5. Bipolar Junction Transistors and Heterojunction Bipolar Transistors

Introduction and Context

The bipolar junction transistor (BJT) remains one of the most important semiconductor devices in electronics. Although field-effect transistors dominate many digital integrated circuits, BJTs continue to be valued for high transconductance, accurate current amplification, low input-referred noise, good matching, and excellent high-frequency performance. They are widely used in precision analog circuits, differential amplifiers, current mirrors, oscillators, power stages, radio-frequency (RF) circuits, and mixed-signal systems.

A BJT is a three-terminal, two-junction device in which both electrons and holes participate in conduction. Its operation is governed by carrier injection across a forward-biased junction, transport through a thin base, and collection at a reverse-biased junction. This physical picture leads naturally to the concepts of current gain, transistor action, charge storage, frequency response, breakdown, and thermal feedback.

The heterojunction bipolar transistor (HBT) extends the BJT by replacing one or more homojunctions with heterojunctions formed from different semiconductor materials. Proper selection of band gaps and material composition permits the emitter and base to be engineered independently. HBTs can achieve high emitter injection efficiency, high current gain, high cutoff frequency, and improved power performance. Silicon–germanium HBTs, for example, are central to many high-speed bipolar CMOS (BiCMOS) technologies, while compound-semiconductor HBTs are used in microwave and millimetre-wave systems.

This chapter develops the physical operation and circuit modelling of BJTs and HBTs. It connects semiconductor transport theory with practical design issues such as frequency limitations, avalanche breakdown, thermal instability, reliability, and analog/RF applications.

Learning Objectives

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

1. Explain carrier injection, transport, recombination, and collection in a BJT.

2. Derive and interpret the relationships among emitter, base, and collector currents.

3. Distinguish common-emitter, common-base, and common-collector operation.

4. Use the Ebers–Moll and Gummel–Poon models for device and circuit analysis.

5. Explain current gain variation, charge storage, frequency response, and transit-time limitations.

6. Analyze avalanche breakdown, punch-through, thermal effects, and safe operating limits.

7. Describe HBT band-engineering principles and compare HBTs with conventional BJTs.

8. Select and apply BJTs or HBTs in analog, RF, and mixed-signal systems.

9. Recognize modelling assumptions, practical nonidealities, and design trade-offs.

The Physical Basis of Bipolar Transistor Action

A conventional npn transistor consists of an n-type emitter, a thin p-type base, and an n-type collector. The emitter is heavily doped, the base is lightly doped and physically narrow, and the collector is generally more lightly doped than the emitter so that it can sustain a large reverse voltage. A pnp transistor has the complementary doping arrangement and operates with reversed voltage polarities and carrier types.

In forward-active operation, the emitter–base junction is forward biased and the collector–base junction is reverse biased. Electrons are injected from the n-type emitter into the p-type base. Because the base is thin and lightly doped, most of these electrons diffuse across it before recombining with holes. On reaching the depletion region of the collector–base junction, they are swept into the collector by the electric field. A small fraction of the injected electrons recombines in the base; this recombination requires a base current.

The emitter current is therefore composed mainly of injected electrons, while the base current represents recombination and other parasitic components. The collector current is nearly equal to the electron component injected by the emitter, but is slightly smaller because of base recombination. This transport process explains why a small base current can control a much larger collector current.

The emitter injection efficiency is the fraction of the total emitter current contributed by carriers injected from the emitter into the base. High emitter doping and relatively light base doping improve this efficiency. The base transport factor is the fraction of injected carriers that cross the base without recombination. The common-base current gain, alpha, is approximately the product of these two factors:

α ≈ γ α_T

where γ is the emitter injection efficiency and α_T is the base transport factor. The common-emitter current gain is related to alpha by

β = α/(1 − α).

Since alpha is close to unity, beta may be large even when the difference between alpha and unity is small. For example, α = 0.99 gives β ≈ 99.

