Topic 4 – Diodes

In the exam you are expected to:

• Be able to use data sheets to look up specifications for devices;

• Describe the characteristics of diodes including the forward voltage drop, forward current, and reverse breakdown voltage;

• Describe the regulation of output voltage with a zener diode;

• Describe the characteristic of the LED;

• Calculate the value of the protection resistor for an LED;

• Describe a photodiode.

Data Sheets

When we design electronic circuits, we need to know the specifications of the components that we are going to use, to ensure that the components can be used in the environment that we require.  For example, if we use a capacitor whose working voltage is 5 V in an environment where there were voltages of 15 V, the capacitor would not survive.  Some transistors might require a base current of 200 mA; if the circuit could only provide a current of 20 mA, the transistor would not work.

We can refer to data sheets that are found in retailers’ catalogues.   As well as price, we might get data on:

·        Current that the component can carry

·        Temperature that the component can tolerate.

·        Case style.

·        Working voltages.

Here is some data for some high power diodes from a catalogue:

If we have a device that takes 10 A, then we would have to use the SKN 2.504, rather than the cheaper 16FR40, which requires a forward current of 16 A.

  Diodes

Diodes are one-way electrical valves that allow current to flow in one direction only.

• In circuit A the diode is forward-biased so the current flows and the bulb will light. 

• In circuit B the diode is reverse-biased, the current will not flow and the bulb will not light.

Diodes have a low resistance when they are forward biased, and a very high resistance when they are reverse-biased.  This means that current can flow one way only.

The diagram below shows the four main kinds of diode:

Question 1  What do you think the difference is between an LED and a photodiode?

ANSWER

The photograph shows a number of different kinds of diodes.

Voltage Characteristic of the Diode

The circuit above can be used to investigate the Voltage current characteristic of a diode.  The voltage and current can be recorded for the forward biased diode.  We can turn the diode round for the reverse biased diode.

A forward biased diode starts to conduct at the junction voltage (for silicon, about 0.6 V).  Thereafter a small increase in the forward voltage leads to a large increase in forward current, as shown on the graph.

Question 2  How might you modify the circuit above to investigate the reverse bias behaviour of a diode? ANSWER

Notice that the axes are calibrated with very uneven scales.  This is done for convenience, otherwise we would not be able to show the characteristic properly.  If we reverse the voltage, we get a tiny leakage current of no more than a few microamps.  At a certain voltage, anything from 10 V to 2000 V, depending on the doping, the insulation of the barrier layer breaks down suddenly and a sudden increase in current occurs.  The breakdown voltage is the voltage at which this happens.  In many diodes, this would result in burn-out.

Germanium diodes have a full forwards voltage of about 1 to 2 V, although they can start to turn on at about 0.2 to 0.3 V.  The breakdown voltage is about 100 V.

Question 3  In a project a student is using a supply voltage of 20 V but uses a diode which has a reverse-bias breakdown voltage of 12 V.  What problems do you think he will have?

ANSWER

  Zener Diode

The zener diode is designed to be used in a reverse biased configuration.  Its behaviour is very like an ordinary diode, but a typical reverse-biased breakdown voltage is  –5.6 V, and there is a very rapid rise in current.

On its own the zener diode is little use, but in its reverse biased configuration, it will hold the output voltage at a constant 5.6 V.  It can be described as a voltage clamp.  If the voltage is above 5.6 V, a current will flow through the diode which results in the voltage being held at 5.6 V.  The higher the voltage, the greater the current.  There will be a limit to the current that the diode can conduct.  They are found in voltage regulators.

The graph shows how diode limits the voltage, which remains steady at 5.6 volts while a small current is taken.  However the diode can only take a limited current, and the output voltage will fall if the current taken is excessive.

LEDs and Photodiodes

Light Emitting Diode

The light emitting diode (LED) uses gallium arsenide phosphide as a semi-conductor.  According to the doping, the diode can emit red, yellow, green, or blue light.  They are small and reliable, so have replaced indicator lamps in many devices.  They can also be used as alphanumeric displays, although liquid crystal displays have taken over many of these functions as they take much less current.  Infrared LEDs are found in remote controllers for TV sets, or as optical fibre lasers.

As the electrons combine with the holes, quanta (packets) of energy are released as photons, some as heat, others as light. The forward voltage is about 2.2 V. 

It is very easy to ruin LEDs with currents in excess of 30 mA.  To prevent this we put a current limiting resistor in series with the LED.

Example

 Question 4 The same LED in the example above is to be used with a 15 volt supply.  What resistor do you need to protect the LED from excessive current?  ANSWER

The voltage current characteristic of the LED has a similar shape to the graph of the ordinary diode.  The LED can only tolerate a small reverse voltage.  A reverse voltage of 20 V will destroy an LED.

The Photodiode

A photodiode is a semiconductor device whose characteristics can be altered by light falling on it.  The photodiode is arranged to be reverse biased like a zener diode.  In the dark, no current flows.  When light falls on a photodiode, it leaks and a current flows.  The current flowing is proportional to the intensity.  Photodiodes respond much more rapidly to changes in light level than light dependent resistors.

The LED is often used as a transmitter of signals, and the photodiode is used as a receiver, for example in an optic fibre system.

In this arrangement the photodiode is reverse biased.   The leakage current is proportional to the light intensity, regardless of the voltage.  The linear response to light level allows the photodiode to produce a faithful copy of the original signal.  An LDR used for this kind of purpose would distort the signal badly, as its response time is slow. Optical fibres transmit data much better than wires.  There is no electrical interference and much less noise.

Another example of a use for the photodiode is the TV remote control, where an infra-red LED in the controller sends signals which are picked up by a photodiode in the TV set.

Question 5 Why is a photodiode used in preference to a light dependent resistor?  ANSWER

Home            Physics A2            Module 9