Electronic Devices

p-n Junction, Depletion Region and Diode Characteristics

The thickness of the depletion region in a p-n junction is typically:

**p-n junction formation** diffusion of holes from p to n and electrons from n to p leaves ionized donors positive on n-side and acceptors negative on p-side, forming space-charge depletion region with electric field directed from n to p (positive to negative), barrier potential V_b ≈0.3 V Ge, 0.7 V Si, opposes further diffusion, drift current due to minority carriers swept by field balances diffusion at equilibrium net current zero. The depletion region, formed by diffusion and drift, is very thin, on the order of one-tenth of a micrometer ( 0.1 μ m ), due to the sharp transition at the junction. Substituting values gives

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Semiconductors, Types and Energy Bands

A germanium diode has a threshold voltage of approximately:

**Semiconductor properties** distinguish from conductors and insulators by temperature dependence and doping response. At 0 K intrinsic acts as insulator, conductivity due to thermally generated electron-hole pairs, number of outer electrons 4 for Si/Ge forming covalent bonds, each atom shares electrons, crystal with N atoms has 4N valence electrons, 2N bonds. The threshold voltage for a germanium diode, where forward current increases significantly, is approximately 0.2 V, lower than silicon due to its smaller energy gap. Substituting values gives 0.2 V, which matches expected behaviour for this semiconductor device configuration, confirming doping, depletion and rectifier principles.

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Semiconductors, Types and Energy Bands

The conductivity of an extrinsic semiconductor increases due to:

**Types of semiconductors** elemental Si, Ge group IV with 4 valence electrons, compound GaAs, InP etc. Intrinsic has n_e = n_h, extrinsic doped with pentavalent donors (P, As) gives n-type excess electrons, trivalent acceptors (B, Al) gives p-type excess holes, resistivity range semiconductors 10⁻⁵ to 10⁶ Ω·m vs insulators 10¹¹ Ω·m. Doping introduces impurities (pentavalent or trivalent) that provide additional charge carriers (electrons or holes), significantly enhancing conductivity compared to intrinsic semiconductors. Substituting values gives Addition of impurities, which matches expected behaviour for this semiconductor device configuration, confirming doping, depletion and rectifier principles.

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p-n Junction, Depletion Region and Diode Characteristics

In a p-n junction, the positive space-charge region is located on:

**p-n junction formation** diffusion of holes from p to n and electrons from n to p leaves ionized donors positive on n-side and acceptors negative on p-side, forming space-charge depletion region with electric field directed from n to p (positive to negative), barrier potential V_b ≈0.3 V Ge, 0.7 V Si, opposes further diffusion, drift current due to minority carriers swept by field balances diffusion at equilibrium net current zero. During junction formation, electrons diffuse from n-side to p-side, leaving behind immobile positive ionized donors on the n-side, forming the positive space-charge region. Substituting values gives N-side, which matches expected behaviour for this semiconductor device

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Semiconductors, Types and Energy Bands

Which property distinguishes insulators from semiconductors?

**Semiconductor properties** distinguish from conductors and insulators by temperature dependence and doping response. At 0 K intrinsic acts as insulator, conductivity due to thermally generated electron-hole pairs, number of outer electrons 4 for Si/Ge forming covalent bonds, each atom shares electrons, crystal with N atoms has 4N valence electrons, 2N bonds. Insulators have a large energy gap ( E_g > 3 eV ), preventing electron excitation, while semiconductors have a smaller gap ( 0.2 eV to 3 eV ), allowing some conduction. Substituting values gives Energy gap, which matches expected behaviour for this semiconductor device configuration, confirming doping, depletion and rectifier principles.

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p-n Junction, Depletion Region and Diode Characteristics

The current in a forward-biased p-n junction diode is primarily due to:

**Diode characteristics** forward bias reduces barrier, width of depletion region decreases, resistance low ~10-100 Ω, current primarily due to majority diffusion, exponential I = I_s(e^{eV/kT}-1), reverse bias widens depletion region, resistance high ~MΩ, dominant current drift due to minority carriers, reverse saturation current small μA-nA, typically 10⁻⁶ A, increases sharply at breakdown Zener/avalanche. In forward bias, the applied voltage reduces the barrier height, allowing minority carriers to cross the junction and diffuse, resulting in a current (in mA) due to diffusion of holes and electrons. Substituting values gives Diffusion of charge carriers, which matches expected behaviour for this semiconductor device configuration, confirming doping, depletion and rectifier principles.

