EXAMPLE 11.1 OBJECTIVE To calculate the internal pinchoff voltage and pinchoff voltage of an...
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EXAMPLE 11.1 OBJECTIVE To calculate the internal pinchoff voltage and pinchoff vo ltage of an n-channel Assume that the p + n junction of uniformly doped silicon n-channel JFET at T = 300 K has doping concentrations of N a = 10 18 cm -3 and N d = 10 16 cm -3 . Assume that that the metallurg ical channel thickness, a, is 0.75 m = 0.75 10 -4 cm. Solution The internal pinchoff voltage is given by Equation (11.3), so we have The built-in potential barrier is The pinchoff voltage, from Equation (11.4), is then found as V 35 . 4 10 85 . 8 7 . 11 2 10 10 75 . 0 10 6 . 1 2 14 16 2 4 19 2 0 s d p ea V V 814 . 0 10 5 . 1 10 10 ln 0259 . 0 ln 2 10 16 18 2 i d a t bi n N N V V V 54 . 3 35 . 4 814 . 0 0 p bi p V V V
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Transcript of EXAMPLE 11.1 OBJECTIVE To calculate the internal pinchoff voltage and pinchoff voltage of an...
EXAMPLE 11.1OBJECTIVETo calculate the internal pinchoff voltage and pinchoff voltage of an n-channel Assume that the p+n junction of uniformly doped silicon n-channel JFET at T = 300 K has doping concentrations of Na = 1018 cm-3 and Nd = 1016 cm-3. Assume that that the metallurgical channel thickness, a, is 0.75 m = 0.75 10-4 cm. SolutionThe internal pinchoff voltage is given by Equation (11.3), so we have
The built-in potential barrier is
The pinchoff voltage, from Equation (11.4), is then found as
CommentThe pinchoff voltage, or gate-to-source voltage, to achieve pinchoff for the n-channel depletion-mode device is a negative quantity, as we have said.
V35.4
1085.87.112
101075.0106.1
2 14
1624192
0
s
d
p
eaV
V814.0
105.1
1010ln0259.0ln 210
1618
2
i
datbi n
NNVV
V54.335.4814.00 pbip VVV
EXAMPLE 11.2OBJECTIVEDesign the channel doping concentration and metallurgical channel thickness to achieve a given pinchoff voltage. Consider a silicon p-channel pn JFET at T = 300 K. Assume that the gate doping concentration is Nd = 1018 cm-3. Determine the channel doping concentration and channel thickness so that the pinchoff voltage is Vp = 2.25 V. SolutionThere is not a unique solution to this design problem. We will pick a channel doping concentration of Na = 2 1016 cm-3 and determine the channel thickness. The built-in potential barrier is
From Equation (11.8), the internal pinchoff voltage must beVp0 = Vbi + Vp = 0.832 + 2.25 = 3.08 V
and from Equation (11.6), the channel thickness can be determined as
CommentIf the channel doping concentration chosen where larger, the required channel thickness would decrease; a very small value of channel thickness would be difficult to fabricate within reasonable tolerance limits.
V832.0
105.1
10102ln0259.0ln 210
1816
2
i
datbi n
NNVV
m446.0
102106.1
08.31085.87.11222/1
1619
142/1
0
a
ps
eN
Va
EXAMPLE 11.3OBJECTIVETo calculate the cutoff frequency of a GaAs JFET. Consider a GaAs JFET with the following parameters: n = 8000 cm2/V-s, Nd = 1016 cm-3, a = 0.50 m, and L = 2 m. SolutionSubstituting the parameters in Equation (11.18), we have
orfT = 110 GHz
CommentThis example shows that GaAs JFETs can have large cutoff frequencies.
2414
241619
1021085.81.132
105.0108000106.1
Tf
