating signal, we eliminate the negative half of the wave. We only let the positive peaks pass through.
- Polarity Protection: Afraid of connecting the battery backward and burning your board? Put a 1N4007 diode in series with the positive input. If you connect the battery correctly, it works (losing 0.7 V). If you connect it backward, the diode blocks and saves your circuit.
- Flyback Diode: in DC coils with unidirectional switching, it limits the overvoltage and provides a recirculation path.
The 1N400x series is designed for slow rectification. In converters and high-frequency PWM, a fast diode or Schottky diode may be necessary.
Schottky Diodes
Schottky diodes use a metal-semiconductor junction. They typically offer lower forward drop and fast switching, at the cost of higher reverse leakage and, depending on the device, lower allowable reverse voltage.
In general, they have a lower voltage drop, around 0.3 V, although this also depends on current, temperature, and the component.
Zener Diode
A Zener diode is designed to operate in the reverse breakdown region. When it reaches this region, it conducts and maintains a roughly stable voltage within a current range.
The Zener works the same as a normal diode in forward bias. Its main characteristic lies in the reverse region. When the reverse voltage reaches a specific point (called the Zener voltage, Vz), the diode starts conducting current backward, but maintains a fixed voltage across its terminals.
Main Uses
Suppose you have a 9 V source (which might vary between 8 V and 9.5 V depending on the load) and you need a stable reference of exactly 5.1 V for a comparator.
We place a resistor in series and a 5.1 V Zener in reverse bias to GND.
- As long as the current stays within the intended range and the power rating is not exceeded, the Zener will maintain a voltage approximately close to 5.1 V. Outside this range, it will stop regulating or be damaged.
The Zener is primarily used for signal references or very low-power regulators. Do not try to power a motor using a Zener to lower the voltage, as the Zener will get extremely hot and burn out. For power applications, we will use regulators.
