ISCET Semiconductor Device Principles 5 — Questions and Answers
Question 1: What is the relationship between the diffusion length (L) and minority carrier lifetime (τ) and diffusivity (D)?
- L = D × τ
- L = √(D × τ) (Correct answer)
- L = D / τ
- L = τ / D
Correct answer: L = √(D × τ)
Diffusion length L = √(Dτ), representing the average distance a minority carrier travels before recombining.
Question 2: An NPN transistor is in saturation when:
- VBE < 0.7 V and VBC > 0
- Both the emitter-base and collector-base junctions are forward biased (Correct answer)
- VCE > VCE(sat) and IB is maximum
- The collector current equals zero
Correct answer: Both the emitter-base and collector-base junctions are forward biased
Saturation occurs when both junctions (E-B and C-B) are forward biased, causing VCE to drop to VCE(sat) ≈ 0.2 V.
Question 3: What causes hot-carrier degradation in short-channel MOSFETs?
- Low drain-to-source voltage creating insufficient carrier energy
- High electric field near the drain accelerating carriers to energies sufficient to cause impact ionization or oxide injection (Correct answer)
- Excessive gate current through the thin oxide
- Body effect shifting the threshold voltage
Correct answer: High electric field near the drain accelerating carriers to energies sufficient to cause impact ionization or oxide injection
In short-channel devices, the high lateral field near the drain accelerates carriers to high energies ('hot'), causing impact ionization, substrate current, and oxide trap creation.
Question 4: The unity-gain frequency (fT) of a BJT is defined as the frequency at which the common-emitter current gain (|hFE|) equals:
- 0.707
- 0
- 1 (Correct answer)
- β₀ / √2
Correct answer: 1
fT is the transition frequency where the magnitude of the short-circuit common-emitter current gain falls to unity (1).
Question 5: Which doping profile produces an abrupt p-n junction as opposed to a graded junction?
- Diffusion doping with a Gaussian profile
- Ion implantation with a step-function profile at the metallurgical junction (Correct answer)
- Epitaxial growth with linearly varying doping
- Thermal diffusion from a limited source
Correct answer: Ion implantation with a step-function profile at the metallurgical junction
A step-function (abrupt) doping change at the metallurgical junction, achievable with ion implantation, defines an abrupt p-n junction.
Question 6: In the MOSFET body effect, applying a reverse body-to-source voltage (VBS < 0 for N-channel) causes VTH to:
- Decrease, making the device easier to turn on
- Increase, requiring a higher VGS to invert the channel (Correct answer)
- Remain unchanged
- Drop to zero
Correct answer: Increase, requiring a higher VGS to invert the channel
A reverse body bias widens the depletion region under the channel, requiring more gate voltage to achieve inversion, thus increasing VTH.
Question 7: What is the primary advantage of using a Darlington transistor pair configuration?
- Lower forward voltage drop than a single BJT
- Very high current gain (β_total ≈ β₁ × β₂) with improved drive capability (Correct answer)
- Faster switching speed than a single transistor
- Lower saturation voltage than a single BJT
Correct answer: Very high current gain (β_total ≈ β₁ × β₂) with improved drive capability
A Darlington pair multiplies the individual transistor beta values (β_total ≈ β₁ × β₂), providing very high current gain ideal for driving heavy loads from small input currents.
ISCET Semiconductor Device Principles 5
What is the relationship between the diffusion length (L) and minority carrier lifetime (τ) and diffusivity (D)?