Types of Diodes: Current Flow, Functionality, and Characteristics

Key Takeaways

● A diode allows current to flow in one direction only. Forward-bias enables current flow, while reverse-bias blocks it entirely.

● The forward voltage drop is the minimum voltage a diode needs before current can pass through, typically around 0.5 volts - 0.7 volts.

● Learn about how changes to diode biasing determine whether they function as insulators or conductors.

A diode’s biasing affects current flow.

The diode is one of the simplest yet most essential components in electronics. It allows current to flow in one direction and blocks it it the other, a straightforward function that makes it incredibly useful across a wide range of circuit designs. Unlike many other components, a diode operates in one of two clear states. It either passes current through or blocks it entirely.

Description of a Diode

As stated, the current flowing through a diode can only go in one direction, and we call this state forward-bias. Since the current can only flow in one direction, we unofficially think of diodes as one-way electronic valves. If the voltage across a diode is negative, then no current will flow; thus, an ideal diode appears as an open circuit.

The conditions or states a typical diode can exist in are forward-bias or reverse-bias. In electronics, we define bias or biasing as a method of establishing a set of currents or voltages at different points of an electronic circuit to set proper operating conditions within an electronic component(s). Although this is a simplified version of the answer, it is still fundamentally correct.

A diode consists of two semiconductor materials, a P-type and N-type; known as the p-n junction. At the boundary between these two materials lies the depletion region, a zone where free charge carriers are absent, creating an internal electric field that governs how and when current can flow. Leads at each end make the diode easy to integrate into virtually any electronic circuit.

Diode Functionality

We call the lead affixed to the N-type semiconductor the cathode. Therefore, the cathode is the negative side of a diode. In contrast, we call the lead connected to the P-type semiconductor the anode, which makes it the positive side of a diode.

When we connect a voltage source to a diode so that the positive side of the voltage source links to the anode and the negative side connects to the cathode, the diode acts as a conductor, thus allowing current to flow. When we connect voltage to a diode in this direction, we call it forward-bias.

However, if we reverse this voltage direction, i.e., connect the negative (-) side to the anode and the positive (+) side to the cathode, the current will not flow. At this time, the diode acts as an insulator. When we connect voltage to a diode in this direction, we call it reverse-bias.

Note: Although while in forward-bias current flows and reverse-bias it does not, there is a maximum limit to the level of current a diode can effectively block.

The Two Regions of a Diode

We briefly discussed the two semiconductor regions in a diode (P and N). However, it is also crucial that you differentiate between the sides or semiconductor regions as well.

Firstly, about the symbol representing a diode schematically, the cathode is on the right side and the anode is on the left side. As a rule, consider the anode side of the schematic symbol as the arrow depicting the standard direction of current flow, i.e., positive (+) to negative (-). Hence, a diode permits the flow of current in the direction of the arrow. And then consider the vertical line on the cathode side as an enormous minus sign (-), depicting which side of a diode is negative for forward-bias.

Diode Current Flow Functionality

A standard diode requires a precise amount of forward voltage before it permits the flow of current. This threshold is typically around 0.5 volts for a standard silicon diode. Until the voltage reaches this level, the diode remains off and no current passes through. Once the threshold is met, current flows freely.

This minimum voltage threshold is called the forward voltage drop. It represents the voltage the circuit loses at the diode during operation. You can measure it directly by placing a multimeter across the diode's leads while it is in forward-bias. The resulting reading will be the diode’s forward voltage drop.

To further illustrate, we can use the above schematic circuit diagram. Measuring across the lamp's terminals with a multimeter will show the difference between the battery voltage and the diode's forward voltage drop. For example, with a battery voltage of 12 volts and a forward voltage drop of 0.8 volts, the lamp receives 11.2 volts.

Note that that the forward voltage drop varies between diode types. Schottky diodes, for instance, have a lower forward voltage drop of around 0.2 to 0.3 volts, making them a better choice in circuit where minimizing voltage loss is a priority. Understanding this difference is important when selecting which type of diode is right for your specific application.

Types of Diodes

Not all diodes are built the same. The most common types of diodes each serve a specific function in circuit design:

Diode Characteristics

Every diode has a maximum reverse voltage that it can handle before breaking down and allowing reverse current to flow. We refer to this reverse voltage as peak inverse voltage (PIV) or peak reverse voltage. This is one of the most critical specifications to check during component selection.

Exceeding the PIV rating even briefly can cause permanent diode failure. To stay safe, designers typically select a diode with a PIV rating well above the maximum voltage expected in the circuit. Along with its PIV and forward voltage drop rating, a diode also has a maximum current rating. This is the highest operating current the diode can handle without sustaining damage.

Pushing beyond this limit causes excessive heat build up inside the diode, which can lead to thermal runaway and ultimately destroy the component. In some cases, a failed diode can also damage surrounding components in a circuit. This is why it is good practice to always build in a safety margin when selecting a diode for a design, rather than operating at the absolute limits of its rating.

Despite its simplicity, a diode delivers the functionality of two distinct components in one. It acts as either a conductor or an insulator depending on how it is biased. The wide array of applications for a diode, from rectifier diodes in power supplies to zener diodes in volage regulator, includes a virtually endless list of electrical device applications. In summary, a diode is a genuinely adaptive component that affords designers optimal control on what function a diode will play in their circuit design.

A set of diodes in various shapes and sizes, but they all share the same current flow characteristics.

Successfully implementing a diode into your design with appropriate diode current flow characteristics requires that you use a high-quality PCB design and analysis software. Allegro X gives you the design and simulation tools to implement diodes and every other component with precision and confidence. To learn more about our innovative solutions, subscribe to our newsletter or our YouTube channel.