3. Specialized Semiconductor and Optoelectronic Devices

Introduction and Context

The conventional p–n junction diode is the starting point for understanding semiconductor devices, but many engineering applications require characteristics that an ordinary rectifier cannot provide. High-speed switching, voltage-controlled capacitance, microwave detection, optical communication, light generation, energy conversion, and precise sensing have led to specialized device structures. Schottky, varactor, tunnel, PIN, photodiode, light-emitting diode (LED), laser diode, solar-cell, and semiconductor sensor devices all exploit particular aspects of carrier transport, junction electrostatics, recombination, or photon interaction.

For postgraduate engineers, device selection cannot be based solely on a symbol or a nominal voltage rating. The designer must relate physical structure to measurable performance: forward drop, reverse leakage, switching speed, breakdown behavior, capacitance, responsivity, quantum efficiency, spectral range, noise, thermal stability, reliability, and cost. The same device may be excellent in one application and unsuitable in another. A Schottky diode may minimize switching loss but suffer excessive reverse leakage at high temperature; a laser diode may provide high-speed communication but demand accurate thermal and current control.

This chapter develops a comparative and application-oriented understanding of specialized semiconductor and optoelectronic devices. It connects device physics with circuit behavior and concludes with a practical selection framework and an illustrative engineering case.

Learning Objectives

After studying this chapter, the reader should be able to:

1. Explain the operating principles and structures of Schottky, varactor, tunnel, and PIN diodes.

2. Relate photodiode, LED, laser-diode, and solar-cell behavior to photon absorption, generation, recombination, and carrier collection.

3. Interpret important device parameters such as turn-on voltage, junction capacitance, responsivity, quantum efficiency, dark current, fill factor, and threshold current.

4. Compare devices for switching, RF, sensing, communication, illumination, and energy-harvesting applications.

5. Select suitable devices using electrical, optical, thermal, reliability, and system-level criteria.

6. Identify limitations, safety issues, manufacturing constraints, and emerging developments in specialized semiconductor devices.

From Junction Physics to Device Function

A semiconductor device is defined not only by its material but also by its geometry, doping profile, contacts, and operating bias. In a p–n junction, diffusion of majority carriers creates a depletion region and a built-in electric field. Forward bias reduces the barrier and permits significant carrier injection; reverse bias widens the depletion region and produces a small minority-carrier current until breakdown or field-assisted transport occurs.

Specialized devices modify one or more of these mechanisms. They may replace the p–n junction with a metal–semiconductor barrier, introduce a very heavily doped and extremely thin junction, insert an intrinsic region between p and n layers, or engineer the semiconductor to interact efficiently with light. These structural changes alter carrier storage, capacitance, tunnelling probability, optical absorption, or recombination.

A useful engineering interpretation is to classify devices by the dominant controlled quantity:

• Charge and barrier control: Schottky and tunnel diodes.
• Capacitance control: varactor diodes.
• Carrier transit and field control: PIN diodes.
• Photon detection: photodiodes.
• Photon generation: LEDs and laser diodes.
• Photon-to-electricity conversion: solar cells.
• Material-property modulation: semiconductor sensors.

Schottky Diodes: Barrier Devices for Fast Switching

A Schottky diode is formed by a metal–semiconductor junction, usually with a metal contacting moderately doped n-type silicon, gallium arsenide, or a wide-bandgap semiconductor. The junction creates a Schottky barrier. Conduction is primarily due to majority carriers crossing the barrier, rather than minority-carrier injection and storage as in a conventional p–n diode.

The absence of substantial minority-carrier storage gives the Schottky diode a very small reverse-recovery time. Its forward voltage is commonly lower than that of a silicon p–n diode, although the exact value depends on barrier height, current density, temperature, and material. The main trade-off is reverse leakage: thermionic emission over the barrier increases strongly with temperature. A practical Schottky device therefore combines low forward loss and high speed with limited reverse-voltage capability and potentially significant leakage.

Schottky diodes are used in switching power supplies, OR-ing circuits, clamp networks, RF detectors, mixers, and logic families. In a power converter, reduced forward voltage improves conduction efficiency, while negligible reverse recovery reduces switching loss. At high reverse voltage, however, leakage and electric-field concentration become serious concerns. Silicon carbide Schottky diodes extend the concept to higher voltage and temperature operation because SiC has a wide band gap and supports high electric fields.

