Semiconductor Electronics – NEET UG PhysicsPractice Questions & PYQs

The voltage gain of an amplifier with negative feedback is related to its open-loop gain by the feedback formula $A' = \frac{A}{1 + \beta A}$. Substituting the given feedback gain of 10 and the feedback factor of 0.09 into this expression generates the algebraic equation $10 = \frac{A}{1 + 0.09 A}$. Solving this equation for the open-loop gain yields a value of $A = 100$.

Logic gates process binary inputs through specific Boolean functions to determine the resulting output state. Evaluating the circuit combination reveals that an input pair where $A$ is 0 and $B$ is 0 yields an output $Y$ of 0. As long as at least one input receives a high signal of 1, the output $Y$ becomes 1, which holds true even when both inputs are simultaneously set to 1.

A cubic crystal system is characterized by the highest degree of symmetry among all crystal systems, possessing equal axial lengths and mutually perpendicular axes. Specifically, all three unit cell edge lengths are identical while the interfacial angles between them are all right angles. This unique geometric arrangement defines the fundamental parameters of a standard cubic lattice structure.

An AND gate functions such that its output is high only when all of its inputs are simultaneously high. Translating the provided voltage waveforms into a truth table reveals that the output $C$ is 1 solely when inputs $A$ and $B$ are both 1, whereas any combination involving a 0 results in a 0 output.

The current gain in a common emitter transistor configuration is defined by the equation $\beta = \frac{\Delta I_{\mathrm{C}}}{\Delta I_{\mathrm{B}}}$, which measures the ratio of the change in collector current to the corresponding change in base current. Computing the variations gives a collector current change of 5 mA and a base current change of 50 $\mu$A. Evaluating the expression $\beta = \frac{5 \times 10^{-3}}{50 \times 10^{-6}}$ yields a final value of 100.

Applying a forward bias to a p-n junction connects the positive terminal of the external source to the p-type region and the negative terminal to the n-type region, directly opposing the internal built-in electric field. This opposition neutralizes a portion of the immobile ions in the depletion region, causing the depletion layer to shrink and the potential barrier to lower. Consequently, the net electric field across the depletion zone is reduced, allowing majority charge carriers to easily cross the junction and establish a significant forward current.

Semiconductors possess a negative temperature coefficient of resistance because thermal agitation excites more electrons across the forbidden energy gap. This process significantly boosts the density of charge carriers available for conduction as the temperature rises. Consequently, electrical conductivity increases and resistivity decreases, making the statement claiming increasing resistivity entirely incorrect.

In a face-centered cubic crystal structure, the interatomic spacing $d$ represents the distance between nearest neighbor atoms, which is connected to the lattice constant $a$ by the geometric relation $d = \frac{a}{\sqrt{2}}$. Rearranging this expression yields the formula $a = d \sqrt{2}$. Substituting the given interatomic spacing into this equation provides the calculation $2.54 × 1.414$, which evaluates to 3.59 angstroms.

Operating in the reverse breakdown region, a Zener diode maintains a nearly constant voltage across its terminals despite wide fluctuations in the input current or load. This unique characteristic makes it exceptionally well suited for voltage regulation and circuit stabilization. Other components like standard p-n junction diodes are typically employed for tasks such as rectification rather than maintaining constant voltage levels.

The energy band gap in group 14 elements strongly depends on the principal quantum number and the overlap of atomic orbitals, becoming smaller as atomic size increases down the group from carbon to germanium. Since carbon has the smallest atoms and tightly bound valence electrons in the second shell, it possesses an exceptionally large band gap of 5.5 eV. In contrast, silicon and germanium have larger atomic radii and looser outer electrons, resulting in significantly smaller band gaps, which makes the band gap of carbon substantially greater than that of silicon.

When monochromatic light illuminates a p-n junction photocell, photons are absorbed near the junction to generate electron-hole pairs via the photovoltaic effect. The accumulation of these photo-generated charge carriers across the barrier establishes a potential difference known as the photo-electromotive force. Because a higher photon influx creates a proportionally larger concentration of separated charges, the magnitude of the resulting photo-electromotive force is directly proportional to the intensity of the incident light.

An OR gate performs logical disjunction, meaning its output evaluates to true whenever at least one of its inputs is high. In binary terms, this translates to an output of 1 if either input line receives a 1, or if both input lines simultaneously receive a 1. Therefore, the output is 1 whenever either or both inputs are active at that level.

A p-n junction diode becomes reverse biased when its p-type semiconductor terminal is maintained at a lower electrical potential compared to its n-type counterpart. Among the provided options, this specific configuration is uniquely present in circuit c, where the applied terminal voltages oppose the internal barrier potential. This setup widens the depletion region and effectively blocks major charge carrier movement.

The direct current component of the output voltage in a half-wave rectifier is given by the formula $V_{dc} = \frac{V_m}{\pi}$, where $V_m$ denotes the peak value of the input sinusoidal voltage. Substituting the given peak voltage of $10$ volts directly into this relationship yields $\frac{10}{\pi}$ volts for the average output.

Thermal agitation at room temperature provides sufficient kinetic energy to valence electrons, enabling them to cross the forbidden energy gap from the valence band into the conduction band. This thermal promotion leaves behind empty states in the previously full valence band while introducing mobile electrons into the previously empty conduction band. As a result of this continuous thermal excitation, the valence band becomes partially empty and the conduction band becomes partially filled.

The barrier potential of a semiconductor junction is fundamentally determined by internal material properties, impurity concentrations, and thermal conditions rather than physical manufacturing geometry. Temperature variations alter the intrinsic carrier concentration and thermal energy of carriers, while doping density modifies the built-in electric field across the depletion region, and external biasing directly modulates this potential barrier. Consequently, microscopic material characteristics and operational conditions dictate the height of this barrier, rendering macroscopic physical design parameters irrelevant to its magnitude.

Applying a reverse bias voltage to a p-n junction connects the external battery terminals in opposition to the built-in electric field, effectively widening the depletion region. This expansion sweeps majority carriers further away from the junction and deepens the potential hill that charges must cross. Consequently, the potential barrier increases significantly, restricting current flow to a tiny drift current of minority carriers.

Bipolar junction transistors require specific biasing polarities across their two internal junctions to establish steady-state carrier injection and collection. For an n-p-n transistor to conduct substantial current, its emitter-base junction must be forward-biased to inject electrons into the base, which requires the emitter to be negative relative to the base. Simultaneously, the collector-base junction must be reverse-biased to sweep those injected electrons across into the collector circuit, requiring the collector to be positive relative to the base.

The fundamental ripple frequency in the output of a full-wave rectifier is twice the frequency of the alternating input mains supply, governed by the relation $f_{ripple} = 2f_{\in}$. Given an input frequency of 50 Hz, multiplying this value by two results in a fundamental ripple frequency of 100 Hz because both half-cycles of the AC input are successfully redirected to the load.

A p-n junction diode changes its electrical resistance dramatically based on how it is connected in a circuit. When placed in forward bias, the diode offers negligible resistance, forcing practically all of the source voltage $V$ to drop across the resistor $R$. In contrast, a reverse-biased diode creates a massive resistance barrier that prevents current flow and shifts the entire voltage drop onto the diode itself.