The ideal forward-active collector current follows the exponential junction relationship:

I_C ≈ I_S exp(V_BE/V_T),

where I_S is the transport saturation current and V_T = kT/q is the thermal voltage. At room temperature, V_T is approximately 25.9 mV. The exponential dependence makes the BJT a transconductance-efficient device: its transconductance is approximately

g_m = I_C/V_T.

Thus, at a specified collector current, a BJT can provide substantial transconductance in a small device area.

Operating Regions and Terminal Characteristics

The four combinations of junction bias define the principal BJT operating regions:

• In cutoff, both junctions are reverse biased or insufficiently forward biased, and the transistor current is very small.
• In forward-active operation, the emitter–base junction is forward biased and the collector–base junction is reverse biased. This is the normal region for amplification.
• In saturation, both junctions are forward biased. Carrier storage is significant, and the transistor is commonly used as a low-resistance switch.
• In reverse-active operation, the collector and emitter exchange functional roles. Performance is poor because the collector is not optimized for injection.

The common-emitter configuration provides current gain and voltage gain and is consequently the most widely used amplifier configuration. The common-base configuration has a current gain slightly below unity, low input resistance, and useful high-frequency characteristics. The common-collector configuration, or emitter follower, has a voltage gain close to unity, high input resistance, and low output resistance.

The base-width modulation effect, often called the Early effect, occurs because increasing reverse bias at the collector–base junction widens its depletion region into the base. The effective neutral base width decreases, increasing the collector current. In the output characteristic, this appears as a finite slope rather than perfectly horizontal curves. A useful parameter is the Early voltage V_A, for which

I_C ≈ I_S exp(V_BE/V_T)(1 + V_CE/V_A).

The small-signal output resistance is approximately r_o = V_A/I_C. A larger Early voltage corresponds to a flatter output characteristic and higher output resistance.

Injection, Recombination, and Charge Storage

The base current is not simply an arbitrary control current; it is a direct consequence of minority-carrier recombination in the base and at semiconductor interfaces. At low and moderate current levels, recombination in the quasi-neutral base is an important component. At higher current densities, high-level injection, emitter crowding, mobility reduction, and conductivity modulation alter the ideal relationships.

Stored charge is particularly important when a BJT is driven into saturation. Both junctions then inject carriers into the base, and the base contains a large excess charge. When the transistor is turned off, this stored charge must be removed before the collector current can fall substantially. The resulting storage delay limits switching speed. Schottky-clamped BJTs and carefully controlled drive circuits reduce deep saturation and improve switching performance.

At high current density, beta may decrease. This phenomenon, often called beta roll-off, results from effects such as increased recombination, emitter current crowding, high-level injection, and resistive voltage drops. At very low currents, beta may also be low because depletion-region recombination and surface leakage become relatively important. Consequently, a transistor's current gain is generally a function of collector current, collector–emitter voltage, temperature, and device history.

Large-Signal Models: Ebers–Moll and Gummel–Poon

The Ebers–Moll model provides a physically meaningful large-signal description of a BJT over all four operating regions. It represents the transistor as two coupled diodes and dependent current sources. In a simplified form, the terminal currents are expressed using forward and reverse junction exponential currents. For an npn device, the forward and reverse transport currents can be written as

I_F = I_ES [exp(V_BE/V_T) − 1]

and

I_R = I_CS [exp(V_BC/V_T) − 1].

The emitter and collector currents are combinations of these currents, weighted by the forward and reverse common-base gains. The model correctly describes cutoff, forward-active, saturation, and reverse-active operation, making it useful for conceptual analysis and elementary circuit simulation.

However, the basic Ebers–Moll model does not adequately represent several important effects: base-width modulation, charge storage, high-current beta degradation, depletion capacitances, and transit-time behaviour. The Gummel–Poon model extends Ebers–Moll by introducing charge-control relationships and voltage-dependent junction capacitances. It is therefore more suitable for practical transistor modelling and SPICE simulation.

