MOSFET Transistors: ON/OFF Switches

Switch-Level Models: Simple ON/OFF Switches

• Positive voltage is often called VDD (or) Power and represents logic 1 in digital circuits.
• Low voltage is called Ground (GND) or VSS and represents logic 0, typically 0 volts.
• MOS transistors can be viewed as simple ON/OFF switches.

nMOS Transistor

• When the gate (G=1) of an nMOS transistor is '1', the transistor is ON.
• When ON, an nMOS acts as a "closed switch" with a conducting path from source to drain.
• When the gate (G=0) is low, the nMOS transistor is OFF.
• When OFF, an nMOS acts as an "open switch" with almost zero current flow from source to drain.

MOSFET Channel

• If the MOSFET channel consists of electrons, it is an nMOS transistor.
• If the MOSFET channel consists of holes, it is a pMOS transistor.
• The channel is formed between the drain and source, with the opposite doping type to the substrate.
• An n-channel is made with a p-type substrate.
• A p-channel is made with an n-type substrate.
• Channel conductivity depends on n-type or p-type channel characteristics (electrons or holes).
• N+ and P+ regions indicate heavily doped n- or p- silicon.
• Since the 1970s, the gate has been formed from polycrystalline silicon (polysilicon).
• The body is typically grounded.

Enhancement Mode MOSFETs

• Symbols for n-channel and p-channel enhancement mode MOSFET transistors are shown in Fig 1.6.
• A broken line represents connection between the source and drain, indicating enhancement mode.
• Conductivity increases by increasing the oxide layer, which adds carriers to the channel. This oxide layer is referred to as the 'Inversion layer'.

Depletion Mode MOSFETs

• Depletion mode MOSFET transistors are generally "switched ON" at zero gate-source voltage (VGS = 0).
• This is equivalent to a "normally closed" switch, requiring VGS to switch OFF the device.
• If gate voltage increases positively, the channel width increases.
• Symbols for depletion mode MOSFETs in both n-channel and p-channel are shown in Fig 1.7.
• The fourth terminal (substrate) is connected to the source terminal.
• A continuous thick line between the drain and source terminals represents the depletion type.
• An arrow symbol indicates the type of channel (n-channel or p-channel).

PMOS Transistor

• A pMOS transistor's behavior is opposite to that of an nMOS transistor.
• It is ON when the gate is low (G = 0) and OFF when the gate is high (G = 1).

Modes of Operation

• In MOSFETs, two operation modes correspond to the transistor being in the ON state or OFF state at zero gate-source voltage (VGS = 0).
• The two modes are enhancement mode and depletion mode.

Enhancement Mode

• The enhancement mode MOSFET is equivalent to a "normally open" switch.
• These devices are OFF at zero gate-source voltage for both nMOS and pMOS.
• These transistors require VGS to switch ON.
• It turns ON by applying a gate voltage higher than the source voltage for nMOS.
• It turns ON by applying a gate voltage lower than the source voltage for pMOS.

Depletion Mode Channel Conductivity

• The conductivity of the channel in depletion MOSFETs is less than that of enhancement type MOSFETs.

MOSFET Applications

• MOSFETs are used in digital integrated circuits (microprocessors).
• Used in calculators.
• Used in memories and logic CMOS gates.
• Used as analog switches.
• Used as amplifiers.
• Used in power electronics and switch mode power supplies.
• Used as oscillators in radio systems.
• Used in sound systems in automobile and reinforcement systems.

MOS Transistor Basics

• The Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is the basic element in VLSI design.
• It's a type of Field-Effect Transistor (FET).
• MOSFET has an insulated gate, whose voltage controls the device's conductivity.
• Changing conductivity with voltage is used for amplifying or switching electronic signals.
• A MOSFET has three terminals: Drain (D), Source (S), and Gate (G).
• There's also a Substrate (or) Body (B) terminal used in circuit connections.
• The gate electrode is insulated from the channel by a thin layer of metal oxide.
• The body is often connected to the source, making it a three-terminal device like FET.

MOSFET Layers

• Metal gate electrode layer
• Insulating oxide (SiO2) layer
• P(or) N type substrate

MOSFET Construction

• Transistors are built on nearly flawless single crystals of silicon.
• These crystals are available as thin flat circular wafers of 15-30 cm in diameter.

MOSFET Working Principle

• A MOSFET acts as a voltage-controlled resistor when no current flows into the gate terminal.
• It works by varying the width of a channel.
• Charge carriers (electrons or holes) flow along this channel.
• The small voltage at the gate controls current flow through the channel between the source and drain (channel length L).

MOSFET Types

• The main types of MOSFET are based on the charge carriers:
• n-channel MOSFET (or) nMOS transistor (or) NMOS transistor.
• p-channel MOSFET (or) pMOS transistor (or) PMOS transistor.