A thin channel of n-type material, called appropriately enough, the n channel, connects
the two islands. A layer of insulating material (such as silicon dioxide, whence the “O” in
MOSFET), mere nanometers thick, is deposited on top and penetrated by the two metal
leads (whence the M) shown in the diagram. On top of the insulating material lies a third
deposit of metal, called the gate G. Because of the insulating layer, no charge can flow
from the gate to the rest of the transistor.
This is a reasonably complex configuration. It is all the more impressive when you
consider that semiconductor manufacturers are now building transistors where the
gates measure 65 nanometers across, and that a single microprocessor chip can
contain 500 million or more transistors. (It is a safe bet that humans have manufactured
more transistors than any other device.)
The diagrams on the right show a transistor as part of a circuit. There is always a
potential difference between the source and the drain. The key to how the transistor
functions resides in whether there is a potential difference between the source and the
gate. We will call this difference the gate voltage.
Let’s consider what happens when there is no potential difference at the gate: In this
case it has no effect on the other parts of the device. The illustration in Concept 2
shows this state. Electrons flow from the negative lead to the source island, then across
the n channel to the drain island and the positive lead, because of the potential
difference between the source and the drain. In short, a current flows through the
transistor when the gate voltage is zero.
Now we assume that there is a negative gate voltage. This is illustrated in Concept 3. (A
signal from another circuit might “turn on” this potential difference.) When it is turned on,
we show the gate as negatively charged. Electrons repel each other, so the field caused
by the electrons in the gate drives the n channel’s electrons into the p-doped substrate
where they find holes to occupy.
The channel is said to be depleted: The n channel has less n, electrons. Another way to
put it is that the channel becomes narrower. This increases its resistance, and with a
strong enough electric field from the gate, no electrons can flow from the source to the
drain at all. This is where the “field effect” in MOSFET comes into play. (The substrate
is too lightly doped and too poor a native conductor to allow current to flow there.) The
entire process can be likened to stepping on a flexible garden hose to cut off the flow of
water.
With a variation of the electric field of the gate, the transistor can be turned from ON to
OFF, from allowing current to flow to preventing it. This simple idea, enabling circuits to
be set to “ON” or “OFF”, to represent “1” or “0”, “true” or “false”, underlies the design of
computer memories and microprocessors.
After all this, you may think: It just turns on and off. Indeed. Just repeat that 500 million
times or so and you have a microprocessor! The sophistication of the manufacturing
process allows many transistors to be packed into a very small region.
The transistor also can function as part of an amplifier. Imagine that the potential
difference between the source and the drain is very large: This is the “power” part of the
amplifier. We show this in Concept 4.
When there is no sound, the microphone creates a negative gate voltage, depleting the
n channel and preventing current from flowing through the transistor. Although the
voltage from the “large emf” source is much greater than the voltage of the microphone,
the microphone is preventing any current from flowing.
Now imagine that a singer starts her song. As the sound of her voice becomes louder,
the microphone creates a smaller negative gate voltage, or even a positive one, and
current is allowed to flow from the drain to the source, unleashing the power of the large
emf and driving the loudspeaker. The transistor functions as a variable resistor that is
controlled by the microphone signal.
The strong current flowing through the channel from the source to the drain is in perfect
synchronization with the amount of charge on the gate, which depends on the voltage
applied by the microphone. The large signal mimics the small one.
Metal gate G is above insulator
How a transistor functions (ON)
When gate voltage = 0 with
Potential difference between source and
drain
·Electrons flow from source to drainHow a transistor functions (OFF)
When gate has negative voltage
Electrons driven out of n channelCurrent cannot flow
·n channel is depleted of electrons
A transistor at work
Source/drain emf is large
Gate emf is small (signal)
Signal determines output of amplifier