Semiconductor electronics
Diode:
A
diode is a basic semiconductor device that allows current to pass in only one
direction. They are formed from a pn
junction, a p-type region of silicon adjacent to an n-type region of
silicon. The p-type region
contains “holes” due to missing electrons in the material lattice, while the n-type
contains extra electrons that have some freedom to move around. The holes in the p-type material
prevent current flow unless the junction is forward biased, caused by a voltage
field with negative field on the n side. A typical diode is forward biased at approximately 0.7 V and has
a voltage drop of approximately 0.7 V (when forward biased).
The
figure below demonstrates one use of diodes in creating a half-wave rectifier
(converting ac voltage to semi-dc voltage):
Light-emitting diodes (LEDs) are a common diode
that emits light when forward biased.
Note: the longer lead is the positive (anode) lead. LED’s have a voltage drop of about 1.5-2
Volts when forward biased, and can handle approximately 100 mA current to avoid
being destroyed. Therefore, be certain
to include an appropriate resistor in an LED circuit to exactly specify the
current (generally, a 250-300 W resistor in a 5V circuit). Remember, the general purpose of resistors
in circuits is to control or set a desired voltage current in a circuit (often
called current-limiting resistors). All
circuits should have a resistor since a circuit without resistance will have
infinite current flow (theoretically).
Transistors:
Transistors
consist of multiple layers of n- and p- silicon, for example in
an npn configuration or a pnp configuration in the bipolar
junction transistor (BJT). A small
amount of current introduced at the base (center) of the transistor will cause
the overall device to be forward biased and allow current to flow (see figure)
The following equations
define transistor voltages and current flow:
IE = IC + IB
VBE = VB – VE
VCE = VC – VE
IC = bIB
with b the current amplification factor for the transistor.
A small amount of current applied to
the base of the transistor allows a larger amount of current to flow through
the body of the transistor, in a somewhat linear relationship as shown in the
equation for IC above.
When the base current is large enough, the transistor is saturated and
the transistor acts as a short circuit with small voltage drop (approximately
.2V for bipolar junction transistors (BJTs)).
It is this form that we will use transistors, as a switching device in
which the junction between the collector and base acts either as an open or
short circuit controlled by the base voltage.
Consider
an npn transistor. The transistor is
forward biased when the base-to-emitter voltage is 0/7 V.
The
common emitter transistor circuit is shown below, where the emitter is attached
to ground. When designing a transistor
switch, the transistor must be fully in saturation when it is on (note that for
BJT’s, bipolar junction transistors, the VCE at saturation is
about .2V).
Common emitter switch:
Example designing a
switch:
Given
a typical signal transistor, 2N3904 in a circuit as shown below with
specifications from the package as: Max. collector current, IC(max)
= 200mA, VCE at saturation = 0.2V, hFE = b = 100 (approximately, amplification factor for
base-to-collector current), find the necessary input voltage to ensure
saturation:
Since
VCE is .2 V, IC is 9.8 mA. Thus, the base current must be 9.8/b or 0.098 mA and i:
Ib=(Vin-.7)/10kW: Vin
= 1.68 V.
Note
that the base resistor is necessary to limit the base current since the BE
junction behaves as a diode.
Remember these guidelines
for a transistor switch:
- The base-to-emitter voltage must be 0.7 V to be
on.
- Maximum values of IC, IB
and VCE must be observed and maintained.
- There must be sufficient base current to ensure
saturation (IB > IC/b, and VCE = 0.2 V))
- Collector current is independent of base current
at saturation.
Other Transistors:
Other
transistors include phototransistors and Mosfets. We will see more of these in the labs.