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Semiconductors, Types and Energy Bands

The number of valence electrons in Si and Ge atoms is:

**Energy bands in semiconductors** consist of valence band filled at 0 K and conduction band empty, gap E_g small ~1 eV (Si 1.1 eV, Ge 0.7 eV), insulators large gap >3 eV (C diamond 5.4 eV), conductors overlapping. Intrinsic semiconductor at 0 K behaves as insulator because no thermal excitation, at T>0 K electrons jump to conduction band leaving holes, conductivity increases with temperature. Si (third orbit) and Ge (fourth orbit) are group IV elements, each with four valence electrons (2s and 2p for Si, 4s and 4p for Ge), forming covalent bonds in their lattice. Substituting values gives 4, which matches expected behaviour for

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Rectifiers, Filters and Applications

In a rectifier with a capacitor filter, the capacitor discharges through:

**Rectifier applications** diode must have reverse breakdown voltage higher than peak inverse voltage, centre-tap transformer provides two opposite phase voltages for full-wave, capacitor discharges through load R_L when diode off, drift current in junction is minority carrier motion due to field, diffusion due to gradient, diode conducts when forward biased anode positive. The capacitor charges to the peak voltage and discharges through the load resistor ( R_L ) during the non-conducting half-cycle, smoothing the output voltage. Substituting values gives Load resistor, which matches expected behaviour for this semiconductor device configuration, confirming doping, depletion and rectifier principles.

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Doping, Charge Carriers and Conductivity

The direction of conventional current in a forward-biased diode is:

**Charge carriers** in n-type majority electrons, minority holes, in p-type majority holes, minority electrons, minority concentration reduced due to recombination n_e n_h = n_i², doping increases majority, conductivity σ = e(n_e μ_e + n_h μ_h), increases with doping. Donor ionization energy small ~0.01 eV, electrons easily promoted to conduction band at room temperature. In forward bias, current flows from p-side (positive) to n-side (negative) conventionally, as indicated by the diode symbol’s arrow, due to the flow of majority carriers. Substituting values gives P-side to n-side, which matches expected behaviour for this semiconductor device configuration, confirming doping, depletion and rectifier principles.

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p-n Junction, Depletion Region and Diode Characteristics

The negative space-charge region in a p-n junction is located on:

**p-n junction formation** diffusion of holes from p to n and electrons from n to p leaves ionized donors positive on n-side and acceptors negative on p-side, forming space-charge depletion region with electric field directed from n to p (positive to negative), barrier potential V_b ≈0.3 V Ge, 0.7 V Si, opposes further diffusion, drift current due to minority carriers swept by field balances diffusion at equilibrium net current zero. Holes diffuse from p-side to n-side, leaving behind immobile negative ionized acceptors on the p-side, forming the negative space-charge region of the depletion layer. Substituting values gives P-side, which matches expected behaviour for this semiconductor

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Rectifiers, Filters and Applications

The diode in a half-wave rectifier conducts during:

**Rectifier applications** diode must have reverse breakdown voltage higher than peak inverse voltage, centre-tap transformer provides two opposite phase voltages for full-wave, capacitor discharges through load R_L when diode off, drift current in junction is minority carrier motion due to field, diffusion due to gradient, diode conducts when forward biased anode positive. In a half-wave rectifier, the diode is forward biased and conducts only during the positive half-cycle of the AC input, blocking the negative half-cycle. Substituting values gives Positive half-cycle, which matches expected behaviour for this semiconductor device configuration, confirming doping, depletion and rectifier principles.

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Doping, Charge Carriers and Conductivity

Which of the following is NOT a pentavalent dopant?

**Charge carriers** in n-type majority electrons, minority holes, in p-type majority holes, minority electrons, minority concentration reduced due to recombination n_e n_h = n_i², doping increases majority, conductivity σ = e(n_e μ_e + n_h μ_h), increases with doping. Donor ionization energy small ~0.01 eV, electrons easily promoted to conduction band at room temperature. Pentavalent dopants (valency 5) include P, As, and Sb. Indium (In) is trivalent (valency 3) and used for p-type doping, not n-type. Substituting values gives Indium (In), which matches expected behaviour for this semiconductor device configuration, confirming doping, depletion and rectifier principles.

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