Semi Conductor

Semi Conductors
Diodes P-N Junction Diodes & Other Diodes Transistor Gates


Semiconductors are a unique class of materials in which the number of charge carriers, the particles that move electric charge, is intermediate between those of insulators and those of conductors. A Conductor is a material that contains a large number of free electrons that are able to move freely when a voltage is applied. An Insulator is a material that has very few free electrons and hence is a poor conductor of electricity.

Two of the most common pure semiconductor materials are Germanium and Silicon. Each of these materials has atoms with four valence electrons that are shared with the valence electrons of its four nearest neighbors in the solid state.

Types of Semi Conductor

There are two main types of semiconductors:

Intrinsic & Extrinsic Semiconductors

For pure semiconductors as the temperature is increased the resistance decreases and so raising the temperature increases the conducting power of these materials. These types of semiconductor materials are called Intrinsic semiconductors.

The conducting power of intrinsic semiconductors is greatly influenced by temperature and sometimes light making them particularly interesting.

The other type of semiconductor is one in which the charge carriers are created by an impurity. These types of semiconductors are known as Extrinsic semiconductors. The addition of impurities, such as arsenic or indium, has a significant influence on the conductive properties of semiconductors. The addition of such impurities is known as Doping.

Doped Semiconductor

A semiconductor crystal is called n-type if the addition of an impurity element results in a large number of free electrons (negative charge carriers) available for conduction. Each impurity atom is called a donor atom since it donates an electron. The electron is free to move and can contribute to an electric current. The positive ion left behind is fixed and cannot take part in conduction.

A semiconductor crystal can be made p-type by doping it with a different element so that there are a large number of positive charge carriers available for conduction. The positive charge carriers actually correspond to vacancies or deficiencies of electrons in the bonds holding the atoms in the crystal lattice. The positive charges are called holes.

Current flow in Semiconductors

An electric current can flow through a semiconductor as a result of the movement of holes and/or free electrons. There are two important processes that account for current flow in semiconductors. These processes are called Drift and Diffusion.



Diodes that are composed of a combination of semiconductor materials, with p-regions and n-regions, are good conductors when a voltage is applied so that a current is driven in one direction but a poor conductor if the voltage is reversed.

V-I Characteristics of Diodes

Forward Characteristics of Diodes

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 V-I characteristics curve above as the "knee" point.

Reverse Characteristics of Diodes

This results in a wide depletion layer.


P-N Junction Diode

Imagine that a p-type block of silicon can be placed in perfect contact with an n-type block. Free electrons from the n-type region will diffuse across the junction to the p-type side where they will recombine with some of the many holes in the p-type material. Similarly, holes will diffuse across the junction in the opposite direction and recombine.

The recombination of free electrons and holes in the vicinity of the junction leaves a narrow region on either side of the junction that contains no mobile charge. This narrow region which has been depleted of mobile charge is called the Depletion layer. It extends into both the p-type and n-type regions.

Biasing in P-N Junction Diode

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

Zero Biasing

When a diode is connected in a Zero Bias condition, no external potential energy is applied to the PN-junction.

Forward Biasing

If a battery is connected to such a device so that the positive end is connected to the p side of the device and the negative end is connected to the n side of the device, the situation is referred to as a Forward-biased junction. The charges across the p-n junction with ease, since there is an accelerating field setup, with electrons driven towards the positive end of the battery and positive holes driven towards the negative end of the battery, that allows them to overcome the opposing electric field.

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.

Reverse Biasing

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.

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)


Zener Diode

A Zener Diode is a special kind of diode which permits current to flow in the forward direction as normal, but will also allow it to flow in the reverse direction when the voltage is above a certain value - the breakdown voltage known as the Zener voltage.

Zener Effect

With the application of sufficient reverse voltage, a p-n junction will experience a rapid avalanche breakdown and conduct current in the reverse direction.The illustration shows this phenomenon in a Current vs. Voltage graph. With a zener diode connected in the forward direction, it behaves exactly the same as a standard diode - i.e. a small voltage drop of 0.3 to 0.7V with current flowing through pretty much unrestricted. In the reverse direction however there is a very small leakage current between 0V and the Zener voltage - i.e. just a tiny amount of current is able to flow. Then, when the voltage reaches the breakdown voltage (Vz), suddenly current can flow freely through it.


Light Emitting Diodes(LEDs)

LEDs emit light when an electric current passes through them.

An LED must have a resistor connected in series to limit the current through the LED, otherwise it will burn out almost instantly.

LEDs must be connected the correct way round, the diagram may be labelled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is the short lead and there may be a slight flat on the body of round LEDs.

Note: Never connect an LED directly to a battery or power supply!

Colours of LEDs

LEDs are available in red, orange, amber, yellow, green, blue and white. Blue and white LEDs are much more expensive than the other colours. The colour of an LED is determined by the semiconductor material, not by the colouring of the 'package ' (the plastic body).



The transistor is a special semiconductor device discovered by John Bardeen, Walter Brattain (1902-1987), and William Shockley(1910-1989) in 1948. Their discovery revolutionized the world of electronics and the three were awarded the Nobel Prize in physics in 1956.

There are two types of transistors:
p-n-p transistors and n-p-n transistors.

The p-n-p transistor consists of a semiconducting material with a very narrow n region sandwiched between two p regions. The n-p-n transistor consists of a p region sandwiched between two n regions.

The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon.

The leads are labelled Base (B), Collector (C) and Emitter (E).



Logic gates process signals which represent true or false. Normally the positive supply voltage +Vs represents true and 0V represents false. Other terms which are used for the true and false states are shown in the table on the right. It is best to be familiar with them all.

Gates are identified by their function: NOT, AND, NAND, OR, NOR, EX-OR and EX-NOR. Capital letters are normally used to make it clear that the term refers to a logic gate.

NOT Gate(Inverter)

The output Q is true when the input A is NOT true, the output is the inverse of the input:
Q = NOT A .

A NOT gate can only have one input.

A NOT gate is also called an inverter.

AND Gate

The output Q is true if input A AND input B are both true: Q = A AND B.

An AND gate can have two or more inputs,

its output is true if all inputs are true.

OR Gate

The output Q is true if input A OR input B is true (or both of them are true): Q = A OR B.

An OR gate can have two or more inputs,

its output is true if at least one input is true.

NAND Gate (NAND = Not AND)

This is an AND gate with the output inverted, as shown by the 'o' on the output.

The output is true if input A AND input B are NOT both true: Q = NOT (A AND B).

A NAND gate can have two or more inputs,

its output is true if NOT all inputs are true.

NOR Gate (NOR = Not OR)

This is an OR gate with the output inverted, as shown by the 'o' on the output.

The output Q is true if NOT inputs A OR B are true: Q = NOT (A OR B)

A NOR gate can have two or more inputs,

its output is true if no inputs are true.

EX-OR (EXclusive-OR) Gate

The output Q is true if either input A is true OR input B is true, but not when both of them are true:

This is like an OR gate but excluding both inputs being true.

The output is true if inputs A and B are DIFFERENT.

EX-OR gates can only have 2 inputs.

EX-NOR (EXclusive-NOR) Gate

This is an EX-OR gate with the output inverted, as shown by the 'o' on the output.

The output Q is true if inputs A and B are the SAME (both true or both false):

EX-NOR gates can only have 2 inputs.


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