Selection should consider forward voltage at the actual load current, reverse-voltage margin, leakage at maximum junction temperature, parasitic inductance, thermal resistance, and switching frequency. A device with the lowest catalogue forward voltage is not necessarily the most efficient choice if its leakage or thermal behavior is unacceptable.

Varactor Diodes: Voltage-Controlled Junction Capacitance

A reverse-biased p–n junction behaves as a voltage-dependent capacitor. The depletion region contains few mobile carriers and separates the conducting p and n regions. As reverse voltage increases, the depletion width increases and the junction capacitance decreases. For an abrupt junction, the idealized relationship is approximately

Cj = C0/(1 + VR/Vbi)^(1/2),

where C0 is the zero-bias capacitance, VR is reverse voltage, and Vbi is the built-in potential. Graded junctions produce a different voltage exponent, enabling designers to tailor tuning characteristics.

A varactor diode is optimized to exploit this capacitance rather than forward conduction. It is normally operated under reverse bias and placed in resonant circuits, phase-locked loops, voltage-controlled oscillators, tunable filters, frequency multipliers, and electronic tuning networks. The effective capacitance also includes package, wiring, and circuit parasitics; consequently, the useful tuning range may be much smaller than the ideal junction range.

Important parameters include capacitance range, tuning ratio, quality factor, series resistance, reverse-voltage limit, capacitance linearity, and control-voltage range. High-Q devices are preferred in narrowband RF circuits, while broadband tuning may require a compromise between range and loss. Excessive RF amplitude can forward bias the junction or introduce nonlinear distortion. Temperature compensation and bias filtering are often necessary in precision oscillators.

Tunnel Diodes: Quantum Transport and Negative Differential Resistance

A tunnel diode uses extremely heavy doping on both sides of a p–n junction. The depletion region becomes so thin that carriers can pass through the barrier by quantum-mechanical tunnelling, even when their classical energy is insufficient to cross it. The current–voltage curve contains a region in which increasing voltage produces decreasing current. This is negative differential resistance.

At low forward voltage, occupied states on the n side align with available states on the p side, and tunnelling current rises rapidly to a peak. With further voltage increase, the alignment becomes less favorable and current falls to a valley. At still higher voltage, ordinary diffusion current dominates and the curve rises again.

The negative-resistance region can sustain oscillation and amplification when embedded in an appropriate bias and resonant network. Tunnel diodes have been used in microwave oscillators, high-speed switching, pulse circuits, and memory elements. Their very high speed results from tunnelling and the absence of minority-carrier storage. Their disadvantages include limited power handling, small signal amplitude, sensitivity to bias point, and manufacturing variability. Resonator design and bias stability are essential because the operating point must remain within the negative-differential-resistance region.

PIN Diodes: Controlled Transit and RF Switching

A PIN diode contains heavily doped p and n regions separated by a relatively wide intrinsic or lightly doped region. Under forward bias, carriers flood the intrinsic region, reducing its effective resistance. Under reverse bias, the intrinsic region is depleted and the diode presents a high resistance with low capacitance. At radio and microwave frequencies, this bias-dependent resistance allows the device to act as a current-controlled RF switch, attenuator, limiter, or phase-control element.

In photodetection, the intrinsic region is valuable because it provides a thick absorption volume and a strong electric field. Photogenerated carriers are swept across the region, reducing transit-time limitations and improving responsivity-speed trade-offs. Increasing the intrinsic width can improve absorption and reduce capacitance, but it also increases carrier transit time and may require higher reverse bias.

PIN diode selection involves forward current needed for the desired RF resistance, reverse bias and breakdown rating, carrier lifetime, switching speed, junction capacitance, insertion loss, isolation, and thermal management. In optical receivers, bandwidth, dark current, responsivity, and wavelength response are more important than the RF resistance characteristic.

Photodiodes: Converting Optical Power into Electrical Signal

A photodiode operates by absorbing photons with energy at least equal to the semiconductor band gap. Absorption creates electron–hole pairs. A depletion-region electric field separates the carriers, producing photocurrent. The total current can be represented conceptually as the sum of dark current and light-generated current, with the polarity determined by bias and circuit connection.

A photodiode may operate in photovoltaic mode with no external bias or in photoconductive mode under reverse bias. Photovoltaic operation generally gives low noise and low dark current but lower speed. Reverse bias widens the depletion region, reduces junction capacitance, and improves response speed, at the cost of increased dark current and possibly noise.