A Gummel–Poon model typically includes:

• Forward and reverse transport currents.
• Forward and reverse beta values.
• Base-width modulation through Early-effect parameters.
• Diffusion charge associated with stored minority carriers.
• Depletion capacitances of the base–emitter and base–collector junctions.
• High-current effects and recombination parameters.
• Temperature dependence of saturation current, mobility, and junction voltage.

Model parameters are normally extracted from measured current–voltage and capacitance–voltage data. A designer should not assume that a model accurately predicts every operating condition. Parameter extraction quality, wafer process variation, self-heating, layout parasitics, and measurement bias all influence the usefulness of the model.

Small-Signal Operation and Frequency Response

For a transistor biased in forward-active operation, the hybrid-pi model is commonly used for small-signal analysis. Its principal elements include the transconductance g_m, base resistance r_π, output resistance r_o, and junction capacitances C_π and C_μ. The relationship between transconductance and current is

g_m = I_C/V_T,

while the base-emitter small-signal resistance is approximately

r_π = β/g_m.

The base–emitter capacitance C_π contains diffusion capacitance as well as depletion capacitance. In forward-active operation, diffusion capacitance may dominate because it is associated with stored minority-carrier charge. The base–collector capacitance C_μ is primarily a depletion capacitance and produces feedback from output to input.

The forward current gain decreases with frequency. A simplified form is

β(f) ≈ β_0/[1 + j(f/f_β)],

where f_β is a characteristic pole frequency. The unity current-gain frequency, f_T, is the frequency at which the short-circuit common-emitter current gain falls to unity. A useful approximation is

f_T ≈ 1/[2π(τ_F + τ_B + τ_C)],

where the terms represent forward transit time, base transit time, collector depletion-region transit time, and related parasitic delays. In practical devices, base resistance, contact resistance, wiring, and substrate coupling further reduce usable gain.

The Miller effect caused by C_μ can greatly increase the effective input capacitance in a voltage amplifier. Cascode arrangements reduce the voltage swing across the base–collector junction and therefore suppress Miller multiplication. This is one reason cascodes are widely used in wideband amplifiers, current mirrors, and RF integrated circuits.

Noise is another frequency-dependent design issue. BJT noise includes shot noise associated with junction currents, thermal noise in base resistance, generation–recombination noise, and, at low frequencies, flicker noise. The optimum bias current for minimum noise is not necessarily the same as the bias current for maximum f_T or maximum linearity. RF design therefore requires simultaneous consideration of gain, noise figure, power consumption, stability, and dynamic range.

Breakdown, Thermal Effects, and Reliability

Several breakdown mechanisms can limit the voltage range of a BJT. Avalanche breakdown occurs when carriers accelerated by a strong electric field generate additional electron–hole pairs by impact ionization. In a transistor, breakdown may occur at the collector–base junction with the emitter open, or under different conditions when the emitter is connected to a bias source. The specified breakdown voltage depends strongly on terminal configuration and current.

Punch-through occurs when depletion regions expand sufficiently that the neutral base region becomes very narrow or disappears. The collector field can then strongly influence the emitter junction, producing a rapid increase in current before conventional avalanche breakdown is reached. Device geometry and collector doping are selected to balance breakdown voltage against speed and capacitance.

Thermal effects are central to BJT design. The base–emitter voltage required for a given collector current decreases with increasing temperature, while saturation current increases strongly with temperature. In a biased transistor, this can produce a positive feedback loop: temperature rises, collector current increases, power dissipation rises, and the device becomes still hotter. Thermal runaway is especially likely in parallel devices or in circuits with poorly controlled bias.

Emitter degeneration, ballast resistors, active thermal compensation, current limiting, heat spreading, and careful layout improve stability. Safe operating area (SOA) specifications define combinations of voltage, current, pulse duration, and temperature that the device can withstand. Secondary breakdown, caused by localized current concentration and thermal instability, may impose a more restrictive limit than simple power dissipation.

