A basic discussion of bipolar transistors. Germanium type is specifically discussed. Silicon behaves in the same basic manner.
In order to use transistors effectively, a knowledge of the fundamental principles of transistor operation is essential. In transistors, as in many other solid-state devices, the charge carriers move through a solid material, such as germanium or silicon.
The germanium is in the form of a crystal in wich the atoms align themselves in a regular pattern known as a lattice structure. Each germanium atom consists of a nucleus and 32 electrons. the nucleus and 28 of the electrons form an inert core of net charge of +4 units of the charge of an electron. these cores represent most of the mass of the solid but do not contribute directly to the electrical and chemical properties of the element. the remaining 4 electrons, wich constitute the bonds between the atoms, are the valence electrons, and are responsible for the chemical and electrical properties. Two valence electrons, one from each of two neighboring atoms, by virtue of their relative motion cause a binding force to exist between the two atoms. This binding force and the repulsive force of the positive nucleii are in equilibrium, resulting in the specific arrangement of the atoms within the crystal. The electron-pair bonds are called covalent bonds. In the absence of external disturbances, the covalent bonds are stable and the motion of the valence electrons is restricted to their specific bonds. Although there are enourmous numbers of electrons in the crystal, these electrons are bound either in the cores or in the covalent bonds, and are not free to move from one point to another in the crystal under the influence of an electric field. the pure germanium crystal behaves as an insulator with a high dielectric constant.
Suppose that by some means an electron is injected into a perfect germanium crystal. Since this excess electron is situated in an environment of perfect periodicity of electric potential, wave mechanics predicts that the electron will not be affected by the fluctuating electric field inside the crystal. This implies that the electron should either remain at rest or move with a constant velocity through the crystal. In practice, however, the presence of thermal energy in the crystal, wich we can never avoid unless the crystal is kept at absolute zero, causes lattice vibration. This vibration excites the electrons into motion.
The mechanism of thermal excitation may be illustrated by adopting the concept of the phonon. In much the same manner as light energy can be considered to be composed by discrete quanta, photons, the energy of the lattice vibration may be considered as comprised of particles of quantized energy, known as the phonons. we may consider phonons to be uncharged elastic masses moving with random thermal energies. Successive collisions between the phonons and the electron cause the electron to describe a random zigzag motion. the random zigzag motion does not cause a net displacement in any one direction, and therefore does not contribute to the conduction of electricity in the solid. A net displacement of electrons in a solid may happen as a result of drift or diffusion. when an electric field is applied to the solid, the random motion of the excess electron is modified to show a net drift in the direction of the electric field. The resultant motion is the superimposition of the random zigzag motion and the motion due to the electric field.
Up to this point we have been discussing the properties of a perfecly structured crystal. A small number of electrons injected in to the crystal does not disturb the crystal structure nor does it appreciably disturb the electric fiels in the crystal. However, as far as transistors are concerned, the perfect crystal is but an idealized model. The operation of transistors (and many other solid state devices) actually depends on controlled imperfections in the crystal. imperfections in the crystal provide the electric charge carriers and contribute to the control of the flow of these carriers. The three main causes of imperfections are radiation energy, chemical impurities, and disordered atomic arrangements.
The term "imperfections" also refers to the energy imperfections resulting from the disturbance of the normal energy state of the crystal. Imperfections caused by radiation include changes of the electrical qualities of germanium when exposed to light. Light is composed of photons, each a quantum of energy, E=hv where h is planck's constant and v is the frequency of the incident light. when light falls on a crystal, a quantum may be absorbed by the crystal and delivered to onne of the covalent bonds. (heres where it gets interesting) Providid the energy is great enough, (for example, the frequency of the incident light is high enough), an electron may be ejected from the bond. The ejected electron is free to wander in the crystal, like the injected excess electron discussed earlier, and it contributes to the conduction of electricity in the same manner. the empty space in the covalent bond left behind by the ejected electron is called a hole.
It is quite easy for an electron from a neighboring bond to move into the hole, thereby creating a new hole in its place. In this manner, the hole seems to move through the crystal. In the absence of an electric field, the movement is random, similar to that of the excess electron. under the influence of an electric field, however, the hole behaves as an excess electron with a positive electrostatic charge. in fact, hole may be treated as a positive excess electron. Electrons and holes are referred to as the negative and positive carriers, respectively, and are solely responsible for the conduction of electricity in the crystal. the generation of electron-hole pairs by light is the basic principle of operation of phototransistors and photodiodes. In ordinary transistor and crystal diode operation the crystal is shielded to eliminate the effects of light. Thermal energy also causes crystal imperfections, and assumes an important role in generation of electron-hole pairs. The average thermal energy of a phonon at room temperature is too small to cause imperfections in the crystal. However, the distribution of thermal energy among individual phonons follows the Maxwell-Boltzmann distribution function and a small percentage of phonons posess energies high enough to break a covalent bond. These high energy phonons generate electron-hole pairs in the same manner as incident light energy.
