Are screened transistors an endangered species?

Question

In the germanium transistor era, the RF transistors were PNP small signal devices. Some of these small metal cased BJTs had 4 leads instead of the usual 3. The metal can was isolated and the 4th lead was called the screen and was connected to an internal screen. Common european example part numbers of such devices are:

• AF116
• AF117
• AF126

I have even seen such transistors in early solid state MW AM car radios, and the popular Phillips 1960s electronics set had an AF116.

Sure there were some reliability issues with the screens shorting due to tin whiskers but they were not that bad. The long leads of the day would undermine the effectiveness of the screen at HF but the lead could be cut shorter.

• Why don't they make SMD screened Si or GaAs or GaN devices today?
• Is a screen expensive to manufacture?
• Would there still be reliability issues?

Answer 1

The normal approach for small SMT components is to put a single big shield or "can" over a number of components:

(image from this link)

Answer 2

They did make it into the silicon era ... just. Look for any transistor in the TO-72 package for example : it's a 4-lead version of the TO-18, with the extra lead connecting to the case.

A few examples : BF115 (perhaps following the AF116?) but also BF173, and 2N4416 JFET.

And the 2N5179 - still (just?) available from Farnell.

I believe what killed them was the move away from metal cans (TO-18, TO-72, TO-5 etc) to epoxy (TO-92 etc) packaging, where screening would cost extra.

Inside those shiny cans in pjc50's answer you will often find further internal partitions, between stages, so the idea hasn't gone away altogether.

Answer 3

RF screening makes sense if any of the elements come even close to a length where RF of relevant frequencies can couple in.

For SMD components, that would only happen in the millimeter wave regions.

For millimeter wave circuits, you basically need to consider your whole circuit including the circuit board as RF element that you'd want to simulate to know what's happening: you'd not expect any "stray" radiation coming from the outside, since you'd typically RF-encase the whole circuit, and no stray radiation from the circuitry itself, since everything needs to be designed as transmission lines rather than simple current-carrying traces, anyway.