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Diodes

Posted on : Sun , 03 2014 by : virusi

Theory OF OPERATION :


Diode construction :

A PN Junction Diode is one of the simplest semiconductor devices around, and which has the characteristic of passing current in only one direction.

However, unlike a resistor, a diode does not behave linearly with respect to the applied voltage as the diode has an exponential current-voltage ( V-I ) relationship and therefore we cannot described its operation by simply using an equation such as Ohm’s law.

Diode_construction
Fig. 1 Diode construction and representation

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 which has a resistivity value somewhere between (Depletion region) that of a conductor and an insulator.

There are two operating regions and three possible “biasing” conditions for the standard Junction Diode and these are :

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 IF . Likewise, holes generated in the N-type material (minority carriers), find this situation favorable and move across the junction in the opposite direction. This is known as the “Reverse Current” and is referenced as IR. This transfer of electrons and holes back and forth across the PN junction is known as diffusion, as shown below.

Forward bias.
If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it can supply free electrons and holes with the extra energy they require to cross the junction as the width of the depletion layer around the PN junction is decreased. If this external voltage becomes greater than the value of the potential barrier, approx. 0.7 volts for silicon and 0.3 volts for germanium, the potential barriers opposition will be overcome and current will start to flow.

Reverse bias.
By applying a negative voltage (reverse bias) results in the free charges being pulled away from the junction resulting in the depletion layer width being increased. This has the effect of increasing or decreasing the effective resistance of the junction itself allowing or blocking current flow through the diode. This condition represents a high resistance value to the PN junction and practically zero current flows through the junction 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 ).

Diode V-I characteristic :

IdealDiode_Characteristic
Fig.2 Ideal Diode V-I characteristic

As you can see from the figure above that ideal I-V characteristic for a diode would be in forward bias at any voltage applied to the diode current that can flow through the diode is infinite but in the reverse bias at any voltage applied to the diode no current will pass through the diode. But let’s take a look at the real diode V-I characteristic.

diode_characteristics
Fig.3 Diode V-I characteristic

As you can see from the Fig.3 depending on the diode material the I-V characteristic varies, plus this I-V characteristic is temperature dependent so you should be aware when using a diode in your application what is the reverse break down voltage a certain temperature. The most popular diode is the SI ones because they are cheap to produce and the knee voltage is very low (0.7V) and reverse break down voltage is very big.

Since the diode can conduct “infinite” current above this knee point as it effectively becomes a short circuit, therefore resistors are used in series with the diode 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 a very quick failure of the device.

PRACTICAL EXAMPLE :


If you don’t see the example below than you should follow this steps:

– In your browser allow Java SE 7.

– Lower you java security settings (Go to Control Panel >> Java >> Security and set the security level to medium) .

– Edit Site List (Go to Control Panel >> Java >> Security and click on Edit Site List… and add eagerlearning.org in the list).

Let’s connect a LED in series with a resistor to a battery and check what is happening at different resistor and battery values.

Sorry, you need a Java-enabled browser to see the simulation.

If you want to change resistor or battery value than you should double click on the element and insert the desired value. First Diagram is showing Diode IV characteristic, you can play with the resistance value and see how the V-I characteristic is changing. The second diagram shows the I and V that is flowing through the Diode.

Last updated on Mon , 03 2014
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