6. Semi-conductors (Diode)

Diodes are made from a single piece of Semiconductor material which has a positive "P-region" at one end and a negative "N-region" at the other, and has a resistivity somewhere between that of a conductor and an insulator. But what is a "Semiconductor" material?, firstly let's look at what makes something either a Conductor or an Insulator.

Semi-conductors materials such as Silicon and Germanium, have electrical properties somewhere in the middle, between those of a "Conductor" and an "Insulator". They are not good conductors nor good insulators (hence their name semi-conductors). They have very few "fee electrons" because their atoms are closely grouped together in a crystalline pattern called a "Crystal Lattice". However, their ability to conduct electricity can be greatly improved by adding certain "Impurities" to this crystalline structure thereby, producing more free electrons than holes or vice versa. By controlling the amount of impurities added to the semiconductor material it is possible to control its conductivity. This process of adding impurity atoms to semiconductor atoms (the order of 1 impurity atom per 10 million (or more) atoms of the semiconductor) is called Doping.

The most commonly used semiconductor material is Silicon. It has four valence electrons in its outer most shell which it shares with its adjacent atoms in forming covalent bonds. The structure of the bond between two silicon atoms is such that each atom shares one electron with its neighbour making the bond very stable. As there are very few free electrons available to move from place to place producing an electrical current, crystals of pure silicon (or germanium) are therefore good insulators, or at the very least very high value resistors. Silicon atoms are arranged in a definite symmetrical pattern making them a crystalline solid structure. A crystal of pure silicon (silicon dioxide or glass) is generally said to be an intrinsic crystal.

a. The PN-junction

The semiconductor N and P-type materials are electrically neutral, but when we join (or fuse) together these two materials they behave in a very different way producing what is generally known as a P-N Junction.

When the N and P-type semiconductor materials are first brought together some of the free electrons move across the junction to fill up the holes in the P-type material producing negative ions, but because the electrons have moved they leave behind positive ions on the negative N-side and the holes move across the junction in the opposite direction into the region where there are large numbers of free electrons. This movement of electrons and holes across the junction is known as diffusion. This process continues until the number of electrons which have crossed the junction have a large enough electrical charge to repel or prevent any more carriers from crossing the junction. Eventually a state of equilibrium (electrically neutral situation) will occur producing a "Potential Barrier" zone around the area of the junction as the donor atoms repel the holes and the acceptor atoms repel the electrons. Since no free charge carriers can rest in a position where there is a potential barrier it is therefore "depleted" of any free mobile carriers, and this area around the junction is now called the Depletion Layer.

The PN-junction

As the N-type material has lost electrons and the P-type has lost holes, the N-type material has become positive with respect to the P-type. The external voltage required to overcome this barrier potential that now exists and allow electrons to move freely across the junction is very much dependent upon the type of semiconductor material used and its actual temperature, and for Silicon this is about 0.6 - 0.7 volts and for Germanium it is about 0.3 - 0.35 volts. This potential barrier will always exist even if the device is not connected to any external power source.

The significance of this built-in potential is that it opposes both the flow of holes and electrons across the junction and is why it is called the potential barrier. In practice, a PN-junction is formed within a single crystal of material rather than just simply joining or fusing together two separate pieces. Electrical contacts are also fused onto either side of the crystal to enable an electrical connection to be made to an external circuit.

Then the resulting device that has been made is called a PN-junction Diode or Rectifier Diode.

b. The Junction Diode

The effect described in the previous tutorial of PN - junction is achieved without any external voltage potential being applied to the actual PN-junction. However, if we were to make electrical connections at the ends of both the N-type and the P-type materials and then connect them to a battery in the appropriate direction, the depletion layer around the junction can be increased or decreased thereby increasing or decreasing the effective resistance of the junction itself. The behavior of the PN-junction with regards to the potential barrier size produces an asymmetrical conducting device, better known as the Junction Diode.

c. The Basic Diode Symbol and Static I-V Characteristics.

But before we can use the PN-junction as a practical device or as a rectifying device we need to firstly "Bias" the junction, i.e. connect a voltage potential across it. On the voltage axis above "Reverse Bias" refers to an external voltage potential which increases the potential barrier. An external voltage which decreases the potential barrier is said to act in the "Forward Bias" direction.

There are 3 possible "biasing" conditions for the standard Junction Diode and these are:

1. Zero Bias - No external voltage potential is applied to the PN-junction.

2. Reverse Bias - The voltage potential is connected negative, (-ve) to the P-type material and positive, (+ve) to the N-type material across the diode which has the effect of Increasing the
PN-junction width.

