Introduction to Electronics - Diodes

Diodes are semiconductor electronic components. Their main characteristic is that they only pass current in one direction. This is extremely useful in numerous applications, such as for protection or for handling alternating current.

Before we understand what diodes are and how they work, it is good to remember how semiconductors work. We already have an article about this here on the website, just go to this link .

Diode Structure

Diodes are composed of P-type and N-type semiconductor materials side by side, as shown in the image below. It has two terminals, the anode, where the P-type material is located, and the cathode, where the N-type semiconductor is located.

Anode Cathode

Current can only flow from the anode to the cathode; the opposite does not happen. This is due to the order of the semiconductor materials. Diodes are represented by the symbol below in electrical diagrams.

How Diodes Work

As seen in the article on semiconductors, N-type materials have an excess of electrons, and P-type materials have a lack of them. Therefore, the flow of electrons tends to go from the N-type semiconductor (which has excess electrons) to the P-type semiconductor (with a lack of electrons).

The opposite does not happen because there is a blockage to the electric current from a material with few electrons to one that already has an excess of them.

A Confusion in Physics: It is worth noting that electric current and the flow of electrons have opposite directions. Many years ago, before the discovery of electrons, it was believed that charge flowed from the positive terminal to the negative terminal. Years later, it was discovered that electricity was the movement of negative charges, not positive ones. However, the standard for electric current, going from the positive terminal to the negative terminal, remained.

That is, when we say that current is flowing from the anode to the cathode, electrons are going from the cathode to the anode.

Ideal Diodes

The operation of an ideal diode is simple. When a positive voltage is applied between the anode and the cathode, it will act as a short circuit, allowing all the current to pass. Otherwise, it will act as an open circuit, not allowing any current to pass.

The graph below shows this behavior. It represents the Voltage x Current curve of the ideal diode. For any negative voltages, the current is zero. However, when applying a positive voltage, the ideal diode acts as a short circuit, allowing any current to pass without causing a voltage drop across the component.

Real Diodes

Real diodes do not behave as simply as ideal ones. Some differences are:

1. Current does not flow at any voltage: Diodes have a minimum voltage to start conducting current, called Vd . This is usually around 0.7V. In other words, even if positive voltages are applied to the diode, if it is lower than this limit voltage, the current will not flow.

2. Real diodes exhibit voltage drops: Ideal diodes, when exposed to positive voltages, act as short circuits and do not exhibit voltage drops. This does not happen with real diodes. Their voltage drops are equal to Vd . This voltage increases in a non-linear manner as the current passing through the diode increases. The graph below shows this behavior. It is noteworthy that the current only begins to be conducted from a certain voltage, in this case approximately 0.5 V.

   

   
   

3. There is a small opposite current: When applying negative voltages, the diode does not completely block the current in the opposite direction, allowing what is called reverse saturation current ( Is ) to pass. This current is usually in the range of μA or nA.

4. Very high reverse voltages damage the component: If the negative voltage is too high, the diode may break down. This voltage is called breakdown voltage. When this happens, the electric field is so high that electrons begin to flow, and this movement breaks the covalent bonds, releasing even more electrons, causing a snowball effect and making the system unstable. This effect is irreversible, damaging the component and preventing its use.

Other Types of Diodes

1. LEDs: LEDs (Light Emitter Diode) are also diodes.

2. Zener diode: Zener diodes behave in the same way as regular diodes when applied to positive voltages. However, when applying reverse voltages, the breakdown occurs at much lower voltages than in normal diodes. Furthermore, this process is reversible, and the component returns to normal operation after the breakdown.

3. Schottky diode: It has the same behavior as a conventional diode. However, it has a shorter switching time (change of state) and a lower Vd voltage drop.