Diode-connected transistor has negative resistance?

[merlinb]

I needed a CA3086 transistor array but they've got crazy expensive, so I tried subbing a HFA3046. Long story short, it didn't work, but that's not really my question.

I discovered that with the base and collector connected together, the CA3086 behaves like a normal diode like you'd expect.
But the HFA3046 has negative resistance! I have attached my measured DC I/V curves for diode-connected, and for just the BE junction alone.
How is this possible? Has anyone else seen diode-connected transistors exhibit negative resistance? Is there anything on the datasheet I should look for if I wanted to exploit/avoid this feature in future?
Datasheet: https://docs.rs-online.com/8edb/0900766b8002512d.pdf

[RoGeorge]

I've heard of negative resistance when the C and E terminals are swapped (transistor "reversed"):
https://www.eevblog.com/forum/testgear/fooln-around-with-dso-awg/msg4422982/#msg4422982

How does the plot looks like if you reverse C with E?

[mawyatt]

Somethings wrong here, your graph shows a BE only junction conducting ~0.25ma at 400mV Vbe. Unless this was conducted at a very high temperature like 100C, this is not possible with a proper silicon bipolar small junction transistor.

Best,

[2N3055]

Static damage?
These are 8GHz transistors, small geometries, easy to damage.

[merlinb]

Static damage?
These are 8GHz transistors, small geometries, easy to damage.

Not impossible I suppose, though I did get the same behaviour from all four transistors in the package, and I tried two separate packages

[AnalogTodd]

Static damage?
These are 8GHz transistors, small geometries, easy to damage.

Not impossible I suppose, though I did get the same behaviour from all four transistors in the package, and I tried two separate packages

Which curve did you take first? If you are getting as much current as you show for the BE junction, the diode junction should show at least as much current.

[tggzzz]

I needed a CA3086 transistor array but they've got crazy expensive, so I tried subbing a HFA3046. Long story short, it didn't work, but that's not really my question.

I discovered that with the base and collector connected together, the CA3086 behaves like a normal diode like you'd expect.
But the HFA3046 has negative resistance! I have attached my measured DC I/V curves for diode-connected, and for just the BE junction alone.
How is this possible? Has anyone else seen diode-connected transistors exhibit negative resistance? Is there anything on the datasheet I should look for if I wanted to exploit/avoid this feature in future?
Datasheet: https://docs.rs-online.com/8edb/0900766b8002512d.pdf

Show us a picture of your experimental setup, complete with schematic. That can be very informative and help rule out (or in!) some possible causes.

In the absence of a photo, even a written description and hand-drawn schematic are better than nothing.

[PCB.Wiz]

How is this possible? Has anyone else seen diode-connected transistors exhibit negative resistance?

Something is awry in your measurements.
Take the plots in two directions up and down, as the two way sweep can expose hysteresis from effects like self heating.

The using-collector curves should always be lower in voltage / higher in current than base only connection - see their data. Zero negative resistance there .
NPN VBE junctions are very well behaved, but GHz RF parts will be very small and very good at picking up stray RF.