Bsim1 Why?: B S I M

BSIM1 WHY? Industry switched to BSIM models � Berkeley Short channel Igfet Model � About 10 years ago industry quit using MOS level 1 � Was a big impact on students at first job(conversion form one level to another) To make hand calculation easy extract some parameter. 1

How? All parameters have Length and width modifiers Using equation,

Where P’ is BSIM electrical parameter. L-DL is effective channel length W-DW is effective channel width LP is length modifier WP is width modifier all above are electrical parameter assoi with P’ 2

BSIM DC equation

Threshold voltage  The drain current  The subthresod current

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The Thresold voltage

Remember that this equation is calculated using,

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Much worse than the old threshold Equation Not constant for a given device size Not easily worked by hand Simplified equation,

By neglecting the effect of device and lower drain induced barrier resulting from non zero VDS

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BSIM DC equation

Threshold voltage

The drain current The subthresod current

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Drain current Cut off : VGS < VTHN (ID = 0, neglecting STC) Linear : VDS < VGS-VTHN > 0

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U0Z’ and U1z’ accounts for mobility degradation effect X2UO, X2U1 X3U1

sens. of trans. field mobility degrad. eff. subs. bias sens. of velocity saturation effect to substrate bias sens. of vel. sat. eff. on drain bias at Vds=VddL

LX2UO, WX2UO LX2U1, WX2U1 LX3U1, WX3U1 10

Saturated Current model

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This equation does not depend on VDS

(are we wrong?)

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channel length modulation

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MUO , neglecting length and width dependence by using linear interpolation between the mobility at VDS=0(MUZ) and VDS=VDD(MUS)

Output resistance of MOS is given by,

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Parameter λm is called mobility modulation parameter. Thus change in ro MUS( VDS=VDD) < MUZ(VDS=0) 15

For short channel length λc For long channel length λm In digital application we assume both to be zero

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BSIM DC equation

Threshold voltage The drain current

The subthresod current

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Subthresold current VGS< VTHN  The transistor doesn’t turn off suddenly  As Vgs nears Vth, it turns on gradually  Two current components Weak Inversion The current we have ignored Strong Inversion The part we said was 0 before This current is due to diffusion between drain and source 18

Iweak

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 Small currents, used in low power operation  Sensitive to Vgs, exponentially

�Sensitive to Temperature  Gradual change

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If Iexp << Ilimit Intersection of slopes of current curves VA (Early Voltage…)

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The output resistance of a MOSFET, subthresold and SI region ,

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Increases by 10X every 60-100mV change in VGS-VTHN Increase VGS or lower VTHN !!!! S = n(kT/q)ln(10) depends on n and T

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Thanks

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