Unit 2 — Introduction to Modern Physics · Physics. These are 10 questions from the Magmo3 bank on this lesson, from easy to hard. Solve each one, then open "Show answer and explanation". Magmo3 has 54 questions on this lesson, each with an explanation.
When a silicon crystal is doped with a pentavalent element (five electrons in its outermost energy level), the majority charge carriers in the resulting crystal are the ............
Correct answer: (A) electrons
Explanation: A pentavalent donor supplies an extra free electron, so electrons are the majority carriers (n-type).
The main function of a p-n junction diode in an electric circuit is to ............
Correct answer: (A) allow the current to pass in one direction only
Explanation: A diode conducts easily in one direction only, so it is used for rectification.
Adding a small, controlled amount of a suitable impurity to a pure semiconductor crystal in order to increase its electric conductivity is called ............
Correct answer: (A) doping
Explanation: This deliberate addition of impurity atoms to a semiconductor is called doping.
Which of the following elements, when added to a pure silicon crystal, produces an n-type semiconductor?
Correct answer: (D) phosphorus
Explanation: Phosphorus is pentavalent (a donor) and produces an n-type crystal, while the other listed elements are trivalent.
The very small (leakage) current that flows through a p-n junction diode when it is reverse biased is due to the ............
Correct answer: (C) minority charge carriers
Explanation: In reverse bias only the minority charge carriers drift across the junction, giving a tiny leakage current.
The depletion region formed at a p-n junction contains ............
Correct answer: (C) immobile (fixed) charged ions and no free charge carriers
Explanation: Diffusion and recombination near the junction leave only fixed charged ions, forming a region free of mobile carriers.
The energy gap of silicon (about 1.1 eV) is larger than that of germanium (about 0.7 eV). Therefore, at the same temperature, the number of thermally generated free charge carriers in silicon compared with germanium is ............
Correct answer: (A) smaller
Explanation: A larger energy gap makes it harder to free electrons, so silicon has fewer thermally generated carriers than germanium.
In a transistor, the collector current is 9.8 mA and the emitter current is 10 mA. The current factor (alpha = IC / IE) equals ............
Correct answer: (B) 0.98
Explanation: alpha = IC / IE = 9.8 / 10 = 0.98.
A silicon diode of threshold voltage 0.7 V is connected in forward bias in series with a resistor of 400 ohm to a source of e.m.f. (VB) 4.7 V. The potential difference across the resistor equals ............
Correct answer: (A) 4 V
Explanation: VR = VB - V_diode = 4.7 - 0.7 = 4 V.
A pure silicon crystal has an intrinsic carrier concentration nᵢ = 1.5 × 10¹⁶ m⁻³. It is doped with pentavalent atoms of concentration 5 × 10²² m⁻³. The concentration of holes in the doped crystal is approximately…
Correct answer: (C) 4.5 × 10⁹ m⁻³
Explanation: The crystal becomes n-type with n ≈ N_D⁺ = 5 × 10²² m⁻³; by the law of mass action p = nᵢ²/n = (1.5 × 10¹⁶)²/(5 × 10²²) = 4.5 × 10⁹ m⁻³, far fewer holes than in pure silicon.
The first 10 questions in every subject are free, each with an explanation.