Transistor as a Switch

• The transistor can be used for two types of applications viz. amplification and switching. For the amplification applications a transistor is biased in its active region.

• Whereas for switching applications it is biased to operate in the saturation (fully on) or cutoff (fully off) regions.

1. Transistor in cutoff region [open switch] :

• In the cutoff region both the junctions of a transistor are reverse biased and a very small reverse current flows through the transistor.

• The voltage drop across the transistor (VCE) is high. Thus cutoff region the transistor is equivalent to an open switch as shown in Fig. (a).

2. Transistor in the saturation region:

• When Vin is positive, a large base current flows and the transistor saturates.

• In the saturation region both the junctions of a transistor are forward biased. The voltage drop across the transistor (VCE) is very small of the order of 0.2V to IV depending on the type of transistor and collector current is very large.

• In saturation the transistor is equivalent to a closed switch as shown in Fig. (b)

• If we apply a square wave at the input of the circuits then the input and output wave forms when the transistor acts as a switch are as shown in Fig, (c)

Applications of Transistor as a Switch

Some of the applications of the transistor as switch are:

1. In the logic circuit.

2. Temperature controllers.

3. Choppers used for DC motor control.

4. Multivibrators and Schmitt trigger.

5. Relay drivers.

6. Stepper motor controllers.

7. Timers.

8. SMPS and UPS

MOSFET operation

A. Construction

• Following fig. (a) shows the basic structure of N-channel depletion type MOSFET

• It consist of conducting bar N-type material with an insulate gate on the left and P-region on the right.

• Free electrons can flow from source to drain (D) through the N-type material.

• P-region is called as substrate.

• A layer of insulating material (i.e. Sio2) is deposited on left of channel.

B. Working

When VGS=0v

• Usually fourth terminal of MOSFET(i.e. substrate) is made common with source terminal.

• Following fig (b) shows biasing condition for VGS=0v

• In fig (b), gate source and substrate are connected together to ground pin to make VGS=0v.

• A positive voltage VDS is applied between drain to source.

• Due to the positive voltage applied to the drain terminal, free electrons from the channel are attracted to drain and drain current starts flowing

When VGS= negative (depletion mode of operation)

• Due to applied negative voltage between gate and source, gate terminal will repel free electrons towards P-type substrate and attracts holes from substrate.

• These electrons and holes will get combine inside the channel and reduce width of channel and amount of current

• Thus with increase in negative value of VGS there is decrease in amount of current.

• If gate voltage is made positive with respect to source, this will increase the number of free electron passing through channel.

• The greater gate voltage greater is number of free electrons passing through channel.

Characteristics of D-MOSFET

Symbol

• Circuit symbol for N-channel D MOSFET is as given below.