SEMICONDUCTORS

1. Introduction

Semiconductors play an immense role in modern electronics. Without them, you would not be using this computer! The utility of semiconductors is due in a large measure to the fact that the conductivity of a semiconductor is extremely sensitive to the presence of impurities.

Solid-state devices such as transistors, diodes, photocells and integrated circuits are widely used in all sorts of familiar objects, such as digital cameras, TV receivers, computers, and mobile phones. All depend on semiconductor technology. The following will serve as an introduction to the science of semiconductors.

2. Intrinsic semiconductors

When an electron from the valence band of a semiconductor (shown as a red dot in the diagram on the right) jumps into a higher energy conduction band, a deficiency in the number of electrons arises in the covalent bond from which the electron was removed. This leaves, as it were, a positive "hole" (which will appear as a white dot when the animation is running), which can be filled by an electron from a neighbouring atom, and so on. In this way, the "holes" move in the semiconductor valence band and contributes, together with the electron in the conduction band, to the observed current. See the animation.

In a pure semiconductor, the number of electrons in the conduction band must equal the number of holes in the valence band, as might well be expected from the mechanism of hole formation. In practice, thermal energy is sufficient to cause a few electrons to move from the valence band to the conduction band. The process leads to the formation of an"electron-hole pair", with the electron free to move in the conduction band, and the "hole" free to move in the valence band. This type of conductor is called an INTRINSIC SEMICONDUCTOR.

The resistance of an intrisic semiconductor decreases with increasing temperature. This is because more electrons from the valence band acquire energy values enabling them to reach the conduction band.

3. Doping

In an intrinsic semiconductor, such as silicon (Si) or germanium (Ge), all the valence electrons (there are four of them) are utilised to form covalent bonds with other Si or Ge atoms, as the case may be. A three-dimensional lattice of atoms is formed (shown here on the left in two dimensions - valence electrons are shown as red dots). The energy state of these electrons confines them to the valence band of the material, which is completely filled. When suitably energised, some of these electrons can "jump" into the conduction band, as we saw above.

If very small amounts of a suitable impurity (about 1 atom of impurity to 106 atoms) are added, either p-type or n-type semiconductors are formed. This process is called DOPING. The semiconductor is then known as an EXTRINSIC SEMICONDUCTOR. A knowledge of the theory behind this process enables semiconductors to be designed so as to possess certain desirable properties.

3.1 n-Type semiconductors

Consider a lattice of pure germanium. It is an intrinsic semiconductor, for the same reason outlined above for silicon. If arsenic, with five valence electrons is added as an impurity, arsenic, due to its similar size to germanium (it is actually a little larger) will be accommodated in the lattice, forming four covalent bonds with the germanium atoms. Now there is one extra electron, which is not bound and therefore free to move around. These electrons are known as MAJORITY CARRIERS, because they are largely responsible for the flow of current through the semiconductor. Such a semiconductor belongs to the n-type (the "n stands for "negative").


The unbound electrons of n-type semiconductors have their own energy band, called the DONOR LEVEL, (shown in light blue in the diagram on the right), with an energy value that places that band just below the conduction band (yellow). This means that at room temperature, electrons in the donor level can easily jump into the conduction band where they are free to move under the influence of an applied electric field.


3.2 p-Type semiconductors:

If an aluminium atom is added as an impurity to silicon, the Al atom can replace an Si atom in the lattice, because the Al atom is roughly the same size as the Si atom (actually, it is a little smaller). The Al atom however has only three valence electrons, and so can only form three covalent bonds with a Si atom. Compared to pure silicon, the region of the lattice shown here is one electron short, in other words, there is a positive "hole", which is free to move in the crystal lattice. These "holes" are referred to as MAJORITY CARRIERS, since the are largely responsible for the flow of current. Such a situation is typical of so-called p-type semiconductors (the "p" stands for "positive").


The positive "hole" moves in the lattice by accepting electrons from neighbouring Si atoms, thus filling the "hole", but creating another one. The electrons that hop from hole to hole have an energy corresponding to a band (the so-called ACCEPTOR BAND, shown in the diagram on the left as a purple line) lying just above the valency band (green). At room temperature, electrons from the valency band can easily jump into the acceptor band, and hence move under the applied electric field.


4. The p-n junction

When a potential difference is applied to the ends of a conductor with a resistance R (see the above diagram), the current through that conductor is directly proportional to the applied voltage (Ohm's law). In the diagram above, left, a negative current simply means that the current flows in the other direction, such as would happen if the poles of the battery were to be interchanged.

If a samples of a p-type and n-type semiconductor are placed in contact, a so-called P-N JUNCTION is formed. The result is a conducting device that does not obey Ohm's law. Instead, current can flow preferentially in one direction. Such a device is called a RECTIFYING DIODE or JUNCTION DIODE.

If a current is caused to flow across a p-n junction, we find the current can flow more readily in the direction p→n than in the direction n→p. Usually, that reverse current is negligible compared to the forward current. A graph of I against V looks something like the diagram on the right (above). The following discussion will help you to visualise why this should happen. It is important that you should understand this, as it is the basis on which transistors operate, a topic that you will learn about in Grade 12.

The p-n junction separates a region of a semiconductor that has excess electrons (shown as red circles in the diagram on the right) (the n-type) from a region that is deficient in electrons ("holes" are shown as open circles in the diagram on the right)(the p-type). At the outset, neither piece is charged, that is, there are no cations or anions in the material.


However, a small quantity of electrons on the n-side of the junction are attracted by the "holes" in the p-side. They migrate across the junction, and fill available holes. This causes a region across the junction to be depleted in carriers of current (that is, electrons and "holes"). This region is called the DEPLETION REGION, because it has been depleted in majority carriers. The depletion zone is a few micrometers thick, and, since it has no majority carriers, acts as an insulating barrier.

It is important to realise that when electrons moves from the n-side of the junction to the p-side, the n-side becomes positively charged (since neutral atoms have lost electrons), while the p-side of the junction now becomes negatively charged (since neutral atoms have now acquired negatively charged electrons). A potential difference now exists across the junction. This potential difference is about 0.6 V for silicon, and about 0.2 V for germanium. This is called the BARRIER POTENTIAL.

If an external potential difference is applied to the junction, the junction is said to be BIASED. Two types of bias can be applied. In the REVERSE BIAS (below, left), "holes" are attracted to the negative terminal of the external cell, and electrons are attracted to the positive terminal of the cell. This causes the depletion region to widen, in other words, increasing the effectiveness of the insulator at the junction. This prevents current from flowing across the junction.


If a FORWARD BIAS is applied (above, right), electrons are repelled by the external negative pole, and move into the depletion region and across the junction. At the same time, the "holes" are repelled by the positive pole of the cell. The depletion region largely disappears, allowing current to pass through the junction.

Diodes exploit the principle of the p-n junction. When forward biased, current will flow through, but when reverse-biased,they act as resistors with a high resistance. Unless a high voltage is applied, no current will flow through:


However, when a diode is forward-biased, a minimum voltage must be applied across the diode in order to overcome the barrier potential. You will learn more about diodes in Grade 12.

5. Additional questions