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GCSE     Semiconductor     >Comparison<     Diode     LED     MOSFET     Photodiode     Rectifier and PSU     Transistor     Zener Diode    

Semiconductor Comparison


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  GCSE    Semiconductors  0 of 16    Question 268    Components 
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MOSFETs are better than bipolar junction transistors in almost all respects except price. For low current moderate speed switching, BJT devices can be cheaper.

 

MOSFET

N anbd P Channel MOSFET symbols

Metal Oxide Semiconductor Field Effect Transistor

BIPOLAR JUNCTION TRANSISTOR

NPN and PNP Bipolar Junction Transistor Symbols

Bipolar Junction Transistor ( BJT )

Output
Current
is controlled by the input gate voltage. is controlled by the input base current.
Cost More Expensive Lower Cost
ESD
Risk
Easily damaged by ESD Electrostatic Discharge. ESD is rarely a problem
Gain Very high current gain which is nearly constant
for varying drain currents.
Lower current gain and it is not constant. It decreases
when the collector current increases.
Input
Resistance
Very high. More than 107 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) Quite a big input current might be needed.
Input
Current
Picoamps (approximately zero). Microamps or Milliamps
Saturation Close to zero Volts.    VDS = 20 mV

Even lower heat dissipation when
saturated (fully turned on).

Close to zero Volts.    VCE = 200 mV

Low heat dissipation when saturated (fully turned on).
Switching
Speed and
Frequency
Response
Faster than Bipolar

Better frequency response

May result in unwanted (parasitic)
supersonic (frequency too high to hear) oscillations
that can destroy your speaker tweeters.
Slower than MOSFETs. Inferior frequency response.
Voltages When fully turned on (saturated), the potential drop across the
device is close to zero or about 20 mV. (between the source and drain).
When fully turned on (saturated), the potential drop across the device
(Vce) is close to zero or about 200 mV. (between the collector and emitter).
Bias (input)
Voltages
N Channel MOSFETS need +2 to +4 volts to turn them on.
The gate current is approximately zero.

Base current starts to flow with an input voltage of about +0.7V.
Relatively large base currents are needed to make transistors operate.

Thermal
Runaway
When MOSFETS heat up, the current flowing
through them decreases.
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.

 

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