# Mosfet or bjt choice for low voltage joule thief application

I make solar energy harvester using joule thief circuit with a small solar cell as the input. The output voltage from the solar cell is 0,5V. But the generic transistor, exactly silicon material need for 0,7V to start conducting.

Is there any transistor type (mosfet or bjt) that work with 0,5V or lower for my project ?

Thanks you,

Indeed 0.5 V is too small to do anything useful with a BJT.

For MOSFETs the threshold voltage $$\V_t\$$ matters, the ones with the lowest $$\V_t\$$ I can find have $$\V_t\$$ between 0.3 V and 1.0 V, for example the Si2334DS.

That 0.3 V sounds good however that is the minimum value, the manufacturer guarantees that $$\V_t\$$ is always 0.3 V or higher (but less than 1.0 V). This does not mean that you can buy a MOSFET which has a $$\V_t\$$ of 0.3 V. Typically the $$\V_t\$$ will be around 0.65 V I guess, so that will not be more usable than a BJT.

There are solutions which can work on a voltage of 0.5 V but these use a dedicated power harvesting IC, for example the BQ25570 but do not that even this IC needs at least 0.6 V to start up. Once started it can work down to 0.1 V at its input.

Do note that a BQ25570 solution is significantly more complex than your "Joule thief" idea.

You really need a bit more voltage 0.5 V to do anything useful at all. Can't you use two solar panels in series?

• There are other integrated solutions that have ultralow startup voltage. For example LTC3108. From datasheet: minimum startup voltage 50mV, but it uses some fancy transformer and I don't even understand how this works (but I'm sure it does work, if engineers from LT said so).
– dim
May 22, 2019 at 15:16
• The threshold voltage of a mosfet is almost entirely arbitrary and can even be trimmed to an extent by the end user. But it’s best to leave that to the manufacturer. You can buy mosfets with very closely controlled threshold voltages that can be positive, zero, or even negative. They cost more, but you get IMHO good value for the money. They are specifically processed for that though, and they allow very low voltage logic, etc. Aug 12, 2022 at 3:00