Key performance measures include:

• Responsivity: photocurrent per unit incident optical power, expressed in A/W.
• Quantum efficiency: collected electron–hole pairs per incident photon.
• Dark current: current in the absence of light.
• Rise and fall time: measures of temporal response.
• Noise-equivalent power: optical power producing a signal equal to noise.
• Detectivity: a normalized measure of sensitivity.
• Spectral response: dependence of sensitivity on wavelength.

The PIN photodiode provides a useful balance between speed and sensitivity. The avalanche photodiode (APD) applies high reverse bias so that carriers create secondary electron–hole pairs by impact ionization. APDs offer internal gain but require high-voltage control, exhibit excess noise, and are more temperature-sensitive.

Photodiodes are used in fiber-optic receivers, encoders, barcode readers, medical instruments, flame detectors, ranging systems, and industrial feedback. Optical filters, transimpedance amplifiers, shielding, and careful grounding are often as important as the detector itself.

LEDs: Efficient and Robust Electroluminescent Sources

An LED emits light when forward-biased carriers recombine in a semiconductor. In a direct-band-gap material, recombination can release energy efficiently as photons. In an indirect-band-gap material such as silicon, radiative recombination is inefficient, which is why common LEDs use compound semiconductors such as gallium nitride or related alloys.

The emitted wavelength is primarily determined by the active-region band gap, while quantum wells and heterostructures improve carrier confinement and radiative efficiency. LEDs are minority-carrier devices: injected electrons and holes recombine in the active region, producing spontaneous emission. Their optical output generally increases with current within a safe operating range, but efficiency droop, self-heating, and wavelength shift may occur at high current.

LED advantages include long service life, mechanical robustness, low operating voltage, fast modulation relative to lamps, and high efficiency. Limitations include finite modulation bandwidth, broad spectral linewidth compared with lasers, temperature-dependent output, and the need for current—not merely voltage—control. A series resistor, constant-current driver, or switching regulator is required.

LEDs serve in indicators, displays, solid-state lighting, optocouplers, automotive lamps, visible-light communication, and optical sensors. Thermal design is crucial even when electrical power appears modest, because junction temperature affects luminous output, color, reliability, and lifetime.

Laser Diodes: Coherent, High-Density Optical Sources

A laser diode is a semiconductor heterostructure designed to produce stimulated emission. The active region is placed between materials of larger band gap, which confine both carriers and photons. Optical feedback is supplied by a resonant cavity, commonly formed by cleaved or engineered semiconductor facets. Above threshold current, optical gain exceeds total cavity losses and coherent laser emission begins.

The threshold condition can be expressed qualitatively as gain equalling internal and mirror losses. Below threshold, the device behaves more like an LED with spontaneous emission; above threshold, output power increases approximately with current according to the slope efficiency. Important parameters include threshold current, slope efficiency, output power, wavelength, spectral linewidth, beam divergence, modulation bandwidth, and temperature coefficient.

Laser diodes enable fiber-optic communication, optical storage, barcode scanning, laser printing, range measurement, spectroscopy, and high-resolution sensing. They provide narrow spectral width, high radiance, and rapid modulation, but require precise current limiting and thermal control. Optical feedback, electrostatic discharge, facet damage, mode hopping, and catastrophic optical degradation are significant reliability concerns. Laser safety classification is mandatory because focused or collimated radiation can damage the eye even when the source appears small.

Solar Cells: The Photovoltaic Junction as an Energy Converter

A solar cell is a large-area p–n junction designed to convert incident radiation into electrical power. Photons generate electron–hole pairs, and the built-in electric field plus diffusion processes separate and collect them. Under illumination, the current–voltage characteristic is shifted relative to the dark diode characteristic.

The principal parameters are short-circuit current, open-circuit voltage, maximum-power voltage and current, maximum power, fill factor, and conversion efficiency. Fill factor is

FF = Pmax/(Voc Isc),

where Voc is open-circuit voltage and Isc is short-circuit current. Series resistance lowers the fill factor and maximum power; shunt leakage also degrades performance. Cell temperature generally reduces voltage and efficiency, while irradiance strongly influences current.

A practical photovoltaic system includes series-connected cells, bypass diodes, blocking elements, maximum-power-point tracking, power converters, protection, and energy storage or grid interfaces. The maximum-power point changes with irradiance and temperature. Therefore, system-level efficiency depends not only on cell efficiency but also on mismatch, shading, wiring, converter loss, thermal conditions, and degradation.

