Of the two most common types of transistors, the Field
Effect transisitor is the simpler to understand. It is a
piece of doped semiconducting material through which a
current naturally flows. The control of this current is
achieved by the addition of one or two regions of doped
semiconducting material containing the opposite type of
carriers, effectively creating two p-n junctions such as
those in a diode. These p-n junctions are used to modulate
the properties of the channel where the current flows from
the drain to the source (the source is actually a source of
electrons which flow into the drain opposite to the
current). If NO voltage is applied to the gate the device
is full ON - that means that when a voltage and resistance
are applied to the source and drain, a current will flow at
its maximum value. As a voltage is applied to the gate the
current will diminish until the channel has been depleted
of all current carrying charges. In this type of device you
use a voltage to turn it OFF.
But as you might guess this story of FETs is far from
complete here. This is the traditional explanation for how
this device works. But it can be fabricated with many
additional features. The channel can be manufactured so
that it is always open and needs a voltage to turn it off -
this is an depletion mode device OR it can be
manufactured so that it needs a voltage to turn it on -
this is an enhancement mode device. In our lab we
will be using an enhancement mode FET so the gate voltage
is used to turn ON the current. The FET we will be using is
also fabricated using a special technique - it is a MOSFET
which stands for Metal Oxide Semiconductor Field Effect
Transistor. The beauty of this device is that it requires
very little power input into the device to control the
switching. The gate voltage controls the flow of current
through the device - no current, or very little current
flows through the gate. The improvement over a traditional
FET is to add a layer of insulation - the oxide - to the
gate to further insulate the gate from the source-drain
channel. This is the common fabrication technique for
building logic gates because so little energy is required
compared to the Bipolar Junction transistor, as you will
see it requires the input of some current to control the
device.
How do we use these devices? As previously mentioned we
will be using the transistors as switches/current
amplifiers. Let's see how we would build such a circuit
from a Field Effect Transistor. The topmost circuit in the
figure below shows the symbol traditionally used to
represent the junction Field Effect transistor (JFET). This
is an N-channel device which means that the channel is made
of the type of doped semiconducting material that has an
excess of electrons - much like a conductor. Since the
characteristics of the channel of this device is controlled
by the voltage at the gate this device can be connected
directly to the biasing voltage source (see middle figure).
When the voltage is zero the device is full on. As the
voltage is increased the current delivered to the load is
decreased - quite opposite of the behavior of the BJT.
Symbol for an N-channel JFET
The biasing (or input) circuit for a JFET
The JFET provides current to the load through the more
robust source Vload