Uncle Josh's NOS Garage

Drawer

Semiconductors

50 parts in this drawer

Silicon does not wear out sitting still. A transistor in a sealed factory reel is the same device it was when it was packed, with no chemistry quietly running down in the dark. That makes them the easiest thing in this catalog to buy with confidence, and moves the risk elsewhere: to whether the part is what the marking says, and whether the package survived storage.

What is in this drawer. Discrete small signal and medium power devices, in surface mount and older leaded packages. The Diodes FZT753TA is representative: a PNP medium power transistor rated 100 volts and 2 amps in SOT223, originally a Zetex part number and still made under the Diodes name after the 2008 acquisition. Parts like it go obsolete because the design that used them ended, not because anything better appeared.

How semiconductors go bad in storage. Three ways, none of them the die. Solderability is the common one: tin plated leads grow oxide, and a part from 1985 can be good silicon that will not wet. Fresh flux fixes most of it and some parts need retinning. The second is moisture in plastic packages, which matters if you reflow parts that sat outside a dry pack, because it flashes to steam in the oven and cracks the body. The third is handling damage, which is why anything MOS gated goes back in the antistatic bag.

Reading the number. On FZT753TA the FZT prefix is the Zetex SOT223 medium power family, 753 is the device, and TA is a packing code meaning tape on a particular reel size. That last pair is packaging and not a different transistor, which is worth knowing before you reject a substitution over it. Older American numbers work differently: a 2N prefix is a JEDEC registration, so the specification belongs to the number and every maker of it meets the same figures.

Condition here. Sealed is the grade most of this drawer carries: factory tape or tube, never opened, date code intact. Open stock means the reel was opened or a quantity was cut off a larger strip. Tested means we put it on a curve tracer and checked gain, breakdown and leakage before listing. Pulls are graded separately and always identified as pulls, because a desoldered transistor has taken a heat cycle nobody documented.

On arrival, check the marking against the datasheet you are working from rather than against the invoice. Look at the leads under a light for oxide and at plastic bodies for cracks at the lead frame. Keep the antistatic packaging. Hand soldering, fresh flux beats any amount of cleaning. Reflowing an opened reel, bake it dry first.

Questions about semiconductors

Are old transistors still good, or does silicon degrade?

The die itself does not degrade in storage at room temperature. What ages is the outside of the part: lead and pad solderability, and moisture absorbed into plastic packages. Both are handling problems with known fixes. A sealed 1990s reel is electrically a new reel.

What does the TA suffix on FZT753TA mean?

It is a packing code, not a device variant. FZT753 is the transistor and TA describes how it was supplied on tape and reel. If a design calls for FZT753 and you are offered FZT753TA, the silicon is the same. Check the manufacturer datasheet for the specific suffix table rather than assuming across makers, because the letters mean different things in different families.

How do I know the marking is genuine?

Provenance and packaging do most of the work. We buy sealed factory packaging where we can and we say plainly when we could not, and the date code, the lot marking and the reel label all have to agree with each other. Where a part arrived loose or in an unlabelled bag we grade it lower and we say what we know about where it came from. We do not decap or X ray, and we will not tell you a loose part is certainly authentic.

Do I need to bake these before reflow?

If the tape or the dry pack has been opened and the part is a plastic surface mount body, yes, treat it as moisture sensitive and bake it per the manufacturer level before you run it through an oven. Parts still in an unopened dry pack with a live desiccant are ready to run. Hand soldering does not put enough heat into the body fast enough to matter.

What is the difference between a pulled part and new old stock?

A pull was desoldered from equipment, so it has taken at least one uncontrolled heat cycle and its leads have been cut and reworked. New old stock was never installed. We never mix the two in one listing, and pulls carry either a tested grade, meaning we verified function, or an untested grade, meaning it is priced as a gamble and described as one.

Can you substitute an equivalent if the exact number is gone?

Tell us the voltage, current, gain range and package you need and we will say what is on the shelf that meets it. What we will not do is relabel a near equivalent as the part you asked for. If the closest thing we have is close but not identical, the listing will say exactly how it differs and the decision stays yours.