Comprehensive PN Diode Analysis with PSpice

Key Takeaways

• What is a PN Diode?

• What are the PN Diode operation modes? 

• Analyzing PN Diodes with PSpice.

The basics are the foundation of success.

In most cases, listening to a professional athlete give a post-game interview is a lot like eating cotton candy. It may occupy a lot of space (or time), but afterward, you still feel empty. With this expectation, I half-heartedly began to listen to NBA Hall of Famer Allen Iverson admonish a reporter for questioning his effort and attendance at team practices. At one point, he stated apparently in amazement, “We're talking about practice, not a game, not a game, not a game, we’re talking about practice.” This departure from the normal bland responses for these interviews was definitely entertaining. Yet, Mr. Iverson had obviously forgotten at that moment that practices are held, at least in part, to reinforce the basics, which are the foundation upon which success in the game, when it counts, depends. 

Designers and engineers do not have the luxury of practice when designing circuit boards. Nevertheless, the success of your design depends upon the utilization of basic electronic circuit design principles. Today, for many PCBAs, this means knowing how to determine and effectively leverage the operational characteristics of fast switching transistors.  Although transistors come in many types and may themselves be implemented as foundational components for other ICs, modules, and devices, their construction and functionality can be traced back to the PN diode. Let’s go back to the basics of this diode’s operation, which will serve only to enhance your design of boards that use this component and its many derivatives.

Diode Basics

When thinking of basic electrical circuit elements, the resistor, capacitor, and inductor are typically the first to come to mind. Yet, for the majority of circuit boards designed and built today, the diode—or one of its derivatives—is almost as common. This is especially true for PCBAs that include digital circuitry that requires switching, which is the primary function of diodes. 

Nevertheless, there are many types of diodes, including Zener, Schottky, Avalanche, Laser, LED, Schockley, and others. Although specific diode types may be used for different applications, their basic functionality (as well as that of transistors) can be traced back to the discovery of the PN junction, shown in the figure below, in the 1940s by Vadem Lashkarev.  

PN diode representations.¹

 ¹Image from Energy Education at https://energyeducation.ca/encyclopedia/Diode_operation. Last accessed 10/19/20.

This discovery included an understanding of hole-electron diffusion, whereby the introduction of carriers into a depletion region between negatively and positively charged metals can create a flow of negatively charged electrons based on the availability of “holes” to receive them. Semiconductors—such as silicon and germanium—that are primarily used today, also exhibit this electron flow, although only outer electrons of the crystal lattice structure flow or move between holes. The opposite of this movement of electrons is what is known as current flow, which proceeds in a positive-to-negative direction. 

The fact that the amount of current flow can be controlled by the amount of injection of carriers is profound. In fact, this is the basis for fabricating different types of PN junction-based devices that exhibit various operating conditions. And in conjunction with the voltage across the device, the mode of operation for the diode is controllable, as discussed below.

Diode Operation Modes

For the basic PN diode, there are two states or modes of operation. These are forward-biased, where current flow occurs almost unimpeded, and reverse-biased, where current flow is blocked. This operation is why diodes are often referred to as switches. During forward bias, the depletion region decreases in width to allow for more transfer of carriers. During reverse bias, the region expands to minimize or stop transfer. Switching from forward to reverse bias, and vice-versa, is controlled by the voltage that is placed across the diode’s cathode to anode, as shown below. 

When analyzing a diode, the relationship between the current through and the voltage across the device is the key attribute. This I-V characteristic defines the operating point of the diode and the range of operation within the modes.  

Using PSpice to Analyze a PN Diode  

The most common method of analysis for a PN diode is to utilize the Shockley diode equation, shown below.

This equation defines the diode current in terms of the saturation current modified by an inverse logarithmic function that changes based on the relationship between the diode voltage and the thermal voltage. This expression assumes that the diode voltage (numerator) is much larger than the thermal voltage multiplied by 1 or 2 (denominator), which is the value of eta for silicon.  The thermal voltage is typically assumed to be approximately 26mV, defined by kT/q, for nominal temperatures where

    k is the Boltzman constant, 1.380649 x 10-23 J/K,

    T is the temperature in Kelvins and

    q is the standard charge, 1.60217662 x 10-19 C.

This result of analyzing this equation is the diode’s I-V characteristic. For silicon diodes, forward bias is achieved when the diode voltage reaches approximately 0.67V. At this point, the diode “turns on” and the current can be increased to a significantly high level, limited only by the diode’s construction. 

The equation above requires that the saturation current be a known quantity or that various characteristics be calculated with Vs varying over a range of likely or possible values. The most efficient and accurate means of solving these equations and obtaining a comprehensive PN diode analysis is by using a graphical simulation tool, such as PSpice. 

Obtaining explicit solutions such as the above require that you have advanced PCB Design and Analysis capabilities. With Cadence’s PCB design software, PSpice is integrated, which makes determining the actual performance of your diode-based components before committing to your schematic design simple and easy.  

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