At a glance
Identify the required function first, then compare ratings, operating behavior, standards and application constraints before selecting a term or component. The table gives the scan-friendly answer; the detailed sections explain the distinctions and exceptions that a short definition can miss.
What is BJT?
A bipolar junction transistor is a three-terminal semiconductor device—emitter, base and collector—in which both electron and hole carrier behavior is involved. In circuit design it is commonly modeled as a current-controlled/current-amplifying device, although precise behavior is described by transistor equations.
What is FET?
A field-effect transistor is a three-terminal or four-terminal device in which an electric field at the gate controls current through a semiconductor channel. MOSFETs are the most common FET family in modern digital and power electronics.
BJT vs FET: comparison table
| Point of comparison | BJT | FET |
|---|---|---|
| Control quantity | Base-emitter voltage/base current establishes collector current | Gate voltage controls channel conduction |
| Input impedance | Lower than MOSFET gate input impedance | Very high for insulated-gate MOSFETs |
| Carrier type | Bipolar conduction | Majority-carrier device in common FET operation |
| Drive power | Needs continuing base drive in many uses | MOSFET gate mainly needs charge during switching |
| Thermal behavior | Can be susceptible to thermal runaway without bias control | Power MOSFETs often parallel more easily in positive-temp-coefficient region |
| Switching | Useful but charge storage can limit saturated switching | MOSFETs dominate many high-speed switching applications |
| Analog strength | High transconductance and predictable small-signal behavior | High input impedance and excellent switching/integration |
| Common examples | NPN, PNP | MOSFET, JFET |
Key differences explained
1. Control quantity
For BJT, the key point is Base-emitter voltage/base current establishes collector current. For FET, it is Gate voltage controls channel conduction. This is often one of the fastest checks when the two terms are being confused.
2. Input impedance
Under input impedance, compare the descriptions directly: BJT — Lower than MOSFET gate input impedance. FET — Very high for insulated-gate MOSFETs. Keeping this dimension separate prevents a similarity elsewhere from hiding an important distinction.
3. Carrier type
The practical split for carrier type is BJT: Bipolar conduction versus FET: Majority-carrier device in common FET operation. Use this point together with the definitions above rather than as an isolated rule.
4. Drive power
If drive power is the question, use the comparison-table wording directly: BJT — Needs continuing base drive in many uses; FET — MOSFET gate mainly needs charge during switching. Context determines how much weight this difference should carry.
5. Thermal behavior
Another separator is thermal behavior. The relevant descriptions are Can be susceptible to thermal runaway without bias control for BJT and Power MOSFETs often parallel more easily in positive-temp-coefficient region for FET. This becomes useful when both terms appear in the same broader subject area.
6. Switching
For this dimension, read the contrast as BJT — Useful but charge storage can limit saturated switching and FET — MOSFETs dominate many high-speed switching applications. Check the surrounding context because jurisdiction, species, standards, product specifications or professional usage may narrow the general rule.
Other practical distinctions
- Analog strength: BJT: High transconductance and predictable small-signal behavior. FET: High input impedance and excellent switching/integration.
- Common examples: BJT: NPN, PNP. FET: MOSFET, JFET.
Similarities
- Both are transistors used for switching and amplification.
- Both have three principal terminals and require correct biasing.
- Both exist in small-signal and power forms.
- Device choice depends on voltage, current, frequency, noise, efficiency and cost.
Practical examples
- A small-signal BJT may be chosen for an analog gain stage where transconductance is valuable.
- A MOSFET is commonly used to switch a motor or DC load because the gate can be driven with little steady-state current.
How to distinguish them in practice
Identify the required function first, then compare ratings, operating behavior, standards and application constraints before selecting a term or component.
Common mistakes to avoid
- Choosing a component or construction method from a label alone instead of the design requirement.
- Ignoring ratings, standards, operating conditions or manufacturer specifications.
- Treating a conceptual comparison as a substitute for an engineered calculation or code requirement.
- Assuming terminology is identical across industries, countries or generations of equipment.
Frequently asked questions
Is a MOSFET a FET?
Yes. MOSFET means metal-oxide-semiconductor field-effect transistor.
Which has higher input impedance?
An insulated-gate MOSFET normally has much higher DC input impedance than a BJT input junction.
Which is better for switching?
MOSFETs dominate many switching applications, but BJTs and IGBTs remain useful in particular voltage, current, cost and analog cases.
Is a BJT literally current controlled?
That is a useful circuit model, but transistor physics is more accurately described through base-emitter voltage and carrier transport. Designers use the model appropriate to the task.
Bottom line
A BJT controls collector current through base-emitter drive and involves both majority and minority carriers, while a FET controls channel current primarily with an electric field produced by gate voltage. FETs generally offer very high input impedance; BJTs often provide high transconductance for a given bias current. The most useful first check is control quantity: BJT — Base-emitter voltage/base current establishes collector current; FET — Gate voltage controls channel conduction.
Sources and further reading
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KnowDifferences Editorial Team
Independent explanations with definitions, practical examples and references. Read our editorial approach.