EE 100 Semiconductor Device Primer
Fall 2007
Diodes
A diode is a two terminal semiconductor device that allows current flow in one direction
but not in the other. Using the water analogy, a diode is equivalent to a check valve that
enables fluid flow in one direction but prevents back flow. The diode is a polar device
that must be inserted into the circuit in the correct direction for the proper operation to be
achieved. The symbol used for a diode is shown below, and the arrow in the diode
symbol points in the direction that current can flow.
+
v diode
-
i diode
The ideal diode is modeled as a device that has infinite idiode for any vdiode > 0. When vdiode
< 0, idiode is equal to 0. Therefore, if the voltage across the device would tend to be
positive, the diode is simply a short circuit (zero resistance wire) and the voltage across
the device is 0 V. When the voltage across the diode is negative, however, the diode acts
as an open circuit (no wire at all), and no current flows. This model is useful for
understanding how circuits function, and often no more detailed model is required.
Outside of your immediate needs in this course, it may be useful to have a more detailed
diode model. Physical diodes require some finite voltage drop before they start
conducting large currents. A more realistic diode model is to say that a diode conducts no
current until vdiode exceeds some threshold. When vdiode is greater than this threshold
voltage, the diode has zero resistance, but it has a fixed voltage drop equal to the
threshold voltage. Most commonly used diodes have a threshold of about 0.7 V, but there
are families of devices that range from 0.2 V (Schottky diodes) to a few volts (some
LEDs). From lab 2, you noticed that very small changes in voltage can change the current
significantly. This shows that the approximation of a constant voltage drop is a good one
for most applications.
MOSFETs
A MOSFET is a Metal Oxide Semiconductor Field Effect Transistor commonly called a
FET or simple a transistor. Understanding what all of those words mean is not required
before you can use one, however, so there’s no need to be intimidated. A MOSFET is a
three terminal device that is a voltage controlled resistor. The three terminals are called
the gate (the control terminal), the source (where the charge carriers enter the device),
and the drain (where the charge carries leave the device). A drawing of the symbol is
shown below.
Gate
+
v gs
-
Drain
+
v ds
Source
The resistance between the drain and the source is controlled by the voltage between the
gate and the source, and the transistor can be viewed as an electrical switch where one
signal on the gate shorts the drain and source together (zero resistance), and a different
signal on the gate makes an open circuit between the drain and source (infinite resistance).
The gate is simply a control terminal, and it has infinite resistance to the other terminals
(no current flows into our out of the gate). The switch is on when vgs exceeds the
threshold voltage of the device, and it turns off when vgs is below the threshold voltage. A
common configuration is to have the source connected to ground, and then the gate
voltage directly controls the state of the switch, and you will see this configuration in lab
and in class. When the gate voltage exceeds the threshold, vds must drop to zero because
the resistance from drain to source is 0. When the gate voltage is less than the threshold,
the voltage of the drain is controlled by the other devices connected to the drain. The
most common addition to this model is to add some resistance between the drain and
source in the on state, and then the transistor can be treated as a resistor when on and an
open circuit when off. The transistor operation is more complex than what is covered
here, but this basic model covers most situations quite well.
This discussion has been for NMOS or N-type devices, and you may come across other
types of devices as an engineer. There are PMOS (or P-type) devices that are
complimentary to NMOS devices. If you need to use or come across one of these parts, a
little reading will get you up to speed.