Understaing parameters of transistors for swtich mode

[anso-engineer]

Hello,
I am trying to investigate it byself, but a some things is not revealed. On the examples of 2N2222 and IRF540 datasheets, they are pretty common.
1) BJT is current contolled devices, but where in datasheet the parameter that allows to understand at which point trasistor goes opened?
View attachment 148571

According this picture, probably it's forward active mode.
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?

 

[rjenkinsgb]

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.

 

[alec_t]

Welcome to ETO!

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

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.

 

[danadak]

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 :