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Junction field-effect transistors (JFET)
43 questions By Tony R. Kuphaldt
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Question 7 of 43
Bipolar junction transistors (BJTs) are considered “normally-off” devices, because their natural state with no signal applied to the base is no conduction between emitter and collector, like an open switch. Are junction field-effect transistors (JFETs) considered the same? Why or why not?
Reveal answerJFETs are “normally-on” devices.
Notes:Ask your students to elaborate on the answer given. Do not accept a mindless recitation of the answer, “JFETs are normally-on devices,” but rather demand that some sort of explanation be given as to why JFETs are normally-on devices.
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Question 8 of 43
Match the following field-effect transistor illustrations to their respective schematic symbols:
Reveal answer
Notes:Be sure to ask your students to identify which symbol is the “P-channel” and which is the “N-channel” transistor!
It might help to review diode symbols, if some students experience difficulty in matching the designations (P-channel versus N-channel) with the schematic symbols.
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Question 9 of 43
Based on these DC continuity tester indications, what type of JFET is this, N-channel or P-channel?
- Resistance with negative test lead on pin 1, positive test lead on pin 2: no continuity
- Resistance with negative test lead on pin 1, positive test lead on pin 3: no continuity
- Resistance with negative test lead on pin 2, positive test lead on pin 1: continuity
- Resistance with negative test lead on pin 2, positive test lead on pin 3: continuity
- Resistance with negative test lead on pin 3, positive test lead on pin 1: continuity
- Resistance with negative test lead on pin 3, positive test lead on pin 2: continuity
Also, to the best of your ability, identify the transistor’s three terminals (source, gate, and drain).
Reveal answerThis is an N-channel JFET. Pin 1 is the gate, and pins 2 and 3 are drain/source or source/drain (interchangeable).
Notes:Advise your students about the risks of using an analog multimeter (in ohmmeter mode) to test semiconductor components. Some inexpensive analog multimeter designs actually switch the polarity of the test leads when in the ohmmeter mode. In other words, the red test lead actually connects to the negative side of the meter’s internal battery, while the black test lead connects to the positive side of the internal battery! If you are used to associating red with positive and black with negative, this switch will be quite a surprise.
Ask your students: what effect would a switch in polarity such as the one just described have on the determination of a transistor’s identity? What if the person thought their meter’s red lead was positive and the black lead negative, when in fact it was just the opposite? Would this affect their ability to accurately identify the transistor’s terminals? Why or why not?
