Wednesday, March 12, 2008

Diodes: what are they?

This post will be a bit technical.


What are diodes?

Diodes are electronic devices capable of rectifying signals to one direction only. They are made by joining two types of semiconductors, a P type and an N type. One is full of free holes, and the other have many free electron carriers.

As far as a practicing electrical engineer is concerned, the internal physics of the operation of a diode may not be too important. The important things to be known however are their limitations, such as its forward-bias voltage drop, reverse breakdown voltage, reverse saturation current, reverse-recovery time, and maximum forward current. All of these parameters are usually provided by manufacturers through datasheets.

Forward-bias voltage drop is the amount of voltage drop that will occur on a diode if it is being forward-biased (i.e. anode more positive than cathode). On silicon diodes, the forward-bias voltage is around 0.6 - 0.7V. While it is very convenient to assume that this value is fixed at 0.6V or 0.7V, in reality however, this parameter is a function of the current flowing through the diode as well as temperature.

Reverse breakdown voltage specifies how much voltage a diode can handle such that current is kept from flowing in reverse-bias conditions. In silicon diodes, it may be in the order of 75V to 100V, but a special kind of diode called zener diodes are actually made to operate on its breakdown region, which is usually practically low. There are two causes of breakdown voltages: avalanche breakdown and zener breakdown. Since I can't remember how they differ, I woudn't explain it here.

Reverse saturation current is the tiny amount of current that flows in reverse bias condition. It's in the order of microamps and is usually not of a concern. But it's amount is crucial in estimating other diode parameters, as stated in the formulas relating diode voltage and its current.

Reverse recovery time is the period when a diode keeps conducting despite being suddenly reverse-biased from a previously forward-biased condition. Experiments shows that its amount is not a constant, but rather affected by the positive signal swing as well as the diode capacitances. However, datasheets normally gives a certain typical value convenient enough for us to use. A special kind of diode, the schottky diode, practically has zero reverse recovery time because the factors contributing to it does not exist in these diodes which are made from a semiconductor-metal junction.

Maximum forward current is the maximum current that may pass a diode without damaging it. Based on this parameter, people usually call diodes with large current capabilities as rectifiers, whereas diodes with smaller current capabilities (but with higher speed specifications) are more commonly known as small-signal diodes.

Wednesday, March 05, 2008

Still not sure how to teach

Today is week 5 of the Elektronika Dasar class that i teach. We've finished discussing diodes 2 weeks ago, so I decided to give the students a quiz (without telling them beforehand).

And the results were almost as predicted: unsatisfactory.

I made it my intention to test them with simple questions but with slight "twists".

Question number one was about the basic rectifying behaviour of diodes. (21 answered wrong, 24 answered right)

Question number two was about zener diodes. The circuit topology was made exactly the same as in a previous assignment, but as I had suspected, not much of them really understood what they handed in. Only six students answered correctly in the quiz, while almost ALL OF THEM answered right in their previous assignments. Amazed? LoL.

The last question was a "twisted" version of a clipper diode. Instead of giving an output as a clipped version of its input, in this question I wanted the students to draw how to put a protection diode that can clip the input to a certain voltage (Vcc=5V). Only a few answered the right answer, but I admit for this question it might be a bit too hard to come up with the "expected answer".

Anyways, the bottom line is that it's very important to accurately know how the students are doing in a big class, and this can be done by giving assignments and quizzes, and teachers SHOULD evaluate the results ASAP (never postpone marking assigments to after exams... never!). It's one good way of getting feedback.