MOSFET V-I Characteristics

Types of Transistors�

MOSFET (Types)

• Four types:
• n-channel enhancement mode
• Most common since it is cheapest to manufacture
• p-channel enhancement mode
• n-channel depletion mode
• p-channel depletion mode

MOSFET

FET = Field-Effect Transistor

A four terminal device (gate, source, drain, bulk)

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Symbols of MOSFET

MOSFET characteristics�

• Basically low voltage device. High voltage device are available up to 600V but with limited current. Can be paralleled quite easily for higher current capability.
• Internal (dynamic) resistance between drain and source during on state, R_DS(ON), , limits the power handling capability of MOSFET. High losses especially for high voltage device due to R_DS(ON) .
• Dominant in high frequency application (>100kHz). Biggest application is in switched-mode power supplies.

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• The transistor consists of three regions, labeled the ``source'', the ``gate'' and the ``drain''.
• The area labeled as the gate region is actually a ``sandwich'' consisting of the underlying substrate material, which is a single crystal of semiconductor material (usually silicon); a thin insulating layer (usually silicon dioxide); and an upper metal layer.
• Electrical charge, or current, can flow from the source to the drain depending on the charge applied to the gate region.
• The semiconductor material in the source and drain region are ``doped'' with a different type of material than in the region under the gate, so an NPN or PNP type structure exists between the source and drain region of a MOSFET.

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• Most important device in digital design

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• Very good as a switch

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• Relatively few parasitics

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• Rather low power consumption

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• High integration density

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• Simple manufacturing

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• Economical for large complex circuits

n-Channel MOSFET�

NMOS Structure�

• MOS (Metal-Oxide-Semiconductor) Nowadays gate is made of poly-silicon

􀂉 Channel length L and width W

􀁺 In most digital design, L is set at the minimum feature size

􀁺 W is selectable by the designer

􀂉 Bulk is connected to the Gnd in NMOS to prevent forward-biased PN junction

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On state

Off state

n-MOSFET Characteristics�

Plots V-I characteristics

of the device for various

Gate voltages (VGS)

At a constant value of VDS , we can

also see that IDS is a function of the

Gate voltage, VGS

The transistor begins to conduct

when the Gate voltage, VGS , reaches

the Threshold voltage: VT

PMOS Structure�

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• PMOS transistor has a negative threshold voltage (Vtp) -0.3v~-1.2v
• A pMOS turns on when Vgs<Vtp

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The terminal characteristics of the device are given by drain-to-source current Ids against drain-to-source voltage Vds for different values of gate-to-source voltage Vgs. All voltages are referenced with respect to the source voltage, which is assumed to be at ground potential.

P-MOSFET Characteristics�

Switch models of MOSFETs�

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