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Insulated Gate Field-Effect Transistors
39 questions By Tony R. Kuphaldt
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Question 10 of 39
Metal Oxide Field-Effect Transistors (MOSFETs) differ in behavior from Bipolar Junction Transistors (BJTs) in several ways. Address each one of these behavioral aspects in your answer:
- Current gain
- Conduction with no input (gate/base) signal
- Polarization
Reveal answerMOSFETs have much greater current gains than BJTs.
BJTs are normally-off devices, whereas a MOSFET may either be normally-on or normally-off depending on its manufacture.
MOSFETs can pass current from source to drain, or from drain to source with equal ease. BJTs can only pass current from emitter to collector in one direction.
Notes:For each one of these behavioral aspects, discuss with your students exactly why the two transistors types differ.
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Question 11 of 39
E-type MOSFETs are normally-off devices just like bipolar junction transistors, the natural state of their channels strongly resisting the passage of electric currents. Thus, a state of conduction will only occur on command from an external source.
Explain what must be done to an E-type MOSFET, specifically, to drive it into a state of conduction (where a channel forms to conduct current between source and drain).
Reveal answerA voltage must be applied between gate and substrate (or gate and source if the substrate is connected to the source terminal) in such a way that the polarity of the gate terminal electrostatically attracts the channel’s majority charge carriers (forming an inversion layer directly underneath the insulating layer separating gate from channel).
Notes:This is perhaps the most important question your students could learn to answer when first studying E-type MOSFETs. What, exactly, is necessary to turn one on? Have your students draw diagrams to illustrate their answers as they present in front of the class.
Ask them specifically to identify what polarity of VGS would have to be applied to turn on an N-channel E-type MOSFET, and also a P-channel E-type MOSFET.
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Question 12 of 39
D-type MOSFETs are normally-on devices just like junction field-effect transistors, the natural state of their channels being passable to electric currents. Thus, a state of cutoff will only occur on command from an external source.
Explain what must be done to an D-type MOSFET, specifically, to drive it into a state of cutoff (where the channel is fully depleted).
Reveal answerA voltage must be applied between gate and substrate (or gate and source if the substrate is connected to the source terminal) in such a way that the polarity of the gate terminal electrostatically repels the channel’s majority charge carriers.
Follow-up question: unlike JFETs, D-type MOSFETs may be safely “enhanced” beyond the conductivity of their natural state. Describe what is necessary to “command” a D-type MOSFET to conduct better than it naturally does.
Notes:This is perhaps the most important question your students could learn to answer when first studying D-type MOSFETs. What, exactly, is necessary to turn one off? Have your students draw diagrams to illustrate their answers as they present in front of the class.
Ask them specifically to identify what polarity of VGS would have to be applied to turn off an N-channel D-type MOSFET, and also a P-channel D-type MOSFET.
