Engineering Reference
BJT Datasheet Parameters Reference
Quick-reference BJT parameters including hFE, VBE, VCE(sat), VCEO, collector current, SOA, power, thermal resistance, fT, capacitance, and operating regions.
- Reading Time
- 12 min
- Format
- Parameter lookup
- Updated
- September 24, 2026
Quick BJT Parameter Lookup
| Need to check | Datasheet entry | Interpretation condition |
|---|---|---|
| Maximum collector-emitter voltage | VCEO | Check polarity, temperature, and transient margin |
| DC current gain | hFE / βDC | Read at the relevant IC, VCE, and temperature |
| Saturation voltage | VCE(sat) | Read with both IC and IB test conditions |
| Maximum collector current | IC / ICM | Separate continuous and pulsed ratings; verify SOA |
| Base-emitter voltage | VBE(on) / VBE(sat) | Read at the stated current and operating region |
| High-frequency current gain | fT | Small-signal unity-gain definition, not maximum switching frequency |
| Thermal path | RθJA / RθJC | Use the stated PCB, case, airflow, and mounting conditions |
| Safe voltage-current combinations | SOA | Check VCE, IC, pulse duration, temperature, and secondary breakdown |
BJT Operating Regions
| Region | Base-emitter junction | Base-collector junction | Behavior | Common use | Caution |
|---|---|---|---|---|---|
| Cutoff | Not strongly forward biased | Reverse biased or not forward biased | Very low collector conduction, with real-device leakage | Off state | IC is not exactly zero |
| Forward active | Forward biased | Reverse biased | IC ≈ βIB is a useful local approximation | Linear amplification | β varies with IC, VCE, temperature, and device |
| Saturation | Forward biased | Forward biased | Collector current is limited by circuit and base drive; linear β relation no longer applies | On-state switching | Use VCE(sat) at stated IC and IB |
| Reverse active | Reverse biased | Forward biased | Emitter and collector roles are reversed with generally poor gain | Rare special cases | Normal ratings and performance may not apply |
| Breakdown / outside normal operation | Device-dependent | A junction exceeds a breakdown rating | Potential avalanche, heating, or damage | Only where explicitly designed and rated | VCEO, VCBO, and VEBO are different limits |
Datasheet Parameter Reference
Notation varies among manufacturers. Interpret each symbol using the specific datasheet definition and test conditions.
| Symbol | Meaning | Condition to retain | Common mistake |
|---|---|---|---|
| VCEO | Collector-emitter breakdown voltage, base open | Maximum-rating table and stated current condition | Not interchangeable with VCBO |
| VCBO | Collector-base breakdown voltage, emitter open | Maximum-rating table | Using it as the normal VCE rating |
| VEBO | Emitter-base breakdown voltage, collector open | Usually a comparatively low reverse junction rating | Reverse-biasing the base-emitter junction without checking it |
| IC / ICM | Continuous / peak collector current | Package, pulse width, duty cycle, and temperature | Treating peak current as continuous |
| IB | Base current | Maximum rating and drive condition | Ignoring driver capability and base dissipation |
| hFE / βDC | DC forward current gain | Specified at stated IC and VCE; often a range or curve | Using a typical value as a guaranteed constant |
| VBE(on) | Base-emitter on voltage | Specified at an operating current and voltage | Assuming exactly 0.7 V |
| VBE(sat) | Base-emitter voltage in saturation | Specified with IC and IB | Confusing it with active-region VBE |
| VCE(sat) | Collector-emitter saturation voltage | Specified with IC, IB, and temperature | Assuming exactly 0.2 V |
| PD / Ptot | Device power dissipation | Ambient- or case-temperature basis and derating | Combining headline PD with unrelated thermal conditions |
| TJ / Tstg | Junction / storage temperature | Absolute temperature limits | Confusing ambient and junction temperature |
| RθJA / RθJC | Junction-to-ambient / junction-to-case thermal resistance | Defined test board, mounting, and environment | Treating RθJA as a package-only constant |
| ICBO / ICEO | Collector leakage with different terminal conditions | Specified voltage and temperature | Projecting room-temperature leakage to hot operation |
| fT | Small-signal transition frequency | Bias point and test frequency conditions | Calling it maximum digital switching frequency |
| Cob / Cobo | Output or collector-base capacitance | Specified reverse bias and frequency | Ignoring bias dependence and notation variation |
Parameter Boundaries
hFE and forced beta
VBE is condition-dependent
VCE(sat) requires IC and IB
Absolute maximum is not recommended operation
Power and thermal estimate
Frequency, capacitance, and leakage
Worked Reference Examples
Base current assumption
Base resistor
Saturation conduction loss
Junction-temperature estimate
Datasheet Lookup Workflow
- 1Identify NPN or PNP and terminal polarity.
