I need to make a test PCB for an LED drive circuit that can drive a variety of different single LEDs. The forward voltage (Vf) of the LED could be anywhere from ~2.2V up to ~6.5V. I'd like the test PCB to be able to simulate the range of possible Vf.
Rather than using switches (likely FETs) and putting a variety of LEDs on the test PCB, it would be great to find something (a device, a circuit) that is a "variable Vf diode" or "voltage controlled diode." Does such a thing exist?
My searching and (limited) EE knowledge so far suggests that an PN junction (a diode) gives a constant Vf, that adding another junction (BJT transistor) allows for current but not Vf control, and that adding a 4th junction (thyristors, SCRs, triacs, ...) doesn't help either. A FET can be thought of as a voltage controlled resistor and a Varicap is a voltage controlled capacitor, both of which are close but not what I need.
For what little I know about device construction, it seems like there should be a device that uses a voltage or current to bias up a PN junction, changing the width of the depletion region and therefore changing the bias required (Vf) to "punch through" with carriers into conduction. Then again, this sounds a lot like a BJT, which I don't think will work.
I guess putting a heater under a diode could give some Vf control, but that seems like it would have poor control (and be inefficient and sort of silly).
It feels like I'm missing something obvious here. Any thoughts on devices or circuits that can function as a voltage-controlled, forward-voltage diode?