Material selection involves a trade-off among band gap, spectral response, manufacturing cost, stability, flexibility, and toxicity. Crystalline silicon remains important, while thin-film, tandem, perovskite, and III–V technologies address specialized efficiency or form-factor requirements. Emerging devices require careful evaluation of long-term stability, encapsulation, environmental impact, and manufacturing scalability.

Semiconductor Sensors

Semiconductor sensors convert a physical, chemical, or biological quantity into an electrical change. The sensed quantity may alter resistance, junction voltage, capacitance, carrier mobility, surface charge, or optical response. Examples include thermistors, photoconductors, Hall sensors, gas sensors, ion-sensitive field-effect transistors, magnetic sensors, and MEMS-integrated semiconductor devices.

A sensor should be evaluated through sensitivity, selectivity, linearity, resolution, response and recovery time, hysteresis, drift, noise, operating range, calibration stability, and environmental compatibility. Signal conditioning is usually required. A bridge circuit may convert resistance change into voltage; a transimpedance amplifier may process photodiode current; an analog-to-digital converter and calibration algorithm may compensate for offset and temperature dependence.

Gas sensors based on metal-oxide semiconductors, for example, rely on surface adsorption and reaction that change conductivity. They can be sensitive but may require heating and may respond to several gases. Hall sensors use the Lorentz force to produce a voltage proportional to magnetic flux density and are widely used for current measurement and position detection. Integrated sensors increasingly combine the sensing element, amplifier, converter, diagnostics, and digital interface on one chip.

A Device-Selection Framework

A rational selection process begins with the required function rather than with a preferred component. The engineer should define the signal or power range, frequency or wavelength, allowable loss, response time, environmental conditions, safety constraints, and reliability target. The following sequence is useful:

1. Identify the physical conversion or switching mechanism required.

2. Establish electrical, optical, thermal, mechanical, and environmental limits.

3. Compare candidate devices using datasheet curves at actual operating conditions.

4. Include parasitic components, driver circuits, packaging, and thermal paths.

5. Estimate failure modes, lifetime, protection requirements, and regulatory constraints.

6. Prototype, measure, calibrate, and validate performance over temperature and production variation.

A communication receiver may prioritize bandwidth, sensitivity, wavelength compatibility, and noise, whereas a power rectifier prioritizes conduction loss, reverse recovery, surge rating, thermal resistance, and leakage. The correct decision is therefore application-specific.

Illustrative Case Study: Selecting an Optical Receiver for Industrial Data Transfer

Background

An industrial automation company must receive a 100-Mb/s optical signal over a short fiber link in a factory environment. The receiver must operate from a low-voltage supply, tolerate temperature variation, and maintain low bit-error rate in the presence of electrical noise.

Situation and Problem

The design team is considering a silicon PIN photodiode, an APD, and a phototransistor. The phototransistor provides internal gain but has limited speed. The APD offers gain but requires a high, temperature-compensated bias. The PIN device has no internal gain but is simple and fast.

Analysis

The required data rate makes junction capacitance, carrier transit time, and transimpedance-amplifier bandwidth important. The optical power budget indicates that adequate signal is available without avalanche multiplication. The factory environment favors low-voltage operation, low electromagnetic susceptibility, and simple maintenance. APD excess noise and high-voltage bias complexity provide no corresponding system benefit. The phototransistor fails the bandwidth requirement.

Approach and Solution

The team selects a reverse-biased InGaAs PIN photodiode if the transmitter wavelength is in the near-infrared telecommunications band. The detector is integrated with a low-input-capacitance transimpedance amplifier, a narrow optical filter, shielding, and a temperature-aware gain-control stage. The PCB minimizes parasitic capacitance and separates the optical signal path from switching-power circuitry. Eye diagrams and bit-error measurements are performed across temperature, supply tolerance, and minimum received power.

Outcome

The PIN solution meets the data-rate requirement with lower circuit complexity and power consumption than the APD alternative. The design also avoids high-voltage safety controls. The result illustrates that the device with the highest intrinsic gain is not automatically the best engineering choice.

Learning Points

Device selection must be based on the complete link budget and system constraints. Bandwidth, noise, bias complexity, temperature behavior, packaging, and compliance can dominate the decision. A component should be evaluated together with its driver, amplifier, layout, and protection network.

Challenges, Risks, and Common Design Errors

A frequent error is comparing nominal parameters measured under different conditions. Forward voltage, leakage, optical power, and sensitivity must be evaluated at the relevant current, temperature, wavelength, and duty cycle. Another error is ignoring thermal feedback: temperature changes leakage, band gap, wavelength, threshold current, and luminous efficiency.

