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 ntype
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.
– Pinchoff will occur where the depletion regions
Pinchoff
encompass the entire ntype 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 pinchoff,
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, pinchoff will
For
occur at lower values of drain voltage and thus the
saturation drain current is lower (see Fig 65)
saturation
•
• • A depletion region is set up in the ntype region
The region is uniform relative to the zero gate voltage,
The
high current case
high Performance parameters that can be calculated
– Pinchoff voltage: Related to doping of nregion
– 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 Pinchoff
– The depletion regions on either side of channel meet
– At pinchoff, 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 ntype 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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 Spring '10
 ZORMAN
 Semiconductors, Transistor, Volt, MOS Capacitors

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