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MEMS1082

Chapter 3-2 Field Effect Transistors - MOSFET

Department of Mechanical Engineering MOSFET

 Junction Field Effect Transistor  Insulated Gate Field Effect Transistor, in which Gate input is electrically insulated from the main current carrying channel and is therefore called IGFET.  The most common type of insulated gate FET or IGFET as it is sometimes called, is the Oxide Field Effect Transistor or MOSFET for short.

Department of Mechanical Engineering MOSFET

MOSFET is one of the cornerstones of modern semiconductor technology. The general structure is a lightly doped p-type substrate, into which two regions, the source and the drain, both of heavily doped n-type semiconductor have been embedded. The symbol n+ is used to denote this heavy doping. The source and the drain are about 1 μm apart. Metallized contacts are made to both source and drain, generally using aluminium. The rest of the substrate surface is covered with a thin oxide film, typically about 0.05 μm thick. The gate is laid on top of the insulating oxide layer, and the body electrode in the above diagram provides a counter electrode to the Gate. The thin oxide film contains dioxide (SiO2), but it may well also contain silicon nitride (Si3N4) and silicon oxynitride (Si2N2O). The p-type doped substrate is only very lightly doped, and so it has a very high electrical resistance, and current cannot pass between the source and drain if there is zero voltage on the gate. Application of a positive potential to the gate electrode creates a strong electric field across the p-type material even for relatively small voltages, as the device thickness is very small and the field strength is given by the potential difference divided by the separation of the gate and body . When the gate electrode is positively charged, it will therefore repel the holes in the p-type region. For high enough electrical fields, the resulting deformation of the energy bands will cause the bands of the p-type region to curve up so much that will begin to populate the conduction band. Department of Mechanical Engineering MOSFET

This isolation of the controlling gate makes the input resistance of the MOSFET extremely high in the Mega-ohms region and almost infinite.

The metal oxide semiconductor field effect transistor (MOSFET)

Department of Mechanical Engineering MOSFET

 As the gate terminal is isolated from the main current carrying channel "NO current flows into the gate" and like the JFET, the MOSFET also acts like a voltage controlled resistor. Also like the JFET, this very high input resistance can easily accumulate large static charges resulting in the MOSFET becoming easily damaged unless carefully handled or protected.

Department of Mechanical Engineering MOSFET

Department of Mechanical Engineering MOSFET

 The P-channel and the N-channel MOSFET is available in two basic forms,  Enhancement type : Normally switched “off”  Depletion type. Normally switched "ON" without a gate bias voltage but requires a gate

to source voltage (Vgs) to switch the device "OFF".

Department of Mechanical Engineering Depletion-mode MOSFET and circuit Symbols

 Depletion-mode MOSFET's are constructed similar to their JFET transistor counterparts where the drain-source channel is inherently conductive with electrons and holes already present within the N-type or P-type channel. This doping of the channel produces a conducting path of low resistance between the drain and source with zero gate bias.

Department of Mechanical Engineering Enhancement-mode MOSFET

 The Enhancement-mode MOSFET is the reverse of the depletion- mode type. The conducting channel is lightly doped or even undoped making it non-conductive. This results in the device being normally "OFF" when the gate bias voltage is equal to zero.

Department of Mechanical Engineering MOSFET

Measured Characteristic Curves for 2N7000

for VG = 2.5 V, 2.75 V, 3 V, ...., 4 V

Department of Mechanical Engineering MOSFET Summary

MOSFET type Vgs = +ve Vgs = 0 Vgs = -ve

N-Channel Depletion ON ON OFF

N-Channel Enhancement ON OFF OFF

P-Channel Depletion OFF ON ON

P-Channel Enhancement OFF OFF ON

Department of Mechanical Engineering Enhancement MOSFET

Department of Mechanical Engineering Enhancement MOSFET as a Switch

 Enhancement-mode MOSFET's make excellent electronics switches due to their low "ON" resistance and extremely high "OFF" resistance and extremely high gate resistance.  Enhancement-mode MOSFET's are used in integrated circuits to produce CMOS type logic gates and power switching circuits as they can be driven by digital logic levels.

Department of Mechanical Engineering Enhancement MOSFET as a Switch

 Example: MOSFET Power Switch

If the resistive load of the lamp was to be replaced by an inductive load such as a coil or solenoid, a "Flywheel" would be required in parallel with the load to protect the MOSFET from any back-emf.

Department of Mechanical Engineering Enhancement MOSFET as a Switch

 Example: MOSFET Analog Switch

Department of Mechanical Engineering Enhancement MOSFET as a Switch

 Example: Circuit to switch Power

Open-collector circuit

npn transistor

Department of Mechanical Engineering Enhancement MOSFET as a Switch

 Example: Simple Power MOSFET Motor Controller

Department of Mechanical Engineering