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Insulated Gate Field-Effect Transistors
39 questions By Tony R. Kuphaldt
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Question 16 of 39
Identify each type of MOSFET (whether it is N-channel or P-channel, D-type or E-type), label the terminals, and determine whether the MOSFET in each of these circuits will be turned on or off:
Reveal answer
Follow-up question: which of these transistors is depleted and which is enhanced?
Notes:It is very important for your students to understand what factor(s) in a circuit force a MOSFET to turn on or off. Some of the information contained in the diagrams is relevant to the determination of each transistor’s status, and some is not.
Be sure to ask your students to explain their reasoning for each transistor’s status. What factor, or combination of factors, is necessary to turn a MOSFET on, versus off?
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Question 17 of 39
Explain why a circuit designer would choose a MOSFET over a bipolar transistor for a certain application. What advantage(s) does a MOSFET have over a bipolar transistor?
Challenge question: prove your point by comparing parametric ratings from two transistor datasheets, one bipolar and the other an insulated-gate field effect. Be sure these two transistors have similar controlled current ratings (maximum collector current and drain current, respectively).
Reveal answerMOSFETs have extremely low input current “drive” requirements.
Notes:Ask your students to explain what “drive current” means in terms of transistor ratings. Also, ask them to explain why MOSFETs do not require as much drive current as bipolar transistors. Challenge them to prove their point by a comparison of datasheets.
Is low drive current the only advantage that MOSFETs enjoy over bipolar transistors? Pose this question to your students, to see if they investigated these respective devices any further than the question demanded.
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Question 18 of 39
What does the term transconductance mean, with reference to a field-effect transistor? Is the transconductance function for an FET a linear or a nonlinear relationship? Explain why, making reference to an equation if at all possible to explain your answer.
Reveal answer“Transconductance” refers to the amount of change in drain current for a given amount of change in gate voltage \((\frac{\Delta I_D}{\Delta V_G})\). The transconductance function for an FET is definitely nonlinear.
Challenge question: what unit of measurement would be appropriate for expressing transconductance in?
Notes:Transconductance is not just a parameter for JFETs, but also MOSFETs (IGFETs) and vacuum tubes. Any voltage-controlled current-regulating device has a transconductance value (though it may change over the operating range of the device, just as β changes over the operating range of a BJT).
