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Put simply, why do some diodes such as most Zeners and Schottky diodes have a glass package as opposed to the more traditional plastic package?

Is it ease of manufacturing, thermal properties, or some other electrical phenomenon?

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2 Answers 2

up vote 35 down vote accepted

Early semiconductor diodes were mostly glass packaged which provided the advantage that they were hermetic and did not depend on passivation of the chip to survive heat and humidity. The glass package also allows a very high operating temperature. Early devices such as the 1N34A (germanium) and the 1N914 as well as the 1N7xx Zener series became very popular and inexpensive.

Plastic-packaged devices were developed to reduce costs where high performance was not so important.

For example, the glass 1N4148 has a maximum junction temperature of 200'C compared to only 150'C for the plastic-packaged 1N4001.

Ceramic packaged diodes have also been produced.

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One think to look out for with glass packaged diodes is their light sensitivity. –  George Herold Jul 28 at 13:38
    
@GeorgeHerold Is it measurable with, say, a 1N4148? I've heard that but never been bitten (yet). I guess now that I use BAVxx all the time it's unlikely to happen. –  Spehro Pefhany Jul 28 at 13:43
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I've never had issues with 1n4148's. (I just stuck one under an incandescent bulb with DMM measuring current, I got ~30nA right at the resolution limit of my meter so a bit suspect, but I'm pretty sure it's real.) (I'm too lazy to pull out the big guns.) I've had the biggest issue with (20V) Zeners that I use in a noise source. I run those right near the knee, and light will just kill all the lovely noise spikes. There the bias currents are in the 1-10 uA range. I'm not sure why the zeners seem to be more sensitive. –  George Herold Jul 28 at 13:55
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I second the realness of light sensitivity, though it is a very small current indeed and significant in only the most sensitive circuits. LEDs make decent photodiodes in a pinch, and I bet (though it's just a guess) that a Zener operating close to the knee shares some mechanism with an avalanche photodiode. –  Phil Frost Jul 28 at 18:48
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@PhilFrost Reportedly a flash photo of an old-style EPROM-based microcontroller (the kind with a quartz window for UV erasure) could cause it to latch up and be destroyed if it was powered at the time. –  Spehro Pefhany Jul 28 at 18:54

Thermal properties. The glass and the semiconductor expand and contract at the same rates. This is for reliability of signal diodes. The expansions or contraction at different rates would cause damage to the semiconductor.

Relevant paper from 1961

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CTE SiO2 = 5e-7, Si = 2.6e-6. In what sense of "Matching" are these the same? –  placeholder Jul 28 at 4:54
    
Sorry, some clarification and some history: electronicdesign.com/archive/… The glass used was of a molecular structure like pyrex. –  Enemy Of the State Machine Jul 28 at 4:59
    
That is from 2001, glass diodes were the first packaging type. So this link is doesn't support what you think it does. –  placeholder Jul 28 at 5:03
    
I must be misunderstanding your comment. I was responding to the thermal expansion difference of the glass used in the process. That is all I was trying to support. –  Enemy Of the State Machine Jul 28 at 5:07
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The paper Is from October 25th 1961. The hypothesis did contain a "may" but the last paragraph states: No devices failed in five months. Diodes tested under 20-V reverse bias and similar environmental conditions did not show any changes after two months of testing, according to IBM researchers." Summing up the results. –  Enemy Of the State Machine Jul 28 at 5:21

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