Junction-Path Shorts

Preventing failures and improving reliability by preventing shorts through semiconductor junctions.


Problem -Failure due to current surges through semiconductor junctions.
Relevance - Occurs in environments of high ionizing radiation, high electromagnetic fields, lighting and transients, energizing circuits through relays, and rapid sequencing of turn-on and turn-off.
Solvability - Identify paths through semiconductor junctions and add impedance.
Solution - Solution details.
Personal - A personal anecdote.
On the Web - Additional information on the Web.
References - One to a few key papers.


Small signal semiconductors and integrated circuits fail due to paths between power sources or power sources and ground that have insufficient resistance or inductance, or rely on back-biased semiconductor junctions to limit current.



This problem occurs most frequently in environments of high ionizing radiation, high electromagnetic fields (circuit in beam of radar transmitter, etc.), and unusual transient environments such as lightning, energizing circuits through relays with contact bounce, or turning a system on and off in a time span less than the normal settling time of the circuit. Large capacitors should be treated the same as power sources.

Ionizing Radiation. During high levels of ionizing radiation, all semiconductor junctions are effectively shorted. If there are only semiconductor junctions between voltage sources, with no resistance or inductance to limit the current, sufficient current can flow to damage circuit components.

Some circuit environments produce the same results.

Transmitters. High fields of radiation (transmitters, radar beams, etc.) can induce energy into a circuit that is rectified by nonlinear circuit elements and then forward-bias normally-off junctions.

Energizing Circuits. Turning a circuit on and off rapidly can forward bias a normally back biased junction and allow current to flow.

Relay Bounce. Can have the same effect of rapidly turning a circuit on and off.

Lightning. Lighting and Electromagnetic Pulses transients can also cause the problem.



The problem is solved by identifying paths without resistance and adding resistance or inductance in series with the path.



Find Paths. Examine the schematic. Find if there are any paths between voltage sources or voltage sources and ground or through semiconductor junctions with no discrete resistance to limit current. Integrated circuits should be considered a semiconductor junction. Capacitors large enough to store sufficient energy to damage connected circuits should be considered a voltage source.

PWM IC. One common sneak path is the transistor inside a PWM controller IC that discharges the timing capacitor. If the timing capacitor has enough energy, it can damage the integrated-circuit transistor if an external current limiting resistance is not used.

Current Limits. If no information is available on damage thresholds or surge currents for the semiconductors in the path, use the peak current on Safe Operating Area (SOA) curves, or if this is not available, limit current to less than 4 times the maximum continuous current rating. The weakest semiconductor in the path determines the resistance.

Avoiding Instability. If resistance is placed in the power lead of an integrated circuit to limit current, care must be taken that it does not cause oscillation. Often, a small decoupling capacitor may be necessary.

Radiation Hardening Correlation to Reliability. These current-limit techniques are used in circuits exposed to high levels of ionizing radiation which can short all semiconductor junctions during the period of radiation. It has been found that there is a strong positive correlation between field reliability and circuits so designed, even if they are never exposed to ionizing radiation.


Personal Anecdote

The first power supply I ever designed for production was a transistor series regulator. Out of the first 100 fielded, 18 failed within three months. The problem was a path through semiconductor junctions that depended on back-biased junctions for impedance.

In an all-parts change, a 10 ohm resistor was added into the collector of a transistor to break up the path. With several thousand units in the field for the ten year life of the product, there was never another failure.

This rather traumatic experience motivated me to become an expert in reliable power supply design. I hope to share much of what I have learned throughout this hypertext. This was one of my first and most important lessons - learned the hard way. Needless to say, looking for low impedance circuit paths through semiconductors is one of the first thing I look for in a design or design review. It has saved myself and many others much grief.

Besides damaging circuit components, inadvertently forward-biased junctions can cause system malfunctions. I received a letter of commendation for solving this problem with a hydrofoil patrol boat. The boat would crash down off the hydrofoils onto the hull anytime the crew transmitted a message or flushed the toilet. The problem with the transmitter was that the high-frequency-ac radiated signal was picked up and rectified by a diode to a dc voltage. This voltage appeared across the gate of an SCR in the hydrofoil control system protection circuit, activating the circuit. The toilet flushing problem was an unsuppressed inductive kick when the pump motor flushing the toilet turned off, thereby activating the control system protections circuit. Desensitizing the control system protection circuit solved both problems. The toilet pump motor was also interesting. An aircraft type toilet, it was suppressed assuming the grounding system used in aircraft, not ships. Adding suppression techniques suitable for ships also solved the problem with the toilet pump motor. The lesson learned? Normally back-biased junctions can be turned on by the environment and you have to protect against the consequences of this happening.


On the Web

To date, I have found no information on this subject on the web.



I learned about current surges through back-biased junctions the hard way (personal perspective above). The only reference I've seen related to these techniques is a 1970-era General Electric transistor manual that gives the ground rule that resistance should be placed in two of the three leads of every transistor. There is a literature on pulse overstress testing and rating of semiconductors that also may contain relevant information.

The hydrofoil patrol boat problem was solved by the application of normal electromagnetic interference (EMI) trouble-shooting techniques described in EMI books such as the book by H. W. Ott and other EMI books in the booklist, combined with a knowledge of the trouble caused by inadvertently forward-biased junctions.

Webmaster and editor: Jerrold Foutz
Original: March 12, 1996, revised March 24, 2004