Question

Perform an internet information search on the terms “Bipolar Junction Transistors” (or “BJT”) and “MOSFET”. Extract relevant information and write a ONE-PAGE explanation on how one can use such devices for building amplifiers and digital logic switches (or “gates”). Which device, BJT or MOSFET, is more suitable for ultra-low power applications? What is the main reason?

Answer 1

Bipolar Junction Transistors:

BJT is three terminal device which tranfer the resistance through bi junctions. Let us take a pnp transistor and see how it opearated as amplifier and switches.

The BJT has two depletion regions/two junctions look like a two PN diodes connected back to back.

Now connect BE depletion in reverse bias by connecting - terminal to Emitter and connect Vcc to base to collector junction. which leads to increase the two depletion regions. (Both junctions are operated in REVERSE bias). Then there is no current flow from input to output then used as OFF state.

Now connect BE depletion in forward bias by connecting + terminal to Emitter and connect Vcc at collector to base junction, which leads to remove deplection regions and flows the current instantly from input to output then used as ON state.

The BJT can be used as switching deviced by operating BJT in cutoff and saturation region.

Amplifier:

Connect BE junction in forward bias and BC junction as reverse bias, like in diagram. The one depletion region increases and one depletion region decreases. Means output current will depends upon the input. Means it can be operated as amplifier in active region.

MOSFET:

The above figure is NMOSFET can be operated in three different regions like BJT. The regions are discussed below

Cutoff:

The Gate to source volatge applied will form the channel. If the Vgs applied is less than Vth,the current flowing is negligible and cann say zero and say it as OFF.

0<Vgs<Vth

Active:

The channel will get start current flow when Vgs>Vth . Then by increasing the voltage at Vds the current start flowing depending upon the channel width (mainly on Vgs) and increases accordingly. This is an active region and used as Amplifier. The Vds can be incresed upto Vds<Vgs-Vth

Saturation:

If the voltage Vds increased beyond Vgs-Vth, there seems to have no effect on current and considered as saturation or ON state. Vds>Vgs-Vth

Finally we can conclude that BJT or MOSFET, cutoff and saturation regions are used in switching applications and amplifier is used in active region.

MOSFET is used in low power applications and BJT is used in high power applications.

FETs are easier to drive (specially at high voltages) than BJTs because the BJT current frequently has to come from the same (high voltage) supply as the load; thus there can be a significant power loss in the base drive alone. At low - medium frequencies, the gate drive requirements of FETs are easier than BJTs.