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Unformatted text preview: ?mes the gate voltage in excess of the threshold voltage. EE105, Fall 2013 Lecture 16 – Slide 13 Prof. Ming C. Wu Charge Density at a Point Q( x) = WCox [VGS − V ( x) − VTH ]
§༊ Let x be a point along the channel from source to drain, and V(x) its poten?al; the expression above gives the charge density (per unit length). EE105, Fall 2013 Lecture 16 – Slide 14 Prof. Ming C. Wu 7 Parabolic ID VDS Rela?onship ID = Q ⋅ v
Q ( x ) = WCox #VGS − V ( x ) − VTH %
$
&
v = +µ n dV
dx dV ( x )
I D = WCox #VGS − V ( x ) − VTH % µ n
$
&
dx
1
W#
2
I D = µ nCox $2(VGS − VTH )VDS − VDS %
&
2
L EE105, Fall 2013 §༊ By keeping VG constant and varying VDS, we obtain a parabolic rela?onship. §༊ The maximum current occurs when VDS equals to VGS VTH. Lecture 16 – Slide 15 Prof. Ming C. Wu ID VDS for Diﬀerent Values of VGS 2 I D ,max ∝ (VGS − VTH ) EE105, Fall 2013 Lecture 16 – Slide 16 Prof. Ming C. Wu 8 Linear Resistance Ron = 1 µ nCox W
(VGS − VTH )
L §༊ At small VDS, the transistor can be viewed as a resistor, with the resistance depending on the gate voltage. §༊ It ﬁnds applica?on as an electronic switch. EE105, Fall 2013 Lecture 16 – Slide 17 Prof. Ming C. Wu Applica?on of Electronic Switches §༊ In a cordless telephone system in which a single antenna is used for both transmission and recep?on, a switch is used to connect either the receiver or transmiJer to the antenna. EE105, Fall 2013 Lecture 16 – Slide 18 Prof. Ming C. Wu 9 Eﬀects of On Resistance §༊ To minimize signal aJenua?on, Ron of the switch has to be as small as possible. This means larger W/L aspect ra?o and greater VGS...
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 Fall '07
 KingLiu

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