The conduction of electricity in these electron-hole pairs is called intrinsic conduction. Once generated a carrier will remain in the crystal for a finite lifetime, before recombining with a second carrier of opposite charge to form a bond again. the average lifetime of the electron-hole pairs in germanium at room temperature is about 0.001 second. the generation and recombination of electron-hole pairs is a continuous process. when a germanium specimen reaches any set temperature, the generation and recombination of E-H pairs reach a dynamic equilibrium, resulting in a definite concentration of carriers in the specimen. (this is what causes "thermal runaway" in germanium power transistors.) The resistivity of pure germanium at room temperatures is about 60 ohm-cm as compared to an insulator such as mica, having a resistivity of 9*10E+15 ohms-cm, while a conductor , such as copper, has a resistivity of about 1.7 *10E-6 ohm-cm. This is why germanium and similar solids are called semiconductors.
Carriers of either kind may be provided in an otherwise perfect germanium crystal by introduction of a minute amount of a foreign element (chemical impurity) whose position in the periodic table is usually either in column III or V. In either case, the impurity atom replaces the germanium atom in the perfect lattice structure. Because the impurity atom has one more or less valence electron than germanium (or silicon, etc..), the result is the existence of one more electron or hole in the structure. If the impurity is antimony or arsenic, wich each have 5 valence electrons, then the carrier will be an electron, loosely attatched to the antimony or arsenic core. the germanium is then known as N-Type, because it has excess electrons, wich are negatively charged. If indium or boron, each having 3 valence electrons, were inserted into the lattice, then there would be a hole in the lattice structure, because the indium or boron, having replaced a germanium atom surrounded with 4 germanium atoms, is short one electron to form covalent bonds with. since holes represent positive chargen this kind of germanium is called P-Type. It should be kept in mind that even though these impure germanium specimens are short of or stuffed with electrons, there is no net charge, since the neutral impurities were added to the also neutral germanium. (Neutral in this case means that there is no net ionic charge.) there are just more electrons or holes wandering around.
The energy band theory of solids should be briefly touched upon: The energy state of an isolated atom is measured by the potential and kinetic energy posessed by each electron in the atom. each electron can posess only cetain exact amounts of energy, and exist only in certain discrete energy levels. In an isolated atom, there are a finite number of energy levels, and only two electrons may exist in any one energy level at the same time. An electron wich occupies a low energy level is strongly attached to the atom, and an electron in a high energy level is feebly attatched to the atom. Between the energy levels are forbidden enegry gaps in wich no electron may remain, but wich it must pass if it should gsain sufficient energy to go from one level to a higher one, or if it should give up enough energy to pass from a given level to a lower one. in absence of disturbances the electrons in an atom fill all energy levels from the ground up. the atom is said to be in the normal state. an atom wich has one or more of it's atoms raised to a higher energy level is said to be in an excited state. when two identical atoms (Identical atoms have identical energy levels) are brought into close proximity, their energy levels shift with respect to one another. In the case of a solid, vast numbers of atoms are packed close together, so that interaction between the atoms causes splitting and merging of energy levels into energy bands, where each band is composed of enormous but finite numbers of energy levels. these bands are separated by forbidden gaps, just as with the single atom. In the case of solid crystaline semiconductor materials under discussion, the valence electrons completely fill the second-highest energy band. this band is called the valence band or filled band. excess electrons, whose existance is a consequence of imperfections in the crystal, are in the highest band, called the conduction band or the empty band. The energy bands below the valence band are filled with the electrons in the inert core, wich do not contribute to the electrical characteristics of the material.
To generate an electron-hole pair in a perfect crystal it is necessary to supply to one electron in the filled band sufficient energy to excite it into the empty band, thus depositing a carrier, the electron, into the empty band, and creating a second carrier, a hole , into the filled band. the energy required for the generation of the electron-hole pair in this manner is equal to the height of the forbidden energy gap, that is, the energy required to break a covalent bond. recombination occurs when an electron drops back into the filled band. Energy in the form of a phonon or photon is released when recombination takes place. note that all atoms with four valence electrons do not behave alike: diamond (crystalized carbon) has 4 valence electrons, yet is a good insulator. germanium, remember is a semiconductor. this difference in conduction is caused by the fact that the distance between the filled band and the empty band is far in diamond, whereas in germanium, the bands are actually overlapping at room temperature.
