Semiconductor Diodes and Special Purpose Diodes
The pn junction diode or semiconductor diode
The semiconductor diode is created by simply joining an n -type and a p -type material together, nothing
more, just the joining of one material with a majority carrier of electrons to one with a majority carrier of
holes.
Formation of pn junction
The common method making pn junction is called alloying. In this method a small block of indium (trivalent
impurity) is placed on n-type germanium slab as shown in Figure below.
The system is then heated at about 5000C. The indium and germanium melt to form a small puddle.
The temperature is then lowered, and the puddle solidifies. Under proper conditions, the atoms of indium
impurity will be suitably adjusted to the germanium slab to form a single crystal. The addition of indium in
the n-type germanium creates a p-type region. As the process goes on, the remaining molten structure
becomes increasingly rich in indium.
Properties of pn junction
At the instant of pn-junction formation the free electrons near the junction in the n-region begin to diffuse
across the junction into the p-region where they combine with holes near the junction. The result is that n-
region loses free electrons as they diffuse into the junction. This creates a layer of positive charges
(pentavalent ions) near the junction. As the electrons move across the junctions, the p-region loses holes as
the electrons and holes combine. The result is that there is a layer of negative charges (trivalent ions) near
the junction. These two layers of positive and negative charges form the depletion region (or depletion
layer). The term depletion is due to the fact that near the junction, the region is depleted (i.e., emptied) of
charge carries(free electrons and holes) due to diffusion across the junction. It may be noted that depletion
layer is formed very quickly and is very thin compared to the n-region and the p-region. Once the p-region
is formed and depletion layer created, the diffusion of free electrons stops. In other words, the depletion
region acts as a barrier to the further movement of electrons across the junction. The positive and negative
charges set up electric field as shown by a black arrow in figure. There exists a potential difference across
the depletion layer and is called barrier potential (V0).
Applying a DC voltage across pn junction or biasing a pn junction
No Applied Bias ( V = 0 V)
The region of uncovered positive and negative ions is called the depletion region due to the “depletion” of
free carriers in the region.
(a)
If a battery is now connected, a two-terminal device results. Three options then become available: no bias
, forward bias , and reverse bias . The term bias refers to the application of an external voltage across the
two terminals of the device to extract a response
In the absence of an applied bias across a semiconductor diode, the net flow of charge in one direction is
zero.
Reverse-Bias Condition ( VD < 0 V)
If an external potential of V volts is applied across the p – n junction such that the positive terminal is
connected to the n -type material and the negative terminal is connected to the p -type material as shown in
Fig. 1.13 , the number of uncovered positive ions in the depletion region of the n -type material will increase
due to the large number of free electrons drawn to the positive potential of the applied voltage. For similar
reasons, the number of uncovered negative ions will increase in the p -type material. The net effect,
therefore, is a
widening of the depletion region. This widening of the depletion region will establish too great a barrier for
the majority carriers to overcome, effectively reducing the majority carrier flow to zero, as shown in Fig.
1.13a .
The current that exists under reverse-bias conditions is called the reverse saturation current and is
represented by I s .
The reverse saturation current is seldom more than a few microamperes and typically in nA, except for
high-power devices.
Forward-Bias Condition ( VD > 0 V)
A forward-bias or “on” condition is established by applying the positive potential to the p -type material
and the negative potential to the n -type material as shown in Fig. 1.14 . The application of a forward-bias
potential V D will “pressure” electrons in the n -type material and holes in the p -type material to recombine
with the ions near the boundary and reduce the width of the depletion region as shown in Fig. 1.14a .
Reduction in the width of the depletion region has resulted in a heavy majority flow across the junction. An
electron of the n -type material now “sees” a reduced barrier at the junction due to the reduced depletion
region and a strong attraction for the positive potential applied to the p -type material. As the applied bias
increases in magnitude, the depletion region will continue to decrease in width until a flood of electrons
can pass through the junction, resulting in an exponential rise in current as shown in the forward-bias region
of the characteristics of Fig. 1.15 (see next page). It can be demonstrated through the use of solid-state
physics that the general characteristics of a semiconductor diode can be defined by the following equation,
referred to as Shockley’s equation, for the forward- and reverse-bias regions:
where I s is the reverse saturation current V D is the applied forward-bias voltage across the diode; n is an
ideality factor, which is a function of the operating conditions and physical construction; it has a range
between 1 and 2 depending on a wide variety of factors. The voltage V T in Eq. is called the thermal voltage
and is determined by
where k is Boltzmann’s constant = 1.38 x 10 -23 J/K
T K is the absolute temperature in kelvins = 273 + the temperature in °C
q is the magnitude of electronic charge = 1.6 x 10 -19 C
Volt- Ampere Characteristics of pn junction
Breakdown Region
There is a point where the application of too negative a voltage with the reverse polarity will result in a sharp change in the characteristics, as shown in Fig. 1.17 . The reverse-bias potential that results in this dramatic change in characteristics is called the breakdown potential and is given the label VBV . Once the breakdown voltage is reached, the high reverse saturation current may damage the junction. The maximum reverse-bias potential that can be applied before entering the breakdown region is called the peak inverse voltage (referred to simply as the PIV rating) or the peak reverse voltage (denoted the PRV rating).
Knee voltage: It is the forward voltage at which the current through the junction starts to increase rapidly. For Si diode, knee voltage is 0.7V and for germanium it is 0.3V.
Temperature Effects
In the forward-bias region the characteristics of a silicon diode shift to the left at a rate of 2.5 mV per centigrade degree increase in temperature.
A decrease in temperature has the reverse effect, as also shown in the figure: In the reverse-bias region the reverse current of a silicon diode doubles for every 10°C rise in temperature. The reverse breakdown voltage of a semiconductor diode will increase or decrease with temperature.
Ideal vs Practical
The semiconductor diode behaves in a manner similar to a mechanical switch in that it can control whether current will flow between its two terminals.
Resistance Levels
The type of applied voltage or signal will define the resistance level of interest
1. DC or Static Resistance
The application of a dc voltage to a circuit containing a semiconductor diode will result in an operating point on the characteristic curve that will not change with time. The resistance of the diode at the operating point can be found simply by finding the corresponding levels of V D and I D as shown in Fig. 1.23 and applying the following equation:
In general, therefore, the higher the current through a diode, the lower is the dc resistance level. Typically, the dc resistance of a diode in the active (most utilized) will range from about 10 ohm to 80 ohm.
2. AC or dynamic resistance
If a sinusoidal rather than a dc input is applied, the situation will change completely. The varying input will move the instantaneous operating point up and down a region of the characteristics and thus defines a specific change in current and voltage as shown in Fig. 1.25 . With no applied varying signal, the point of operation would be the Q -point appearing on Fig. 1.25 , determined by the applied dc levels. The designation Q-point is derived from the word quiescent , which means “still or unvarying.”
A straight line drawn tangent to the curve through the Q -point as shown in Fig. 1.26 will define a particular change in voltage and current that can be used to determine the ac or dynamic resistance for this region of the diode characteristics.
An effort should be made to keep the change in voltage and current as small as possible and equidistant to either side of the Q -point. In equation form,
In general, therefore, the lower the Q-point of operation (smaller current or lower voltage), the higher is the ac resistance.
3. Average AC resistance
If the input signal is sufficiently large to produce a broad swing such as indicated in Fig. 1.28 , the resistance associated with the device for this region is called the average ac resistance. The average ac resistance is, by definition, the resistance determined by a straight line drawn between the two
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