Bipolar Junction Transistors

Introduction and Context

The bipolar junction transistor (BJT) is a three-terminal semiconductor device in which a small input current controls a much larger output current. It is called bipolar because conduction involves both electrons and holes, and junction transistor because its operation depends on two p–n junctions. BJTs remain important in analog amplifiers, signal-conditioning circuits, switching systems, voltage references, sensor interfaces, and integrated circuits.

A BJT can be understood in two complementary ways. As a physical device, it consists of a thin semiconductor base between an emitter and a collector. As a circuit element, it is a current-controlled device whose behavior depends strongly on biasing, terminal voltages, temperature, and frequency.

Learning Objectives

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

• describe NPN and PNP construction and transistor action;
• explain the relationships among emitter, base, and collector currents;
• identify common-base, common-emitter, and common-collector configurations;
• interpret input and output characteristic curves;
• distinguish cutoff, active, saturation, and breakdown regions;
• analyze BJT switching and amplification;
• calculate bias currents, voltages, gain, and power dissipation;
• explain thermal effects, ratings, and safe operating limits;
• select appropriate BJT configurations for practical applications.

Construction and Physical Operation

A transistor contains three differently doped regions: emitter, base, and collector. In an NPN transistor, the sequence is n-type–p-type–n-type. In a PNP transistor, it is p-type–n-type–p-type. The emitter is heavily doped so that it can inject carriers efficiently. The base is very thin and lightly doped, allowing most injected carriers to cross it. The collector is moderately doped and physically larger, enabling it to withstand voltage and dissipate heat.

The emitter–base junction and collector–base junction are the two internal p–n junctions. In normal active operation of an NPN transistor, the emitter–base junction is forward biased and the collector–base junction is reverse biased. Electrons are injected from the emitter into the base. Because the base is thin, most electrons diffuse across it and are swept into the collector by the reverse-biased collector junction. A small fraction recombines in the base; this recombination produces the base current.

For a PNP transistor, holes are the principal injected carriers and all voltage and current polarities are reversed. Conventional current flows from emitter to collector in a PNP device, whereas in an NPN device it generally flows from collector to emitter.

The circuit symbol contains an emitter arrow. The arrow points outward for NPN (“not pointing in”) and inward for PNP. The arrow indicates conventional emitter-current direction, not electron flow.

Current Relationships and the Common-Emitter Model

Kirchhoff’s current law gives the fundamental transistor relationship:

where I_E is emitter current, I_C is collector current, and I_B is base current. All currents are measured in amperes, with signs defined according to the chosen transistor convention.

The common-base current gain is

where α is usually slightly less than 1 in active operation. The common-emitter current gain is

where β is also commonly denoted h_FE for a specified test condition. Combining the two definitions with the current relationship gives

and

These equations assume steady operation in the forward-active region. Transistor gain is not a universal constant: it varies with collector current, temperature, device sample, and collector–emitter voltage.

A useful approximation for a silicon transistor in forward-active operation is

This is a practical design approximation, not a fundamental constant. The actual value may be approximately 0.6–0.8 V depending on current and temperature.

The exponential junction relationship is more physically accurate:

Here, I_S is the scale or saturation current, V_BE is base–emitter voltage, n is an ideality factor, and V_T is thermal voltage. At room temperature, V_T ≈ 25.9 mV. This equation applies in forward-active operation over an appropriate current range and neglects several nonideal effects.

Transistor Configurations

A configuration is named for the terminal that is common to the input and output circuits. The common terminal is usually at AC ground, although its DC voltage need not be zero.

Common-Base Configuration

In the CB circuit, the base is common to input and output. The current gain is approximately α, which is less than unity, but the voltage gain can be high because the output resistance is large compared with the input resistance. The output voltage is generally in phase with the input voltage.

Common-Emitter Configuration

The CE circuit is the most widely used configuration. A change in base current produces a much larger change in collector current. For a resistive collector load, an increase in collector current increases the voltage drop across the load and decreases collector voltage. Thus, the voltage output is 180° out of phase with the input.

For a small-signal CE amplifier with an unbypassed emitter resistor, emitter degeneration improves linearity and stabilizes bias, but reduces voltage gain. If the emitter resistor is bypassed for AC signals, gain increases over the bypassed frequency range, while DC stabilization remains provided by the resistor.

Common-Collector Configuration

The CC circuit is also called the emitter follower because the emitter voltage approximately follows the base voltage:

Its voltage gain is slightly less than unity, but its current gain is high. It is therefore useful as a buffer between a high-resistance source and a low-resistance load.

