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.
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 100 Ω in daylight-used in automatic streetlight circuits
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
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.
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 .) The PN Junction Diode Anode (+) Cathode (-)
Diode ratings include maximum ratings and electrical characteristics.
Typical ratings are
Breakdown Voltage Rating, V BR
Average Forward-Current rating, I O
Maximum Forward-Surge Current Rating, I FSM
Maximum Reverse Current, I R
Diode Ratings Rating Abbreviation Designated As Significance Breakdown Voltage V BR PIV, PRV, V BR , or V RRM Voltage at which avalanche occurs; diode is destroyed if this rating is exceeded. Average Forward-Current I O I O Maximum allowable average current. Maximum Forward-Surge Current I FSM I FSM Maximum instantaneous current. Maximum Reverse Current I R I R Maximum reverse current.
The circuit shown is called a half-wave rectifier .
When the top of the transformer secondary voltage is positive, D 1 is forward-biased, producing current flow in the load.
When the top of the secondary is negative, D 1 is reverse-biased and acts like an open switch. This results in zero current in the load, R L .
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
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.
LED The 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!
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 R F .
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.