Reverse-bias ratings must include transients, not merely steady-state voltage. Photodiode amplifiers can become unstable because detector capacitance and input capacitance create a pole. Varactors can distort RF signals when their voltage excursion is too large. LEDs and laser diodes can be destroyed by uncontrolled current or electrostatic discharge. Solar-cell measurements are meaningful only when illumination spectrum, intensity, temperature, and test method are specified.

Packaging introduces series inductance, capacitance, optical coupling loss, and thermal resistance. Reliability analysis should address electromigration, bond-wire fatigue, humidity, radiation, optical degradation, and repeated thermal cycling where applicable.

Professional, Ethical, and Regulatory Considerations

Engineers must report device performance honestly, including test conditions, uncertainty, calibration method, and degradation. Optical products require laser classification, labeling, interlocks, and appropriate exposure controls. Photovoltaic and power systems require electrical isolation, overcurrent protection, fire safety, and compliance with applicable standards. Sensor systems used in medical, industrial safety, or environmental monitoring require traceability and validation because inaccurate measurements can cause physical or economic harm.

Sustainability also affects device choice. Material toxicity, rare-element dependence, recyclability, embodied energy, expected lifetime, and end-of-life handling should be considered rather than optimizing only initial efficiency. Data-producing sensor systems raise additional concerns about cybersecurity, privacy, calibration ownership, and responsible use of automated decisions.

Current Developments and Best Practices

Current research emphasizes wide-bandgap power devices, silicon-carbide Schottky diodes, gallium-nitride optoelectronics, integrated photonics, high-speed silicon photodetectors, tandem photovoltaic cells, perovskite absorbers, quantum-well emitters, and heterogeneous integration. Sensor development increasingly combines MEMS structures, nanomaterials, optical readout, edge processing, and machine-learning-assisted calibration.

Best practice is to use physics-based preliminary estimates followed by datasheet verification, simulation, controlled measurement, and accelerated reliability testing. Designers should maintain parameter margins, characterize complete assemblies rather than isolated components, provide protection against abnormal conditions, and document calibration and operating limits. Compact models and electrothermal simulation are valuable, but experimental validation remains essential because packaging, defects, and environmental conditions often determine real performance.

Chapter Summary

Specialized semiconductor devices achieve application-specific behavior by modifying barriers, doping, depletion width, carrier storage, transit regions, and optical interaction. Schottky diodes provide low-loss, fast majority-carrier conduction but have higher leakage. Varactors exploit voltage-dependent depletion capacitance for electronic tuning. Tunnel diodes use quantum tunnelling and negative differential resistance. PIN diodes provide bias-controlled resistance and efficient high-speed photodetection.

Photodiodes convert light into current and are selected using responsivity, bandwidth, noise, dark current, and wavelength. LEDs generate spontaneous emission efficiently and robustly, whereas laser diodes provide stimulated, narrow-spectrum, high-radiance output but demand tighter current, thermal, and safety control. Solar cells convert optical power into electrical power, with fill factor, resistance, temperature, and system mismatch determining practical efficiency. Semiconductor sensors translate physical or chemical variables into measurable electrical changes.

Sound engineering selection considers the complete operating environment, not one headline specification. Electrical and optical performance, thermal behavior, parasitics, reliability, safety, regulation, sustainability, and system integration must be evaluated together.

Key Terms

Schottky barrier: Metal–semiconductor potential barrier responsible for rectification in a Schottky diode.

Reverse recovery: Time required for a diode to cease conducting reverse current after forward conduction.

Varactor: Reverse-biased semiconductor diode used as a voltage-controlled capacitor.

Tuning ratio: Ratio describing the change in varactor capacitance over its control-voltage range.

Tunnel diode: Heavily doped diode in which quantum tunnelling produces a negative differential-resistance region.

Negative differential resistance: Region where current decreases as voltage increases.

PIN diode: Diode containing a lightly doped or intrinsic region between p and n regions.

Photodiode: Semiconductor device that converts incident optical power into electrical current.

Responsivity: Photocurrent generated per unit incident optical power.

Quantum efficiency: Ratio of collected charge carriers to incident photons.

Dark current: Current flowing through a photodetector when no light is incident.

Avalanche photodiode: Photodiode using impact ionization to provide internal multiplication.

LED: Semiconductor source producing spontaneous electroluminescence under forward bias.

Laser diode: Semiconductor source producing stimulated, coherent optical emission above threshold.

