8.5 Semiconductor devices
(see also textbook sections 14.5 and 14.6)
The need for devices is driven by applications such as computer memory storage, fast switching
at low power levels, etc. It has led to development of nanometer size devices in some cases, e.g.
as integrated circuits on a chip.
Three basic devices will be described here:• p-n junctions
—
these can rectify signals (allow current flow in just
one direction)
• junction transistors
—
these can amplify signals and provide switching
• MOSFET (transistors)
—
these can amplify signals and provide switching
(a) p-n junction
This acts as a diode — it is a 2-terminal device where the current flow is easier in one direction
compared with the other. So it can be used to rectify ac signals.
Basically it consists of a single piece of a semiconductor material where one half is doped p-type
and the other half is doped n-type.
A voltage supply can be connected in an external circuit in two ways, either
forward bias — (+ve battery terminal to p side, −ve battery terminal to n-side) or
reverse bias — (the opposite to the above)
In forward bias,
In reverse bias,
electrons and holes move towards each other and mostly recombine
(annihilating each other):
el + hole → energy.
A current can flow in the external circuit.
electrons and holes move apart, leaving a region with almost no charge
carriers, so it acts like an intrinsic semiconductor or like an insulator (so
very little current flows).
This effect (and the rectification process) can be illustrated by the following diagrams.
For a p-n rectifying junction in:
─ forward bias, and
─ reverse bias.
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The current-voltage characteristics
of a p-n junction for forward and
reverse biases. The effect of
“breakdown” under large
magnitude voltage is also shown.
(a) Voltage vs. time for the input to a
p-n rectifying junction.
(b) Current vs. time showing
rectification of the above voltage
signal by a p-n junction with the I-V
characteristics shown previously.
(b) junction transistor
If we form more than two doping regions in a semiconductor sample, we can achieve other more
complicated functions. In a transistor there are 3 terminals (3 regions) — it can be used (e.g.) as
a switch or as an amplifier.
In a junction transistor we sandwich an n-type layer between two p-type layers (or vice-versa) to
form:
p–n–p
(or n – p – n)
Schematic diagram of a p-n-p
junction transistor and its
associated circuits, including input
and output voltage-time
characteristics for voltage
amplification.
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The mode of operation is to make one p-n (or n-p) junction (labeled 1) forward biased and the
other (labeled 2) reverse biased. This makes holes from p-region on the left (called the emitter)
move into the centre n-region (called the base). The base region is designed to be sufficiently thin
(and with just the right doping) that most of the hole charges are swept right through to the pregion on the right (called the collector). Lots of charges are accumulated in the collector and this
produces an amplification effect.
For a p-n-p junction transistor, the
distribution and movement of
electrons and holes (a) when no
potential is applied and (b) with
appropriate bias for voltage
amplification.
(c) MOSFET
This is an example of a Field Effect Transistor. The MOS stands for Metal-Oxide Semiconductor.
In a p-type device there are two p-type regions formed near a surface and surrounded by n-type
material. A thin connecting channel of p-type material is formed between the p-type regions. An
oxide insulating film is deposited on the surface and metal contacts are formed (using etching) to
give contacts to the p-type regions. Another metal contact (called the gate) on top of the
insulating film, but adjacent to the channel, can have a bias voltage applied either to attract or
repel holes. In the one case the channel will be conducting (have lots of charges) and in the other
it will intrinsic-like (depleted of charges).
Schematic cross-section
view of a p-type Si
MOSFET transistor
8.6 Other electrical properties
(see also textbook section 14.8)
Two other electrical effects that can occur in a relatively few compound materials, that have low
crystalline symmetry or are distorted, are ferroelectricity and piezoelectricity.
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Ferroelectricity
This is somewhat analogous to ferromagnetism (where a magnetic moment or magnetization
occurs spontaneously below a critical temperature TC but is zero at higher temperature).
The difference is that, in ferroelectric materials, there is a spontaneous electric-dipole moment
below some temperature TC. It vanishes for T > TC.
The effect is associated with a distortion of the crystal lattice in an ionic material (so oppositely
charged ions move relative to one another) — it sets in below some critical temperature and is
absent (destroyed by the thermal energy) above that temperature.
Common examples are:
BaTiO3 with TC ~ 120 C
and
KH2PO4 (known as KDP)
These materials (typically insulators) can have extremely high dielectric constants and are useful
(e.g.) in capacitors.
A unit cell of BaTiO3 in two
views: (a) in perspective view
and (b) looking at one face and
showing the displacements of
the Ti and O charged ions.
Piezoelectricity
In these materials, if a mechanical force is applied (in order to compress or extend the lattice),
then an electric polarization and an electric field are produced,
or vice-versa (i.e., applying an electric field will cause a mechanical strain in the material).
(a) Electric dipoles within a
piezoelectric material.
(b) A voltage is generated when the
material is subjected to a
compressive stress σ.
They have device applications as basic transducers (converters of electrical energy to mechanical
strain, or vice versa),
e.g., as in microphones, loudspeakers, ultrasonic imagers, etc.
A common example of a piezoelectric material is quartz.
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