Electronic Devices Questions and Answers

Answer: D

The forward bias characteristic is an exponentially increasing curve.

The diode current follows \(I = I_{0}\left(e^{eV/kT} - 1\right)\). In forward bias, the current stays very small until the voltage approaches the barrier potential (about 0.7 V for silicon), and then rises steeply, growing exponentially with the voltage.

A straight line would mean an ohmic conductor, which the diode is not. The curve is not parabolic or sinusoidal, and it increases rather than decreases with forward voltage.

Answer: B

A p-n junction rectifies because it blocks current in reverse bias.

A diode conducts easily when forward biased and passes only a tiny current when reverse biased. Fed with an alternating voltage, it lets through only the half cycles that forward bias it, turning ac into a one-way (pulsating dc) current. That is rectification.

If it conducted equally in both directions, or simply let ac pass, there would be no rectification. A diode does not amplify a signal (a transistor does), and rectification has nothing to do with a phase difference between voltage and current.

Answer: C

In a p-type semiconductor, \(n_{h} \gg n_{e}\).

A p-type semiconductor is made by doping with trivalent atoms (such as boron), each of which accepts an electron and creates a hole. Holes become the majority carriers and electrons the minority carriers, so the hole concentration greatly exceeds the electron concentration. At equilibrium \(n_{e}n_{h} = n_{i}^{2}\).

\(n_{e} = n_{h}\) holds for an intrinsic semiconductor, and \(n_{e} \gg n_{h}\) for n-type. \(n_{e} = 2n_{h}\) and \(n_{h} + n_{e} = n_{h}n_{e}\) are not physical relations.

Answer: B

The numbers of states are 4N and 4N.

In a crystal of N atoms of silicon or germanium, the outer s and p levels (2N + 6N = 8N states) split into two bands of 4N states each: a lower valence band and an upper conduction band.

At absolute zero all 4N valence electrons fill the 4N valence states and the 4N conduction states are empty. The options with 0 confuse the number of states with the number of electrons occupying them.

Answer: C

A capacitor in parallel with the load filters the ripples.

A capacitor connected across (in parallel with) the load charges up to the peak voltage and then discharges slowly through the load while the rectified voltage falls, smoothing the output. It offers a low-impedance path to the ac ripple and blocks the dc.

A capacitor in series with the load would block the dc entirely. An inductor works as a filter when placed in series with the load, opposing changes in current, not in parallel with it. The combinations listed put both elements on the same side, which is not how an LC filter is arranged.

Answer: C

When forward biased, the width of depletion layer is reduced.

The forward bias voltage opposes the junction's built-in potential. Majority carriers (holes from the p-side, electrons from the n-side) are pushed towards the junction, the depletion region narrows, the barrier falls, and a large current flows.

So the majority carrier current is large, not zero, and the junction resistance is small, not large. The depletion layer widens under reverse bias, not forward bias. The minority carrier current is small in either bias.

Answer: E

The incorrect statement is the energy band gap of Ge is greater than 4 eV.

Germanium's band gap is only about 0.7 eV, which is what makes it a semiconductor. A gap above about 3 eV is typical of insulators; diamond's is about 5.5 eV.

The other statements are correct: germanium and silicon crystallise in the diamond-like lattice, silicon has four electrons in its outermost shell, semiconductors have band gaps below about 3 eV, and in pure (intrinsic) germanium every electron freed into the conduction band leaves a hole, so the numbers of free electrons and holes are equal.

Answer: B

Such a solid is a metal.

When the conduction band and the valence band overlap, there is no energy gap: electrons can move into empty states with no extra energy, so the material conducts freely. That is the band picture of a metal.

Semiconductors, whether elemental, inorganic or organic, have a small gap (about 1 eV for silicon) that electrons must be given energy to cross. Insulators have a large gap (several eV). Neither has overlapping bands.

Answer: E

The correct statement is The potential barrier in a forward biased p-n junction gets reduced.

In forward bias, the applied voltage opposes the built-in potential across the depletion layer, so the barrier falls and majority carriers can cross the junction; current flows.

The others are wrong. An intrinsic semiconductor is pure; it is an extrinsic one that has trivalent or pentavalent impurities, and so pure Ge or Si is intrinsic, not extrinsic. A semiconductor's conductivity increases with temperature, so reducing the temperature lowers it. And rectifiers are built from p-n junction diodes.

Answer: A

The change in current is 300 mA.

The forward bias (dynamic) resistance relates a change in voltage to the change in current it produces: \(r = \frac{\Delta V}{\Delta I}\).

\[ \begin{aligned} \Delta I &= \frac{\Delta V}{r} = \frac{0.9 - 0.6}{1} \\ &= 0.3 \text{ A} = 300 \text{ mA} \end{aligned} \]

750 mA is the average of the two voltages divided by 1 Ω, not the change, and the other values do not come from \(\Delta V / r\).