A
diode generally refers to a two-terminal solid-state semiconductor device that
presents a low impedance to current flow in one direction and a high
impedance to current flow in the opposite direction. These properties allow the
diode to be used as a one-way current valve in electronic circuits. Rectifiers
are a class of circuits whose purpose is to convert ac waveforms (usually
sinusoidal and with zero average value) into a waveform that has a significant
non-zero average value (dc component). Simply stated, rectifiers are ac-to-dc
energy converter circuits. Most rectifier circuits employ diodes as the
principal elements in the energy conversion process; thus the almost inseparable
notions of diodes and rectifiers. The general electrical characteristics of common
diodes and some simple rectifier topologies incorporating diodes are discussed.
Most
diodes are made from a host crystal of silicon (Si) with appropriate impurity
elements introduced to modify, in a controlled manner, the electrical
characteristics of the device. These diodes are the typical pn-junction (or bipolar)
devices used in electronic circuits. Another type is the Schottky diode (unipolar),
produced by placing a metal layer directly onto the semiconductor [Schottky,
1938; Mott, 1938]. The metal semiconductor interface serves the same function
as the pn
semiconductor materials such as gallium-arsenide (GaAs) and silicon-carbide
(SiC) are also in use for new and specialized applications of diodes. Detailed
discussion of diode structures and the physics of their operation can be found
in later paragraphs of this section. The electrical circuit symbol for a bipolar
diode is shown in Fig.1. The polarities associated with the forward voltage
drop for forward current flow are also included. Current or voltage opposite to
the polarities indicated are considered to be negative
values with respect to the diode conventions shown.
The
characteristic curve shown in Fig.2 is representative of the currentvoltage dependencies
of typical diodes. The diode conducts forward current with a small forward
voltage drop across the device, simulating a closed switch. The relationship
between the forward current and forward voltage is approximately
given by the Shockley diode equation [Shockley, 1949]:
where
Fig 2 |
Is
is the leakage current through the diode, q is the electronic charge, n is a
correction factor, k is Boltzmann’s constant, and T is the temperature of the
semiconductor. Around the knee of the curve in Fig.2 is a positive voltage that
is termed the turn-on or sometimes the threshold voltage for the diode. This
value is an approximate voltage above which the diode is considered turned “on”
and can be modeled to first degree as a closed switch with
constant forward drop. Below the threshold voltage value the diode is
considered weakly conducting and approximated as an open
switch. The exponential relationship means that the diode
forward current can change by orders of magnitude before there is a large
change in diode voltage, thus providing the simple circuit model
during conduction. The nonlinear relationship also provides a
means of frequency mixing for applications in modulation circuits. Reverse
voltage applied to the diode causes a small leakage current (negative according
to the sign convention) to flow that is typically orders of magnitude
lower than current in the forward direction. The diode can withstand reverse
voltages up to a limit determined by its physical construction and the
semiconductor material used. Beyond this value the reverse voltage
imparts enough energy to the charge carriers to cause large increases in
current.
The mechanisms by which this current increase occurs are impact ionization
(avalanche) [McKay, 954] and a tunneling phenomenon (Zener
breakdown) [Moll, 1964]. Avalanche breakdown results in large1power dissipation
in the diode, is generally destructive, and should be avoided at all times.
Both breakdown regions are superimposed in Fig .2 for
comparison of their effects on the shape of the diode characteristic curve.
Avalanche breakdown occurs for reverse applied voltages in the range of volts
to kilovolts depending on the exact design of the diode. Zener breakdown occurs
at much lower voltages than the avalanche mechanism. Diodes specifically
designed to operate in the Zener breakdown mode are used extensively as voltage
regulators in regulator integrated circuits and as discrete components in large
regulated power supplies. During forward conduction the power loss in the diode
can become excessive for large current flow. Schottky diodes have an inherently
lower turn-on voltage than pn -junction diodes and are therefore more desirable
in applications where the energy losses in the diodes are significant (such as
output rectifiers in switching power supplies). Other considerations
such as recovery characteristics from forward conduction to reverse blocking
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