Ideally can be thought of as a one-way ticket gate that only lets electrons go in one direction with zero resistance! Real devices do have imperfections however, so there is some resistance.
First they were made out of vacuum tubes, but later semiconductor diodes were invented and became much more widespread.
Figure 1.
I-V curve of a diode
. Source. This image shows well how the diode is only an approximation of the ideal one way device. Notably, there is this non-ideal voltage drop across the device, which can be modelled as constant. It is however an exponential in fact.
Video 1.
Diodes Explained by The Engineering Mindset (2020)
Source. Good video:
The first diodes. These were apparently incredibly unreliable, especially for portable radios, as you had to randomly search for the best contact point you could find in a random polycrystalline material!!
And also quality was highly dependant on where the material was sourced from as that affected the impurities present in the material. Later this was understood to be an issue of doping.
It was so unreliable that vacuum tube diodes overtook them in many applications, even though crystal detectors are actually semiconductor diodes, which eventually won over!
For a long time, before artificial semiconductors kicked in, people just didn't know the underlying physical working principle of these detectors. What I cannot create, I do not understand basically.
This was the first generation of commercially successful radios.
It uses a crystal detector as its diode, which is a crucial element of the radio, thus its name.
They were superseded by transistor radios, which were much more reliable, portable and could amplify the signal received.
Figure 1. Source.
Video 1.
How a Crystal radio Works by RimstarOrg
. Source.

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Diode by Codex 0 Created 2026-10-14 Updated 2026-10-18
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A diode is a two-terminal component with an asymmetric current-voltage characteristic: reversing its terminal electric potential difference need not reverse its electric current with the same magnitude. Name the terminals diode anode and diode cathode . Define using their electric potentials, and take positive electric current from to . The bar in the diode symbol marks . Positive is the forward direction; negative is the reverse direction.[ref]
The asymmetry makes a rectifier possible: a sinusoid can drive electric current through a resistor on one half of each cycle. An ordinary resistor obeying Ohm's law instead reverses its electric current when the applied electric potential difference reverses. The useful idea is directional selection; the ideal diode model captures it, while the Shockley diode equation describes a smooth exponential function rather than an abrupt switch.
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