Question

   ketch labelled diagrams of a field effect transistor and an NPN transistor and describe how the bias of each can be controlled for applications such as switches and amplifiers.

Answer 1

1) N chanell MOSFET:

As shown in figure below, n+ material is doped in p- type substrate to form a N chanell MOSFET. MOSFET I normally OFF, biasing is required to operate MOSFET in particular region. As we can see that chanell is absent in mosfet, when positive gate to source voltage is applied at the gate terminal then electrons from n+ region is attracted towards gate to form channel through which drain current flow.

Mosfet in cutoff region- in this region of operation, gate to source voltage is less than threshold voltage ( it is a parameter design at the time of manufacturing) i.e. VGS < VT and hence drain current is zero. This region is used to operate MOSFET as a switch in digital circuit as we can get only two conditions ON or OFF.

MOSFET in Triode region- in this region VGS > VT I.e. channel is formed but drain to source voltage is less than (VGS- VT). In this region is called linear region as drain current linearly increases due to VGS and VDS. This region is called ohmic region. This region is also used in digital circuit as a switch.

Saturation Region- Keeping gate to source voltage constant and increasing drain to source voltage makes channel tapper at drain point at which pinched off occurs and MOSFET enters into satirasatu region, which is called non linear region. In this VGS> VT and VDS>(VGS-VT). Increasing VDS further does not affect drain current, this region is used as a amplifier.

2) NPN transistor:

npn bjt is formed by sandwitching two n material with p material as shown in figure below. In n type material electron is a majority carriers and hole is a minority carriers, and in p type material holes is a majority carriers and electrons is a minority carriers. There is a three region of operation.

Cutoff Region: When both base emitter and collector emitter junction is Reversed biased then there will be no current flow at the output. This region is used as a switch, minute current flow due to reverse biased.

Saturation region- When both BE junction and CB junction is forward biased then high current flow from emitter to collector which acts as normally ON switch. This region is also used as a switch.

Active region- when BE junction if forward biased and CB junction is reversed biased current will be in the range of 0<IC<ICmax. Hence it is also called regulator, which is used as a amplifier. Bjt is a current control device, and MOSFET is called as voltage control device.

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