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Electronics I Laboratory Manual

Special Purpose Diodes

Besides the PN diodes that we studied in the last experiment there are many more types of diodes.  Each one is used in applications that can take advantage of its behavior and characteristics.  What follows is a short statement explaining features of some of these special purpose diodes.

Zener Diode
A zener diode is similar to the PN diode, but with special fabrication techniques it can be built with a specific breakdown voltage.  The breakdown voltage of these diodes is called the Zener voltage, VZA typical PN diode normally has a very large breakdown voltage.  A zener diode, on the other hand, can have practically any value for the breakdown voltage.  You can find zener diodes with VZ ranging from a few volts to several hundred volts.  Another important feature that makes the zener diode different from the normal PN diode is that at breakdown the characteristic curve is approximately a vertical line, indicating that in breakdown the Zener diode produces nearly constant voltage.  Thus a Zener diode operating in its breakdown region will produce a constant voltage equal to VZ.  Because of this fact, the Zener diode is normally used in the reverse-bias direction.  Figure 6.1 shows the characteristic curves of a typical PN diode and a typical Zener diode with VZ = 5.1 V.  Notice the difference in the breakdown regions of both devices.

LED
The Light-Emitting Diode (LED) produces light when forward biased.  In the LED the electrical energy applied to the junction raises the energy level of the electrons.  As they return to their original energy level, they emit photons (light particles).  The light is visible.  The LED is one of most versatile electro-optical devices, forming the displays of practically all electronic devices such as VCR’s, clock receivers, TV sets, remote controls, etc.


LEDs light up when forward biased.  The typical forward voltage (VF) and forward current (IF) for LEDs are as follows:


IF   :  < 20 mA for AC operation and < 10 mA for DC operation.

                                                                                                                                                   

Caution:

LEDs typically have reverse breakdown voltage of the order of 5V.  This means you must be very careful not to use LEDs with a high level of reverse bias because the current could be very high.

                                                                                                                                                   

The color of the light given off by an LED depends on the material used in its construction and on the construction technique.

Schottky Diode 
The standard PN diodes are built with P- and N-type material.

The Schottky diode is constructed by bending a metal (aluminum or platinum) to, for example, N-type silicon.  In other words we form the diode’s anode with metal rather than with P-type material.  The threshold voltage (Von) of the Schottky diode is around 0.3V like the germanium diode. 
    
The characteristic behavior of this diode is very similar to the characteristic of the standard PN diode.  The main difference is the fact that the Schottky diode can switch ON and OFF much faster than the PN junction diodes.  Another difference is that the Schottky diode produces less unwanted noise than either the silicon or germanium diodes.  These two characteristics make the Schottky diode very useful for applications where high-speed switching power circuits are required.

The Schottky diode and the germanium diode have both the same turn-on voltage, as we have seen (0.3V).  The Schottky diode, however, has the additional advantage of speed.  As a result, you rarely see a germanium diode in use today.
           
Figure 6.2 shows the layout of the construction of this diode, the symbol, and a typical characteristic curve.  Notice that there is only one semiconductor piece (the N-type material).  The anode side of the Schottky diode is a piece of metal, as you can see.

Tunnel Diode
These type of diodes have the unique feature that for a particular voltage range they act as a negative resistance!  Figure 6.3 shows the symbol used to represent the tunnel diode and a typical characteristic curve.

In the I-V curve we can distinguish three important regions:  In regions A and C the diode behaves like a typical device with positive resistance; this means that as the voltage increases the current will increase as well.  In the second region (region B), however, you can see that as the voltage increases, between VP and Vu, the current decreases.

Negative resistances are useful in oscillator construction and in high frequency applications, and this is where sometimes you will find a tunnel diode.
                                                                                                                                                   

Note:

The negative resistance of the tunnel diode only manifests itself for AC signals.  For DC, a positive voltage will produce a positive current.
                                                                                                                                                   

Varactors
As you know, when a PN diode is biased by applying a voltage to the junction the physics of the device results in a negative charge on the P side and positive charge on the N side.  The region these positive and negative charges form, known as the depletion region, does not contain any moving charges.

The result of the above statements is that we end up with charges separated by an insulator.  This is exactly a capacitor.  In fact, all PN junctions have an associated capacitance (Cj).  When voltage is applied to the diode, the depletion region decreases (forward bias) or increases (reverse bias), changing the value of the PN junction capacitance.

Some diodes are specially manufactured so that the PN junction capacitance has a known and controllable relation to the applied diode voltage.  This produces a device known as a varactor, which has a voltage controlled capacitance.  These diodes are operated normally in reverse bias only.  Figure 6.4 shows the layout, the symbol, and the curve that shows the relationship between the applied reverse bias voltage and the capacitance.

Note that the reverse bias voltage (VR) increases the capacitance decreases.  The quality CT is the capacitance of the device when there is no applied voltage.  The relation between the reverse bias voltage and the capacitance is given by the following formula.

                                                                       

                                                                                                                                                   

Notes:
  • For some varactors the denominator of the above equation is cube root rather than a square root.
  • The useful range of the varactor is approximately 1/2 to 1/3 of the zero-bias value (Cj).

                                                                                                                                                   

Photo Diodes

A photo diode converts light energy into electrical energy.  Devices that convert from one form of energy to another are known as transducers, sensors, or detectors.

The photo diode works as follows:  You know that the reverse bias current of a diode is very small.  If we can construct a diode so that light (photons) can reach the junction, then the energy imparted by the photons to the atoms in the junction will create more free electrons (and more holes).  These extra electrons will create a large reverse bias current.  As the incident light is increased, the reverse-bias current increases.

If we use the photo diode in reverse bias, then we have a device that its current output is dependent on the intensity of the incident light.  Used in this fashion, the photo diode is operating in the photoconductive mode.  If we use the photo diode in the forward bias region, we have a device that produces an output voltage in response to light illumination; used in this manner, the photo diode is operating in the photovoltaic mode.


                                                                                                                                                   

Note:

It is not the purpose of this experiment to study photo diodes in particular.  To know more about this topic, the reader should read any introductory electronic book.

                                                                                                                                                   

Figure 6.5 shows the symbols used for the different diodes that we have presented here.

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