A diode could be a two-terminal electronic material that conducts the current initially initio in one direction (asymmetric conductance); it’s low resistance in one direction, and high resistance within the different. A diode vacuum tube or thermionic diode is a gap tube with two electrodes, a heated cathode, and a plate, in which electrons can stream in only one direction, from cathode to plate. A semiconductor diode, the most commonly used type today, is a crystalline fragment of semiconductor material with a p–n junction attached to two electrical terminals. Semiconductor diodes were the first semiconductor electronic devices. Semiconductor diode consists of two, differently doped semiconductor crystals – “p” and “n” types.
A diode made of semiconductor components, usually silicon. The cathode, which is negatively charged and has an excess of electrons, is placed adjacent to the anode, which has an inherently positive charge, carrying an excess of holes. At this junction a depletion region forms, with nor holes neither electrons. A positive voltage at the anode makes the depletion region small, and current flows; a negative voltage at the anode makes the depletion region large, preventing current flow.
Construction of Semiconductor Diode:
Semiconductor diode consists of two, differently doped semiconductor crystals – “p” and “n” types. Together, they form the so-called “p-n junction”, where the “n” layer has an excess quantity of electrons, which are the majority carriers there (we have more electrons (-) than electron holes (+)). However, in the “p” layer the majority carriers are electron holes (+) rather than electrons (-), so we have more holes “to fill”, than electrons available. The electron-hole is a vacancy created by the electron “traveling” from its primary place to some other location in that crystal. In reality, there is no such thing as “a hole”, but that lack of electron kind of makes it a positively charged particle, which attracts negative electrons to form a pair again.
After they combine, a proportional ordination of electrons begins. Electrons, which previously lacked in “p” layer are transferred therefrom “n” layer, where were too many of them. So, “n” layer is a good friend for “p” layer, right? And this is where the so-called depletion region is formed, which prevents the flow of the electric current.
Fig:1 P-N junction in state of thermodynamic equilibrium
To allow the flow of the electric current through the “p-n junction”, the external positive electric voltage must be applied to “push” and help a large group of electrons and holes to meet together (forward bias of the diode). After they are “pushed” through the depletion region with enough force (VF = 0,7V) diode starts conducting current, so it starts to flow through it.
Fig:2 P-N junction forward-biased (electric valve on)
To make sure, that the electric current won’t flow (electric valve off), it is needed to apply an external negative voltage to the semiconductor diode (reverse bias) to make the depletion region even larger.
Fig.3 P-N junction reverse-biased (electric valve off)
With passing time, technological requirements were increasing what resulted in the development of new types of diodes. When a semiconductor is combined with the corresponding metal, we acquireMS junction (Metal-Semiconductor), which also possesses rectifying properties (current conduction in one direction) – it is used for example in fast Schottky diodes.
MS junctions can have one of two current-voltage characteristics:
# Unsymmetrical non-linear.
# Symmetrical linear.
MS junction properties depend mainly on the surface state of semiconductors and on the output work difference of electrons from metal and semiconductors itself. Schottky diode is mainly used in systems that require fast switching time(small junction capacitance Cj of the diode has a decisive impact) with frequencies up to several tens of GHz.
Semiconductor Diode – Current-voltage characteristic:
The graph under shows the current-voltage characteristic of the semiconductor diode. This is an emblematical characteristic for semiconductor diodes used in electronics (VF = 0,7V). The semiconductor diode starts conducting current after exceeding the threshold of the forward voltage value specified by the producer in the datasheet. Semi-thermal diodes are mainly used to defend other electronic components.
SEMICONDUCTOR DIODES AND DIODE SYMBOL:
What is a diode in general?
A two-electrode device is called a ‘diode‘. It’s simply a P-N junction with connecting leads or terminal on the two sides of the P-N junction. Such diodes are also called a “crystal diode” because the junction is raised out of a crystal. A diode allows unidirectional flow of current and it restrains the flow in opposite direction.