3. Forward Bias - The voltage potential is connected positive, (+ve) to the P-type material and
negative, (-ve) to the N-type material across the diode which has the effect of Decreasing the
PN-junction width.

d. Zero Bias.

When a diode is connected in a Zero Bias condition, no external potential energy is applied to the PN-junction. However if the diodes terminals are shorted together, a few holes (majority carriers) in the P-type material with enough energy to overcome the potential barrier will move across the junction against this barrier potential. This is known as the "Forward Current" and is referenced as I_F

Likewise, holes generated in the N-type material (minority carriers), find this situation favourable and move across the junction in the opposite direction. This is known as the "Reverse Current" and is referenced as I_R, as shown below.

e. Zero Biased Diode.

An "Equilibrium" or balance will be established when the two currents are equal and both moving in opposite directions, so that the net result is zero current flowing in the circuit. When this occurs the junction is said to be in a state of "Dynamic Equilibrium".

This state of equilibrium can be broken by raising the temperature of the PN-junction causing an increase in the generation of minority carriers, thereby resulting in an increase in leakage current.

f. Reverse Bias.

When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material. The positive voltage applied to the N-type material attracts electrons towards the positive electrode and away from the junction, while the holes in the P-type end are also attracted away from the junction towards the negative electrode. The net result is that the depletion layer grows wider due to a lack of electrons and holes and presents a high impedance path, almost an insulator. The result is that a high potential barrier is created thus preventing current from flowing through the semiconductor material.

A Reverse Biased Junction showing the Increase in the Depletion Layer.

This condition represents the high resistance direction of a PN-junction and practically zero current flows through the diode with an increase in bias voltage. However, a very small leakage current does flow through the junction which can be measured in microamperes, (μA). One final point, if the reverse bias voltage Vr applied to the junction is increased to a sufficiently high enough value, it will cause the PN-junction to overheat and fail due to the avalanche effect around the junction. This may cause the diode to become shorted and will result in maximum circuit current to flow, Ohm's Law and this shown in the reverse characteristics curve below.

Reverse Characteristics Curve for a Diode.

Sometimes this avalanche effect has practical applications in voltage stabilising circuits where a series limiting resistor is used with the diode to limit this reverse breakdown current to a preset maximum value thereby producing a fixed voltage output across the diode. These types of diodes are commonly known as Zener Diodes and are discussed in a later tutorial.

g. Forward Bias.

When a diode is connected in a Forward Bias condition, a negative voltage is applied to the N-type material and a positive voltage is applied to the P-type material. If this external voltage becomes greater than the value of the potential barrier, 0.7 volts for Silicon and 0.3 volts for Germanium, the potential barriers opposition will be overcome and current will start to flow as the negative voltage pushes or repels electrons towards the junction giving them the energy to cross over and combine with the holes being pushed in the opposite direction towards the junction by the positive voltage. This results in a characteristics curve of zero current flowing up to this "knee" voltage and high current flow through the diode with little increase in the external voltage as shown below.

Forward Characteristics Curve for a Diode.

This results in the depletion layer becoming very thin and narrow and which now represents a low impedance path thereby producing a very small potential barrier and allowing high currents to flow. The point at which this takes place is represented on the static I-V characteristics curve above as the "knee" point.

h. Forward Biased Junction Diode showing a Reduction in the Depletion Layer.

This condition represents the low resistance direction in a PN-junction allowing very large currents to flow through the diode with only a small increase in bias voltage. The actual potential difference across the junction or diode is kept constant by the action of the depletion layer at about 0.3v for Germanium and about 0.7v for Silicon diodes. Since the diode can conduct "infinite" current above this knee point as it effectively becomes a short circuit, resistors are used in series with the device to limit its current flow. Exceeding its maximum forward current specification causes the device to dissipate more power in the form of heat than it was designed for resulting in failure of the device.

h. Summary

1) Semiconductors contain two types of mobile charge carriers, Holes and Electrons.

2) The holes are positively charged while the electrons negatively charged.

3) A semiconductor may be doped with donor impurities such as Antimony (N-type doping), so that it contains mobile charges which are primarily electrons.

4) A semiconductor may be doped with acceptor impurities such as Boron (P-type doping), so that it contains mobile charges which are mainly holes.

5) When a diode is Zero Biased no external energy source is applied and a natural Potential Barrier is developed across a PN-junction which is about 0.7v for Silicon diodes and about 0.3v for Germanium diodes.

6) When a diode is Forward Biased the PN-junction is "reduced" and current flows through the diode.

7) When a diode is Reverse Biased the PN-junction is "increased" and zero current flows, (only a very small leakage current).

Next topic will be on Bipolar Transistor