Thursday, October 14, 2010

Failed Tests and The Hall Effect

Ok guys,

Just got back my Semiconductor materials test, very low grade (like most of the class, actually). :(
Talked with some of the students from last year, and they said it was a similar situation then. The tests this teacher give out are monsterously difficult, nothing like the homework or examples, and hardly any information is given about each problem. One person scored 100, and she actually memorized dozens of physical constants as well as nearly all the parameters of intrinsic Silicon and intrinsic Gallium Arsenide semiconductors at 300 yeah. She did a great job (and she's rather cute, too). We'll have to put in as many hours as she does if we're going to pass this class. I'm still upset the teacher refused to give us most of the physical constants and none of the equations we needed, but it really made me think during the test.

My burning academic career aside, let's talk about the Hall Effect. Here's a nifty picture taken from my professor's lecture (forgive my larcenous ways!)

Let's say we have a chunk of semiconductor material that's p-type (doped so that the carrier concentration favors holes left by absent electrons).

Let's say we jam some wires on the left and right side and run a voltage through that thing positive side on the left, negative on the right. What will happen? The voltage will push the holes through the semiconductor torwards the right (x-axis).

Now let's say we get an electromagnet and place a pole on the long, close side and another pole on the far side(z-axis). Now what happens? The Hall Effect.

A magnetic field perpendicular to the hole's path forms on the z-axis, inducing an electric field downwards through the semiconductor material. This electric field applies an upwards force on the hole. Like in the picture, the hole curves up under the effect of two forces (one right, one up). This leads to a collection of negative charge on the bottom of the material, and a collection of positive charge on the top of the material.

Now if you were to break out a meter and read the top and bottom of the material, you'd get a voltage reading! Magnet off, nothing. Magnet back on, Voltage!

Just by applying a voltage on either end and a magnetic field, a new voltage can be created. This new voltage can even help you calculate how many atoms of 'doping' material are in the semiconductor, a difficult feat if you don't know it and it isn't labeled.

There are several pages of equations (I have to memorize like everything else -_-) and derivations to actually prove all this. I could touch on it later if anyone wants me to go through that torture.

Later guys


  1. Fuckin magnets!
    Better luck next time with your future tests though bro :)

  2. I can't run away from this ... learn it on college, on the internet ELECTRICITY IS EVERYWHERE ! :D

  3. btw: good luck with learning all that :D

  4. aww hah, well there is always next text.

  5. this should be related with rocket science

  6. i was never s good with physics