NEET Physics Notes Modern Physics-Electronic Devices-Light Emitting Diode (LED)
Light Emitting Diode (LED)
It is a specially designed diode made of GaAsP, GaP etc. When used in forward biased, it emits characteristic, almost monochromatic light.
In reverse biased, it works like a normal diode.
I-V Characteristics of LED
LEDs are current dependent devices with its forward voltage drop (VF) depending on the forward biased LED current. Light emitting diode I-V characteristics as given below:
Photo Diode
It is a special diode used in reverse bias which conducts only when light of suitable wavelengths is incident on the junction of diode. The energy of incident light photon must be greater than the band gap of semiconductor. Materials used are Cds, Se, Zns.
Solar Cell
It is a special p-n junction, in which one of the semiconductors is made extremely thin, so that solar radiation falling on it reaches junction of diode without any absorption. A solar cell directly converts, solar energy into electrical energy. Popularly used solar cells, Ni-cd, PbS cell.
Transistor
A transistor is a combination of. .two p-n junctions joined in series. A junction transistor is known as Bipolar Junction Transistor (BJT). It is a three terminal device.
Transistors are of two types
- n-p-n transistor,
- p-n-p transistor
A transistor has three regions:
- An emitter (E), which is most heavily doped, and is of moderate size. It supplies large number of charge carriers, which are free electrons in a n-p-n transistor and holes in a p-n-p transistor.
- A base (B), which is very lightly doped and is very thin (thickness = 10_5m).
- A collector (C), which is moderately doped and is thickest.
A transistor is symbolically represented as shown in figures.
For proper functioning of a transistor, the emitter-base junction is forward biased but the collector-base junction is reverse biased. In a n-p-n transistor, electrons flow from emitter towards the base and constitute a current IE. Due to larger reverse bias at base-collector junction, most of these electrons further pass into the collector, constituting a collector current Ic. But a small percentage of electrons (less than 5%) may combine with holes present in base. These electrons constitute a base current IB.It is self evident, that IE = Ic+IB.
Action of p-n-p transistor is also same but with one difference that holes are moving from emitter to base and then to collector.
A transistor can be connected in either of the following three configurations
- Common Emitter (CE) configuration
- Common Base (CB) configuration
- Common Collector (CC) configuration.
Generally, we prefer common emitter configuration because power gain is maximum in this configuration.
Characteristics of a Transistor
In common emitter configuration, variation of current on the input side with input voltage (IE versus VBE ) is known as the input characteristics, and the variation in the output current with output voltage (Ic versus VCE) is known as output characteristics. From these characteristics, we obtain the values of following parameters.
The current gain for common-emitter configuration p ranges from 20 to 200.
A transistor can be used as an amplifier. The voltage gain of an amplifier will be given by
Where, Rc and RB are net resistances in collector and base circuits, respectively.
In common base configuration, AG current gain is defined as
Transistor as an Amplifier
A transistor consisting of two p-n junctions, one forward biased and the other reverse biased can be used to amplify a weak signal. The forward biased junction has a low resistance path, whereas the reverse biased junction Has a high resistance path. The weak input signal is applied across the forward biased junction, and the output signal is taken across the reverse biased junction. Since, the input and output currents are almost equal, the output signal appears with a much higher voltage. The transistor thus acts as an amplifier. Common-emitter configuration of transistor amplifier is given below:
Transistor as an Oscillator
An electronic oscillator is a device that generates electrical oscillations of constant amplitude and of a desired frequency, without any external input. The circuit providing such oscillation, is known as a tank oscillator, is using positive feedback.
Some of the properties of the oscillator are
1. Oscillator is using positive feedback.
2. To work as an oscillator,