Characteristic Curves and Operating Regions

For a CE transistor, output characteristic curves plot I_C against V_CE for several fixed values of I_B. The curves demonstrate that collector current is mainly controlled by base current in the active region, while V_CE has a smaller effect due to the Early effect.

The principal operating regions are as follows:

In cutoff, both transistor junctions are effectively off, although small leakage currents remain. In saturation, both junctions are forward biased and the transistor no longer obeys the simple active-region relationship I_C = βI_B. The collector current is then limited primarily by the external circuit.

For a saturated silicon transistor, a design approximation is

The exact value depends on collector current, base drive, and device type. A transistor should not be assumed to be saturated merely because its base current is large; the collector circuit must also permit the required current.

DC Biasing and the Operating Point

Biasing establishes a suitable quiescent, or Q-point, for a transistor. In an amplifier, the Q-point is normally placed in the active region so that the signal can vary in both directions without excessive cutoff or saturation distortion.

For a simple fixed-bias NPN circuit with emitter grounded and a base resistor connected to a supply V_CC,

and, if active operation is verified,

The collector voltage is

These expressions assume negligible supply resistance, emitter voltage equal to zero, and operation outside saturation and breakdown.

Worked Example: Fixed-Bias Analysis

An NPN transistor has V_{CC = 10 V, R_B = 470 kΩ, R_C = 2 kΩ, and β = 100. Assume V_{BE = 0.7 V. Then

Therefore,

The collector–emitter voltage is

Because the resulting voltage is well above a typical saturation voltage, the active-region assumption is self-consistent. The emitter current is approximately I_E = 1.98 mA + 0.0198 mA = 2.00 mA.

Fixed bias is simple but poorly stabilized because β varies widely. Practical circuits often use an emitter resistor and a voltage-divider base network. The emitter resistor introduces negative feedback: if current increases, emitter voltage rises, reducing V_{BE and opposing the original increase.

For a voltage-divider bias circuit, when divider current is sufficiently larger than base current, the base voltage can be approximated by

where R₁ connects to V_CC and R₂ connects to ground. Then

and

A more accurate analysis replaces the divider by its Thevenin equivalent and includes base current. Bias design must also check the resulting V_CE and transistor power.

BJT Amplification

A BJT amplifier uses a DC bias point and a small AC signal. The transistor’s nonlinear exponential behavior is approximated locally by a linear small-signal model. The intrinsic emitter resistance is approximately

where I_E is the quiescent emitter current in amperes and V_T ≈ 25.9 mV at room temperature. Thus, increasing bias current reduces the intrinsic emitter resistance.

For a CE amplifier with an emitter resistor fully bypassed for the signal and a collector load represented by R_C′, an approximate voltage gain is

The negative sign indicates phase inversion. Here R_C′ is the effective AC collector resistance, including the collector resistor and any load in parallel. This formula assumes a small signal, active-region operation, negligible transistor output conductance, and adequate bypassing.

If the emitter resistor is not bypassed, a useful approximation is

This lowers gain but improves linearity and reduces sensitivity to device gain variation.

The CE amplifier has finite bandwidth. At low frequencies, coupling and bypass capacitors have high reactance and reduce gain. At high frequencies, transistor junction capacitances and carrier transit time reduce gain. The gain–bandwidth trade-off is central to amplifier design.

BJT Switching

A transistor used as a switch is intentionally driven between cutoff and saturation. In cutoff, base drive is removed and the load current is approximately zero. In saturation, sufficient base drive is applied and the transistor exhibits a low collector–emitter voltage.

For a low-side NPN switch controlling a resistive load,

where R_L is the load resistance. To design reliable saturation, engineers often use a forced beta lower than the transistor’s nominal β:

Typical design values of β_forced may be 5–20, depending on switching speed and device requirements. The base resistor is then approximately

The base-drive source must be able to supply this current. A transistor switch also has storage and transition delays. Excessive base drive can increase stored charge and slow turn-off. A diode or clamp network may be used in specialized high-speed circuits.

Applied Scenario: Relay Driver

Consider an illustrative relay-driver design using a 12 V supply. The relay coil resistance is 240 Ω, so the required coil current, assuming V_{CE(sat) = 0.2 V, is

Choose a conservative forced beta of 10:

If a 5 V logic output drives the base and V_{BE(sat) = 0.8 V,

A standard 820 Ω resistor provides slightly more base current, approximately 5.1 mA. The logic output must be rated for that current. A flyback diode must be connected across the relay coil, reverse-biased during normal operation, to suppress the inductive voltage generated when the transistor turns off. Without the diode, the transistor may experience a destructive voltage spike.

