Wednesday, April 28, 2010

You can’t melt an action figure with an LED

When I was a kid, my brother and I were playing with our action figures; G. I. Joes to be exact. We got distracted and turned our attention elsewhere for a while until we started smelling burning plastic. My mother’s keen nose helped her quickly sniff out the problem. My (not very bright) brother stuck his action figure in the lamp as a hiding place and the heat from the light bulb was melting poor Joe’s backside. Childhood tales like this will soon no longer be possible, not because G.I. Joe is no longer popular, but because of the heat source that melted his nether-regions. The goal of a “light bulb” is to produce light, not heat. Any heat production is an unwanted byproduct and represents conspicuous inefficiency. Australia has already banned the sale of incandescent light bulbs because of their inherent inefficiency and other countries, including the US, are following suit.

The two big alternatives to incandescent bulbs are compact fluorescent lamps (CFLs) and light emitting diodes (LEDs). You can read a lot online about the pros and cons of each. Since we are stuck with these technologies for the foreseeable future, we might as well understand how they work. As I’m a big fan of beating a dead horse, I’ll explain their operation in the same context as my last blog – through dance!

In my last blog, I talked about semiconductor physics and how they generally do not conduct electricity until they are given a stimulus. When stimulated, the electrons can move in the semiconductor but after the stimulus is removed the electrons transition back down to their natural non-conductive state. When electrons transition from “conductive” to “insulating” states, light is often emitted in the process. Now picture the semiconductor dancer from my previous blog. When stimulated with the spot light, she dances freely, but when the light turns off she falls to the floor as an insulator. In a dramatic move right before collapsing to the floor, she gives off a burst of light (radiative recombination to physics folk) – this is basically how a light emitting diode works. Taking this analogy way beyond its limit, her heritage determines the color of light that she emits. For example, a Latin dancer will produce a red light, a Russian dancer a blue light, an Asian dancer a green light. Translated from the stage back to semiconductor reality, this means that the color of light emitted from an LED depends on what materials are used (amaze your friends by saying, “an LED’s wavelength is determined by the material’s electronic band structure”). I often joke that LEDs come in any color you want as long as it’s red, amber, green or blue. These colors happen to correspond to semiconductor materials that are readily mass producible and illustrates the point that LEDs emit one color only. Next time you look at multi-color LED Christmas lights, you will notice the aforementioned LED colors because they are the least expensive to produce.

Anyway, since the light from LEDs is produced by electrons transitioning between states, there is very little waste energy, therefore they generally do not get “hot” like a light bulb and are more energy efficient. I know what you are thinking; because the light in an LED is produced through radiative recombination events it is inherently monochromatic, so how do you explain white LEDs? More on this later…..

Wednesday, April 21, 2010

Interpretive Dance and Semiconductor Physics

Greetings! I am beginning this blog to help explain some of the mysteries of technology because it is difficult to keep pace with all of today’s advances. I want to provide an understanding of the esoteric scientific underpinnings that surround our everyday life. I’ll take on hard-hitting topics from nanotechnology and chip fabs to Mentos and Diet Coke. I will try to explain seemingly unrelated topics from Hudsons voyages to the latest lyrics, all from a scientific viewpoint. As humans, we all see the world differently. Scientists see the world as simply reflected electromagnetic radiation with wavelengths between approximately 400 nm and 750 nm. This blog will explain ideas from this point of view without boring you with scientific jargon and long-winded descriptions. It will hopefully be fun, relevant and educational “info”tainment.

For my very first post, I thought it might be useful to explain some of the science behind semiconductors as the Capital Region is arguably becoming a major hub of semiconductor activity. To me, a semiconductor is one of the most misunderstood materials. The mention of the word ‘semiconductor’ often conjures up images of computers; the two always seem to be intertwined. But why? While semiconductors are used in computers, they are also used in LEDs, solar cells, sensors, and zillions of other electronic gadgets.

A scientific explanation of semiconductors involves looking at the electronic band structure and noticing a characteristic band-gap between conduction and valance band. However, explanations like this are tough stuff. As such, I’ve come up with my own analogy to explain semiconductor behavior through interpretive dance. That’s right, interpretive dance. I figure this explanation will stick with you and you can use it at your next cocktail party to look like a genius.

Imagine a stage with a dancer laying motionless on the floor. This motionless dancer is ‘the insulator’. Glass, plastic and ceramics are all traditional insulators because they don’t pass heat or electricity very easily. Now picture a hyper-active dancer jumping all over the stage with a flowing ribbon in tow. This dancer is ‘the conductor’ and simulates the behavior of electrons in a conductor. When most people think of conductors, they think of metals because they conduct electricity and heat very well - that’s why we make frying pans and wires from them. So we have one dancer laying motionless on the stage (‘the insulator’), one dancer leaping across the stage with flowing ribbon (‘the conductor’), and now enters ‘the semiconductor’ as a third dancer. Picture a dancer, motionless on the floor like ‘the insulator’, until a laser spotlight shines on her and she starts leaping around like ‘the conductor’. Once the laser spotlight moves away from her, she falls motionless to the floor again. She remains motionless until the laser shines upon her to excite her once more into dancing. This is ‘The Dance of the Semiconductor’. A semiconductor is inherently an insulator but can be made into a conductor given a stimulation; hence the name ‘semi’ conductor. The stimulation can come as an applied voltage, as heat, or in this case via laser excitation.

Which is what makes a semiconductor such an interesting material; they conduct like wire but without stimulation revert back to being an insulator. Because of this ability, they make really good on-off switches. A fancy version of an on-off switch is called a transistor, one of the key building blocks for computer logic which is why people map semiconductor materials to computers. Now, however, we have learned ‘The Dance of the Semiconductor’ and understand this “Dancing Queen” can be used in many ways. I think ABBA summed up the potential of semiconductors best, “See that girl, watch that scene, dig in the dancing queen”.

There are several resources online that explain more about semiconductor applications:

and my favorite site which explains nano-sized semiconductor particles:

Full-Disclosure, I’m the founder and CEO of Evident Technologies so I’m a bit biased.