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Transistors and FETs

Bipolar Junction Transistor Tutorial



The Bipolar Junction Transistor (BJT) is a three layer device constructed form two semiconductor diode junctions joined together, one forward biased and one reverse biased.

There are two main types of bipolar junction transistors, the NPN and the PNP transistor.

Transistors are "Current Operated Devices" where a much smaller Base current causes a larger Emitter to Collector current, which themselves are nearly equal, to flow.

The arrow in a transistor symbol represents conventional current flow.

The most common transistor connection is the Common-emitter configuration.

Requires a Biasing voltage for AC amplifier operation.

The Base-Emitter junction is always forward biased whereas the Collector-Base junction is always reverse biased.

The standard equation for currents flowing in a transistor is given as:   IE = IB + Ic

The Collector or output characteristics curves can be used to find either Ib, Ic or β to which a load line can be constructed to determine a suitable operating point, Q with variations in base current determining the operating range.

A transistor can also be used as an electronic switch to control devices such as lamps, motors and solenoids etc.

Inductive loads such as DC motors, relays and solenoids require a reverse biased "Flywheel" diode placed across the load. This helps prevent any induced back emf's generated when the load is switched "OFF" from damaging the transistor.

The NPN transistor requires the Base to be more positive than the Emitter while the PNP type requires that the Emitter is more positive than the Base.


Field Effect Transistor Tutorial

Field Effect Transistors, or FET's are "Voltage Operated Devices" and can be divided into two main types: Junction-gate devices called JFET's and Insulated-gate devices called IGFET´s or more commonly known as MOSFETs.


Insulated-gate devices can also be sub-divided into Enhancement types and Depletion types. All forms are available in both N-channel and P-channel versions.


FET's have very high input resistances so very little or no current (MOSFET types) flows into the input terminal making them ideal for use as electronic switches.


The input impedance of the MOSFET is even higher than that of the JFET due to the insulating oxide layer and therefore static electricity can easily damage MOSFET devices so care needs to be taken when handling them.


When no voltage is applied to the gate of an enhancement FET the transistor is in the "OFF" state similar to an "open switch".


The depletion FET is inherently conductive and in the "ON" state when no voltage is applied to the gate similar to a "closed switch".


FET's have very large current gain compared to junction transistors.


They can be used as ideal switches due to their very high channel "OFF" resistance, low "ON" resistance.


To turn the N-channel JFET transistor "OFF", a negative voltage must be applied to the gate.


To turn the P-channel JFET transistor "OFF", a positive voltage must be applied to the gate.


N-channel depletion MOSFETs are in the "OFF" state when a negative voltage is applied to the gate to create the depletion region.


P-channel depletion MOSFETs, are in the "OFF" state when a positive voltage is applied to the gate to create the depletion region.


N-channel enhancement MOSFETs are in the "ON" state when a "+ve" (positive) voltage is applied to the gate.


P-channel enhancement MOSFETs are in the "ON" state when "-ve" (negative) voltage is applied to the gate.


The Field Effect Transistor Family-tree


Biasing of the Gate for both the junction field effect transistor, (JFET) and the metal oxide semiconductor field effect transistor, (MOSFET) configurations are given as:


Junction FET Metal Oxide Semiconductor FET
Type Depletion Mode Depletion Mode Enhancement Mode
N-channel 0v -ve 0v -ve +ve 0v
P-channel 0v +ve 0v +ve -ve 0v


Differences between a FET and a Bipolar Transistor

Field Effect Transistors can be used to replace normal Bipolar Junction Transistors in electronic circuits and a simple comparison between FET's and transistors stating both their advantages and their disadvantages is given below.


Field Effect Transistor (FET) Bipolar Junction Transistor (BJT)
1 Low voltage gain High voltage gain
2 High current gain Low current gain
3 Very input impedance Low input impedance
4 High output impedance Low output impedance
5 Low noise generation Medium noise generation
6 Fast switching time Medium switching time
7 Easily damaged by static Robust
8 Some require an input to turn it "OFF" Requires zero input to turn it "OFF"
9 Voltage controlled device Current controlled device
10 Exhibits the properties of a Resistor
11 More expensive than bipolar Cheap
12 Difficult to bias Easy to bias

Reproduced with permission from Wayne Storr

Written by Wayne Storr

Wayne Storr