Semiconductors

• A semiconductor is a substance whose resistivity lies between that of a conductor and an insulator. The resistance of a semiconductor decreases as the temperature increases.

• Semiconductors are usually made from Silicon or Germanium

Conduction in semiconductors• A Si atom has 4

electrons in its outermost shell. In a piece of Si at low temp. (near 0 K) each of these electrons forms covalent bonds with 4 other Si atoms

• As the temp. of the semiconductor is increased, these electrons gain more energy. Some gain enough energy to break free of their bonds, and wander through the piece of material.

• Once an electron moves out of a bond, it leaves behind a ‘hole’ in that bond

• This hole is positive, and so can attract nearby electrons which then move out of their bond etc.

• Thus, as electrons move in one direction, holes effectively move in the other direction

Electron moves to fill hole

As electron moves in one direction hole effectively moves in other

• The difference between conduction in metals and semiconductors, in metals conduction is due solely to movement of electrons, in semiconductors it is due to movement of negative electrons and positive holes.

Intrinsic semiconductors

• When conduction in a semiconductor is due to electrons moving from - to + and an equal no. of holes moving from + to -, it is called intrinsic conduction

• The resistance of an intrinsic semiconductor, even at room temp. is quite high

To increase conduction in a semiconductor• (a) Increase the temperature

As the temp. increases the no. of electrons which have enough energy to break free of their bonds increases, and so the conduction increases.

A thermistor is a device whose resistance changes rapidly as the temperature changes

• (b) Shine light on surface.• When light shines on some

semiconductors, (e.g. cadmium sulphide) it gives sufficient energy to the valence electrons for them to break free of their bonds, and so increases conduction. Such a semiconductor is called an LDR (Light Dependent Resistor)

Its resistance varies from several M Ω in darkness, to 100Ω in daylight-used in automatic streetlight circuits

• (c ) Adding small amounts of impurities to the semiconductor can also effect its conduction. This process is known as doping

Doping

• Si has 4 valence electrons. • B has 3 valence electrons. • If a small amount of B is

introduced into the Si lattice, at each location that a B atom occurs in the bond, there will be one electron missing, in other words, a positive hole exists.

• This means that even before you raise the temperature there are already holes in existence, which, if a voltage is applied across the material,it will cause conduction to occur.

• Thus, in a semiconductor doped with B, there are more holes than electrons.

• Positive holes are thus the majority charge carriers, and the system is called a p-type semiconductor

N-type semiconductors

• If, instead, the semiconductor has small quantities of phosphorus (which has 5 valence electrons) added to it, then at each location where P occurs, there is an extra electron.

• Thus the semiconductor has more electrons than holes.

• The electrons are the majority charge carriers and the system is called an n-type semiconductor

Extrinsic conduction

• -increased conduction due to the presence of impurity elements is called extrinsic conduction

• It is important to note that even though there are extra FREE electrons or holes, the piece of material is still electrically neutral. Overall there are still the same TOTAL no. of electrons as protons

P-N Junction• If a piece of p-type an n-type semiconductors

are placed next to each other, at the boundary some of the electrons from the n-type jump to the boundary to fill some of the holes in the p-type.

• Effects:• Firstly, the p-type has now

gained extra electrons, and so is negatively charged.

• Similarly the n-type is now positively charged.

• Thus there is a voltage across the material – this is known as the junction voltage.

• For Si the junction voltage =0.6V, for germanium = 0.2V

Depletion layer

• Also, at the boundary region (junction) there are no free electrons and no holes (as they have cancelled each other)

• Thus, in this region there are no free charge carriers, so no conduction can occur. This insulating layer is called the depletion layer

PN junction (diode)

• Symbol

Depletion layer forms an insulator between the 2 sides

P type

N type

A popular semiconductor device called a diode is made by joining p- and n-type semiconductor materials

• In order to get current to flow through the diode it is necessary to break down the depletion layer (i.e. drive free electrons and free holes into the layer)

PN junction (diode)

Forward biased pn junction

• In this case the + of the battery is connected to the p-type and the – of the battery to the n-type, the free electrons and the free holes are repelled by the battery and driven into the depletion layer.

• Once the applied voltage of the battery is greater than the junction voltage, the depletion layer breaks down and the diode conducts

• Forward-biasing a diode allows current to flow easily through the diode.

The term bias is defined as a control voltage or current.

Reverse biased pn junction

• If the + of the battery is connected to the n-type and the – terminal to the p-type, the free electrons and free holes are attracted back towards the battery, hence back from the depletion layer, hence the depletion layer grows. Thus a reverse biased pn junction does not conduct current

Reverse biased pn junction

When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less.

all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown.

