domingo, 30 de mayo de 2010

Field Effect Transistor

Field Effect Transistor

In 1945, Shockley had an idea for making a solid state device out of semiconductors. He reasoned that a strong electrical field could cause the flow of electricity within a nearby semiconductor. He tried to build one, then had Walter Brattain try to build it, but it didn't work.

Three years later, Brattain and Bardeen built the first working transistor, the germanium point-contact transistor, which was manufactured as the "A" series. Shockley then designed the junction (sandwich) transistor, which was manufactured for several years afterwards. But in 1960 Bell scientist John Atalla developed a new design based on Shockley's original field-effect theories. By the late 1960s, manufacturers converted from junction type integrated circuits to field effect devices. Today, most transistors are field-effect transistors. You are using millions of them now.

Most of today's transistors are "MOS-FETs", or Metal Oxide Semiconductor Field Effect Transistors. They were developed mainly by Bell Labs, Fairchild Semiconductor, and hundreds of Silicon Valley, Japanese and other electronics companies.

Field-effect transistors are so named because a weak electrical signal coming in through one electrode creates an electrical field through the rest of the transistor.  This field flips from positive to negative when the incoming signal does, and controls a second current traveling through the rest of the transistor. The field modulates the second current to mimic the first one -- but it can be substantially larger.  

How it works

On the bottom of the transistor is a U-shaped section (though it's flatter than a true "U") of N-type semiconductor with an excess of electrons.  In the center of the U is a section known as the "base" made of P-type (positively charged) semiconductor with too few electrons. (Actually, the N- and P-types can be reversed and the device will work in exactly the same way, except that holes, not electrons, would cause the current.) 
Three electrodes are attached to the top of this semiconductor crystal: one to the middle positive section and one to each arm of the U. By applying a voltage to the electrodes on the U, current will flow through it. The side where the electrons come in is known as the source, and the side where the electrons come out is called the drain. 

If nothing else happens, current will flow from one side to the other.  Due to the way electrons behave at the junction between N- and P-type semiconductors, however, the current won't flow particularly close to the base.  It travels only through a thin channel down the middle of the U. 

There's also an electrode attached to the base, a wedge of P-type semiconductor in the middle, separated from the rest of the transistor by a thin layer of metal-oxide such as silicon dioxide (which plays the role of an insulator).  This electrode is called the "gate."  The weak electrical signal we'd like to amplify is fed through the gate.  If the charge coming through the gate is negative, it adds more electrons to the base.  Since electrons repel each other, the electrons in the U move as far away from the base as possible. This creates a depletion zone around the base – a whole area where electrons cannot travel.  The channel down the middle of the U through which current can flow becomes even thinner.  Add enough negative charge to the base and the channel will pinch off completely, stopping all current.  It's like stepping on a garden hose to stop the flow of water. (Earlier transistors controlled this depletion zone by making use of how electrons move when two semiconductor slabs are put next to each other, creating what is known as a P-N junction. In a MOS-FET, the P-N junction is replaced with metal-oxide, which turned out to be easier to mass produce in microchips.)

Now imagine if the charge coming through the gate is positive.  The positive base attracts many electrons – suddenly the area around the base which used to be a no-man's-land opens up.  The channel for current through the U becomes larger than it was originally and much more electricity can flow through. 

Alternating charge on the base, therefore, changes how much current goes through the U. The incoming current can be used as a faucet to turn current on or off as it moves through the rest of the transistor. 

On the other hand, the transistor can be used in a more complex manner as well -- as an amplifier.  Current traveling through the U gets larger or smaller in perfect synch with the charge coming into the base, meaning it has the identical pattern as that original weak signal.  And, since the second current is connected to a different voltage supply, it can be made to be larger.  The current coming through the U is a perfect replica of the original, only amplified.  The transistor is used this way for stereo amplification in speakers and microphones, as well as to boost telephone signals as they travel around the world.

Footnote on Shockley
Shockley watched as Silicon Valley grew but could not seem to enter The Promised Land he had envisioned. He never was able to make field effect transistors, while other companies designed, grew, and prospered. Fred Seitz called Shockley "The Moses of Silicon Valley." 

Freddy Vallenilla EES Sec 2

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