Semiconductor Devices II Physics 355

Zener Diodes With the application of sufficient reverse voltage, a p-n junction will experience a rapid avalanche breakdown and conduct current in the reverse direction. Valence electrons that break free under the influence of the applied electric field can be accelerated enough that they can knock loose other electrons and the subsequent collisions quickly become an avalanche. When this process is taking place, very small changes in voltage can cause very large changes in current. The breakdown process depends upon the applied electric field, so by changing the thickness of the layer to which the voltage is applied, zener diodes can be formed which break down at voltages from about 4 volts to several hundred volts.

Zener Diodes When forward-biased, Zener diodes behave much the same as standard rectifying diodes: they have a forward voltage drop which follows the "diode equation" and is about 0.7 volts. In reverse-bias mode, they do not conduct until the applied voltage reaches or exceeds the so-called Zener voltage, at which point the diode is able to conduct substantial current, and in doing so will try to limit the voltage dropped across it to that Zener voltage point. So long as the power dissipated by this reverse current does not exceed the diode's thermal limits, the diode will not be harmed.

Tunnel Diodes

NDC region

Tunnel Diodes: Applications In the NDC region, the diode can be used as either an oscillator, as in the case of the Gunn diode, or as an amplifier. Low-noise, tunnel-diode amplifiers represent an important microwave application of tunnel diodes. Tunnel-diode amplifiers with frequencies up to 85 gigahertz have been built in waveguides, coaxial lines, and transmission lines. The low-noise generation, gain ratios of up to 30 dB, high reliability, and light weight make these amplifiers ideal for use as the first stage of amplification in communications and radar receivers.

Transistors Dr. John Bardeen, Dr. Walter Brattain, and Dr. William Shockley discovered the transistor effect and developed the first device in December, 1947, while the three were members of the technical staff at Bell Laboratories in Murray Hill, NJ. They were awarded the Nobel Prize in physics in 1956.

Transistors Transistors are the main components of microprocessors, which are essential to many of the products we use every day such as televisions, cars, radios, home appliances, and, of course, computers. Transistors are miniature electronic switches. They are the building blocks of the microprocessor which is the brain of the computer. At their most basic level, transistors may seem simple. But their development actually required many years of painstaking research. Before transistors, computers relied on slow, inefficient vacuum tubes and mechanical switches to process information. In 1958, engineers (one of them Intel co-founder Robert Noyce) managed to put two transistors onto a silicon crystal and create the first integrated circuit, which led to the microprocessor.

Transistors The bipolar junction transistor acts as a current amplifier, having many applications for amplification and switching.

Unbiased Transistors

Transistors

When a negative voltage is applied to the base (point B), electrons in the base region are pushed ('like' charges repel, in this case both negative) back creating insulation boundaries. The current flow from point E to point C stops. The transistor's state has been changed from a conductor to an insulator.

Transistors We begin with the transistor acting as an insulator. In order to have it conduct, positive voltage must be applied to the base (point B). As opposite charges attract (in this case, positive and negative), electrons are 'pulled' out of the insulating boundaries and flow out of the base region at point B. The barriers that once restricted flow of electrons from the emitter to the collector are diminished. Electrons begin to flow in at the emitter (point E), through the base to the collector (point C). The transistor's state has been changed from an insulator to a conductor.

Transistors: Amplifier Apply different values of the base bias voltage and the collector bias voltage to the npn transistor and see what happens. With two ammeters (shown as red dots) and a voltmeter to measure V CE we can determine the characteristics of the transistor.

Transistors: Amplifier The amount of base current is determined by the base bias voltage.