Transistor

A useful electrical property results at a boundary where p-type material abuts on n-type material in the same semiconductor crystal. The result is termed a p-n junction diode (or simply junction diode). A junction diode may be thought of as a one-way valve for electricity; it will carry current in one direction much more easily than in the opposite direction. Understanding of the transistor—especially that type termed the bipolar junction transistor or BJT—begins with knowledge of the p-n junction diode. A BJT is, in effect, a back-to-back pair of p-n diodes within a single crystal.

In either p-type or n-type semiconductors, there are two types of charge carriers that carry current: majority carriers and minority carriers. Electrons are the majority carriers in n-type material, due to the extra electrons donated by pentavalent impurity atoms, and holes are the majority carriers in p-type semiconductors. The minority carriers are the relatively few, oppositely charged carriers, electrons in p type and holes in n-type semiconductor, which cannot be eliminated entirely. Heat, ionizing radiation, and unintended impurities in the original intrinsic crystal produce minority carriers. Practical diodes do not behave ideally because minority carriers allow a small reverse current, that is, a trickle of charges leaking backward through a diode, whereas an ideal diode would present a total block to current in that direction. Leakage current occurs in transistors as well as diodes, and these currents can have important consequences for circuit performance.

If voltage is applied across a p-n junction diode with polarity that causes the p region of the diode to be more positive than the n region, the majority carriers in both p and n regions will be pushed toward each other, meeting at the boundary. A diode polarized in this way is said to be forward biased. A forward-biased diode conducts quite well. If the voltage polarity is reversed causing the n-type material to be more positive than the p-type material, the two types of majority carriers will be pulled away from each other. This condition is called reverse bias or back bias. The small current leak through a back-biased diode is the result of minority carriers moving in the opposite direction compared to majority carriers.

There is a very thin volume at the boundary where n-type semiconductor materials interfaces with p-type material, termed the depletion region. In the depletion region electrons tend to fill adjacent holes, depleting the crystal of carriers. When majority carriers from each region are pushed toward each other, hole-electron pairs continually annihilate each other. As each hole is filled by an electron, a new hole and a new electron will be injected into the crystal at the ohmic connections to the crystal (i.e., those places where metal contacts are applied). In this way current can continue to flow through the diode as long as the circuit is energized.

If the reverse-biasing voltage across a diode increases above a critical threshold the diode will suddenly break into heavy conduction when the electric field in the depletion region between the n and p materials is so strong that electrons are torn from their bonding roles. This condition is called Zener breakdown. Usually transistors are operated at voltages low enough so that this type of breakdown doesn't take place. Unless the breakdown current is limited by the external circuitry the transistor or diode may easily destroyed when excess voltages or voltages with the wrong polarity are applied.