Ch-3: Semiconductor Diodes
Compiled by:Prof. G.B.Rathod
EC Dept, Birla Vishvakarma Mahavidyalaya
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n -TYPE AND p -TYPE MATERIALS
A semiconductor material process that has been subjected to the doping is called an extrinsic material.
There are two extrinsic materials of immeasurable importance to semiconductor device fabrication: n -type and p -type materials. Each is described in some detail in the following subsections.
n -Type Material:
An n -type material is created by introducing impurity elements that have five valence electrons ( pentavalent ), such as antimony , arsenic , and phosphorus.
Diffused impurities with five valence electrons are called donor atoms.
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n -TYPE AND p -TYPE MATERIALS
The p -type material is formed by doping a pure germanium or silicon crystal with impurity atoms having three valence electrons. The elements most frequently used for this purpose are boron , gallium , and indium .
Note that there is now an insufficient number of electrons to complete the covalent bonds of the newly formed lattice.
The resulting vacancy is called a hole and is represented by a small circle or a plus sign, indicating the absence of a negative charge. Since the resulting will readily accept a free electron:
The diffused impurities with three valence electrons are called acceptor atoms.
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Electron versus Hole Flow
In an n-type material the electron is the majority carrier and the hole the minority carrier.
In a p-type material the hole is the majority carrier and the electron is the minority carrier.
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n -TYPE AND p -TYPE MATERIALS
The n - and p -type materials represent the basic building blocks of semiconductor devices.
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SEMICONDUCTOR DIODE
Now that both n - and p -type materials are available, we can construct our first solid-state electronic device:
At the instant the two materials are “joined” the electrons and the holes in the region of the junction will combine, resulting in a lack of free carriers in the region near the junction, as shown in Fig.
This region of uncovered positive and negative ions is called the depletion region due to the “depletion” of free carriers in the region.
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Reverse-Bias Condition ( VD < 0 V)
The current that exists under reverse-bias conditions is called the reverse saturation current and is represented by Is.
The reverse saturation current is seldom more than a few microamperes and typically in nA, except for high-power devices.
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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.
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SEMICONDUCTOR DIODE
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 Is is the reverse saturation current VD 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 ( n = 1 will be assumed throughout this text unless otherwise noted).
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SEMICONDUCTOR DIODE
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SEMICONDUCTOR DIODE
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Atomic structure of silicon,germanium,and gallium arsenide.
The three semiconductors used most frequently in the construction of electronic devices are Ge, Si, and GaAs.
Atoms that have four valence electrons are called tetravalent , those with three are called trivalent , and those with five are called pentavalent .
SEMICONDUCTOR DIODE
A plot of Eq. ID with Is 10 pA is provided in Fig. as the dashed line. If we expand Eq. into the following form, the contributing component for each region of Fig. can be described with increased clarity:
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SEMICONDUCTOR DIODE
The current increases at a very rapid rate in a direction opposite to that of the positive voltage region. The reverse-bia potential that results in this dramatic change in characteristics is called the breakdown potential and is given the label VBV .
As noted on the curves, the center of the knee (hence the K is the notation VK ) of the curve is about 0.3 V for Ge, 0.7 V for Si, and 1.2 V for GaAs
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PN Junction Breakdown
An electrical breakdown of any material such as the conductor, semiconductor, and insulator can occur due to two different phenomena known as a) Zener Breakdown b) Avalanche Breakdown.
What Is Zener Breakdown?
When the diode is reverse biased, the kinetic energy of the electrons increases, and they move at a high velocity. The high-velocity electrons collide with other atoms and give rise to free electrons. These free electrons, in turn, give rise to a high value of reverse saturation current. This is known as Zener breakdown.
What Is Avalanche Breakdown?
The avalanche breakdown occurs when a high reverse voltage is applied across the diode. As we increase the applied reverse voltage, the electric field across the junction increases. This electric field exerts a force on the electrons at the junction and frees them from covalent bonds. These free electrons start moving with high velocity across the junction and collide with the other atoms, thus creating more free electrons. This results in a rapid increase in net current. Both these breakdowns occur in Zener diodes.
