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Build Your Own Combat Robot
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are called the Field Effect Transistor (FET) and the Metal Oxide Semiconductor
Field Effect Transistor (MOSFET). For the following discussions, FET will be
used as a generic term to represent both MOSFETs and FETs.
Field Effect Transistor

An FET works something like a semiconductor implementation of a relay. An FET
has two leads, known as the source and the drain, connected to a channel of semi-
conductor material. The composition of the material is such that current cannot
normally flow through it. A third lead, called the gate, is connected to a conductive
electrode that lies on top of the semiconductor junction but is insulated from it by a
thin non-conducting layer. When voltage is applied to the third electrode, it creates
an electric field that rearranges the electrons in the semiconductor junction. With
the field present, current is able to flow between the source and drain pins. When
the gate is driven to a low voltage, the electric field reverses and current is unable to
flow. The FET acts as a voltage-controlled switch, where an applied voltage to the
gate will control the current flow between the drain and source.
The layer of insulation between the gate and the source/drain channel must be
very thin for sufficient field strength to reach from the gate into the semiconductor
channel. This thinness makes the FET vulnerable to being damaged by too high a
voltage. If the voltage between either the drain or source and the gate exceeds the
breakdown voltage of the insulation layer, it will punch a hole through the layer
and short the gate to the motor or battery circuit. This can be caused by connect-
ing the FET up to too high a voltage, or simply by zapping the FET circuit with
static electricity. You should be careful when handling FETs and attached elec-
tronics to avoid accidentally discharging static electricity into them. It is also good
practice to use FETs with a voltage rating of twice the battery voltage you wish to
run your motors on to avoid the possibility of inductive spikes momentarily ex-
ceeding the FET breakdown rating.
When using an FET as a high-current PWM switch, it is important that you
switch the gate from the off voltage to the on voltage as quickly as possible. When at
an intermediary state, the FET will act as a resistor, conducting current inefficiently
and generating heat. Commercial PWM FET-based controllers use specialized
high-current driver chips to slam the FET gates from low to high voltage and back
as quickly as possible, minimizing the time spent in the lousy intermediary state.
The power that can be switched by an FET is fundamentally limited by heat
buildup. Even when fully in the on state, an FET has a slight resistance. Heat buildup
in the FET is proportional to the resistance of the semiconductor channel times the
square of the current flowing through it. The resistance of the semiconductor
channel increases with its temperature—so once an FET begins to overheat, its ef-
ficiency will drop; and if the heat cannot be sufficiently carried away by the envi-
ronment, it will generate more and more heat until it self-destructs. This is known as
thermal runaway. A FET’s power-switching capacity can be improved by removing