Heterojunction Bipolar Transistors

An HBT uses a heterojunction—an interface between two semiconductor materials with different band gaps—to improve transistor operation. In a common npn HBT, the emitter has a wider band gap than the base. This band-gap difference creates a barrier that suppresses hole injection from the base into the emitter while allowing efficient electron injection from the emitter into the base. The result is a higher emitter injection efficiency and higher current gain for a given base doping.

Because the base can be doped more heavily without severely reducing injection efficiency, HBTs can have lower base resistance and improved high-frequency performance. A compositionally graded base can create a quasi-electric field that assists electron transport, reducing base transit time. In SiGe HBTs, germanium content is often graded across the base to produce both band-gap engineering and transport-field benefits while retaining compatibility with silicon manufacturing.

Important HBT material systems include SiGe/Si, AlGaAs/GaAs, InP/InGaAs, and related compound-semiconductor combinations. Material selection determines band alignment, lattice matching, thermal conductivity, breakdown behaviour, cost, and integration capability. Compound-semiconductor HBTs may provide excellent microwave performance, whereas SiGe HBTs offer strong integration with CMOS logic and digital signal processing.

HBT advantages include high current gain, low base resistance, high f_T, high maximum oscillation frequency, good noise performance, and improved high-temperature operation in some applications. Their limitations include more complex epitaxial growth, material defects, process sensitivity, thermal management requirements, and potentially higher manufacturing cost. HBT design is therefore an exercise in balancing speed, breakdown voltage, linearity, power density, yield, and integration.

Analog and RF Applications

BJTs are especially attractive in analog circuits because their exponential current–voltage relationship is predictable and because transconductance is directly proportional to collector current. Differential pairs exploit matched emitter junctions to convert differential voltage into current. Current mirrors use the exponential relationship to replicate currents, although finite output resistance, base-current error, mismatch, and temperature gradients limit accuracy.

Operational amplifiers and instrumentation amplifiers frequently use bipolar input stages where low voltage noise and high transconductance are important. Emitter degeneration improves linearity by applying local negative feedback, at the cost of reduced gain. Band-gap reference circuits combine junction voltage and proportional-to-absolute-temperature voltage components to obtain a relatively temperature-stable reference.

In RF systems, HBTs are used in low-noise amplifiers, frequency mixers, voltage-controlled oscillators, power amplifiers, frequency dividers, and high-speed data transmitters. Their high f_T and f_max support operation at microwave frequencies, while good current density permits compact power stages. Differential and cascode topologies improve isolation, gain, and output resistance. At high output power, thermal resistance, load-line design, and breakdown voltage become as important as small-signal gain.

The choice between a BJT, MOSFET, and HBT depends on the application. BJTs generally offer high transconductance per unit current and low noise at moderate frequencies. MOSFETs provide very high input resistance and scalable digital integration. HBTs offer a particularly strong combination of speed, gain, and RF power capability, but may require more specialized processing and thermal design.

Illustrative Design Example

Consider a common-emitter amplifier biased at I_C = 1 mA and room temperature. The approximate transconductance is

g_m = 1 mA/25.9 mV ≈ 38.6 mS.

If β = 100, then

r_π ≈ 100/38.6 mS ≈ 2.59 kΩ.

Suppose the transistor has an Early voltage of 100 V. Its small-signal output resistance is approximately

r_o ≈ 100 V/1 mA = 100 kΩ.

These values show why a BJT can provide substantial voltage gain with a collector load of only a few kilo-ohms. They also show why the finite output resistance should not be ignored in precision gain calculations. If the circuit uses a collector resistor of 5 kΩ, the effective load is the parallel combination of 5 kΩ and approximately 100 kΩ, not exactly 5 kΩ.

If the collector–base capacitance is significant, the voltage gain may produce Miller multiplication of that capacitance at the input. A cascode transistor can reduce the voltage variation at the first transistor's collector and preserve bandwidth. This example illustrates a general design principle: DC bias determines transconductance, while geometry, capacitance, parasitic resistance, and topology determine whether that transconductance remains useful at high frequency.

