MOSFET NOTES

Metal Oxide Semiconductor FET

Introduction.

Symbol:

·         The Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is one type of  FET transistor.

·          In MOSFET the gate terminal is electrically insulated from the current carrying channel so it is called as IG-FET.

·         Due to the insulation between gate and source terminals the input resistance of MOSFET may be in mega ohms (MΩ).

·          MOSFET also acts as a voltage controlled resistor when no current flows into the gate terminal.

·          The small voltage at the gate terminal controls the current flow through the channel between the source and drain terminals. 

·           MOSFET  terminals are, Drain (D), Source (S), Gate (G) and substrate.

·         The MOSFETs are  available in both types, N-channel (NMOS) and P-channel (PMOS).

·         The MOSFETs are basically classified in to two forms they are Depletion type and Enhancement type transistors.

Working Principle of MOSFET:

·         The aim of the MOSFET is to control the voltage and current flow between the source and drain. It works almost as a switch.

·         The working of MOSFET depends upon the MOS capacitor. The MOS capacitor is the main part of MOSFET.

·         The semiconductor surface at the below oxide layer  is located between source and drain terminal.

·         It can be  inverted from p-type to n-type by applying a positive or negative gate voltages .  

·         When we apply the positive gate voltage the holes present under the oxide layer with a repulsive force and holes are pushed downward with the substrate.

·         if a voltage is applied between the drain and source, the current flows freely between the source and drain and the gate voltage controls the electrons in the channel.

·         Instead of positive voltage if we apply negative voltage , a hole channel will be formed under the oxide layer.

In general, any MOSFET is  operated in three regions :

1.    Cut-Off Region
In this region MOSFET will be in OFF state. No current flow through it.  It behaves as open switch and is used as electronic switches.

2.    Ohmic or Linear Region
In this region the current I_DS increases with increasing of V_DS. This region, is used as amplifiers.

3.    Saturation Region
In this region,  I_DS  is constant by increase in V_DS and once V_DS exceeds the value of pinch-off voltage V_P. the device will act as closed switch.

Construction of an EMOSFET:

1.    The main difference between the construction of DE-MOSFET and  E-MOSFET, is that E-MOSFET substrate extends all the way to the silicon dioxide (SiO₂) and no channels are doped between the source and the drain.

2.    Channels are electrically induced in these MOSFETs, when a positive gate-source voltage V_GS is applied to it.

Operation of an EMOSFET:

1.    This MOSFET operates only in the enhancement mode and has no depletion    mode.

2.     It  operates with large positive gate voltage only. It does not conduct  

  when the gate-source voltage V_GS ⁼ 0,  is called MOSFET –off.

3.    Drain current I_D flows only when V_GS exceeds V_GST [gate-to-source threshold voltage].

4.    When drain is positive w.r.to source and no potential is applied to the gate.

5.    Then two P-N junctions connected back to back with a resistance of the P-substrate.

6.    A  very small drain current i.e, reverse leakage current flows.

7.    If P-type substrate is connected to the source terminal, then zero voltage lies across the source substrate junction, and the^–drain-substrate junction remains reverse biased.

8.    When the gate is made positive w.r.to the source and the substrate, negative (i.e. minority) charge carriers within the substrate are attracted to the positive gate and accumulate close to the-surface of the substrate.

9.     As the gate voltage is increased, more and more electrons accumulate under   the gate.

10. These electrons flow across the insulated layer to the gate, so they accumulate at the surface of the substrate just below the gate.

11. These accumulated minority charge carriers N -type channel stretching from drain to source.

12. Then an inversion layer (N-type) is induced in channel. Now a drain current start flowing.

13. The strength of the drain current depends upon the channel resistance and number of charge carriers attracted to the positive gate. Thus drain current is controlled by the gate potential.

14. The conductivity of the channel is enhanced by the positive bias on the gate so this device is called the enhancement MOSFET or E- MOSFET.

15. E-MOSFET is classified as an enhancement-mode device because its conductivity depends on the action of the inversion layer.

16. Depletion-mode devices are normally ON when V_GS = 0, whereas the enhancement-mode devices are normally OFF when V_GS = 0.

Characteristics of an EMOSFET.

Drain Characteristics-EMOSFET

1.     When V_GS  < V_GST  then I_D =0. When V_GS is greater than V_GST, then device turns- on and  I_D is controlled by the gate voltage.

2.     The characteristic curves have almost vertical and almost horizontal parts.

3.     Thus E-MOSFET can be used as a variable-voltage resistor (WR) or as a constant current source.

EMOSFET-Transfer Characteristics

1.    The current IDSS at VGS <=0 is very small, of a few nano-amperes.  

2.    When V_GS is positive, then I_D increases slowly at first, and then much more rapidly with an increase in V_GS.

The equation for the transfer characteristic of E-MOSFETs is given as:

I_D=K(V_GS-V_GST)²

·         For zero value of V_GS, the E-MOSFET is OFF because there is no conducting channel between source and drain.

·         The symbols has broken channel line to indicate the normally OFF condition.

·          V_GS exceeding the threshold voltage V_GST, an N-type inversion layer, connecting the source to drain, is created.

Depletion Mode MOSFET

·         The depletion mode MOSFETs are generally known as ‘Switched ON’ devices,

·         These  are generally closed when there is no bias voltage at the gate terminal.

·          If the gate voltage increases in positive, then the channel width increases in depletion mode.

·         The drain current I_D through the channel increases.

·         If the applied gate voltage more negative, then the channel width is very less and MOSFET may enter into the cutoff region.

·         The depletion mode MOSFET is rarely used type of transistor in the electronic circuits.

·         V-I characteristic mainly gives the relationship between drain- source voltage (V_DS) and drain current (I_D).

·         The small voltage at the gate controls the current flow through the channel.

·         The channel between drain and source acts as a good conductor with zero bias voltage at gate terminal.

·         The channel width and drain current increases if the gate voltage is positive and these two (channel width and drain current) decreases if the gate voltage is negative

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