Ch 14Semiconductors — Class 11 Physics, concept-first.

Almost every modern electronic gadget -- a cell phone, a smart watch, a computer, even an LED lamp -- shares one common ingredient: semiconductor devices.

Electrical conduction in any solid happens through the transport of charge carriers, and how well a solid conducts depends on several factors: its temperature, the number of charge carriers available,…

To understand why semiconductors conduct the way they do, it helps to start from how electron energies are distributed in a single, isolated atom.

A pure semiconductor -- pure silicon or pure germanium, with no deliberately added impurity -- is called an INTRINSIC semiconductor.

The electrical conductivity of an intrinsic (pure) semiconductor is far too low at room temperature to build practical electronic devices out of.

When silicon or germanium is doped with a PENTAVALENT impurity -- phosphorus, arsenic, or antimony -- the result is an N-TYPE semiconductor.

When silicon or germanium is doped with a TRIVALENT impurity -- boron, aluminium, or indium -- the result is a P-TYPE semiconductor. Each of these dopant atoms has only three valence electrons (Fig.

Even though an n-type semiconductor has an 'excess' of electrons and a p-type semiconductor has an 'excess' of holes, both remain ELECTRICALLY NEUTRAL overall, and it is worth being precise about why.…

When n-type and p-type semiconductor materials are brought together (fused), the resulting structure is called a P-N JUNCTION (Fig. 14.15).

When n-type and p-type semiconductor materials are first fused together, the number of free electrons on the n-side is initially far larger than on the p-side, and, symmetrically, the number of holes…

The diffusion of carriers across the junction, and the resulting build-up of fixed positive (donor) ions on the n-side and fixed negative (acceptor) ions on the p-side, sets up a potential difference…

Because of the potential barrier that exists across the depletion region, charge carriers need some extra energy to overcome it and cross the junction; a suitable external voltage applied to the junct…

In the earlier description, a p-n junction was described, for simplicity, as though it were made by physically fusing together two separately-grown p-type and n-type crystals.

A p-n junction fitted with a metallic connector on each side is called a JUNCTION DIODE, or simply a DIODE -- the name reflecting that it is a device with two ('di-') electrodes. Fig.

A diode is FORWARD BIASED when its anode (p-side) is connected to the positive terminal of the external voltage source and its cathode (n-side) to the negative terminal.

A diode is said to be ZERO BIASED when it is connected in a circuit -- for instance, with its two terminals simply shorted together -- but with no external potential energy applied to the junction fro…

A diode is REVERSE BIASED when its anode (p-side) is connected to the negative terminal of the external voltage source and its cathode (n-side) to the positive terminal.

One of the most practically important properties of a diode is its resistance -- and this differs sharply between forward bias and reverse bias.

Semiconductor devices have found their way into an enormous variety of fields, and their popularity comes from a genuine set of practical advantages -- but, like any technology, they also carry some r…

Semiconductor devices offer several practical advantages that explain their dominance in modern electronics: (1) their electronic properties can be controlled and tuned to suit a specific requirement,…

Semiconductor devices also carry some genuine disadvantages: (1) they are sensitive to electrostatic charges, which can damage them; (2) they are not very well suited to controlling high electrical po…

A p-n junction diode is the basic building block of a large number of semiconductor devices, and a single semiconductor device can in fact contain more than one junction; the electrical properties of…

A THERMISTOR is a temperature-sensitive resistor -- its electrical resistance changes as its temperature changes. There are two types.