4-Legged SMD Transistors

[ParkingLotLust]

I took apart an old Starchoice Satellite LNB and came across some 4-legged smd can transistors, so I removed them, and now Im looking for specs (or even an idea as to what they are!). Whats the fourth leg for?

The only markings on the black ones are a "D" and on the silver ones there are two dots, and a half-circle indicating a certain pin. Also, one leg is wider than the other three.

Best closeup I could get:
**broken link removed**

 

[Nigel Goodwin]

Dual-gate MOSFET's used to be a common RF component - also many RF transistors used to have four connections, but two of them were to the same terminal, it was done to improve stability and make layout less critical.

 

[ParkingLotLust]

What would be the easiest way to test if its a BJT or a type of FET? I know how to test bipolar transistors with the diode function of a DMM, but what would a reading for a J-FET or MOSFET look like?

 

[kchriste]

For bipolars, you can check with an ohm meter with diode check:
Two of the contacts, on opposited sides, should be the emitters and are joined internally. The longer lead (or cut at an angle) is usually the collector. The remaining one is the base.