What Are Diodes, GTO And Transistors?

What Are Diodes, GTO And Transistors?

Diodes:

It is a semiconductor device with two terminals that allows flow of current in one direction. It has low resistance to current in one direction and a very high resistance on the other. It is   vacuum tube that has two electrodes – a plate (anode) and a heated cathode.

A semiconductor diode was the first semiconductor electronic devices. Most diodes are made of silicon, while selenium or germanium is also used sometimes. Because a diode allows electric current to pass one way but not the other, it is used as a rectifier to convert AC to DC current.
Diodes are also used as signal limiters, voltage regulators, switches, signal modulators, signal mixers etc.

The most common type of diode uses a p–n junction. In this type of diode, one material (n) in which electrons are charge carriers abuts a second material (p) in which holes (places depleted of electrons that act as positively charged particles) act as charge carriers. At their interface, a depletion region is formed across which electrons diffuse to fill holes in the p-side. This stops the further flow of electrons.

When this junction is forward biased (that is, a positive voltage is applied to the p-side), electrons can easily move across the junction to fill the holes, and a current flows through the diode. When the junction is reverse biased (that is, a negative voltage is applied to the p-side), the depletion region widens and electrons cannot easily move across. The current remains very small until a certain voltage (the breakdown voltage) is reached and the current suddenly increases.

Gate Turn-Off Thyristor (GTO):

Gate Turn-Off Thyristor, is a high powered semi-conductor device. It differs from a normal Thyristor. Unlike them a GTO has a fully controllable switch that can be tuned off and on by a third lead i.e. the Gate lead.

The device turn on is accomplished by a “positive current” pulse between the gate and cathode terminals. As the gate-cathode behaves like PN junction, there is a relatively small voltage between the terminals.

The turn on phenomenon in GTO is however, not as reliable as that of a standard thyristor and small positive gate current must be maintained even after turn on to improve reliability.

A normal thyristor does not have a fully controllable switch i.e., which can be turned on and off at will. Once it has been turned on or fired, it remains turned on till a turn off state occurs. Which could be anything like a reverse voltage or if the current flowing falls below a certain threshold value.

A GTO can be turned on by a gate signal and can be turned off by one which has negative polarity.

Transistors:

This is semi-conductor device that is used for amplifying and switching electronic signals and power. A transistor has 3 terminals. When voltage or current is applied to one pair of terminal it changes through the other pair. The output power could be higher than the input, a transistor can amplify signals. Most of the transistors are found in integrated circuits.

The semiconductor material is given special properties by a chemical process called doping. The doping results in a material that either adds extra electrons to the material (which is then called N-type for the extra negative charge carriers) or creates “holes” in the material’s crystal structure (which is then called P-type because it results in more positive charge carriers). The transistor’s three-layer structure contains an N-type semiconductor layer sandwiched between P-type layers (a PNP configuration) or a P-type layer between N-type layers (an NPN configuration).