Applied Case: SiGe HBT Receiver Front End

Background

Consider an illustrative telecommunications company developing a compact receiver front end for a multi-gigahertz wireless instrument. The design must provide low noise, adequate gain, linearity under strong interferers, and compatibility with a mixed-signal silicon platform.

Situation and Problem

A conventional CMOS input stage satisfies integration requirements but exhibits insufficient transconductance at the available current and has a less favourable noise match at the target frequency. A discrete compound-semiconductor solution provides excellent RF performance but increases cost and complicates system integration.

Analysis

The design team evaluates a SiGe HBT process. The HBT offers high f_T, relatively low base resistance, and good current density. However, the team must account for collector–base capacitance, device mismatch, substrate coupling, self-heating, and limited breakdown voltage. The input transistor is biased near the current density that provides a favourable compromise among noise figure, gain, and linearity. A cascode device improves reverse isolation, but its voltage headroom and breakdown constraints are included in the analysis.

Gummel–Poon models are first used for circuit simulation. Electromagnetic extraction is then applied to inductors, transmission lines, bond pads, and interconnects. Large-signal simulations assess compression and third-order distortion, while corner and Monte Carlo analyses evaluate process variation.

Approach and Solution

The final front end uses a differential HBT low-noise amplifier with emitter degeneration for controlled linearity, a cascode for isolation, and an integrated matching network. Bias circuitry includes temperature monitoring and current limiting. Layout places sensitive RF nodes away from digital switching regions and uses symmetrical routing to preserve matching.

Outcome

The HBT design meets the required gain and noise targets with improved integration compared with a discrete solution. Its success depends not only on the transistor's nominal f_T but also on model quality, accurate passive extraction, thermal design, supply-noise control, and production test procedures.

Learning Points

The case demonstrates that device selection is a system-level decision. A high-speed transistor does not automatically produce a high-performance RF circuit. Bias point, topology, parasitics, thermal conditions, packaging, and manufacturing variation must be treated as part of the same design problem.

Challenges, Modelling Limitations, and Common Design Errors

Several mistakes recur in BJT and HBT analysis. Treating beta as constant can produce substantial bias errors, particularly over temperature and current. Assuming V_BE is a fixed 0.7 V obscures its dependence on current, temperature, and process. Ignoring Early effect leads to overestimated gain and inaccurate current-source behaviour.

Another common error is to use a low-frequency model at RF. Junction capacitances, base resistance, package inductance, substrate loss, and feedback paths can dominate the response. Similarly, a transistor's data-sheet f_T should not be interpreted as the maximum frequency at which a circuit can provide useful power gain. Stability, matching, noise, and available gain must also be evaluated.

Simulation models have finite validity ranges. Extrapolation beyond characterized voltage, current, temperature, or frequency can be misleading. Measurement fixtures introduce de-embedding uncertainty, and thermal conditions may differ between pulsed and continuous operation. Professional practice requires correlation among analytical estimates, simulation, measurement, and reliability qualification.

Professional and Ethical Considerations

BJT and HBT designs can be used in communications, medical equipment, transportation, industrial control, and safety-critical systems. Designers must therefore respect voltage, current, thermal, electromagnetic compatibility, and reliability requirements established by applicable standards. Device ratings must not be exceeded merely because short-term simulation appears acceptable.

Accurate reporting of model limitations and measurement uncertainty is an ethical engineering obligation. In RF and semiconductor development, undocumented process assumptions or optimistic corner selection can create field failures. Design reviews should include independent checks of SOA, thermal margins, breakdown protection, electrostatic-discharge robustness, and production variability.

Current Developments and Best Practices

Current development trends include SiGe HBTs for millimetre-wave transceivers, BiCMOS platforms for high-speed networking, advanced thermal substrates, self-aligned device structures, and three-dimensional interconnect technologies. Research continues on improved heterostructure band engineering, reduced base resistance, higher breakdown–speed products, and integration with photonic and quantum technologies.

