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Intoduction of MOSFET

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Topics To Cover. Device Structure which is Gate, Drain and Source and their physical operation. Then, I-V characteristics of MOSFET. After that, how MOSFET work as inverter and switch.

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									Analogue Electronics I
      EAB 2014



      Jan 2009



 Lecture 7 - MOSFET
      Introduction

                         1
      Topics To Cover

•   Device Structure and Physical Operation
•   V-I Characteristics
•   MOSFETs as Inverter and Switch
•   Refer to Chapter 6 Boylestad




                                              2
 Basic Structure of MOSFETs
 Types of MOSFETs

   Depletion n-MOSFET    Enhancement n-MOSFET




• Why MOSFETs?
 Smaller in size and fewer processing steps.

                                                3
   Basic Structure of MOSFETs

        Depletion n-MOSFET       Enhancement n-MOSFET

         S       G         D            S       G         D

 SiO2                            SiO2

         n       n         n            n                 n
             p-substrate                    p-substrate




SiO2 is insulator referred to as dielectric – sets up opposing
 electric field. Responsible for very high input impedance.

                                                              4
   Basic Structure of MOSFETs

        Depletion n-MOSFET     Enhancement n-MOSFET

         S       G         D          S       G         D

 SiO2                          SiO2

         n       n         n          n                 n
             p-substrate                  p-substrate



No direct electrical connection between gate terminal and
     channel. M – Metal, O – Oxide, S – Semiconductor.

                                                            5
  Basic Structure of MOSFETs

       Depletion n-MOSFET     Enhancement n-MOSFET

        S       G         D          S       G         D

SiO2                          SiO2

        n       n         n          n                 n
            p-substrate                  p-substrate




Enhancement type does not have ‘channel’ connecting
                 drain and source.

                                                           6
Physical Operation of MOSFETs
    Depletion n-MOSFET

       ID=IS=IDSS
                    S                         D
                             G
                                                  +
                n
                    e   e        e    e
                                          n           VDD
                                                  _

                        p-substrate



For example: gate-to-source voltage (VGS) is set to
zero and voltage is applied across the drain-to source
terminals (VDS)

                                                            7
Physical Operation of MOSFETs
   Depletion n-MOSFET

The gate is insulated from the channel. Therefore,
the gate current is negligible regardless of the gate-
to-source voltage.
The channel has finite conductivity and majority
carriers (electrons) allows the current to flow from
drain to source via the channel when the drain is at
a positive potential with respect to the source.
This make it as a normally ON device.


                                                     8
Physical Operation of MOSFETs
   Depletion n-MOSFET

By controlling the voltage at gate terminal, this
enables us to control the current in the device until
the gate voltage reaches a value at which the device
will turns OFF.
Actually, voltage at the gate controls the width of the
channel which also determines the concentration of
the majority carriers.
The more negative the gate-to-source (VGS) voltage
is, the smaller is the channel width and the smaller is
the drain current

                                                          9
Physical Operation of MOSFETs
   Depletion n-MOSFET
 The channel width disappears when VGS reaches
 its pinch-off value, VP and enters the cut-off
 mode.
 The decrease in the width of the channel is
 viewed as the depletion in the majority-charge
 carriers in the channel.
 It is for this reason, MOSFET is said to operate in
 the depletion mode when VGS is less than or
 equal to zero. When VGS is larger than zero, it is
 said to be operating in enhancement mode.

                                                       10
   V-I Characteristics
Triode
Region   Saturation Region




                             11
Physical Operation of MOSFETs
      Enhancement n-MOSFET
                                VGS




                                   +
                              -
           IS = I D                                     ID
                          S         G               D
                                +++ +++
                                                             +
                              e e e e e e e
                      n                         n                VDS
                              + + + + + + +                  _
                                  p-substrate




For example: Both (VGS) and (VDS) is set to certain value > 0.


                                                                       12
Physical Operation of MOSFETs
  Enhancement n-MOSFET

Voltage at gate controls the flow of current
between drain and source.
When VGS = 0, no current flows between drain and
source  MOSFETs is said to be in cut-off mode.
Transistor will start to activate at its threshold
voltage: VT = the value of VGS when the drain
current just begins to flow.



                                                     13
Physical Operation of MOSFETs
  Enhancement n-MOSFET

 When VGS> 0, holes will repel under the gate and
 attracts electrons from source and drain. This will
 create a channel connecting both terminals.
 At VGS= VT the n-channel under the gate will
 complete the circuit. This channel is also called
 inversion layer.
 As VGS is increased slowly above threshold, a
 deeper channel is induced to allow more current
 to flow.

                                                       14
Physical Operation of MOSFETs
  Enhancement n-MOSFET
 For a small value of VDS and a constant VGS,
 current will flow from drain to source (ID = IS)
 proportionally to VDS. At this point, the MOSFET
 acts in triode region and behaves like a voltage-
 controlled resistor.
 Further increase in VDS,will create a voltage drop
 across the channel and cause a taper in the
 channel depth. When VDS reaches (VGS – VT),
 the channel pinches off. This leads to saturation
 so that there is no further increase in current
 even when VDS continues to increase.

                                                      15
    Operating Region
      Enhancement MOSFET
NMOS: (VDS and VGS normally positive values)
  – VGS < VT → Cut off mode, IDS=0 for any VDS
  – VGS > VT (transistor is turned on)
       • VDS < VGS – VT → Triode Region
       • VDS > VGS – VT → Saturation Region
PMOS: (VDS and VGS normally negative values)
  – VGS > VT → cut off mode, IDS = 0 for any VDS
  – VGS < VT (transistor is turned on)
       • VDS > VGS – VT → Triode Region
       • VDS < VGS – VT → Saturation Region

                                                   16
Inverter Circuit
       This circuit is designated as
       the       common       source
       configuration. It is used in
       digital circuits.
       When Vin is low, the
       transistor is off and Vout is
       high.
       When Vin is high, the
       transistor is on and Vout is
       low.

                                       17
    Circuit Model For Switching




VGS controls the switch in the model and ron is defined as:
                              1
               ron =
                       K [2(VGS − VT )]

                                                          18
Question & Answer ?




                      19

								
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