Semiconductors - Diodes and Transistors

| |



But we've already discussed diodes. They are a simple form of vacuum tube aren't they?

Well - yes and no.

While diodes existed in tube form for many years vacuum tube diodes had their problems, and the electronics industry would try to find a way around those problems. Vacuum tube diodes did a fine job of rectifying (turning ac into dc), but they wasted a lot of electrical energy in the process, which made them inefficient and costly to operate. Quite a bit of power was lost just keeping the filament warm! They also had the problem of being physically fragile, and tended to be the main cause of an electrical equipment failure.

As early as 1874, researchers noted that a metal-lead sulfide junction had rectifying properties. They found that it would conduct electrical current in one direction, but if they reversed the current, it would not flow in the opposite direction. This "junction" was "semi-conductive" in nature. They had, without knowing it, invented the semi-conductor. The problem was, they had no practical application for it, and to be honest, didn't understand how it worked. However, In 1926, P.H. Geiger and L.O. Grondahl discovered the rectifying properties in a semiconducting copper oxide-copper junction. Armed with W. Schottky's theoretical explanation of how it worked, this was the first practical diode that didn't involve a vacuum tube.

Other materials that involved semi-conductive junctions included silicon, germanium, and selenium.

Click for Speach from ShockleyOn Tuesday, December 16, 1947, Physicists John Bardeen and Walter Brattain, while working for William B. Shockley at Bell Laboratories, invented the semiconductor transistor. With the single statement "This thing's got gain!", Brattain announced the discovery of a SOLID-STATE device that could actually amplify electrical current!. By 1956 Bardeen, Shockley, and Brattain, shared the Nobel Prize for jointly inventing the transistor. It was a grand time for the electronics industry. The invention of the transistor would mean the disappearance of the tube... or would it?

Still to this day, there are applications where the transistor isn't practical. Tubes tend to work better for high power or high frequency applications. It is a tube that you look at on your computer monitor (CRT = Cathode Ray Tube). Your TV set and microwave ovens employ tube technology. Most Television Transmitters use a tube as their final output stage. "Nanotubes" are the latest craze in electronic innovation. Audiophiles swear by the rich, full, warm, reproductive sound that can only tubes can make in audio circuits. It is my belief that we need to further understand both tube and transistor technologies, and use them in tandem. It is for this reason that I taught tubes first, and that I will emphasize them throughout the rest of the course.

But now let's take a look at semiconductors: 


The Basics of Semiconductive Materials







Without going into complete details of how a semiconductor works, there are certain things we must know about them in order to use them. Semiconductors are chemical elements, that when compounded with other elements, have certain electrical characteristics.


¿what?

There are several types of semiconductive chemicals, to include but not limited to, Silicon, Germanium, Selenium, and Copper Oxide. Semiconductors do not normally conduct electrical current. But when they are combined with other chemicals, like Boron or Arsenic, can be made to partially conduct. When we combine the second chemical to the semiconductive chemical, we say that the semiconductor is "doped". Doping can either be negative doping, or positive doping. The real magic occurs, though, when we put the two types together.

In the diagram to the right, we see a device with both a positively and negatively doped section joined. The point at which the two sections join, we will call the N - P junction ( or simply the junction ).

Think of the junction as a hill, and the electrons flowing through it as a ball. As long as the ball is rolling down hill, it is easy to push along. But try pushing the ball uphill, and it is much harder to do. The same goes with the flow of electricity through a junction of P and N doped semiconductor material.



Consider, for a moment, what happens if we connect a negative DC voltage to the N doped side, and a positive DC voltage to the P doped side. According to the electron theory of current flow, electrons move from negative to positive. The electrons leave from the negative side of the battery, moving toward the positive. They come into contact with the diode, which acts like a "hill" to the electrons. The electrons flow "downhill", and current flows easily. But what happens if we reverse the direction of current flow by reversing the battery?



If the battery is reversed, the polarity applied to the diode also changes. Electrons still try to flow from negative to positive, however, going through the diode, is more like rolling a ball up-hill. It takes much more effort to push the ball uphill. The hill is steepest at the point of the P - N Junction, where it is nearly impossible to push the ball up the hill.

Because of this, a semiconductor diode acts much like a vacuum tube diode, as it conducts in one direction, while having a high resistance to current flow in the opposite direction.


It is possible, however, to make a diode conduct electricity in reverse. If a high enough voltage is applied across the junction ( which is also sometimes called the "depletion region" ), it will conduct in the reverse direction. Just like if you kick the ball hard enough, it will eventually go over the top of the hill. However, when this happens, the diode is no longer acting quite like a normal diode.

Some diodes are actually designed to be operated in this manner. These are called " ZENER DIODES ". When an exact given reverse bias voltage ( 12 Volts for instance ) is reached, the junction of the diode begins to " break down ", and act like a piece of wire. It does not conduct electricity only in one direction, but in both directions at this point.


The semiconductor diode ( usually just called a " diode " ), is one of the most important building blocks of modern electronics.

To the right is a picture and the schematic symbol for the diode. Note that there is always a line, or band that circles around one end of the diode. This line indicates which end of the diode the cathode, or positive end is on. Sometimes instead of a line, there is a dot or some other kind of indicating marker, but the cathode is always indicated, as the proper polarity of a diode is crucial in a circuit.

In addition to what you've already learned about semiconductor diodes, most semiconductor devices have other properties that we have not discussed. For instance, many semiconductor materials will radiate light when electricity is passing through it. This is the principle behind how a Light Emitting Diode, or LED (schematic to the right) works. The first case of this may be by H. J. Rounds, an employee of Marconi Labs, who in 1907 made note of a light coming from a Cat's Whisker connected to a crystal of silicon carbide.

While knowledge of this wasn't kept a secret, and experimentation was done on it for years, it wouldn't be until 1962 when General Electric employee Nick Holonyak made a practical LED public, and from there it took off. Light Emitting Diodes generate more light than heat, so unlike a standard incandescent lamp, the LED used less electricity. This made it ideal for battery operated devices that needed some kind of light display. In addition, because it was so small, several of them could be put in an array, making a display for characters (numbers and letters) more readily available.

This gives rise to the infamous "Seven Segment Display". The seven segment display is used on all kinds of electronic equipment to display basic letters and numbers.

It works by grounding one common leg of the display, while supplying voltage (typically 5 volts) to other various legs. For instance, in the display to the left, if we wanted to create the number 3, we would ground pins 4 & 12, while supplying power to pins 2,7,8,13, and 14, which would light up segments A, B, C, D, and G.

Another interesting feature of semiconductors, although not un-related, is that it will conduct and/or generate electricity when light is applied to it. this is the basis for the photoelectric cell (photocell - you may relate to the Solar Cell). We will discuss this in more detail in the section on transistors.

<