Best practice in BJT and HBT design includes:

• Establishing the required current, voltage, frequency, noise, and linearity ranges before selecting a device.
• Using a physically appropriate large-signal model and checking its validity range.
• Extracting or verifying parasitic elements rather than assuming ideal interconnects.
• Evaluating temperature, process, mismatch, ageing, and supply variations.
• Checking breakdown, SOA, self-heating, and electrostatic-discharge limits.
• Using layout symmetry, guard structures, isolation, and thermal paths deliberately.
• Correlating simulation with measurements across bias and frequency, not only at one nominal point.

Chapter Summary

A BJT operates through carrier injection from the emitter, transport across a thin base, and collection at a reverse-biased collector junction. Emitter injection efficiency and base transport determine alpha and beta, while the exponential collector-current relationship gives the BJT high transconductance. The device may operate in cutoff, forward-active, saturation, or reverse-active regions.

The Ebers–Moll model provides a unified large-signal representation, whereas the Gummel–Poon model includes charge storage, capacitances, Early effect, and other practical nonidealities. Small-signal performance is governed by transconductance, base resistance, junction capacitances, transit times, and Miller feedback. Breakdown, punch-through, thermal runaway, and secondary breakdown establish important reliability limits.

HBTs use heterojunction band engineering to improve emitter injection efficiency, permit heavier base doping, reduce base resistance, and achieve high-speed operation. BJTs and HBTs remain important in precision analog, RF, microwave, mixed-signal, and BiCMOS systems. Successful design requires simultaneous attention to device physics, circuit topology, parasitics, thermal behaviour, modelling accuracy, manufacturing variation, and professional reliability practice.

Key Terms

Bipolar junction transistor: A three-terminal semiconductor device in which both electrons and holes contribute to current conduction.

Emitter injection efficiency: The fraction of emitter current produced by carriers injected from the emitter into the base.

Base transport factor: The fraction of injected carriers that cross the base without recombination.

Common-base current gain: The ratio of collector current to emitter current, conventionally denoted alpha.

Common-emitter current gain: The ratio of collector current to base current, conventionally denoted beta.

Forward-active region: The operating region with a forward-biased emitter–base junction and reverse-biased collector–base junction.

Saturation: The region in which both transistor junctions are forward biased and substantial stored charge exists.

Early effect: Collector-current increase caused by base-width modulation under changing collector–base reverse bias.

Ebers–Moll model: A large-signal BJT model based on coupled forward and reverse diode transport.

Gummel–Poon model: An extended BJT model incorporating charge storage, capacitances, Early effect, and high-current behaviour.

Transconductance: The change in output current per unit change in input voltage, approximately I_C/V_T for a BJT.

Unity current-gain frequency: The frequency at which the short-circuit current gain of a transistor becomes unity.

Miller effect: Effective multiplication of a feedback capacitance by voltage gain.

Avalanche breakdown: High-field breakdown caused by impact ionization and carrier multiplication.

Punch-through: Loss of the neutral base region when depletion regions expand toward one another.

Heterojunction bipolar transistor: A BJT containing junctions between semiconductor materials with different band gaps.

Band-gap engineering: Deliberate control of semiconductor composition and energy-band structure to obtain desired device behaviour.

Self-heating: Temperature rise produced by dissipated electrical power within a device.

Safe operating area: The permitted combinations of voltage, current, pulse duration, and temperature for reliable operation.

Emitter degeneration: Introduction of emitter resistance or impedance to provide local negative feedback and improve linearity.

Review Questions

Short Answer Questions

1. Why is the base of a conventional BJT made thin and lightly doped?

2. Define emitter injection efficiency and base transport factor.

3. What distinguishes forward-active operation from saturation?

4. What physical mechanism produces the Early effect?

5. Why does stored charge limit BJT switching speed?

Long Answer Questions

1. Explain the operation of an npn transistor in forward-active mode, including carrier injection, transport, recombination, and collection.

2. Describe the Ebers–Moll model and explain why the Gummel–Poon model is more suitable for practical circuit simulation.

3. Derive the relationship between alpha and beta and discuss the factors that cause beta to vary with current and temperature.

