Diodes Appendix A

Factors and Considerations

The Depletion Region

Reverse Voltage and the Depletion Region explained.

Reverse Breakdown Voltage

Mechanisms for typical planar and deep-diffused diode junctions.

Reverse Leakage

Factors influencing the amplitude of the reverse current.

Creating High-Voltage Diodes

Three approaches...

Multi-junction Reverse Recovery Time

Preventing fastest diode from "seeing" all reverse voltage while other diodes are recovering.

Conclusion

HIGH VOLTAGE! The sight of this familiar warning jars the senses and triggers cautious movements in even the most experienced engineers. High voltage is a relative term, though, and warnings are similar whether applied to 100 volts or 100 kilovolts.

High voltage rectification is also a relative term. It requires understanding from designers, as it surrounds our lives in CRT supplies, TWT supplies, X-Ray supplies and a host of other devices.

The ideal high-voltage rectifier is a single-junction diode that blocks the required voltage quickly without leaking reverse current, generating no heat from forward power losses. Real world rectifiers, however are complex trade-offs. The basic choice is between multi-junction high voltage silicon diodes and strings of single-junction silicon diodes.

Before a decision between single and multiple-junction diodes can be made, it is necessary to examine the fundamental characteristics of PN junctions. Two common techniques for fabricating high-voltage PN junctions- planar and deep-diffused- are depicted in Figure 1 below.

Many variations of each technique exist. Severe angles, for example, are often present on a deep-diffused junction, creating a "mesa" appearance. Also, epitaxially-grown silicon may be used in either technique and a variety of dopants can be used.