Junction-type microelectronic device

Microelectronic devices on silicon chips are typically made from layers of n-type and p-type silicon. Films of silica act like the plastic sheath on copper cable, since silica is insulating. A layer of p-type silicon back to back with a layer of n-type, called a p-n junction, allows a current moving across the junction to flow in one direction but not the reverse. This one-way behaviour is the fundamental characteristic of a device called a diode. Early diodes in electronics were made from metal plates sealed inside evacuated glass tubes, which could be seen glowing in the innards of old radio sets. Diodes made from doped silicon can be much smaller and more robust since they are made from solid materials, they are components of solid-state electronics. 

When the direction of electron flow is into n-type material or out of p-type material, it increases the majority carrier concentration of the semiconductor. This increases the conductivity of the semiconductor near the junction and is described as ohmic because the junction acts only as a resistor. Such junctions are desired for contacts to microelectronic devices. Unfortunately, it is often impossible to find a metal of sufficiently high or low work function to arrange the electron flow in the correct direction to obtain an ohmic contact, especially to wide-gap semiconductors and organic electronic materials. 

Transistors, which are extremely important semiconducting devices in today s microelectronic circuitry, are capable of two primary types of function. First, they can perform the same operation as their vacuum-tnbe precursor, the triode—that is, they can amplify an electrical signal. In addition, they serve as switching devices in computers for the processing and storage of information. The two major types are the junction (or bimodal) transistor and the metid-oxide-semiconductor field-effect transistor (abbreviated as MOSFET). 

The bipolar transistor, which began the microelectronics revolution, consists of two diodes joined back to back by a thin common semiconductor layer (the base). Thus, bipolar transistors are referred to as p-n-p or n-p-n depending upon whether the common base layer is n or p type. Consider a typical n-p-n bipolar junction transistor, shown schematically in Figure 3.24 along with the electrical coimections for its operation. When turned on the emitter junction is forward biased. This injects electrons into the base and holes into the emitter. The holes injected into the emitter recombine there and are of no value to operation of the device. The electrons emitted into the base may either recombine, contributing to base current, or transit the base. If the base is thin, most of the electrons pass through it without recombining, reach... 

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