If diodes keep turning up on your schematics, you may wonder what is a diode. It is a semiconductor device that lets current pass easily one way and blocks it the other way. Here is why that works.
Diodes are one of the basic parts in the wider family of electronic components, and this guide covers how they work, what voltage they need and where they are used.
| Item | Value |
|---|---|
| Silicon diode forward voltage drop (barrier potential) | About 0.7 V |
| Germanium diode forward voltage drop | About 0.3 V |
| LED forward voltage drop | 1.5 to 3 V, partly depending on color |
| Growth of reverse saturation current with temperature | Roughly doubles for each 10 °C rise |
| Shockley equation quality factor (n) | Typically between 1 and 2 |
These figures come from the Semiconductor Devices: Theory and Application textbook chapter on the PN junction. They are estimates that can change with the exact part and conditions, so check the source and the data sheet of your own diode before you rely on them.
The semiconductor building blocks
A diode starts with a semiconductor, a material whose electrical conductivity sits between that of conductors and insulators. Silicon and germanium are the most common semiconductor materials used to make diodes, according to Wevolver’s explainer on how a diode works.
Both elements have four valence electrons, which form covalent bonds with neighboring atoms in a crystal lattice. A pure crystal on its own does not behave like a diode, so makers change it on purpose, as the same Wevolver article explains.
That change is called doping: introducing impurities into the pure semiconductor to alter its electrical properties. There are two kinds:
- N-type: doping adds elements with five valence electrons, such as phosphorus, which supply extra electrons as charge carriers.
- P-type: doping uses elements with three valence electrons, such as boron, which create “holes” (electron deficiencies) that act as charge carriers.
A diode brings an N-type region and a P-type region together. The place where they meet is where all the interesting behavior happens.
How the p-n junction creates one-way flow
The p-n junction forms when P-type and N-type semiconductors are brought into contact. In its basic form, a diode is just a PN junction, as the textbook chapter on the PN junction puts it.
Right at the boundary, free electrons from the N material meet the excess holes in the P material and recombine. The area where the two materials abut is then depleted of available electrons and holes, so it is called the depletion region.
This region has an effect on current. In the textbook’s words, it creates an “energy hill” that has to be overcome before current can flow through the device.
That hill is the key to the whole component. You can make it smaller or larger by choosing which way you connect the voltage, and that choice decides whether the diode conducts or blocks.
Forward bias and reverse bias
Forward bias occurs when a positive voltage is applied to the P-type region relative to the N-type region. This reduces the built-in potential barrier, narrows the depletion region and allows significant current to flow, according to Wevolver. In a forward-biased diode, current flows from the anode to the cathode.
The textbook walks through the same case with a battery. Electrons flow from the negative terminal toward the N material, cross into the P material and carry on to the positive terminal, completing the circuit. Note that this electron flow runs opposite the direction of conventional current, and a resistor in the example limits the maximum current, as the chapter describes.
Reverse bias swaps the battery polarity. The electrons in the N material are drawn toward the positive terminal and the P material holes toward the negative terminal, which creates a small, short-lived current.
That movement widens the depletion region, and once it reaches the supplied potential, the flow of current ceases. Ideally, the junction then acts like an open circuit, so the energy hill simply gets bigger.
Forward voltage drop for silicon, germanium and LEDs
For current to flow, the supplied voltage must be large enough to overcome the depletion region. That required potential is called the barrier potential or forward voltage drop, and the precise value depends on the material used, per the same textbook chapter.
The chapter gives these typical values:
- Silicon devices: usually estimated at around 0.7 volts.
- Germanium devices: closer to 0.3 volts.
- LEDs: in the vicinity of 1.5 to 3 volts, partly depending on the color.
Treat these as starting estimates. The textbook uses words such as “usually estimated” and “may exhibit”, and it has a whole section on diode data sheet interpretation, so the data sheet of your actual part is the number to trust.
One more way to picture it: the added voltage source “flattens” the energy hill of the junction. Once the forward voltage is at least as big as the hill, current can flow easily.
What does the Shockley equation tell you?
The behavior of a PN junction can be quantified with an equation derived by William Shockley, as the textbook notes. You do not need to solve it to use a diode, but its terms show what matters.
- The diode current and the voltage across the diode.
- The reverse saturation current.
- The temperature in kelvin.
- A quality factor, n, typically between 1 and 2.
The reverse saturation current is not a constant. It increases with temperature, approximately doubling for each 10 °C rise, according to the same chapter.
In practice, that means a diode’s leakage behavior is not fixed. Temperature is part of the picture whenever you read a data sheet.
Where diodes are used
The one-way property makes diodes essential in applications ranging from power supplies and signal processing to digital logic and optoelectronics, as Wevolver notes. Many of the first circuits you meet use a diode in one of these roles.
The textbook’s unit on diode applications covers three classic circuit types, listed in its contents as rectification, clippers and clampers. Here is how that grouping looks next to the broader uses:
- Power supplies, where current must travel in one direction only.
- Signal processing circuits.
- Digital logic.
- Optoelectronics, which includes LEDs.
To compare diodes with other parts, read the guides on what is an inductor and what is a potentiometer.
Frequently asked questions
Which way does current flow through a diode?
Show answer
When a diode is forward-biased, current flows through the device from the anode to the cathode, as Wevolver explains.
Why does a silicon diode need about 0.7 volts?
Show answer
The supplied voltage has to overcome the depletion region. For silicon devices the barrier potential is usually estimated at around 0.7 volts, according to the textbook chapter.
Does a diode block reverse current completely?
Show answer
Ideally, the PN junction acts like an open circuit under reverse bias. In the textbook’s description, a small, short-lived current appears first, and the flow ceases once the widened depletion region reaches the supplied potential.
Does temperature change how a diode behaves?
Show answer
Yes. The reverse saturation current increases with temperature, approximately doubling for each 10 °C rise, per the same chapter.
