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Semiconductor Devices Flashcards

7 cards from real ISCET practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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  1. What is thermal runaway in a BJT circuit, and what component is commonly used to prevent it?

    Answer: Uncontrolled increase in IC due to temperature rise; prevented by emitter resistor (RE)

    Thermal runaway occurs because rising temperature increases IC, which further heats the device; an emitter resistor provides negative feedback to stabilize the operating point.

  2. The SCR (Silicon Controlled Rectifier) will turn OFF when:

    Answer: The anode current drops below the holding current (IH)

    Once triggered, an SCR latches on and only turns off when the anode current falls below the minimum holding current (IH), regardless of the gate signal.

  3. In a common-emitter BJT amplifier, what is the phase relationship between the input and output voltages?

    Answer: 180° phase inversion

    The common-emitter configuration inverts the signal — a positive-going input produces a negative-going output — resulting in a 180° phase shift.

  4. What is the primary difference between a depletion-mode and an enhancement-mode MOSFET?

    Answer: Depletion-mode has a physical channel at VGS = 0; enhancement-mode does not

    Depletion-mode MOSFETs have a physically doped channel that exists at VGS = 0, while enhancement-mode MOSFETs require gate voltage to induce a channel.

  5. Which semiconductor device is a four-layer PNPN structure that acts as a bidirectional switch?

    Answer: TRIAC

    A TRIAC is a bidirectional thyristor that can conduct in both directions and is triggered by either polarity of gate signal, making it ideal for AC power control.

  6. The knee voltage of a diode's I-V characteristic represents:

    Answer: The voltage at which forward current increases rapidly

    The knee voltage (approximately 0.7 V for silicon) is the forward bias point where the diode transitions from high to very low resistance and current increases sharply.

  7. In a UJT (Unijunction Transistor), conduction begins when the emitter voltage reaches:

    Answer: The peak-point voltage VP, which equals ηVBB + VD

    The UJT fires (turns on) when the emitter voltage reaches VP = ηVBB + VD, where η is the intrinsic standoff ratio and VD is the diode drop (~0.7 V).