Understaing parameters of transistors for swtich mode

Both bipolar transistors and FETs are proportional devices.

The collector current in a bipolar transistor will be the base current * the gain (hFE) of the device.
(The hFE is given in the data, for a typical collector current. It tends to fall at high currents).

If that level of current is great enough to apply close to the full supply voltage across the load, the transistor will "saturate", so more base current will have little to no effect.

eg. If a transistor had a gain of 10 at 100mA, operating on a 10V circuit with a 100 Ohm collector load:

With 5mA base current, the collector current would be around 50mA, so about 5V across the load resistor and 5V between emitter and collector.

For switching applications, such as in switched-mode PSUs or logic, using anything up to 10x more base current is not uncommon, to ensure the lowest voltage drop across the transistor while it is conducting - though a saturated transistor switches off slower than one operated in its linear range.

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anso-engineer said:

where in datasheet the parameter that allows to understand at which point transistor goes opened?

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There is no single point. For a BJT, base current, and hence collector current, vary non-linearly as Vbe changes. For a FET, drain and source current vary non-linearly as Vgs changes.

Saturation behavior in bipolar looks similar to this, varies transistor to transistor,
but general behavior as follows :




A bipolar transistor is fundamentally a V controlled device, although most often used model
is as a current controlled device.

The defining equation for collector current is :

anso-engineer said:

2) MOSFET, mainly the same question in respect that they are voltage controlled. How I can understand how much voltage should be applied to the gate for it opens?

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For the MOSFET to be fully conducting as a switch you want the minimum on-resistance.
This is shown below (bottom row) for the IRF540 as 0.077Ω with a Vgs (gate-source) voltage of 10V, so 0V and that voltage are typically used as a switching control voltage.
Logic-level type MOSFETs that have a lower Vgs(th) can be switched with a lower voltage (typically 5V or less).