(Semiconductor) Diode Basics

Overview

Semiconductor Diodes or Solid-State Diodes are devices that are capable of producing unidirectional current flow using the solid properties of semiconducting materials. In contrast, vacuum and gas diodes achieve this effect by having electrons travel outside of the solid. In the modern day, solid-state diodes completely superseded other designs in practical applications.

Semiconductor diodes were actually discovered very early in the history of electronics, before the term "semiconductor" was even used to describe semiconductors and before the first vacuum tube was even built. Ferdinand Braun (more commonly known for being the CRT guy) observed semiconductor-like properties in lead and copper ores all the way back in 1874, which led to the development of crystal detectors in the early 1900s [1]. Back then, no one understood how semiconductors worked. The principle of operation is inherently tied to quantum mechanics, which was just starting to develop in parallel with vacuum electronics. It took all the way until the 1940s for first proper semiconductor devices to be developed.

TODO: picture

Nowadays, we have many many different kinds of diodes filling various roles. Most prominently, diodes perform many tasks in power supply circuits - rectification, regulation, indication, protection etc. In RF circuits, we primarily care about the Schottky Diode, which allows us to build discrete modulators and demodulators.

Identification

Diodes are polarized two-terminal devices; the positive terminal is called the anode (A) and the negative terminal, the cathode (K). If you are familiar with those terms, this might seem wrong at first, but the terminals are defined from the perspective of the circuit and not the device. The electrons (from the circuit) flow into the cathode, and emerge (to the circuit) from the anode, which is now consistent with the usage of the terms in chemistry.

The generic diode circuit schematic symbol looks like this:

Generic Diode Symbol

I have labeled the cathode and anode terminals on the drawing, although typically the labels are not shown. Instead, the direction of the diode is inferred based on the cathode mark - the perpendicular (to the direction of current) bar located at the cathode terminal.

TODO: real diode photo

The cathode mark is also drawn on physical diode packages to tell us the polarity of the device. On through-hole packages (on the left) this will literally look like the bar from the schematic diagram, while for SMD packages (on the right) you will need to check the datasheet to find out where the cathode mark is located.

SMD LED Package Example [2]

This is an example of a rather ambiguous SMD diode package. If you did not read the datasheet, you would be forgiven for thinking the zener die is actually the cathode mark (although you would not be forgiven for being lazy).

Modeling

Ideal Diode

The Ideal Diode is a mathematical model for how a general-purpose diode is supposed to behave. The characteristic curve is shown below, with the forward direction being defined to be from anode to cathode:

Ideal Diode I-V Characteristic

The plot is a piecewise function of two linear relationships:

Forward bias: the diode behaves as a short circuit.
Reverse bias: the diode behaves as an open circuit.

Note that the ideal diode cannot conduct reverse current and cannot develop a forward voltage. These assumptions never hold true for real diodes, which is why this model is purely mathematical. We can use the ideal diode model to figure out conceptually how a circuit behaves, but you should never bring this model into the lab.

Practical Model (UCVD)

The I-V characterstic of a realistic diode looks something like this:

Realistic Diode I-V Characteristic (loosely based on [3])

The diode actually has exponential I-V charactersitics in many parts of the plot, therefore creating a good linear model becomes a challenge. Thankfully, when precision is not essential, and we are not dealing with small voltage increments, the constant voltage drop (CVD) model is a practical approximation of diode behavior.

In contrast to the ideal diode, real diodes will not conduct much current until some forward voltage is developed. On the other side of the curve, real diodes do end up conducting backwards after entering various breakdown conditions past the breakdown voltage.

To develop the forward CVD model, we place a DC voltage source after the cathode of the ideal diode. The negative leg of the voltage source becomes the new "real" cathode. By flipping the polarity of both elements in the forward CVD, we get the breakdown CVD model. Connect the two CVD models in parallel produces, what I will call, the unified CVD (UCVD) model (no academic name for this as far as I can tell).

UCVD Model

Despite voltage sources being present, this is still a model of a passive devices. The ideal diode causes the voltage sources to only be capable of absorbing power and not creating it. The model gives plots similar to the I-V plot of the ideal diode, offset on the x-axis by the voltage source values:

Realistic Diode I-V (overlayed with UCVD model)

Sources

[1]

T. H. Lee, Planar Microwave Engineering: A Practical Guide to Theory, Measurements and Circuits. Cambridge: Cambridge University Press, 2004.

[2]

Harvatek Corporation, “Harvatek Surface Mount CHIP LED Data Sheet: E1221NB--05D000814U1930.” Oct. 23, 2020. Available: https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/4998/E1221NB--05D000814U1930.pdf. [Accessed: Dec. 27, 2025]

[3]

Fairchild Semiconductor Corporation, “1N4001 - 1N4007.” Oct. 2014. Available: https://www.mouser.com/datasheet/2/149/1N4001-81693.pdf. [Accessed: Oct. 18, 2025]