1. Its main advantages are cheapness, smaller size, robustness and high efficiency
2. Major application is rectification which is conversion of ac to dc
A semiconductor diode can be built of either silicon or Germanium. Both differnts in size and properties.
Properties of a Silicon diode:
# Smaller in shaped compared to Ge diodes
# Peak Inverse Voltage is of 1000 Volts
# Silicon diodes can be operated over temperatures 200 degree celsius
# Forward voltage drop is 0.7 volt
# Reverse current of a Silicon diode, for a given voltage effectively doubles for every 8 degree celsius rise in temperature.
Properties of Germanium Diode:
# Larger shaped compared to Si diode
# PIV is of 400 volts
# Operated on at temperatures in range 75 to 80 degree celsius
# Forward voltage drop is .3 volts
# Reverse current of a Ge(germanium) diode doubles for every 10 degree celsius rise
Circuit and Graphical symbol of a diode:
In the figure given over, first symbol represents the circuit symbol of a semiconductor pn junction diode. The ‘P’ side of diode is always positive terminal and is yclept as anode for forward bias. Other side which is negetive is designated as cathode and is the ‘N’ side of diode.
In the figure over, 2nd symbol represents the Graphical symbol of a diode. Direction of the arrow represents the direction of conventional current flow, when the diode is forward biased.
DIFFERENT TYPES OF DIODES:
These are of different types and pass the electric current based on the properties and specifications of that particular diode. These are of mainly P-N junction diodes, Photo sensitive diodes, Zener diodes, schottky diodes, Varactor diodes. Photo sensitive diodes include LED’s, Photo diodes and Photo voltaic cells. Some of these are explained briefly in this article.
1. P-N Junction Diode:
P-N junction is a semiconductor device, which is formed by P-type and N-type semiconductor component . P-type has high intentness of holes and N-type has high concentration of electrons. Holes pervasion is from p-type to n-type and electron diffusion is from n-type to p-type.
The donor ions in the n-type region become positively(+) charged as the gratis electrons move from the n-type to p-type. Hence, positive charge is built on the N-side of the junction. The free electrons across the junction is the negative acceptor ions by fulfillment in the holes, then negative charge established on the p-side of the junction is shown in figure.
An electric field made by the positive ions in the n-type region and negative ions in p-type regions. This region is said diffusion region. Since the electric field quickly sweeps free bearer out, hence the region is depleted of free carriers. A built-in potential Vbi due to Ê is made at the junction is shown in figure.
FUNCTIONAL DIAGRAM OF P-N JUNCTION DIODE:
Forward Characteristics of P-N Junction:
When positive terminal of source is connected to P-type and negative terminal is connected to N-type is called forward bias of P-N junction is shown figure below.
If this external voltage becomes greater than the value of the potential barrier, approximately 0.7 volts for silicon and 0.3V for Ge, the potential barrier is crossed and the current starts flowing due to movement of electrons across the junction and same for the holes.
Applications of P-N Junction Diode:
P-N junction diode is a two terminal polarity sensorial device, diode conducts when in forward bias and diode not conducts when reverse bias. Due to these feature P-N junction diode is used in many applications like
# Rectifiers in DC power supply
# Demodulation circuits
# Clipping and clamping networks
Photodiode is a kind of diode which generates current proportional to the Event light energy. It is a light to voltage/current converter that search applications in security systems, conveyors, automatic switching systems etc. The photodiode is same to an LED in construction but its p-n junction is highly sensitive to light. The p-n junction may be manifested or packaged with a window to enter light into the P-N junction. Under the forward biased state, current transfers from the anode to cathode, while in the reverse biased state, photo current flows in the reverse direction. In most incident, the packaging of Photodiode is similar to LED(Light emitting diodee) with anode and cathode leads projecting out from the case.
There are two types of Photodiodes – PN and PIN photodiodes. The difference is in their acting. The PIN photodiode has an own layer, so they must be reverse biased. As a result of reverse biasing, the width of the depletion region increased and the capacitance of the p-n junction decreased. This allows the creation of more electrons and holes in the depletion region. But one disadvantage of the reverse biasing is that, it generates noise current that may minimize the S/N ratio. So reverse biasing is fitting only in applications that require higher band width. The PN photodiode is ideal for lower light applications since the operation is unbiased.