In N-TYPE germanium, the excess electrons occupy a narrow band closely below the empty band. In P-TYPE germanium, theholes occupy a region closely above the filled band. As it was pointed out, there is no net charge in p-type or n-type germanium, although there is a predominance of negative carriers (electrons) in the former and positive carriers (holes) in the latter. if a piece of p-type germanium and a piece of n-type germanium are brought together by intimate contact to form a junction, while still preserving continuity of the lattice structure at the junction, regions wich exhibit net charges will be created. The junctions are usually formed by introducing donor impurities (wich supply excess electrons) into one side of the crystal, and introducing acceptor impurities (atoms wich readily accept an excess electron) so as to form 2 regions in a single crystal.
The excess electrons in the n-type region tend to diffuse across the junction into the p-type region. the holes tend to spread in the oppostie direction, from the p-type region to the n-type region. The system soon reaches a state of equilibrium when the established potential caused by the migration of the holes and the electrons into opposite sides of the juncion prevents any further carrier migration. The potential gradient formed by this "potential barrier" affects the charges nearer the actual junction more than those some distance from it, and pushes them away, both polarities. The P-N junction allows elecrical current to flow in one direction only, This current is the flow of electrons & holes across the junction. to describe the operation of a diode, I will discuss the conduction of the holes only, since the electrons' conduction is exactly the same, except for the sign reversal. At a given temperature, a number of holes in the p-type region gain enough energy to climb over the potential barrier to combine with electrons in the n-type region. consider this "forward current". At the same time, thermal agitation generates electron-hole pairs in the n-type region.
Some of the holes so generated will quickly recombine with excess electrons in the vicinity. Others will wander toward the junction and slide downhill over the potential barrier into the p-type region. this part of the hole current will be considered "reverse current". At any given temperature, the two currents balance each other and there is no net current flow across the junction. Now if a reverse bias (voltage) is applied, the potential barrier is raised by the amount of bias and relatively few holes gain enough energy to climb over the barrier, and forward current is reduced. however, reverse current, wich represents holes sliding down the potential hill, is not affected by the height of the barrier, and thus remains constant. This explains the saturation of the reverse current of a junction diode. If forward bias is applied, the potential barrier is lowered accordingly. Now more holes will have the energy to go over the barrier, resulting in a larger forward current, while the reverse current remains constant as explained before. Now I can adress actual transistor action, having covered the underlying physical concepts.
A germanium (or silicon, or GaAs, or whatever) crystal with p-type and n-type regions may be used to amplify the power of an elecric signal. Let us consider the example of the N-P-N junction transistor, wich consists of a germanuin crystal with two n-type regions separated by a thin layer of p-type region. large-area metal contacts serve to connect each region to external circuit elements. The contacts to the N, P, and N layers are refferred to as the EMITTER, BASE, and COLLECTOR, e, b, and c, superrespectively. the two junctions are reffered to as the base-emitter junction, and the collector-base junction. Now consider the case where the transistor is connected to amplify a small-signal alternating-current input (like the signal from your tape deck). In a typical amplifier circuit, the emitter is grounded through a source of bias Vee, and a positive voltage is applied through the load to the collector, so that the collector junction is reverse-biased to saturation. (remember we are discussing electron and hole action both, so the polarities seem reversed to some) The input signal is applied to the base.
Let us take the simplified case where volume and surface recombination of holes and electrons in the base region is negligible. In the absence of an input signal the current flow in the transistor may be taken in three parts. First is the current Ico wich is the collector-base current with the emitter terminal disconnected, wich is the saturated reverse current of a junction diode, as explained before. Ico is highly temperature dependent and requires serious consideration in pracitcal circuit design. Second there is a current I1, composed of electrons flowing from the emitter through the base to the collector. This current is the forward electron current across the emitter-base junction, and is mostly a function of barrier height at the e-b junction. third, there is a current I2 composed of holes flowing from the base to the emitter. the magnitude of this flow is also controlled by the height of the e-b potential barrier. In a practical transistor, the geometry of the junction is such that the ratio of I1/I2 is large. usually 60 to 200 in common transistors. When a signal is applied to the base, the signal voltage modulates the potential barrier of the e-b junction. if a positive signal is applied to the base, the voltage makes the base region more positive wich effectively lowers the emitter-base juncion potential barrier and allows more electrons to flow over the potential hump and go down to the collector, resulting in an increase in current I1. At the same time the lowering of the emitter junction potential barrier increases the hole current I2 by the same proportion. thus for small amounts of input signal, the signal flowing through the load varies by the same proportion.
I hope that explains it.
go back