Diodes Have Polarity

(They must be installed

correctly.) }Diodes

Cathode Lead

Anode Lead

}Diodes

Cathode LeadCathode Lead

Anode LeadAnode Lead

The PN Junction DiodeThe PN Junction Diode

Anode (+)

Cathode (-)

Volt-Ampere Characteristic Curve

• The figure is a graph of diode current versus diode voltage for a silicon diode.

• The graph includes the diode current for both forward- and reverse-bias voltages.

• The upper right quadrant of the graph represents the forward-bias condition.

• Beyond 0.6 V of forward bias the diode current increases sharply.

• The lower left quadrant of the graph represents the reverse-bias condition.

• Only a small current flows until breakdown is reached.

Diode Ratings

• Diode ratings include maximum ratings and electrical characteristics.

• Typical ratings are• Breakdown Voltage Rating, VBR

• Average Forward-Current rating, IO• Maximum Forward-Surge Current Rating,

IFSM

• Maximum Reverse Current, IR

Rating Abbreviation Designated As Significance

Breakdown Voltage VBR PIV, PRV, VBR, or VRRM

Voltage at which avalanche occurs; diode is destroyed if this rating is exceeded.

Average Forward-Current

IO IO Maximum allowable average current.

Maximum Forward-Surge Current

IFSM IFSM Maximum instantaneous current.

Maximum Reverse Current

IR IR Maximum reverse current.

Diode Ratings

Rectifier Diodes The circuit shown is called a half-wave rectifier. When the top of the transformer secondary voltage is positive, D1 is forward-biased, producing current flow in the load. When the top of the secondary is negative, D1 is reverse-biased and acts like an open switch. This results in zero current in the load, RL. The output voltage is a series of positive pulses, as shown in the figure in the right.

Signal Diodes (small current)Signal diodes are used to process information (electrical signals) in circuits, so they are only required to pass small currents of up to 100mA.

General purpose signal diodes such as the 1N4148 are made from silicon and have a forward voltage drop of 0.7V.

Germanium diodes such as the OA90 have a lower forward voltage drop of 0.2V and this makes them suitable to use in radio circuits as detectors which extract the audio signal from the weak radio signal.

Rectifier Diodes (large current)Rectifier diodes are used in power supplies to convert alternating current (AC) to direct current (DC), a process called rectification.

They are also used elsewhere in circuits where a large current must pass through the diode.

All rectifier diodes are made from silicon and therefore have a forward voltage drop of 0.7V.

The 1N4001 is suitable for most low voltage circuits with a current of less than 1A.

Bridge Rectifier

They have four leads or terminals: the two DC outputs are labelled + and -, the two AC inputs are labelled .

converts AC to DC

Special Diodes

• Besides rectification, a semiconductor diode has many other useful applications.

• Semiconductor diodes can be manufactured to regulate voltage or emit different colors of light.

• Examples of two special purpose diodes are• Light-emitting diode• Zener diode

• A light-emitting diode (LED) is a diode that emits a certain color light when forward-biased.

• The color of light emitted by an LED is determined by the type of material used in doping.

• A schematic symbol of an LED is shown in the figure.

Special Diodes

LEDThe cathode is the short lead and there may be a slight flat on the body of round LEDs

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

It will be destroyed almost instantly because too much current will pass through and burn it out.

LEDs must have a resistor in series to limit the current to a safe value, for quick testing purposes a 1k resistor is suitable for most LEDs if your supply voltage is 12V or less. Remember to connect the LED the correct way round!

A zener diode is a special diode that has been optimized for operation in the breakdown region. Voltage regulation is the most common application of a zener diode.

Special Diodes

Using an Analog Meter to Check a Diode

When using an analog meter, check the resistance of the diode in one direction, then reverse the meter leads & measure the resistance of the diode in the other resistor. A good diode has a very high resistance ( towards infinity) in one direction and a low resistance in the other direction.If the diode is shorted, it will measure low resistance in both directions.If the diode is open, it will measure a high resistance in both directions.When using analog meter, do not use the R x 1 range because the current forced through the diode by the meter may exceed the current rating of the diode. R x 100 or R x 1000 range is usually the best range.

Using a Digital Multimeter (DMM) to Check a Diode

When using a DMM, a special range, called the “diode range”, is used for diode testing. When the DMM forward-biases the diode being tested, it measures the forward voltage drop across the diode & not the forward resistance RF.A good silicon diode shows a voltage between 0.6 and 0.7 volts for one connection of the meter leads & an overload (OL) condition for the opposite connection.An open diode shows OL for both connections of the meter leads.A shorted diode shows a very low or zero reading for both connections of the meter leads.

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