Electronic Devices Questions and Answers

Answer: B

The peak output current is 0.07 A.

250 V is the rms value, so the peak voltage is \(250\sqrt{2} \approx 353.6\) V. In a full-wave rectifier one diode conducts at a time, in series with the load:

\[ \begin{aligned} I_{0} &= \frac{V_{0}}{r_{d} + R_{L}} = \frac{353.6}{10 + 5000} \\ &\approx 0.0706\ \text{A} \approx 0.07\ \text{A} \end{aligned} \]

Using 250 V as the peak gives 0.05 A, the trap in option A.

Answer: E

Reverse biasing increases the potential barrier.

In reverse bias the p side is connected to the negative terminal, so the applied voltage adds to the built-in barrier. The depletion layer widens and the barrier height rises, which stops majority carriers from crossing.

Biasing does not change the number of carriers: majority and minority carrier concentrations are set by doping and temperature. Decreasing the barrier is what forward bias does.

Answer: A

The band gap of silicon is about 1.1 eV.

At room temperature silicon has an energy gap of about 1.1 eV between the valence and conduction bands, small enough for some thermal excitation, which makes it a semiconductor.

Germanium's gap is about 0.7 eV, which is the trap in option B (0.7 V is also the familiar forward voltage of a silicon diode). Values near 2 eV belong to wider-gap materials, and 0.5 eV is not silicon's gap.

Answer: A

When a diode is forward biased, the p-n junction provides very low resistance.

In forward bias the p side is connected to the positive terminal. The applied voltage opposes the built-in potential, so the potential barrier is lowered and the depletion layer becomes narrower. Majority carriers (holes from the p side, electrons from the n side) then cross the junction easily, giving a large current, of the order of milliamperes, for a small voltage: the junction offers low resistance.

The other statements describe reverse bias: a wider depletion layer, a higher potential barrier, and a tiny current of the order of microamperes carried by minority carriers.

Answer: C

Semiconductors are made to conduct more by adding impurity elements.

Doping a pure semiconductor with a small amount of a group 15 element (n-type) or a group 13 element (p-type) adds free electrons or holes and raises its conductivity by orders of magnitude. This is the basis of all semiconductor devices.

The other statements are wrong: semiconductors have a negative temperature coefficient of resistance; their band gap is small, about 1 eV (a gap above 3 eV marks an insulator); silicon and germanium are in group 14 (IV), not VI; and their carriers are electrons and holes, not ions.

Answer: E

The keyed statement is that a centre tapped transformer is used.

The standard two-diode full-wave rectifier uses a centre-tapped transformer: each half of the secondary feeds one diode, so one diode conducts in the positive half cycle and the other in the negative half cycle, and the load receives current in both halves.

That rules out the other statements: output appears in both half cycles, not the positive one only; the two diodes never conduct together; and each diode is forward biased in one half cycle and reverse biased in the other, not permanently. (A bridge rectifier reaches full-wave output without a centre tap, using four diodes, but the statement fits the two-diode circuit the question describes.)

Answer: B

Without any bias the diode acts as a resistor.

With no voltage applied, the junction is in equilibrium: the depletion layer and its barrier potential are set up, and no net current flows. To a small test signal it simply presents a resistance, the high resistance of the depletion region.

Rectification is what the diode does when an alternating voltage biases it forward and reverse in turn, and voltage regulation needs a Zener diode under reverse bias. A diode generates no ac and cannot transform voltages, so the remaining options describe other devices.

Answer: B

The resistance of a semiconductor decreases with increase in temperature.

As temperature rises, more electrons gain enough energy to cross the band gap into the conduction band, creating more electron-hole pairs. The number of charge carriers increases rapidly, so the resistance falls: semiconductors have a negative temperature coefficient of resistance.

Increasing resistance with temperature is the behaviour of metals. The resistance does not become independent of temperature, infinite or zero.

Answer: E

The total number of outer electrons is 4 N.

Germanium is in group 14 and has four valence electrons ([Ar]3d¹⁰4s²4p²). In the crystal each atom shares these four electrons in covalent bonds with four neighbours, so N atoms contribute 4N outer electrons.

The other multiples correspond to elements with 1, 2, 3 or 6 valence electrons.

Answer: A

The donor level lies just below the conduction band.

In an n-type semiconductor, the pentavalent impurity atoms have a fifth electron that is only loosely bound. Its energy level lies a little below the bottom of the conduction band (about 0.01 eV in Ge and 0.05 eV in Si), so at room temperature it is easily excited into the conduction band.

Acceptor levels in p-type material lie just above the valence band. The donor level is not in the middle of the gap, on or below the valence band, or above the conduction band.