Bipolar Junction Transistor Parameters and How They Relate

1. Static Parameters (DC)

Parameter Symbol Description Example for BC547 Example for 2N3055
DC Current Gain (Base) hFE, β Ratio of DC collector current to DC base current in active mode. Depends on current and temperature. 100-400 (typ. 200) 20-70 (typ. 50)
DC Current Gain (Emitter) α Ratio of collector current to emitter current: α = β/(β+1) 0.99-0.997 0.95-0.986
Collector-Emitter Saturation Voltage Vce(sat) Voltage drop between collector and emitter in saturation mode 0.2 V at Ic=10 mA 0.5-1 V at Ic=3 A
Base-Emitter Saturation Voltage Vbe(sat) Voltage drop across the B-E junction in saturation mode 0.7-0.9 V 1.2-1.5 V
Cut-in Voltage Vbe(on) B-E junction voltage required to start conduction ~0.6-0.7 V ~0.7 V
Collector Cut-off Current Icbo Leakage current through the reverse-biased collector-base junction with emitter open. Increases with temperature. < 15 nA (at 25°C) < 1 mA (at 25°C)
Emitter Cut-off Current Iebo Leakage current through the emitter-base junction with collector open. Usually very small. < 100 nA -

2. Dynamic Parameters (Small-Signal)

Parameter Symbol Description Example for BC547
AC Current Gain hfe, β Ratio of small changes in collector current to base current 200-400
Input Impedance hie AC signal resistance between base and emitter 5 kΩ at Ic=1 mA
Collector-Base Capacitance Ccb, Cc, Cob Barrier capacitance of the reverse-biased C-B junction 3-5 pF
Transition Frequency fT Frequency at which the current gain drops to unity 300 MHz

3. Maximum Ratings

Parameter Symbol Description BC547 2N3055
Collector-Emitter Breakdown Voltage Vceo Voltage between C and E with base open 45 V 60 V
Maximum Collector Current Ic(max) Maximum allowable continuous collector current 100 mA 15 A
Maximum Power Dissipation Pc(max), Pd Maximum power the package can dissipate (at T=25°C). Decreases with rising temperature! 500 mW 115 W (with heatsink)
Maximum Junction Temperature Tj(max) Absolute maximum crystal temperature +150°C +200°C

4. Temperature Parameters

Parameter Symbol Description Example
Vbe Temperature Coefficient ΔVbe/ΔT Change in B-E voltage with heating −2 mV/°C
Junction-to-Ambient Thermal Resistance Rθja Thermal resistance from the crystal to ambient 200 °C/W (TO-92)
Junction-to-Case Thermal Resistance Rθjc Thermal resistance from the crystal to case 83.3 °C/W (TO-220)

Interdependence of Bipolar Transistor Parameters

Parameter Interaction

Influencing Parameter Affects Nature and Consequences of Influence
Current Gain (hFE, β)

Saturation Voltage Vce(sat)

Input Impedance hie

Temperature Stability

Gain Spread

Low β increases Vce(sat), requires higher base current.

High β increases hie.

High β enhances temperature dependence.

Large technological spread of β (e.g., from 100 to 400) complicates mass production of circuits, requires worst-case design or use of negative feedback.
Collector Current (Ic)

Input Impedance hie

Transition Frequency fT

hie decreases with increasing Ic.

Relationship is nonlinear. Typically fT increases with Ic up to an optimum, after which it begins to fall.
Collector-Emitter Voltage (Vce)

Output Impedance roe

Capacitance Ccb

Power Dissipation Pc

roe increases with Vce (Early effect).

Ccb decreases with increasing reverse voltage.

Pc = Ic × Vce, the main factor for heating.
Junction Temperature (Tj)

Voltage Vbe

Gain β

Leakage Currents Icbo

Vbe decreases by ~2 mV/°C (risk of thermal runaway).

β increases by approximately 0.5% / °C.