The design process is therefore: determine load current, select a transistor with suitable voltage and current ratings, select conservative base drive, verify logic compatibility, add inductive-load protection, and check power dissipation and thermal limits.

Ratings, Power, and Thermal Effects

Important BJT ratings include maximum collector–emitter voltage, collector current, power dissipation, junction temperature, base current, and safe operating area. Ratings are not independent. For example, the maximum permissible collector current usually decreases as collector–emitter voltage increases because of power and secondary-breakdown limitations.

Transistor power dissipation is approximately

The second term is often small, so designers frequently use P_D ≈ V_CEI_C. Units are watts when voltage is in volts and current is in amperes.

The junction temperature can be estimated by

where T_J is junction temperature in °C, T_A is ambient temperature in °C, and θ_JA is junction-to-ambient thermal resistance in °C/W. With a heatsink, a more detailed thermal path is

The symbols represent junction-to-case, case-to-sink, and sink-to-ambient thermal resistances. The calculated junction temperature must remain below the manufacturer’s maximum rating, with an appropriate engineering margin.

Temperature affects BJTs in several ways. V_BE generally decreases by about 2 mV/°C near ordinary operating currents. Leakage current increases strongly with temperature. Under poorly stabilized bias, increasing temperature can increase collector current, which increases power dissipation and causes further heating. This positive feedback is thermal runaway. Emitter resistance, feedback, current limiting, heat sinking, and proper bias design reduce the risk.

Thermal Example

Suppose a transistor dissipates 1.5 W in an ambient temperature of 35 °C, and θ_JA = 40 °C/W. Then

If the maximum junction temperature is 150 °C, the estimated temperature is acceptable under the stated conditions. The result must still be checked against transient operation, enclosure conditions, manufacturing tolerances, and the manufacturer’s derating curves.

Practical Design and Safety Considerations

A transistor should be selected using the actual circuit conditions rather than nominal current alone. Check:

• maximum steady and transient V_CE;
• load and fault current;
• required base-drive current;
• power dissipation and thermal resistance;
• switching speed and storage time;
• gain at the intended current and temperature;
• package mounting and insulation requirements.

Never exceed absolute maximum ratings. Ratings specify damage limits, not recommended continuous operating points. For inductive loads, use a flyback diode, transient suppressor, or an appropriate snubber. For high-energy switching, consult the safe operating area (SOA) graph because simultaneous high voltage and high current can cause secondary breakdown even when each value appears individually acceptable.

BJTs are also subject to manufacturing variation. A robust design should not depend on one exact value of β. Feedback and current sensing are preferable to relying solely on nominal gain. Laboratory measurements should use current-limited supplies and suitable probes; a short circuit or incorrectly connected transistor can cause rapid overheating.

Contemporary Relevance

Although MOSFETs dominate many low-voltage digital switching applications, BJTs remain valuable where high transconductance, predictable analog behavior, low noise, matched pairs, or high current gain is advantageous. Bipolar devices are integrated with CMOS in BiCMOS technologies for high-speed drivers, precision analog circuits, data converters, and radio-frequency systems. Modern transistor selection increasingly involves simulation models, thermal simulation, automated PCB design checks, and reliability analysis. These tools assist engineering judgment but do not replace verification against data-sheet limits and physical measurements.

Chapter Summary

A BJT is a three-terminal, bipolar semiconductor device in which emitter injection and base transport control collector current. The emitter is heavily doped, the base is thin and lightly doped, and the collector is designed for voltage handling and heat dissipation. The basic current relationship is I_E = I_C + I_B, while α and β describe common-base and common-emitter current gain.

Common-base, common-emitter, and common-collector configurations provide different combinations of input resistance, output resistance, voltage gain, and current gain. Characteristic curves distinguish cutoff, active, saturation, reverse-active, and breakdown operation. Active operation is used for amplification; cutoff and saturation are used for switching.

Bias networks establish the Q-point, and emitter feedback improves thermal and parameter stability. Amplifier gain depends on bias current, collector resistance, emitter degeneration, loading, and frequency. Switch design requires adequate base drive, verification of saturation, attention to switching transients, and protection for inductive loads.

Finally, voltage, current, power, thermal resistance, junction temperature, and safe operating area must be considered together. Reliable BJT design uses conservative ratings, feedback, thermal analysis, manufacturer data, and appropriate protection.

Key Terms

Bipolar junction transistor: A three-terminal semiconductor device using both electrons and holes.

NPN transistor: A transistor with n-type emitter and collector regions and a p-type base.

PNP transistor: A transistor with p-type emitter and collector regions and an n-type base.

Emitter: Heavily doped region that injects majority carriers.

