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How the PN Junction Depletion Region Impacts Your Board Design

Key Takeaways

  • What is the PN junction depletion region? 

  • What factors affect the PN junction depletion region?

  • How does the PN junction depletion region affect your board layout?

A man playing the accordion

Synchronizing the accordion.

As a fan of all kinds of music, I am often curious as to why some musicians choose to play less common instruments. Take the accordion, for example; the heavier ones weigh nearly as much as tubas, which weigh 13.6 kg on average. In addition to this physical burden, the accordion player must also independently play two sets of keys or buttons and move the bellows back and forth. 

Mastering these synchronizations seems to makes learning to play other more common instruments easy.  It may appear to a layperson that varying the bellows is the least significant action; however, this is not the case. Moving the bellows in and out actually controls the volume from the instrument. 

Interestingly, the flow—or amount of current—that proceeds from some of the most commonly used electronic components is similarly controlled.  For semiconductor-based devices used in electronic circuitry, current flow is controlled by varying the size of the PN junction depletion region.  In this article, we examine this region, its operation, and how it can influence your PCBA design. 

The PN Junction Depletion Region

Probably, the simplest semiconductor device used on circuit boards is the PN junction diode, which is comprised of p-type (positively charged) and n-type (negatively charged) sections. This junction may be created by the methods listed below.

PN Junction Creation Methods

  • Implanting ions

Injecting impurities into a crystal structure, or doping.

  • Diffusion

The forced movement of a higher concentration of electrons or holes to a region of less concentration.

  • Deposition

Growing one layer on top of another that has a different doping profile. 

The result of the above actions is the formation of an interface between the p-type and n-type areas known as the PN junction depletion region. 

How PN Depletion Regions Operate

The PN junction depletion region may be referenced using a myriad of terms, such as depletion zone, depletion layer, junction region, or space charge region. These monikers all refer to the area within a PN junction that separates the p-type and n-type sections, which due to the free holes and electrons are capable of carrying current. The depletion region, however, is characterized by the lack of these “free” charge carriers. Typically, these carriers are removed as electron flow from negative to positive, or conversely for circuit analysis, current flow in the opposite direction due to an electric field, as shown in the figure below.

Electrical operation of PN junction

Operation of a PN junction depletion region.

The width of this insulation area is defined by the strength of the electric field, material type(s), and the level of doping, as shown below:

PN depletion width equations.

Where xis the distance from the region center in the n-type concentration direction,

      is the distance from the region center in the p-type concentration direction,

    𝜺s  is the material permittivity,

    q is the electron charge = 1.60217662  x 10-19 C,

    NA  is the acceptor (p-type dopant) concentration,

    ND  is the donor (n-type dopant) concentration

and ΔV is the voltage potential across the region, which is related to the electric field, E, as shown below. 

For a specific component, the PN junction region depletion width can be changed by modifying the voltage or E-field strength, which in turn affects trace current density and, thus, impacts your board layout design.        

Using PN Junction Depletion to Aid in PCBA Layout

One of the most important attributes when analyzing PN diodes is the operation mode. The mode or transition of operation is indicative of the state or change in the E-field strength, voltage, and current flow. And, all of these parameters should be considerations for your PCBA layout, as listed below.  

PN Junction Depletion Region Attributes That Impact PCBA Design

  • E-field 

Electric fields are a source of radiation. Depending upon their strength and the proximity of the radiating component to other board elements, they can contribute to interference, such as stray capacitance. This is more of an issue for high-speed board designs.

  • Voltage

The depletion region voltage typically defines the barrier that must be exceeded to change from an OFF to an ON state for PN junction components. For example, simple silicon diodes have a depletion region voltage of approximately 0.6-0.7V and will block current flow in the forward direction until the voltage across the component exceeds this level. Additionally, the voltage is dropped across the device and is not available for any attached load.

  • Current

Ideally, a PN junction can conduct any amount of current once biased appropriately, which means that the depletion region width effectively reduces to zero. Practically, an infinite current is not possible, however, depending upon the material and dopant concentrations, the current flow through a PN junction device can be quite large. This limit has to be a factor when choosing trace parameters such as copper weight, width, and length.

As the list below shows, the PN diode depletion region attributes help in defining limitations or constraints for your board layout. 

 Allegro’s Constraint Manager menu

PCBA layout constraints management.

The selection and implementation of the right rules and constraints for your PCBA layout is critical and requires advanced tools, such as those found in the Allegro Constraint Manager shown above, in order to optimize the process and ensure board manufacturability. 

This tool is standard with Cadence’s PCB Design and Analysis Software. In addition to real-time design manufacturability checking with Allegro PCB Editor, you can examine the electrical behavior and impact of linear and nonlinear circuits and components whose operation is based on PN junction depletion region attributes as you prepare to create your PCB layout.

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