Semiconductor and Tube Components Flashcards
7 cards from real GROL practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Semiconductor and Tube Components flashcards as text
A magnetron tube generates microwave energy through:
Answer: The interaction of a rotating electron cloud with surrounding resonant cavities in a crossed DC electric and magnetic field
Electrons emitted from the central cathode spiral outward under crossed radial electric and axial magnetic fields, coupling energy to the resonant anode cavity slots to produce high-power microwave oscillations.
A traveling-wave tube (TWT) amplifier achieves wide bandwidth because:
Answer: Its helical slow-wave structure matches the electromagnetic wave's phase velocity to the electron beam velocity, enabling continuous energy transfer over a broad frequency range
The helix slows the wave to approximately the electron beam velocity; since no narrow-band resonant cavities are used, TWTs can amplify across 10–100% bandwidth around the center frequency.
Light emission from an LED is produced by the process of:
Answer: Electroluminescence — the release of photons when electrons and holes recombine at the forward-biased P-N junction
In direct-bandgap semiconductors such as GaAs, GaN, and InGaAsP, forward-bias injection causes electron-hole recombination that releases the bandgap energy directly as photons.
Operating a photodiode under reverse bias rather than zero bias provides:
Answer: A wider depletion region that improves light absorption, photocurrent linearity, and response speed
Reverse bias widens the depletion region to capture more photons, reduces junction capacitance for faster response, and improves linearity — this is the standard photoconductive operating mode.
An optocoupler (opto-isolator) achieves electrical isolation between two circuits by:
Answer: Transmitting the signal as light from an LED to a photodetector with no direct electrical path between input and output circuits
An LED and a photodetector are packaged together; the light path carries the signal while the physical gap provides thousands of volts of electrical isolation between the two circuits.
In a cascaded microwave receiver chain, the overall system noise figure is dominated primarily by the first stage because:
Answer: Per the Friis formula, noise contributions of subsequent stages are divided by the gain of all preceding stages when referred back to the input
The Friis formula shows each succeeding stage's noise is attenuated by the cumulative gain of prior stages; therefore a high-gain, low-noise first stage (LNA) has the greatest impact on system noise figure.
A Gunn diode generates microwave oscillations due to:
Answer: The transferred-electron (Gunn) effect in bulk GaAs, where high electric fields transfer electrons to a lower-mobility conduction valley, creating negative resistance
Above a threshold field in GaAs, electrons scatter from the high-mobility Γ valley to the low-mobility L valley; current then decreases as voltage increases, producing the negative resistance that sustains oscillation.