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Insulated Gate Field-Effect Transistors
39 questions By Tony R. Kuphaldt
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Question 37 of 39
A special type of insulated-gate field-effect transistor is the dual gate MOSFET, shown here:
Draw a schematic diagram using normal (single-gate) MOSFETs, equivalent to this dual-gate MOSFET.
Reveal answer
Notes:A pretty simple answer to this question, but the real purpose is to challenge students to think of complex circuit elements in terms of equivalent circuits comprised of simple, idealized components.
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Question 38 of 39
∫f(x) dx Calculus alert!
A potential problem for power MOSFETs is \(\frac{dv}{dt}\) induced turn-on. Explain why a MOSFET may turn on when it’s not supposed to, given an excessive \(\frac{dv}{dt}\) condition.Reveal answerIf the drain voltage rate-of-change over time \(\frac{dv}{dt}\) is excessive, the transistor may turn on due to the coupling effect of gate-to-drain capacitance (CGD).
Challenge question: draw an equivalent schematic diagram showing the parasitic CGD capacitance, and write the equation relating capacitive current to instantaneous voltage change over time.
Notes:This question is a good review of capacitor theory and calculus notation. Ask your students to explain exactly what [dv/dt] means, and how it relates to current in a circuit containing capacitance.
The problem of dv/dt induced turn-on is not unique to power MOSFETs. Various thyristors, most notably SCRs and TRIACs, also exhibit this problem.
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Question 39 of 39
Find one or two real insulated-gate field-effect transistors and bring them with you to class for discussion. Identify as much information as you can about your transistors prior to discussion:
- Terminal identification (which terminal is gate, source, drain)
- Continuous power rating
- Typical transconductance
Note: be careful to keep your transistors in anti-static foam as much as possible, to avoid damage to the gate from electrostatic discharge.
Reveal answerIf possible, find a manufacturer’s datasheet for your components (or at least a datasheet for a similar component) to discuss with your classmates. Be prepared to prove the terminal identifications of your transistors in class, by using a multimeter!
Notes:The purpose of this question is to get students to kinesthetically interact with the subject matter. It may seem silly to have students engage in a “show and tell” exercise, but I have found that activities such as this greatly help some students. For those learners who are kinesthetic in nature, it is a great help to actually touch real components while they’re learning about their function. Of course, this question also provides an excellent opportunity for them to practice interpreting component markings, use a multimeter, access datasheets, etc.
