Mumbai University > EXTC > Sem 3 > Analog Electronics 1

Marks: 7 M

Year: May 2014

• Depending upon the voltage applied to MOSFET we can define three regions of operation of MOSFET
• Cut-off region$(V_{GS} \lt V_{th})$
• Linear region or triode region$(V_{GS} \gt V_{th} ,V_{DS} \lt (V_{GS} - V_{th}))$

• Saturation or active mode$(V_{GS} \gt V_{th} ,V_{DS} \gt (V_{GS} -V_{th}))$

  Where,$V_{th}⟹$It is the minimum gate to source voltage required to turn transistor ON or to induce a channel.

• Cut-off region$(V_{GS} \lt V_{th})$

Fig 1 Cut-off region operation

• In cut-off region the transistor is off.
• Depending upon the gate-to source voltage cut-off region can be divided into three category
• $V_{GS} =0$: when $V_{GS} =0$ no channel is formed between drain and source and hence no current flows through the MOSFET even we apply VDS
• $V_{GS} \lt 0$: When negative gate to source voltage is applied holes from p-substrate are attracted towards the gate hence holes are accumulated near gate region so channel is not formed.

• $0 \lt V_{GS} \lt V_{th}$: when positive gate to source voltage is applied minority carriers from substrate flows towards the gate and they recombine with holes present in drain-source channel hence depletion region is formed and no channel is induced

• Linear region or triode region$(V_{GS} \gt V_{th} ,V_{DS} \lt (V_{GS}-V_{th}))$

  • In this region the transistor is ON.
  • In this region, positive gate to source voltage is applied and minority carrier from p-substrate flow towards the gate. And the concentration of minority carrier is more than the holes in the drain-source channel thus channel is formed having electrons and the current starts to flow on the application of positive $V_{DS}$ i.e. drain to source voltage.
  • In this region MOSFET is linear device. It obeys ohm law and resistance is controlled by gate-to source voltage hence mosfet behave as voltage controlled resistor.

*Fig2 (a) linear region (b) saturation region *

• Saturation or active region $(V_{GS} \gt V_{th} ,V_{DS} = \gt (V_{GS}- V_{th}))$
  • In this region the transistor is ON
  • The drain-source channel is already formed and the current is flowing through channel when drain-to-source voltage is applied.
  • The channel is broader and hence maximum current flows through the drain-source channel.
  • Current in this region primarily depends on gate-to source voltage and weakly on drain-to-source voltage since channel near drain pinches-off as shown in Fig 2(b).