Transistors and FETs

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:   I_E = I_B + I_c

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 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.