MOSFET

BIPOLAR

Voltage operated

Current operated

Gain

The mutual conductance (like gain) is fairly constant once the device is conducting. It is not altered much by temperature.

Current gain is not constant

Gain decreases when collector current increases

Gain increases as temperature increases.

Input Resistance

Very high 10⁹ Ohms (usually a good thing)

For AC signals this figure can be much lower due to the capacitance of the device.

Low (often a bad thing)

Input Current

Pico or Microamps (close to zero)

Milliamps

Saturation

V_DS = 2 Volts

Higher heat dissipation when saturated

V_CE = 0.2 Volts

Low heat dissipation when saturated

Switching Speed and Frequency Response

Much faster than Bipolar

Much better frequency response

May result in unwanted high frequency oscillations (these can destroy your speakers)

Much slower than MOSFET

Inferior to MOSFETS

Voltages

When fully turned on (saturated), the potential drop across the device is about 2 volts. (between the source and drain).

When fully turned on (saturated), the potential drop across the device (V_ce) is about 0.2 volts. (between the collector and emitter).

Bias (input) Voltages

N Channel MOSFETS need +2 to +6 volts to turn them on. The gate current is approximately zero.

Base current starts to flow with an input voltage of about +0.6V. Relatively large base currents are needed to make transistors operate.

Thermal Runaway

When MOSFETS heat up, the current flowing through them decreases. Although they run hotter than bipolar transistors, they are less likely to be destroyed by overheating.

When bipolar transistors heat up, the gain increases and so the current through them increases too. This in turn causes further heating and yet more gain and current. This can cause catastrophic failure called thermal runaway.

Negative feedback helps to prevent this.