Amateur Radio Quiz: The Transistor, Our Three-Legged Friend
By H. Ward Silver, N0AX
ARRL Contributing Editor
What piece of active equipment in the ham shack doesn't have a transistor in it somewhere? Could you troubleshoot that equipment? While transistors are nearly ubiquitous, we often don't understand their inner workings as well as we might like. Here are some sharp questions to put an edge on your solid-state know-how.
1) What parameter is used to specify the current gain of a bipolar transistor in a common emitter configuration?
a. Alpha (A)
b. Beta (B)
c. Mu (M)
d. Rho (P)
2) Adding impurities to a pure semiconductor, creating p-type or n-type material is called:
c. Stagger tuning
3) In the acronym "MOSFET," "MOS" stands for
a. Metal Oxide Semiconductor
b. Metal On Sapphire
c. Metal Over Silicon
d. Metallic Organic Source
4) In a transistor fails from excessive voltage, which is the most likely cause of failure?
a. Thermal runaway
b. Miller effect
c. Avalanche breakdown
d. Parasitic oscillation
5) What is the term that describes FET gate bias reaching the point at which current flow from drain to source stops?
c. Charge starvation
6) True or False: To cause current to flow in an NPN transistor, electrons must flow into the base terminal.
7) True or False: In an FET, increasing device temperature causes drain current to decrease.
8) True or False: Bipolar transistors for RF power amplifiers require impedance matching to drive a 50 Ω load because their collector impedance is very low.
9) Match the bipolar and field-effect terminals that have similar functions:
a) Emitter d) Drain
b) Base e) Source
c) Collector f) Gate
10) Which parameter specifies the maximum voltage that a bipolar transistor can withstand from the collector to emitter with the base terminal open?
11) Which instrument is used to determine the linear region for a transistor?
a. Network analyzer
c. Curve tracer
d. Beta tester
12) In a bipolar transistor, the charge carriers are
a. electrons and positrons.
b. electrons and holes.
d. torts and writs.
Bonus: Which type of transistor was invented first -- bipolar or field-effect?
2. d -- The "dopants" are elements whose atoms have one more or one less electron than those of the semiconductor in their outer shell.
3. a -- The gate of the MOSFET is a deposit of metal on a layer of insulating silicon oxide over the underlying source-drain channel.
4. c -- The avalanche is of electrons that are driven across the transistor's internal potential barriers, causing current to flow unchecked through the transistor.
5. a -- Viewed in cross-section through the source-drain channel, gate bias gradually reduces the area through which current flows, eventually reducing it to zero, as in pinching off water flow through a hose.
6. False -- Conventional current (the flow of positive charge) flows into the NPN base, but electronic current (the flow of electrons) must flow out of the base.
7. True -- Because channel resistance increases with temperature, reducing current, this helps prevent the thermal runaway effect present in bipolar transistors.
8. True -- Sometimes the output impedance of a large transistor can be a fraction of an ohm!
9. a-e, b-f, c-d
10. a -- "BV" stands for breakdown voltage, "CE" stands for collector-to-emitter and "O' stands for remaining terminal open, which is the base.
11. c -- Curve tracers present a graph of many combinations of current and voltage through a device.
12. b -- See also the answer to questions 2 and 6.
Bonus: Field-effect devices based on copper-oxide were first described in the 1920s and 1930s, while the bipolar device was invented in 1948. The bipolar device proved practical to fabricate, though, and became widely used.
Author's Note: The Wikipedia entries for the bipolar transistor and field-effect transistor go into considerable detail about the transistor's functions. You can also learn more about circuit design with transistors in The ARRL Handbook for Radio Communications.