Photo Credit Xeusy
Have you ever wondered how your scale acquired its magic number? Or why it always seems to be three pounds off? It is all because of a special electronic device called a transistor. Transistors are the main building block of all computer processors and are used in almost all electronic devices. A modern computer processor can contain billions of transistors. So you might ask what is a transistor and how do they measure your weight?
A transistor is made of silicon because silicon has four valence electrons and does not conduct electricity when it is pure. The silicon is doped with phosphorus to give it a negative charge because phosphorus has five valence electrons allowing for the movement of extra electrons throughout the silicon. The silicon can also be doped with boron to give it a positive charge because boron has only three electrons and causes there to be an electron hole. When silicon has a positive charge it is called P-type and when it has a negative charge it is called N-type. The combination of N-type and P-type silicon make up a transistor.
When a silicon wafer is doped with half being P-type and half N-type the wafer becomes a diode. In the center of the diode, some of the electrons from the negative side will flow across the middle to fill the holes in the positive side making part of the N-type side positively charged and part of the P-type negatively charged. The resulting neutral space in the center of the diode creates a boundary keeping any more electrons from flowing across the middle until an electric charge is applied. When a battery is hooked up with the positive side to the P-type and the negative side to the N-type then electricity can flow if there is enough energy to overcome the depletion barrier. If the connections are switched the electrons will flow to either end of the silicon wafer, away from the middle, so no electricity can pass through and makes the depletion layer much bigger. Transistors use this basic principle to function.
A transistor consists of a two N-types that sandwich a narrow P-type. No matter how the main power source is connected to either side of the wafer the electricity cannot flow because of there would be a depletion layer on one of the two sides. When a power supply is
added to the top of the P-type wafer that is correctly configured to match the positive side with the P-type and the negative side with the N-type then there will be a weak flow of electricity. The small trickle of electrons crossing the depletion barrier allows for the main power source’s electrons to pass through the entire depletion layer and becomes amplified.
A series of transistors connected together are called cells or logic gates depending on the function. A cell normally has a practical purpose, like a load cell that measures weight or pressure. Then there are gates that control the flow of signals such as, AND gates or OR gates. A AND gate lets a signal pass if both gates are powered and an OR works only when both signals do not match. Common cells that many people use on a daily basis are load cells that are found in electronic scales.
|Photo Credit: Michele M. F.
The Left and right pins are N-types and the middle pin a is P-type.
A load cell has a straight rod with transistors on either side of the rod. Pressure is then applied to the end of the rod causing the rod and the transistors to flex. The top transistor stretches while the bottom transistor compresses. The stretching and compressing of the transistors cause the voltage passing through them to change which is then sent to a basic computer that calculates weight based on the change in voltage. Over time after many uses, the scale can slowly become uncalibrated or have varying weights because the battery is dying. The differences in voltage from the battery skew the computer’s algorithm in the scale and can cause a high variation in the weight shown on the scale’s LED even if the weight being applied remains the same.
Michele M. F. “2N2222a – Small Signal Switching Transistor” Flickr.com, June 29 2014. https://goo.gl/R8za6b
Xeusy “Transistors.” Flicker.com, December 19 2012. https://goo.gl/A5yEo6