EC ELECTRIC CIRCUITS AND ELECTRON DEVICES the equivalent circuit of UJT and explain its operation. Write short notes on light activated SCR. 2nd Semester CSE Notes: 1. Basic civil & Mechanical Notes and Question answers – View. 5. Electron Devices & Electric circuits (EC) – Unit 1 – View /. PH Engineering Physics 2 Anna University Notes: PH Engineering notes (EM notes – ME Notes) – DOWNLOAD EC

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The process that an electron from n-side of a pn diode directly penetrates through the junction into the p-side of diode is called tunneling. It is a quantum —mechanical behaviors.

Once the SCR is turned ON, it starts to conduct and remains in conduction state even when the gate signal is removed. This ability of the SCR to remain conducting, even when the gate signal is removed, is known as latching. Where is the Menu? How to request Study Material? Electric Circuit and Electron Device Year: This diode has negative resistance region. Due to which it is used as an oscillator.

This diode is uses the tunneling phenomenon. Tunneling The process that an electron from n-side of a pn diode directly penetrates through the junction into the p-side of diode is called tunneling. Operation When a tunnel diode is under unbiased condition then there will not transfer of electrons from n-side to p-side hence the net current will be zero. When the diode is reverse biased under this condition the electrons from n-side are attracted by the positive plate and hence move away from the junction.

As a result the energy level in the n-side decreases when compared to the unbiased state. Now, there will be some empty state in valence band of p-side quite opposite to the empty conduction band.

Hence tunneling takes place from p to n-side. As reverse bias is increased this current increase. Advantages High speed operation Ease of operation Low noise Low cost Low power Disadvantages It is ec211 terminal device, there is no isolation between the input and output circuit.

Voltage range over which it can be operated is 1 V or less. PIN diodes are used in microwave switches. This results in improved switching time.

Varactor diode Varactor diode is a specially manufactured reverse biased PN junction diode with a suitable impurity concentration profile. It is also called as varicap or voltacap. It is used as a variable reactance capacitance.

Characteristics of Varactor diode The diode conducts normally in the forward direction. At relatively low voltage the reverse current saturates and then remains constant. It is rising rapidly at avalanche point. At the saturation point the maximum junction capacitance is obtained and a point just ec215 avalanche the minimum junction capacitance is obtained. Therefore there are two conditions which are limiting the reverse voltage swing and the capacitance variation.


There are three terminals called anode Acathode K and botes gate G. The anode terminal is taken out from P 1 layer, and the gate G terminal from the P 2 layer. It conducts the current in forward notees only.


The applied forward voltage is increased, a certain critical voltage called forward break over voltage V BO. It reaches at the junction J 2 breakdown. It has found that most of the voltage will drop across junction J 1 only. SCR characteristics It is the relationship between the anode —cathode voltage and anode current at different gate current. Adjust the gate current to zero value by keeping the switch open. Increase the applied voltage across the SCR in small suitable steps at each step.

Second Semester Anna University lecturer notes

Reverse Characteristics The reverse characteristic is obtained by reversing the connections of the d. Adjust the gate current to any suitable value. Increase the reverse applied voltage in suitable steps. Note the anode current for each steps. Now we plot a ntoes with anode current and anode cathode voltage. The second method is anode current interruption.

Changing anode current by means of momentarily series or parallel switching arrangement. Third method is forced commutation.

In this, the current through SCR is reduced below the holding current. SCR size is very compact. Switching speed is high. It differs from ordinary PN diode in the sense that it has three terminals namely Emitter, Base1 and Base 2.

However, it has ability to control large ac power with a small signal. It also exhibits a negative resistance characteristic which allows it to be used as an oscillator. The two leads are connected to p-region of silicon separated by an n-region. It consists of two 4-layer diodes connected in parallel in opposite direction. It has two main terminals namely Main terminal 1 and Main terminal 2. The gates are connected together. It has a window and lens which focuses light on the gate junction area.

It can be triggered ON by a light input on the gate area, but does not turn OFF, when light source is removed. To reduce the holding current, it can be turned OFF. It can be handled by a photo transistor or a photo diode. This photon multiplication is the key physical mechanism of lasing. The carrier inversion is the first requirement of lasing.

It is achieved at the PN junction by providing the conduction bandwidth electrons from the N-doped side and the Valence band with the holes from the P-doped side. The photon energy is given by the band gap, which depends on the semiconductor material. The optical feedback and the confinement of photon in an optical resonator are the second basic requirement of lasing.


Second Semester Anna University lecturer notes – Site Title

Photodiode It is a light sensitivity device used to convert light signal into electrical signal. It is also called Photo detector. The light energy fall on the junction through lens, when, the PN photodiode junction is reverse bias. The hole-electrons pairs are created. The movement of the hole-electron pairs in a properly connected circuit results in current flows.

The current is proportional to the intensity of light and the frequency of the light nottes on the junction of the photo diode. It is used in demodulator, encodes and light detectors systems. Phototransistor The photo transistor ecc2151 a light detector.

It combines a photodiode and phototransistor.

The phototransistor cannot be directly used in control applications. Because, it produces a very low current.

How to study this subject

Before applying to control circuit the current should be amplified. A lens focuses the energy on the base-collector junction. It has three terminal, but only two leads are generally used emitter and collector. The base current is supplied by the current created by the light falling on the Base-collector photodiode junction.

In ec215, the current is dependent mainly on the intensity of light entering into the lens and the voltage applied to the external circuit. It is made of thin layer of semiconductor material cadmium sulfide. There is no light falls on the sensor the resistance is very high and the current is low. Hence, the voltage drop across R is high. It is used in control circuits to control the current.

Photovoltaic sensors It is a light-sensitive semiconductor device, and it produces a voltage, when the voltage increases and the intensity of light falling notse the semiconductor junction of this photovoltaic cell increases. It consists of a piece of semiconductor material silicon or germanium. The photovoltaic cells are produced more power, as in solar cells. These are called photovoltaic devices. It is used in light meters.

It emits light in both visible and IR region. The amount of light output is directly notess to the forward current. LED structure can be divided into two categories.

When LED is forward biased, the majority charge carriers moves from p to n and similarly from n to p region and becomes excess minority carriers. These excess minority carriers diffuse through the junction and recombines ev2151 the majority carriers in noges and p ec211 respectively to produce light. The light thus produced is emitted from the p-n junction of the diode.

Advantages of LED They are smaller in size. Its cost is very low. It has long life time.