Semiconductor Electronics – NEET UG PhysicsPractice Questions & PYQs

An ideal diode acts as a closed switch when forward-biased and an open switch when reverse-biased. Upon examining the circuit, the upper diode is forward-biased and permits current flow, whereas the lower diode is reverse-biased, cutting off any current in its respective path. Therefore, the total current delivered by the battery only encounters the active branch containing the 10 ohm resistance. Applying Ohm's law via the equation $I = \frac{V}{R}$, the current is evaluated as $I = \frac{5}{10}$, resulting in a value of 0.5 A.

The current gain in a common emitter configuration is defined by the equation $\beta = \frac{\Delta I_C}{\Delta I_B}$, which relates the change in collector current to the corresponding change in base current. The change in collector current is calculated as $20\text{ mA} - 10\text{ mA} = 10\text{ mA}$, and the change in base current is evaluated as $300\mu\text{A} - 100\mu\text{A} = 200\mu\text{A}$. Substituting these computed changes into the formula gives $\beta = \frac{10\times 10^{-3}\text{A}}{200\times 10^{-6}\text{A}} = 50$.

A Zener diode voltage regulator maintains a steady output voltage by absorbing excess current, governed fundamentally by Ohm's law ($I = \frac{V}{R}$) applied across each branch of the circuit. The parallel load resistor experiences a constant voltage equal to the Zener breakdown voltage of 15 V, drawing a current of 15 mA through its 1 kohm resistance. Subtracting this voltage from the 20 V input supply leaves a voltage drop of 5 V across the 250 ohm series resistor, which drives a total source current of 20 mA into the network. The current passing through the Zener diode is finally determined by subtracting the load current from this total source current, yielding a remaining value of 5 mA.

The mass-action law for semiconductors states that the product of electron and hole concentrations equals the square of the intrinsic carrier concentration, expressed as $n_e n_h = n_i^2$. Introducing a trivalent impurity like indium increases the hole concentration, creating a p-type material. Substituting the given values into the rearranged relation yields $n_e = \frac{(1.5 \times 10^{16})^2}{4.5 \times 10^{22}} = \frac{2.25 \times 10^{32}}{4.5 \times 10^{22}} = 5 \times 10^9 \text{ m}^{-3}$.

Doping a tetravalent semiconductor like germanium with a pentavalent impurity such as antimony introduces excess valence electrons into the crystal lattice. Each antimony atom donates a free electron, transforming the material into an n-type semiconductor where electrons outnumber holes. Consequently, the concentration of free electrons becomes significantly greater than the concentration of holes acting as the minority carriers.

Forward biasing a p-n junction requires connecting the positive terminal of an external power source to the p-type semiconductor and the negative terminal to the n-type semiconductor. This external voltage polarity directly opposes the built-in potential barrier of the junction. As a result, the barrier height is reduced, which narrows the depletion region and allows charge carriers to flow easily across the junction.

A semiconductor diode is forward biased whenever its p-side is connected to a higher potential relative to its n-side. Inspecting the provided diagrams shows that this exact condition is met in circuits (A) and (C) where the p-terminal sits at a greater voltage than the n-terminal. Consequently, the diodes in these two particular setups are the ones operating in forward bias.

Logic gates are fundamental digital building blocks identified by distinct schematic symbols that dictate their specific Boolean functions. Recognizing these shapes reveals that symbol (ii) corresponds to the AND operation, symbol (iv) depicts the combined function of a NAND gate, and symbol (iii) represents the single-inverting action of a NOT gate.

Proper transistor operation requires specific doping profiles and biasing configurations to ensure efficient charge carrier transport from the emitter to the collector. The base region must be kept very thin and lightly doped to minimize the recombination of electrons and holes, allowing most injected carriers to successfully reach the collector. Furthermore, standard active-mode operation demands that the emitter-base junction remain forward-biased to inject carriers while the base-collector junction stays reverse-biased to sweep them across the barrier.

