Diode Current Flow: Functionality and Characteristics

October 29, 2020 Cadence PCB Solutions

Key Takeaways

● Learn about diode functionality.

● Gain a greater understanding of the characteristics of diode current flow.

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

 A diode in an electronic circuit board

A diode’s biasing affects current flow.

In comparison to the myriad of electronic components we encounter in the field of electronics, the diode is a relatively simple component. Fundamentally, a diode is a component that permits current to flow in a single direction and blocks it in the other direction. Diodes allow current to flow in one direction without the effects of any impedance, while entirely blocking all flow of current flow in the other. Furthermore, there is a clear designation between these two states of operation.

The 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 (forward-bias), 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 is an electronic component consisting of a P-type and N-type semiconductor material; we call the p-n junction. It also has leads connected to these two ends that afford ease of implementation 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. In general, the specified amount of voltage a diode requires before allowing the current to flow is minute. Typically, this equates to 0.5 volts. Until it achieves this voltage amount, the current will not flow. However, upon attaining the forward voltage, the current flows with ease through the diode.

We call this minimum voltage threshold in the forward direction the diode’s forward voltage drop. The reason for this is because a circuit loses or drops this voltage at the diode. We can examine this by using a multimeter and measuring across the leads of a diode while it is in forward-bias. The resulting reading will be the diode’s forward voltage drop.

circuits schematics showing diode current flow in forward-bias and reverse-bias

To further illustrate, we can use the above schematic circuit diagram. When we utilize a multimeter to measure across the lamp’s terminals, the voltage will be the difference between the battery’s voltage (12 volts) and the forward voltage drop of the diode in the circuit. For instance, if the forward voltage drop of our diode is 0.8 volts and the battery voltage is precisely 12 volts, then the voltage across the lamp will be 11.2 volts.

Diode Characteristics

A diode has a maximum reverse voltage that it can endure before it breaks down, thus allowing the reverse flow of current through the diode. We refer to this reverse voltage as peak inverse voltage (PIV) or peak reverse voltage. Furthermore, it is an essential specification of a diode in terms of circuit functionality. It is imperative that you do not subject any diode in your circuit design to voltages that exceed this limit.

Along with its PIV and forward voltage drop rating, a diode also receives a maximum current rating. As this rating suggests, it is the peak operating current for the diode, and exceeding it will result in irreparable damage to the diode and possibly the circuit as a whole.

The diode as a component is relatively simple, but it affords the functionality of two distinct components in one. The wide array of applications for a diode 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

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, by Cadence, is one such software with a plethora of robust features for layout as well as testing and simulation. 

If you're looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.


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