4. Explain the frequency response of a BJT using the hybrid-pi model, including diffusion capacitance, Miller effect, and f_T.

5. Compare conventional BJTs and HBTs in terms of band structure, injection efficiency, base resistance, speed, breakdown, and manufacturing complexity.

Analytical / Application Questions

1. A BJT is biased at I_C = 2 mA, β = 80, and room temperature. Estimate its transconductance and small-signal base-emitter resistance. Explain how the results influence amplifier design.

2. An RF amplifier shows adequate simulated low-frequency gain but poor measured bandwidth. Identify at least four parasitic or modelling factors that could explain the discrepancy and propose verification methods.

3. A parallel BJT bias circuit exhibits increasing current in one device as temperature rises. Analyze the feedback mechanism and recommend circuit and layout techniques to prevent thermal runaway.

22. Multiple Choice Questions with Answers

Question 1

In forward-active operation of an npn BJT, which condition is correct?

A. Both junctions are reverse biased

B. The emitter–base junction is forward biased and the collector–base junction is reverse biased

C. The emitter–base junction is reverse biased and the collector–base junction is forward biased

D. Both junctions are forward biased

Correct Answer: B. The emitter–base junction is forward biased and the collector–base junction is reverse biased

Question 2

If the common-base current gain is α = 0.98, the approximate common-emitter current gain β is:

A. 0.02

B. 0.98

C. 49

D. 98

Correct Answer: C. 49

Question 3

The Early effect primarily results from:

A. Base-width modulation caused by collector–base reverse bias

B. Emitter breakdown caused by forward bias

C. Thermal noise in the base resistance

D. Carrier trapping in the substrate only

Correct Answer: A. Base-width modulation caused by collector–base reverse bias

Question 4

Which model extends Ebers–Moll by including charge storage, capacitances, and additional nonidealities?

A. Ideal diode model

B. Hybrid-pi model only

C. Gummel–Poon model

D. Small-signal resistor model

Correct Answer: C. Gummel–Poon model

Question 5

At room temperature, the approximate transconductance of a BJT carrying 1 mA collector current is:

A. 0.038 mS

B. 0.386 mS

C. 3.86 mS

D. 38.6 mS

Correct Answer: D. 38.6 mS

Question 6

A major reason for the high current gain of a typical npn HBT is that the wider-band-gap emitter:

A. Suppresses hole injection from the base into the emitter

B. Eliminates all collector–base capacitance

C. Makes the collector junction forward biased

D. Prevents electron injection into the base

Correct Answer: A. Suppresses hole injection from the base into the emitter

Question 7

Why does a cascode arrangement often improve the high-frequency performance of a BJT amplifier?

A. It eliminates the need for biasing

B. It reduces voltage variation across the base–collector capacitance and hence Miller feedback

C. It forces the transistor into saturation

D. It increases stored charge in the base

Correct Answer: B. It reduces voltage variation across the base–collector capacitance and hence Miller feedback

Question 8

Thermal runaway in a BJT can occur because increasing temperature generally:

A. Increases the required V_BE for a fixed collector current and reduces current

B. Decreases saturation current and increases stability

C. Increases current, thereby increasing power dissipation and temperature further

D. Eliminates base recombination

Correct Answer: C. Increases current, thereby increasing power dissipation and temperature further

Question 9

Punch-through occurs when:

A. The emitter becomes heavily doped

B. The neutral base region is greatly reduced or disappears as depletion regions expand

C. The transistor is operated at zero collector voltage

D. The base resistance becomes exactly zero

Correct Answer: B. The neutral base region is greatly reduced or disappears as depletion regions expand

Question 10

For an RF HBT design, selecting a bias point solely for maximum f_T is inadequate because the design must also consider:

A. Only the colour of the package

B. Noise, linearity, power, breakdown, stability, and thermal behaviour

C. The absence of any parasitic capacitance

D. The requirement that the transistor operate in saturation

Correct Answer: B. Noise, linearity, power, breakdown, stability, and thermal behaviour