Comparison of BJT and FET

Comparison of BJT and FET

1. Control element:

BJT: Current controlled device. Input current I_B controls output current I_C.

FET: Voltage controlled device. Input voltage V_GS controls drain current I_D.

2. Device type:

BJT: Current flows due to both, majority and minority carriers and hence bipolar device.

FET: Current flows only due to majority carriers and hence unipolar device.

3. Types:

BJT: n‒p‒n and p‒n‒p

FET: n‒channel and p‒channel.

4. Symbols:

BJT: 

FET: 

5. Configurations:

BJT: CE, CB, CC

FET: CS, CG, CD

6. Input resistance:

BJT: Less compare to JFET.

FET: High compare to BJT.

7. Size:

BJT: Bigger than JFET.

FET: Smaller in construction than BJT, thus making them useful in integrated circuits (IC).

8. Sensitivity:

BJT: Higher sensitivity to changes in the applied signals.

FET: Less sensitivity to changes in the applied voltage.

9. Thermal stability:

BJT: Less

FET: More

10. Thermal runaway:

BJT: Exists in BJT, because of cumulative effect of increase in I_C with temperature, resulting increase in temperature in the device.

FET: Does not exist in JFET, because drain resistance r_d increases with temperature, which reduces I_D reducing the I_D and hence the temperature of the device.

11. Relation between input and output:

BJT: Linear

FET: Non‒linear

12. Ratio of o/p to i/p :

BJT: ΔΙ_C / ΔΙ_Β = β

FET: ΔΙ_D / ΔV_GS = g_m

13. Thermal noise:

BJT: More in BJT as more charge carriers cross junctions.

FET: Much lower in JFET as very few charge carriers cross the junction.

14. Gain bandwidth product:

BJT: High

FET: Low

Review Question

1. Compare BJT and FET.