Diodes are one of the most common semiconductor devices used in electronic equipment today.
They’re also one of the most misunderstood.
After all, diodes are often referred to as “one-way gates” or “steal gates” when talking about their operation.
When a diode is cut off from outside voltage, electrons within it become trapped inside and cannot escape again.
As such, this traps current flowing through that particular part of the circuit inside with no way out except through the opposite terminal or return path (thus the name by-passing the name).
However, when diodes are mentioned in conjunction with electronics they can be confusing.
This is because many people think of them as linear devices—when in fact they possess nonlinear behavior which makes them much more versatile than just a simple on/off switch.
Much like how a musical instrument has multiple uses beyond playing notes, a diode serves many purposes beyond just switching electrical current on and off.
Let’s take a look at how diodes work so you understand how they can be used and what unique properties they possess that make them such useful pieces of electronic circuitry.
What is a diode?
Diodes are one-way electrical shunts.
A diode is an electronically controlled two-way switch that allows current to flow in one direction only under certain conditions.
When current flows in only one direction through a diode, its two semiconductor “fingers” are connected together.
When current is flowing the other way, the two fingers are isolated from each other and no current flows.
Diodes are made from two semiconducting materials that are usually arranged in a “sandwich” fashion to block electrons from flowing in both directions.
A small amount of current under certain conditions can dissipate its excess energy as heat, enabling electrons to flow through the diode in one direction—even if the voltage across the diode is much higher than the voltage applied to the other side.
Because the diode’s active region only allows electrons to flow in one direction while the outer region blocks them from flowing back, it is described as a one-way electrical shunt.
Diodes have positive and negative terminals
A diode’s two ends are labeled with + and – to indicate that it has no internal polarity.
When a voltage is applied to a diode’s ends, this is called short-circuit or “negative” testing.
Diodes are not polarized like normal polarized electrical wiring—the ends are used for testing only and the middle of the diode is neutral (“no polarity”) and is connected to circuit elements.
In electronics, the positive terminal of a diode is usually the anode and the negative terminal is the cathode.
However, the convention isn’t set in stone.
In some circuits, the negative terminal is the cathode and the positive terminal is the anode.
For example, in an LED circuit, the negative terminal is the cathode, but in a battery circuit, the negative terminal is the anode.
There are many types of diodes
There are many different types of diodes available for use in electronics.
Most diodes are of the semiconductor variety, but there are also rectifiers, photodiodes, and transistors that function like diodes.
Selecting the proper type of diode for a particular circuit is important to get the desired results.
Some important diode types include: – Fast Rectifiers: These diodes conduct electricity very quickly, allowing for high-frequency applications.
– Standard Rectifiers: These diodes conduct electricity more slowly, allowing for low-frequency applications.
– Schottky Barrier Rectifiers: These diodes have a built-in Schottky diode that prevents them from conducting backward.
– Photodiodes: These devices convert light into electricity, making them useful in sensing applications.
Diodes have different voltage thresholds, characteristics, and breakdown voltages
Although diodes remain one-way electrical shunts, they typically have a very high breakdown voltage (greater than 1 megavolt) and a breakdown voltage threshold (decreased voltage required to start the breakdown) that make them suitable for certain types of applications.
These threshold parameters are dependent on the type of diode being used and can be altered to create various types of diodes.
As an example, a fast rectifier diode has a breakdown voltage threshold of about 0.3 volts.
This means that if the voltage across the diode is lower than 0.3 volts, the diode won’t conduct and the circuit will remain in its original state.
If the circuit tries to draw more current and the voltage across the circuit is increased, the diode’s breakdown voltage threshold is met and the diode starts conducting current in the opposite direction.
Diodes can be used in linear or nonlinear applications
One unique feature of diodes is that they can be used in linear or nonlinear applications.
When used in linear applications, the diode is used as a switch.
In other words, it conducts current in one direction depending on the voltage applied to the circuit.
When a voltage is applied across a circuit, the electrons begin to flow through the diode and the circuit is powered.
The diode can be thought of as a “one-way switch”.
When the circuit is powered, the diode conducts current, turning the circuit on.
When no voltage is applied across the circuit, the diode doesn’t conduct, and the circuit is powered off.
In nonlinear applications, the diode is used to amplify or increase the amplitude or strength, of a signal.
For example, if a circuit uses a low-frequency signal to control something (like turning a motor on or off), the circuit itself may be powered off by the signal.
But if the signal is high enough (like a telephone dial tone or music from a radio station), the diode can be used to amplify and turn the circuit power on, allowing it to be controlled by the higher-frequency signal.
How Do High Voltage Diodes Work?
When a high voltage is applied across a diode, it begins to conduct.
However, because the voltage is too high, the electrons trapped within the diode can’t release their energy in enough quantities to break free of their confinement.
As a result, the diode conducts a little bit, but not enough to power the circuit.
When a low voltage is applied to the gates of a pair of transistors that control the voltage applied across a circuit (called a ladder circuit), the signal is allowed to pass through unregulated.
However, when there’s too little voltage across the ladder circuit and the diodes are not conducting enough current, the signal isn’t allowed through and the circuit is powered off.
This can be used to power simple circuits and can be useful for sorters, computers, and timers.
How to Calculate the Voltage Threshold for a Diode
Suppose you connect a diode to a 12-volt power source and want to know if it will conduct (provide power) at a low voltage.
The equation for calculating the breakdown voltage (VOM) of a semiconductor device is as follows: In this equation, “VOH” is the voltage across the device when it breaks down, “VOHSC” is the threshold voltage of the diode when it conducts, “I” is the current through the diode, “E” is the voltage of the electric field across the diode and “n” is the number of electrons in the diode.
To determine the voltage threshold of the diode, you need to know the breakdown voltage of the diode.
You can find this value by using the equation above.
The breakdown voltage of a typical silicon p-n junction diode is 1.5 volts.
This means that when the voltage across the diode is 1.5 volts, the diode will break down and begin conducting current.