The power gain of an amplifier is determined by the product of its voltage gain and current gain, which can also be expressed using the square of the voltage gain multiplied by the input-to-output resistance ratio as $A_p = A_v^2 \left(\frac{R_i}{R_o}\right)$. Substituting the given parameters into this relation gives $A_p = (50)^2 \left(\frac{100}{200}\right) = 2500 \times 0.5 = 1250$.

Semiconductor conductivity relies on charge carriers whose types and distributions depend on thermal excitation and specific chemical doping. In an n-type semiconductor created by adding pentavalent impurities, thermal energy and donor atoms ensure that electrons serve as the majority charge carriers while holes function as the minority carriers. Therefore, stating that holes are the majority carriers in an n-type material contradicts fundamental semiconductor physics.

The logical operation of the given circuit is governed by the Boolean expression $Y = (A + B) \cdot C$. To achieve an output of $Y = 1$, both the OR term $(A + B)$ and the input $C$ must evaluate to $1$. Substituting the values $A = 1$, $B = 0$, and $C = 1$ yields $1 + 0 = 1$ for the first part, which then multiplies by $1$ to successfully produce the required output.

A NAND gate produces a low output exclusively when all of its inputs are high, while yielding a high output for any other combination of input states. Inspecting the provided voltage waveforms reveals that the output wave remains high whenever input A, input B, or both are low, dropping to zero only when both input waveforms are simultaneously high. This distinct behavior matches the characteristic truth table of a NAND gate.

Miniaturization in modern electronics allows entire electronic systems containing numerous transistors, resistors, and capacitors to be fabricated onto a single continuous semiconductor chip. This multifunctional microelectronic assembly is known as an integrated circuit, capable of performing complex processing and signal conditioning tasks independently. Unlike discrete components such as individual diodes or transistors, this compact architecture embodies a complete operational circuit within one package.

Logic gates are fundamental building blocks of digital circuits, and each type is instantly recognizable by its unique standardized graphical symbol. The standard OR gate is depicted by symbol (iv) with its characteristic curved input face, the NOT gate is represented by the triangular inverter shape in symbol (ii), and the NAND gate appears as symbol (i) featuring an AND profile followed by a negation bubble. Recognizing these distinct structural features allows you to correctly identify each gate arrangement within a circuit diagram.

A semiconductor photodiode can detect incident radiation only when the photon energy is greater than or equal to the band gap of the material. The maximum detectable wavelength is given by the formula $\lambda = \frac{12400}{E}$, where energy is in electron-volts and the resulting wavelength is in Angstroms. Substituting the band gap value yields a threshold wavelength of 4960 Å, meaning any wavelength shorter than this value carries enough energy to excite the material. Therefore, a signal of 4000 Å can be successfully detected by the photodiode.

The current gain of a common-emitter transistor amplifier represents the ratio of the change in collector current to the corresponding change in base current, given by the formula $\beta = \frac{\Delta I_C}{\Delta I_B}$. Substituting the specified increments into this relationship yields the calculation $\beta = \frac{5 \times 10^{-3}}{100 \times 10^{-6}}$, which simplifies directly to a current gain of 50.

The atomic radius in a face-centered cubic lattice is determined by the geometric relation $r = \frac{a}{2 \sqrt{2}}$, where $a$ represents the edge length of the unit cell. Substituting the lattice parameter value of 3.6 Å into this expression gives the calculation $r = \frac{3.6}{2 \sqrt{2}}$, which evaluates to 1.27 Å.

The minimum frequency of radiation required to excite a photodiode is determined by relating the photon energy to the semiconductor band gap through the governing expression $\nu = \frac{E}{h}$. Alternatively, determining the maximum threshold wavelength via the relation $\lambda = \frac{12400}{E}$ and substituting it into the wave equation $\nu = \frac{c}{\lambda}$ results in the evaluated frequency of $\nu = 5 \times 10^{14}\text{ Hz}$.

A NOR gate is realized by cascading an OR gate with a NOT gate, which inverts the cumulative OR output of the input signals. The resulting Boolean expression for this configuration is represented as $Y = \overline{A + B}$, yielding a high output exclusively when both inputs are low. Tracing the signal through the individual logic components of the given circuit confirms that it replicates this exact truth table behavior.