Chapter 6 Lecture - Part 1

Junction fet junction this increases the resistance

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Unformatted text preview: channel and voltage variation along the channel with VG = 0 and small ID. Junction FET Junction – this increases the resistance in the conducting n-type this region region – this leads to a voltage drop across the channel which this further increases the depletion region near the drain, causing an asymmetry in the channel cross sectional area. area. – Pinch-off will occur where the depletion regions Pinch-off encompass the entire n-type region, leaving a small cross sectional area that is completely depleted. Current saturation is achieved at this point. saturation • • Ohm’s law says V=IR, or R = V/I, so dR = dV/dI If dR is increasing rapidly as the device approaches pinch-off, If either dV is getting very large or dI is getting very small. Its dI that gets small and I reaches saturation. dI Depletion regions in the channel of a JFET with zero gate bias for several values of VD : (a) linear range; (b) near pinchoff; (c) beyond pinchoff. Junction FET Junction • Control of a JFET: – For negative values of gate voltage, pinch-off will For occur at lower values of drain voltage and thus the saturation drain current is lower (see Fig 6-5) saturation • • • A depletion region is set up in the n-type region The region is uniform relative to the zero gate voltage, The high current case high Performance parameters that can be calculated – Pinch-off voltage: Related to doping of n-region – ID and ID(sat) Effects of a negative gate bias: (a) increase of depletion region widths with VG negative; (b) family of current–voltage curves for the channels as VG is varied. JFET JFET • Summary of Pinch-off – The depletion regions on either side of channel meet – At pinch-off, ID does not increase linearly with VD, but but does so more slowly does – Depletion regions are loaded with fixed charges with Depletion associated electric field associated • In n-type channel, the fixed charge is positive • Electrons are swept across the region by the electric field – With respect to Ohm’s Law • • • V = IR dR = dV/dI which gets very high IR So as V increases, I increases very little To a good approximation, we can assume that I do...
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