Ref: https://byjus.com/physics/difference-between-zener-breakdown-and-avalanche-breakdown/
Temperature Effects
Temperature can have a marked effect on the characteristics of a semiconductor diode, as demonstrated by the characteristics of a silicon diode shown in Fig: 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.
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IDEAL VERSUS PRACTICAL
An analogy often used to describe the behavior of a semiconductor diode is a mechanical switch. In Fig.a the diode is acting like a closed switch permitting a generous flow of charge in the direction indicated. In Fig. b the level of current is so small in most cases that it can be approximated as 0 A and represented by an open switch.
Ideal versus actual semiconductor characteristics.
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RESISTANCE LEVELS
As the operating point of a diode moves from one region to another the resistance of the diode will also change due to the nonlinear shape of the characteristic curve. It will be demonstrated in the next few paragraphs that the type of applied voltage or signal will define the resistance level of interest.
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.
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RESISTANCE LEVELS
Ex.1: Determine the dc resistance levels for the diode of Fig.
a. ID = 2 mA (low level)
b. ID 20 mA (high level)
c. VD 10 V (reverse-biased)
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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.
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Average AC Resistance
The average ac resistance is, by definition, the resistance determined by a straight line drawn between the two intersections established by the maximum and minimum values of input voltage.
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DIODE EQUIVALENT CIRCUITS
Simplified Equivalent Circuit
For most applications, the resistance r av is sufficiently small to be ignored in comparison to the other elements of the network. Removing rav from the equivalent circuit is the same as implying that the characteristics of the diode appear as shown in Fig.
DIODE EQUIVALENT CIRCUITS -
Ideal Equivalent Circuit
Now that rav has been removed from the equivalent circuit, let us take the analysis a step further and establish that a 0.7-V level can often be ignored in comparison to the applied voltage level.
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Click here to learn the practical of VI characteristic of PN Junction diode on online simulator.
SEMICONDUCTOR DIODE NOTATION
The notation most frequently used for semiconductor diodes is provided in Figure . For most diodes any marking such as a dot or band, as shown in Figure.
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SERIES DIODE CONFIGURATIONS- Diode as a switch
The forward resistance of the diode is usually so small compared to the other series elements of the network that it can be ignored.
In general, a diode is in the “on” state if the current established by the applied sources is such that its direction matches that of the arrow in the diode symbol, and VD >= 0.7 V for silicon, VD >= 0.3 V for germanium, and VD >=1.2 V for gallium arsenide.
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Example of Series Circuit using diode
For the series diode configuration of Fig., determine VD , VR , and ID .
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PARALLEL AND SERIES–PARALLEL CONFIGURATIONS
Determine Vo , I1 , ID1 , and ID2 for the parallel diode configuration of Fig.
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PARALLEL AND SERIES–PARALLEL CONFIGURATIONS
Determine Vo , I1 , ID1 , and ID2 for the parallel diode configuration of Fig.
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Various types of junction diodes
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Light Emitting Diode
Light Emitting Diode
Common Semiconductor Materials by Color
Photo Diode
A photodiode is a semiconductor device with a P-N junction that converts photons (or light) into electrical current. The P layer has an abundance of holes (positive), and the N layer has an abundance of electrons (negative).
It is a form of light sensor that converts light energy into electrical energy (voltage or current). Photodiode is a type of semi conducting device with PN junction. Between the p (positive) and n (negative) layers, an intrinsic layer is present. The photodiode accepts light energy as input to generate electric current.
It is also called as Photodetector, Photo Sensor or Light Detector. Photodiode operates in reverse bias condition i.e., the p – side of the photodiode is connected with negative terminal of battery (or the power supply) and n – side to the positive terminal of battery.
Photo Diode
Typical photodiode materials are Silicon, Germanium, Indium Gallium Arsenide Phosphide and Indium gallium arsenide.Internally, a photodiode has optical filters, built in lens and a surface area. When surface area of photodiode increases, it results in less response time.
Few photo diodes will look like Light Emitting Diode (LED). It has two terminals as shown below. The smaller terminal acts as cathode and longer terminal acts as anode.
Tunnel Diode
Tunnel Diode
Tunnel Diode Applications
Varactor Diode
Shockley Diode
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