Lecture 15

As vg increases above the threshold voltage vth a

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Unformatted text preview: FormaNon (QualitaNve) §༊  As the gate voltage (VG) is increased, holes are repelled away from the substrate surface. VG < VTH –  The surface is depleted of mobile carriers. The charge density within the deple&on region is determined by the dopant ion density. §༊  As VG increases above the threshold voltage VTH, a layer of conducNon electrons forms at the substrate surface. VG ≥ VTH –  For VG > VTH, n > NA at the surface. à༎ The surface region is “inverted” to be n- type. à༎ The electron inversion layer serves as a conducNve path (channel) for current to flow between the heavily doped (i.e. highly conducNve) source and drain regions. EE105, Fall 2013 Lecture 15 – Slide 5 Prof. Ming C. Wu Voltage- Dependent Resistor §༊  In the ON state, the MOSFET channel can be viewed as a resistor. §༊  Since the mobile charge density within the channel depends on the gate voltage, the channel resistance is voltage- dependent. EE105, Fall 2013 Lecture 15 – Slide 6 Prof. Ming C. Wu 3 Channel Length & Width Dependence §༊  Shorter channel length and wider channel width each yield lower channel resistance, hence larger drain current. –  Increasing W also increases the gate capacitance, however, which limits circuit [email protected] speed (frequency). EE105, Fall 2013 Lecture 15 – Slide 7 Prof. Ming C. Wu Comparison: BJT vs. MOSFET §༊  In a BJT, current (IC) is limited by diffusion of carriers from the emitter to the collector. –  IC increases exponentially with input voltage (VBE), because the carrier concentration gradient in the base is proportional to eVBE / VT §༊  In a MOSFET, current (ID) is limited by drift of carriers from the source to the dra...
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This note was uploaded on 01/22/2014 for the course EE 105 taught by Professor King-liu during the Fall '07 term at Berkeley.

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