- 2Check required VCEO and transient margin.
- 3Check continuous and pulsed collector current.
- 4Check SOA at actual VCE, IC, pulse, and temperature.
- 5Read hFE at relevant IC and VCE.
- 6For switching, read VCE(sat) at specified IC and IB.
- 7Verify base-drive capability and dissipation.
- 8Estimate power and junction temperature.
- 9Check RθJA/RθJC under the real mounting conditions.
- 10Check fT and capacitances when speed matters.
- 11Check leakage over the full temperature range.
- 12Confirm manufacturer curves and guaranteed limits.
Common Interpretation Mistakes
- Treating hFE as constant.
- Designing saturation from typical hFE alone.
- Treating VBE as exactly 0.7 V.
- Treating VCE(sat) as exactly 0.2 V.
- Ignoring VCE(sat) IC and IB conditions.
- Using ICM as continuous current.
- Combining absolute maximum ratings.
- Ignoring SOA and secondary breakdown.
- Treating fT as maximum switching frequency.
- Ignoring hot leakage.
- Treating RθJA as a universal package constant.
- Confusing current gain with voltage gain.
- Assuming cutoff means zero leakage.
- Treating typical values as guaranteed limits.
Support reference
FAQ
Is transistor beta constant?
No. DC gain varies with collector current, collector-emitter voltage, temperature, manufacturing spread, and individual device.
What is hFE?
hFE commonly denotes DC forward current gain at a stated operating point, approximately IC divided by IB. Datasheets may specify a minimum, range, typical value, or curves.
Is hFE the same as forced beta?
No. Forced beta is the design ratio IC/IB chosen for a switching condition. It is not the transistor's measured active-region hFE.
Is VBE always 0.7 V?
No. About 0.7 V is only a rough estimate for some silicon BJT operating points. VBE changes with current, temperature, construction, and region.
What does VCE(sat) mean?
VCE(sat) is collector-emitter voltage under a stated saturated test condition. It must be read together with collector current, base current, and temperature.
Is VCE(sat) always 0.2 V?
No. A 0.2 V value is only a possible example assumption. Actual VCE(sat) depends on device, IC, IB, forced beta, and temperature.
What is the difference between VCEO and VCBO?
VCEO is measured between collector and emitter with the base open; VCBO is collector-to-base with the emitter open. They represent different breakdown conditions.
Can IC(max) be used continuously?
Not automatically. Continuous current also depends on SOA, power, junction temperature, package, board cooling, and voltage-current combination.
What is transistor SOA?
Safe Operating Area describes permitted combinations of collector current, collector-emitter voltage, pulse duration, and temperature, including secondary-breakdown limits where applicable.
Is fT the maximum switching frequency?
No. fT is a small-signal unity-current-gain metric at a stated bias. Practical switching also depends on charge storage, capacitance, drive, load, and circuit topology.
How does temperature affect BJT parameters?
Temperature changes VBE, gain, leakage, power capability, and thermal margin. Use temperature-specific limits and curves rather than room-temperature typical values alone.
What should I check when using a BJT as a switch?
Check VCEO and transients, load current and SOA, available base drive, VCE(sat) at relevant IC/IB, power, junction temperature, leakage, and switching speed.
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