Threshold current: Minimum approximate current at which laser gain balances cavity losses.

Solar cell: Photovoltaic semiconductor junction that converts light into electrical power.

Fill factor: Ratio of maximum photovoltaic power to the product of open-circuit voltage and short-circuit current.

Sensor: Device that converts a physical, chemical, or biological quantity into a measurable signal.

Detectivity: Normalized measure of a photodetector’s ability to detect weak optical signals.

Review Questions

Short Answer Questions

1. Why does a Schottky diode generally switch faster than a p–n diode?

2. How does reverse voltage affect a varactor’s junction capacitance?

3. What physical mechanism produces negative differential resistance in a tunnel diode?

4. Define photodiode responsivity and dark current.

5. What is the role of the intrinsic region in a PIN diode?

Long Answer Questions

1. Explain the structure, operation, advantages, and limitations of Schottky diodes, and discuss their selection in a switching power supply.

2. Describe photodiode operation in photovoltaic and photoconductive modes, including the trade-offs among speed, noise, bias, and sensitivity.

3. Compare LED and laser-diode emission mechanisms, performance parameters, applications, and safety requirements.

4. Derive the significance of fill factor in solar-cell evaluation and explain how temperature, series resistance, and shunt leakage affect photovoltaic output.

5. Discuss a systematic framework for selecting specialized semiconductor devices for an industrial electronic system.

Analytical / Application Questions

1. A converter operates at high frequency and requires low conduction loss, but its ambient temperature is high. Which Schottky-device parameters should be examined before selection, and why?

2. An optical receiver must operate at high data rate but has sufficient received optical power. Compare a PIN photodiode and an APD for this application and justify a choice.

3. A voltage-controlled oscillator shows poor frequency stability and excessive distortion. Analyze how varactor capacitance range, Q factor, RF voltage swing, bias filtering, and temperature compensation may contribute to the problem.

22. Multiple Choice Questions with Answers

Question 1

Which characteristic primarily enables a Schottky diode to have very fast switching?

A. Large minority-carrier storage

B. Majority-carrier conduction with negligible charge storage

C. Very high junction capacitance

D. Operation only in avalanche breakdown

Correct Answer: B. Majority-carrier conduction with negligible charge storage

Question 2

A varactor diode is normally operated under reverse bias because reverse bias

A. maximizes minority-carrier injection

B. creates a controllable depletion capacitance

C. eliminates all leakage current

D. produces coherent optical emission

Correct Answer: B. creates a controllable depletion capacitance

Question 3

The negative differential-resistance region of a tunnel diode results mainly from

A. thermal runaway

B. impact ionization

C. quantum tunnelling and changing state alignment

D. minority-carrier storage

Correct Answer: C. quantum tunnelling and changing state alignment

Question 4

Increasing reverse bias on a PIN photodiode generally tends to

A. increase junction capacitance and reduce speed

B. reduce junction capacitance and improve speed

C. eliminate dark current

D. cause spontaneous light emission

Correct Answer: B. reduce junction capacitance and improve speed

Question 5

Which parameter expresses photodiode output current per unit incident optical power?

A. Fill factor

B. Responsivity

C. Threshold current

D. Tuning ratio

Correct Answer: B. Responsivity

Question 6

Compared with an LED, a laser diode normally provides

A. broader spectrum and lower radiance

B. coherent, narrow-spectrum emission above threshold

C. only spontaneous emission at all currents

D. operation without current control

Correct Answer: B. coherent, narrow-spectrum emission above threshold

Question 7

Which solar-cell parameter is defined as Pmax divided by VocIsc?

A. Quantum efficiency

B. Detectivity

C. Fill factor

D. Responsivity

Correct Answer: C. Fill factor

Question 8

An APD differs from a PIN photodiode because it

A. uses impact ionization for internal gain

B. cannot detect optical radiation

C. operates only without reverse bias

D. has no temperature sensitivity

Correct Answer: A. uses impact ionization for internal gain

Question 9

For a high-frequency oscillator using a varactor, a high quality factor is desirable mainly because it

A. increases optical absorption

B. reduces RF loss

C. raises dark current

D. causes negative differential resistance

Correct Answer: B. reduces RF loss

Question 10

In the illustrative optical receiver case, the PIN photodiode was preferred over the APD primarily because

A. the system required high-voltage avalanche bias

B. adequate optical power was available without internal gain

C. the PIN device emits coherent light

D. the APD has no excess noise

Correct Answer: B. adequate optical power was available without internal gain