The photodiode works in two manner namely Photo voltaic mode and Photo conductive mode. In the photo voltaic mode (also called Zero bias mode), the photocurrent from the device is limited and a voltage build up. The photo diode is now in the Forward biased state and a “Dark current” starts effluent across the p-n junction. This flow of dark current happen opposite to the direction of the photo current. Dark current generates in the default of light. The dark current is the photo current induced by the mounting radiation plus the saturation current in the device.
The Photo conductive mode happens when the photo diode is reverse biased. Finally of this, the width of the depletion layer increases and leads to a reduction in the capacitance of the p-n junction. This increases the response time of the diode. Responsivity is the ratio of the photo current generated to the Event light energy. In the Photo conductive mode, the diode generates only a small current said Saturation current or back current along its direction. The photo current remains same in this condition. The photo current is all-time proportional to the luminescence. Even though the Photo conductive mode is faster and faster than the Photo voltaic mode, the electronic noise in higher in photo conductive mode. Silicon founded photo diodes generate less noise than germanium founded photo diodes since the silicon photo diodes have greater band gap.
3. Zener Diode:
Zener diode is a type of Diode that allows the flow of current in the forward direction similar to a rectifier diode but at the same time it can permission the reverse flow of current also when the voltage is over the breakdown value of the Zener. This is generally one to two volts higher than the rated voltage of the Zener and is known as the Zener voltage or Avalanche point. The Zener was founded so after Clarenze Zener who discovered the electrical properties of the diode. Zener diodes find applications in voltage regulation and to defend semiconductor devices from voltage fluctuations. Zener diodes are widely used as voltage references and as shunt regulators to regulate the voltage across circuits.
The Zener diode uses its p-n junction in the reverse bias mode to give the Zener Effect. During Zener effect or Zener breakdown, the Zener holds the voltage close to a constant value known as the Zener voltage. The common diode also has the property of reverse bias, but if the reverse bias voltage is go beyond, the diode shall be subjected to high current and it shall be damaged. The Zener diode on the other hand is specially schematic to have a reduced breakdown voltage called Zener voltage. The Zener diode also exhibits the property of limited breakdown and allows the current to keep the voltage across the Zener diode close to the breakdown voltage. For example, a 20 volt Zener will drop 20 volts across a wide range of reverse currents.
When the Zener diode is reverse biased, its p-n junction will experience an Avalanche breakdown and the Zener conducts in the reverse direction. Under the influence of the applied electric field, the valance electrons will be accelerated to knock and release other electrons. This ends in the Avalanche effect. When this occurs, a small change in the voltage will results in a large current flow. The Zener break down depends on the applied electric field as well as the thickness of the layer on which the voltage is applied.
The Zener diode requires a current limiting resistor in series to it to restrict the current flow through the Zener. Generally the Zener current is fixed as 5 mA. For example, if a 10 V Zener is used with 12 volt supply, a 400 Ohms is ideal to keep the Zener current as 5 mA. When the supply is 12 volts, there is 10 volts across the Zener diode and 2 volts across the resistor. With 2 volts across the 400 ohms resistor, then the current through the resistor and Zener will be 5 mA. since as a rule 220 Ohms to 1K resistors are used in series with the Zener depending upon the supply voltage. If the current though the Zener is insufficient, the output will be unregulated and less than the nominal breakdown voltage.
The following formula is useful to determine the current through the Zener:
The value of the Resistor R obiously satisfy two conditions.
# It must be a low value to permit sufficient current through the Zener
# Power rating of the resistor must be high enough to protect the Zener.
4. BACKWARD DIODE:
This pattern of diode is also called the back diode. The backward diode is a PN-junction diode that is samely to the tunnel diode in its process. It required a few special applications where its specific properties can be used.
5. BARITT Diode:
The short term of this diode Barrier Injection Transit Time diode or BARITT Diode. It is applicable in microwave applications and allows many difference to the more widely used IMPATT diode.
