Understanding the Breakdown Voltage of a Diode
Diodes are fundamental semiconductor components in electronics that exhibit rectifying behavior.
The electrical characteristics of a diode depend on the semiconductor material used to create the component.
The reverse breakdown voltage of a diode is one electrical property that depends heavily on doping and material properties in the semiconductor.
Here’s how to calculate the breakdown voltage of a diode like this simple pn diode.
Modern electronics wouldn’t be possible without semiconductors, and one of the most fundamental semiconductor devices is the diode. Anyone who remembers their electronics 101 classes should remember the function of a diode: to force current to flow in one direction. However, all diodes have a breakdown voltage, beyond which the diode will allow current to flow in the opposite direction (from cathode to anode). This is actually quite important in some applications like bridge rectification, ESD protection, voltage regulation, and much more.
The question now becomes, how high can a diode be driven into reverse bias before breakdown occurs? If you’re a component designer or you’re building a system to interface with other components over a unique signaling standard, the breakdown voltage of a diode becomes quite important. Here’s how the breakdown voltage arises and what it means for your components.
Equations for Breakdown Voltage of a Diode
All diodes exhibit rectification when driven in forward bias, and they exhibit a breakdown behavior when driven at high voltage in reverse bias. At high reverse bias, the exact mechanism that creates breakdown and drives current flow is different in various diodes (pn diode, Schottkey, or Zener). When looking at the equation describing current in a diode, only the forward current is considered; the reverse current resulting from breakdown in the diode is not present in this equation.
Therefore, it’s fair to ask, what is the breakdown voltage of a diode? The answer is normally found in datasheets, but how can this value be calculated? Unfortunately, there is no single breakdown equation for a diode that is applicable to all devices. There are a few factors that determine the breakdown voltage of a semiconductor diode:
Physical mechanism: Two different physical mechanisms dominate in different diodes: avalanche and tunneling. Both mechanisms can occur simultaneously during breakdown but with only one mechanism being dominant.
Doping profile: The distribution of dopants and any grading in the diode will affect the breakdown voltage.
Geometry and structure: The geometry of the diode affects the field distribution, particularly near the edges of the diode.
Because the exact breakdown voltage will depend on these complex factors, there is no single breakdown voltage formula that applies to all diodes. However, there are some empirical results and an important equation from quantum mechanics that can help you understand the breakdown voltage in a diode.
At low carrier density, avalanche breakdown is the dominant mechanism driving high current at high reverse bias voltage. Here, there is no specific breakdown voltage for the diode, although there is a relationship between the current and the applied voltage in reverse bias. This is formulated as a multiplication factor and the total current is the product of this factor and the saturation current.
Multiplication factor for avalanche breakdown in a diode.
In the above equation, n ranges from 2 to 6. At high carrier density, quantum tunneling becomes the dominant mechanism governing charge transport through the diode. This type of transport in reverse bias is called tunneling breakdown.
Tunneling always occurs when a charge carrier (electron) encounters a potential barrier, such as the barrier between p-type and n-type regions in a Zener diode. Once the carrier concentration in a semiconductor heterojunction exceeds ~1017 cm-3, tunneling will become the dominant breakdown mechanism. Here, you can calculate the tunneling current in terms of the applied voltage, but there is no specific breakdown voltage. By calculating the tunneling probability across the junction as a function of the doping profile, one can calculate the current density across the heterojunction with the equation below.
Tunneling current due to breakdown in a diode.
The definition of these symbols can be found in many semiconductor devices textbooks. Note that the potential energy across the junction V(x) depends on the doping profile and the applied voltage, so this equation nicely captures the distribution of charge carriers within the diode. Because this type of breakdown occurs due to higher carrier concentration in the junction region, the breakdown voltage is lower than that corresponding to the avalanche breakdown.
Including Diode Breakdown Voltage in SPICE Simulations
Basic circuit simulators do not always include the breakdown voltage in their electrical models for diodes. The forward current equation is normally used with a typical ideality factor and saturation current value. This is done with the standard diode equation, which can be defined numerically in simple analog circuit simulators.
SPICE simulators take a different approach and use a set of standard electrical parameters to comprehensively describe the behavior of any diode. The values of these parameters can be determined from datasheets or measurements. These electrical parameters include:
Parasitic series resistance
Emission coefficient (ranges from 1 to 2)
Zero-bias junction capacitance
Built-in voltage across the junction
Junction graded doping coefficient (0.33 for linear grading, 0.5 for abrupt grading)
Saturation current temperature exponent
Forward bias depletion capacitance coefficient
Reverse breakdown voltage and current
The best SPICE-based circuit simulators will give you access to standard models for real diodes, such as 1NXXXX diodes. These model files include predefined electrical parameter values. When creating a component model for a different diode, the electrical parameters need to be copied into the model file from measurements or datasheets before simulation. After this is done, the model can be attached to a new component model for standard simulations, such as transient analysis or parameter sweeps. In these models, the reverse breakdown voltage is explicitly included and does not need to be calculated directly using the equation given above.
When you need to perform circuit simulations that include the breakdown voltage of a diode, the front-end design software from Cadence and the PSpice Simulator application let you run a variety of analyses for your circuits. You’ll have a complete set of tools to create and simulate your circuits in the time domain and frequency domain. Once you’ve designed your circuits, you can capture your schematics in a blank layout and start creating your PCB.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.