Icbo increases exponentially with temperature.
Maximum Operating Temperature

Parameter drift over time

Threshold voltage changes

Connection reliability

Long-term operation at elevated temperature causes irreversible changes in characteristics.

Heating alters the band gap, affecting Vbe and leakage currents.

Thermal cycling leads to material fatigue and contact degradation.
Transition Frequency (fT)

High-Frequency gain

Switching speed

Determines maximum amplification frequency: fmax ≈ fT/√2.

High fT correlates with short switching time.
Collector-Base Capacitance (Ccb)

Frequency response

Switching speed

Acts as an internal feedback capacitance. Provides negative feedback from output to input, reducing HF gain and potentially causing oscillation in an amplifier.

Large Ccb increases the base charge storage time during turn-off (t_off), slowing down the transistor switch.
Saturation Voltage Vce(sat)

Power dissipation in switching mode

Heat generation

Losses in the on-state: P_loss = Ic × Vce(sat).

High Vce(sat) requires a larger heatsink.
Thermal Resistance (Rθja)

Actual maximum power

Reliability and lifetime

Determines temperature rise: ΔT = Pc × Rθja.

Poor heat dissipation → high Tj → accelerated degradation and reduced transistor lifetime.
Noise Figure (NF)

Dynamic range

Input stage sensitivity

Bias point stability

High noise figure limits the minimum detectable signal in low-noise amplifiers.

Depends on collector current and frequency, has an optimum value at a certain Ic.

In RF circuits, the noise figure is influenced by parasitic lead capacitances and inductances.
Emitter-Base Capacitance (Ceb)

HF input impedance

Switching speed

Amplifier stability

Together with base resistance forms a low-pass filter, limiting bandwidth.

In switching mode, affects the turn-on delay time (td).

Large Ceb can lead to oscillation via internal feedback.
Lead Inductance

HF characteristics

Switching noise spikes

High-frequency stability

Together with parasitic capacitances forms resonant circuits at HF.

Causes voltage spikes during fast switching of large currents.

Limits the maximum effective switching frequency.
Base Resistance (rb')

Noise characteristics

Heating with HF signals

Non-uniform turn-on

Causes thermal noise, proportional to √rb'.

Power dissipated in the base region, P = Ib² × rb'.

In power transistors, leads to non-uniform current distribution across the die.
Storage Time (ts)

Maximum switching frequency

Dynamic power losses

Pulse edge shaping

Determines the minimum control pulse width for guaranteed turn-off.

During ts the transistor remains partially on, increasing losses.

Affects the output signal shape in pulse circuits.
Power Gain

Stage efficiency

Thermal regime

Gain stability

Determines the number of stages needed to achieve a given output power.

Low gain requires higher quiescent current or additional stages.

Depends on input and output impedance matching.
Breakdown Voltage with Open Base

Voltage margin in switching mode

Protection against voltage spikes

Reliability in inductive circuits

Determines the need for snubber circuits when switching inductive loads.

Affects the choice of supply voltage for reliability.

Low breakdown voltage requires additional protective elements.

Practical Consequences of Interdependencies

Amplifier Design

The operating point (Ic, Vce) determines hie and voltage gain. The product Ic × Vce must be controlled to comply with thermal limits.

Switching Mode

Requires a compromise between parameters:

  • High β and large Ib for low Vce(sat)
  • High fT and low Ccb for fast switching
  • Low Vce(sat) to minimize losses

Temperature Stability

Temperature rise causes a chain reaction:

  • Increase in β and Icbo
  • Decrease in Vbe
  • Avalanche increase in Ic (thermal runaway)

Solution: emitter degeneration and DC negative feedback.

High-Frequency Applications

Junction capacitances (Ccb, Cbe) become dominant, shunting the input and creating feedback. High fT and specialized package topology are important.

Key Takeaways

Transistor parameters are closely interrelated and require trade-offs in design. Understanding these interdependencies allows for correct transistor selection and the development of stable, efficient circuits.

Always consult the official datasheet for the specific transistor model, as parameters can vary significantly.