The acronym of the BARITT diode is “Barrier Injection Transit Time diode”, bears numerous difference to the more generally used IMPATT diode. This diode is used in the microwave signal generation kind of the more common IMPATT diode and also this diode is frequently used in burglar alarms and where it can simply made a simple microwave signal with a comparatively low noise level.
This diode is very similarly with respect to the IMPATT diode, however the important difference between these two diodes is that the BARITT diode utilizes thermionic emission rather than multiplication of avalanche.
One of the important advantages of using this kind of emission is that the procedure is less noisy. As a result, the BARITT diode does not experience from the similar noise levels like an IMPATT. essentially the BARITT diode comprises of two diodes, which are placed back to back. Whenever potential is applied across the device, most of the potential drop event across the reverse biased diode. If the voltage is then ancient until the ends of the depletion area meet, then a state known as punch through happens.
Gunn diode is a P-N junction diode, this kinds of diode is a semiconductor device that has two terminals. Basically, it is used for producing microwave signals.
GUNN DIODE: WORKING, CHARACTERISTICS & APPLICATIONS:
A diode is a two-terminal semiconductor electronic element that exhibits nonlinear current-voltage characteristics. It allows current in one direction at which its resistance is very low during forward bias. Similarly, in the other direction, it does not allow the flow of current – as it offers a very-high resistance (infinite resistance acts as open circuit) during reverse bias.
The diodes are classified into various types based on their working principles and characteristics. These include Schotty diode, Shockley diode, Constant-current diode, Zener diode, Light emitting diode, Photodiode, Tunnel diode, Varactor, Vacuum tube, Laser diode, PIN diode, Peltier diode, Gunn diode, and so on. On a special case, this article discuss about Gunn diode’s working, characteristics and applications.
The laser diode is not the similar as the ordinary LED because it generates coherent light. These diodes are extensively used in many applications like DVDs, CD drives and laser light pointers for PPTs. though these diodes are inexpensive than other types of laser generator, they are much more costly than LEDs.
8.Light Emitting Diode:
The term LED stands for light emitting diode, is one of the most valuable types of the diode. When the diode is attached in forwarding bias, then the current flows through the junction and generates the light.
This size of diode is characterize by its construction. It has the valuable P-type & N-type regions, but the area between the two regions namely intrinsic semiconductor has no doping. The region of the intrinsic semiconductor has the effect of increasing the area of the depletion region which can be beneficial for switching applications.
A Schottky diode has a lower forward voltage drop than ordinary Si PN-junction diodes. At low currents, the voltage drop may be between 0.15 & 0.4 volts as opposed to 0.6 volts for a Si diode. To accomplish this performance they are designed in a different way to compare with normal diodes having a metal to semiconductor contact. These diodes are widely used in rectifier application, clamping diodes, and also in RF applications.
The tunnel diode is used for microwave applications where its performance crossed that of other devices of the day.
12.VARACTOR DIODE OR VARICAP DIODE:
A varactor diode is one sort of semiconductor microwave solid-state device and it is used in where the changeable capacitance is chosen which can be attained by controlling voltage. This diodes are also called as variceal diodes. Even though the o/p of the variable capacitance can be exhibited by the general PN-junction diodes.But, this diode is approved for giving the preferred capacitance changes as they are different types of diodes. These diodes are exactly designed and enhanced such that they allow a high range of changes in capacitance.
SEMICONDUCTOR DIODE TASKS:
Vt = 0,7V
rd = 20Ω
R = 100Ω
VIN = 5V
VD(t) = VOUT(t)
VD(t) = Vt + (id(t) * rd) = 0,7 + (36m * 20) = 0,7 + 0,72 = 1,42V
Vt = 0,7V
R = 100Ω
VIN = 5V
VD(t) = VOUT(t)
VD(t) = Vt + (id(t) * rd) = 0,7 + (43m * 0) = 0,7 + 0 = 0,7V
Semiconductor diode formula: