vb.net barcode reader source code Figure 938 Depletion MOSFET characteristic curves in Software

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Figure 938 Depletion MOSFET characteristic curves
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Allows negative as well as positive gate voltages Can be in the saturation region for vGS = 0
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Figure 938 depicts the characteristic curves of a depletion-mode MOSFET Once again, note the similarity to the enhancement-MOSFET curves: The primary difference results from the fact that, since a channel exists even for zero gate voltage, the device can operate for both positive and negative gate-source voltages Junction Field-Effect Transistors The last member of the FET family that will be discussed in this chapter is the junction eld-effect transistor (JFET) The construction of a JFET and its circuit
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symbol are shown schematically in Figure 939 The n-type JFET consists of an n-type bulk element, with heavily doped p-type regions forming the gate Metal contacts at the gates and at the ends of the n-type material provide external circuit connections If the material composition is reversed, a p-type JFET can be similarly constructed To understand the operation of a JFET we refer to Figure 940 When the gate-source and drain-source voltages are both zero, the transistor will not conduct any current, and is said to be in the cutoff region (see Figure 940(a)) The JFET will remain in cutoff until the gate-source voltage exceeds a threshold or pinch-off voltage, VP We shall give a de nition of this voltage shortly As the gate-source voltage is increased above VP for small values of the drainsource voltage, the pn junction between gate and channel becomes more reverse biased, and the width of the depletion region increases, thus narrowing the channel This has the effect of increasing the channel resistance, resulting in the voltage-controlled resistor behavior that is characteristics of the ohmic region As shown in Figure 940(b), the channel will become narrower towards the drain end of the device, because the reverse bias of the pn junctions is larger near the drain (because the drain is at higher voltage than the source) If we now increase the drain-source voltage, the reverse bias will increase to the point where the channel is pinched off This condition is shown in Figure 940(c) Now any further increase in vDS will not result in an appreciable increase in drain current, leading to operation in the saturation region This corresponds to a attening of the curve of drain current versus drain-source voltage When the drain-source voltage is increased above a breakdown voltage, VB , the drain current will increase very rapidly due to avalanche conduction, leading to excessive heat generation and device destruction This is the breakdown region of the JFET
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Gate Source p Channel n p Drain
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D iD + vDS _ S
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Figure 939 JFET construction and circuit symbol
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Gate Source p Channel n p Drain Source
Gate Drain p n Channel p depletion regions Source
Gate Drain p n Channel p Pinched-off channel (c)
When the gate-source voltage is lower than VP, no current flows This is the cutoff region
For small values of drain-source voltage, depletion regions form around the gate sections As the gate voltage is increased, the depletion regions widen, and the channel width (ie, the resistance) is controlled by the gate-source voltage This is the ohmic region of the JFET
As the drain-source voltage is increased, the depletion regions further widen near the drain end, eventually pinching off the channel This corresponds to the saturation region
Figure 940 JFET operation
The behavior just described can also be visualized in the JFET characteristic curves of Figure 941 Note the similarity between the curves of Figure 941
9
Transistor Fundamentals
3 05 V Drain current, mA 2 10 V 2 15 V 08 20 V 0 0 25 V VGS = 3 V 90 100
30 40 50 60 70 Drain-source voltage, V
Figure 941 JFET characteristic curves
and those of Figure 938, which describe the depletion MOSFET Because of this similarity, the two families of devices can be described by the same set of equations, given in the following subsection The operation of p-channel JFET is identical to that of an n-channel JFET, with the exception that the polarities of the voltages and the directions of the currents are all reversed Depletion MOSFET and JFET Equations The equations describing the operation of depletion MOSFETs and JFETs are summarized in the Table 92 We note that the equations describing depletion MOSFETs and JFETs are identical if we recognize that the depletion MOSFET threshold voltage and the JFET pinch-off voltage play the same role In Table 92 we use the symbol VP to represent both the depletion MOSFET threshold voltage and the JFET pinch-off voltage The equations of Table 92 are also valid for p-channel devices if one substitutes vSG for vGS and vSD for vDS The following examples illustrate analysis and biasing methods and an application of JFETs
Table 92 Regions of operation and equations of n-channel depletion MOSFET and of JFET Cutoff region: vGS < VP Ohmic or triode region: vDS < 025(vGS + VP ), vGS > VP 2 VP (equivalent drain-to-source resistance) RDS = 2IDSS (vGS + VP ) vDS iD RDS Saturation region: vDS vGS + VP , vGS > VP IDSS (vGS + VP )2 2 VP Breakdown region: vDS > VB iD =
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