Base: Thin, lightly doped control region between emitter and collector.

Collector: Region that collects carriers and commonly supports substantial voltage.

Forward-active region: Operating region used for amplification.

Cutoff: State in which transistor current is approximately zero.

Saturation: State in which both junctions are forward biased and V_CE is low.

Breakdown: Operation beyond a specified reverse-voltage limit.

Common-emitter gain: Ratio of collector-current change to base-current change, commonly β.

Q-point: Quiescent DC operating point of a biased transistor.

Emitter degeneration: Negative feedback produced by an emitter resistor.

Forced beta: Conservative design ratio used to calculate switch base current.

Thermal runaway: Self-reinforcing increase in temperature and current.

Thermal resistance: Temperature rise per unit power dissipation, measured in °C/W.

Safe operating area: Permitted combinations of transistor voltage, current, and time.

Flyback diode: Diode that suppresses inductive voltage when current is interrupted.

Review Questions

Short Answer Questions

1. Why is the BJT called a bipolar device?
2. State the relationship among emitter, collector, and base currents.
3. What conditions define forward-active operation?
4. Why is a common-collector circuit called an emitter follower?
5. What is the purpose of a flyback diode across a relay coil?

Long Answer Questions

1. Explain the construction and carrier operation of an NPN transistor.
2. Compare common-base, common-emitter, and common-collector configurations.
3. Explain cutoff, active, saturation, and breakdown regions using junction bias conditions.
4. Describe how an emitter resistor improves bias stability and affects amplifier gain.
5. Discuss BJT thermal behavior, power dissipation, thermal resistance, and thermal runaway.

Analytical / Application Questions

1. An NPN transistor has V_CC=15 V, R_B=680 kΩ, R_C=3.3 kΩ, and β=120. Assuming V_BE=0.7 V, calculate I_B, predict I_C, and verify whether active operation is possible.
2. Design the base resistor for a 5 V logic-driven NPN switch that must carry 80 mA from a 9 V supply. Use V_CE(sat)=0.2 V, V_BE(sat)=0.8 V, and forced beta of 10.
3. A transistor dissipates 2 W at an ambient temperature of 40 °C. If θ_JA=30 °C/W and the maximum junction temperature is 150 °C, determine the estimated junction temperature and discuss whether additional cooling is required.

Multiple Choice Questions with Answers

Question 1

Which transistor region is normally used for linear amplification?

A. Cutoff

B. Forward active

C. Saturation

D. Breakdown

Correct Answer: B. Forward active

Question 2

For a BJT, which equation expresses Kirchhoff’s current law at the terminals?

A. I_C = I_E − I_B

B. I_B = I_C + I_E

C. I_E = I_C + I_B

D. I_E = I_C − I_B

Correct Answer: C. I_E = I_C + I_B

Question 3

In an NPN transistor operating in the forward-active region, the collector–base junction is:

A. Forward biased

B. Reverse biased

C. Short-circuited

D. Unbiased in all cases

Correct Answer: B. Reverse biased

Question 4

A common-collector amplifier is commonly called a:

A. Collector follower

B. Base follower

C. Emitter follower

D. Source follower

Correct Answer: C. Emitter follower

Question 5

If β=100 and I_B=20 μA, the approximate active-region collector current is:

A. 0.2 mA

B. 2 mA

C. 20 mA

D. 200 mA

Correct Answer: B. 2 mA

Question 6

A transistor switch is normally driven into saturation to obtain:

A. Very high collector–emitter voltage

B. Low collector–emitter voltage

C. Zero base current

D. Reverse-active operation

Correct Answer: B. Low collector–emitter voltage

Question 7

Why is a flyback diode used with a transistor driving a relay coil?

A. To increase coil resistance

B. To suppress the inductive voltage transient at turn-off

C. To forward bias the transistor collector junction

D. To increase the transistor’s beta

Correct Answer: B. To suppress the inductive voltage transient at turn-off

Question 8

If a transistor dissipates 2 W and has θ_JA=25 °C/W, its approximate junction temperature rise above ambient is:

A. 12.5 °C

B. 23 °C

C. 50 °C

D. 100 °C

Correct Answer: C. 50 °C

Question 9

The emitter of a BJT is generally heavily doped primarily to:

A. Prevent all current flow

B. Inject carriers efficiently

C. Increase collector breakdown voltage only

D. Eliminate the base region

Correct Answer: B. Inject carriers efficiently

Question 10

A common-emitter amplifier normally produces an output voltage that is:

A. In phase with the input

B. 180° out of phase with the input

C. Always zero

D. Independent of collector current

Correct Answer: B. 180° out of phase with the input