intrinsic and extrinsic (ntyme ptype

Section A4: Intrinsic & Extrinsic Semiconductors There is one more thing we need to talk about before getting into current flow. Although currents may be induced in pure, or intrinsic, semiconductor crystal due to the movement of free charges (the electron-hole pairs, remember?), these currents are too small to be of real use. This limitation is due to the relatively small number of electrons that obtain the required energy to jump the bandgap (typically on the order of 1010 electrons/cm3 at room temperature for silicon). Now, while this may seem to be a very large number, you must keep in mind that silicon has on the order of 5x1022 atoms/cm3, so this effect is essentially nonexistent. What is important here is that, although the intrinsic concentration, ni, is a function of bandgap, temperature, and physical constants through

ni = CT 3 e where C T Eg k

− E g / kT

,

is a material constant that depends on the effective density of states in the conduction and valence bands (C=5.4x1031 for silicon) is temperature in degrees Kelvin (oK) is the bandgap (Eg=1.12eV for silicon) is Boltzmann’s constant (k=8.62x10-5eV/oK)

For intrinisic silicon, n=p=ni•1.5x1010cm-3 at room temperature (•300oK). Or, in words… the number of free electrons is equal to the number of free holes and is also equal to the intrinsic carrier concentration (since the only free carriers in an intrinsic material are due to EHP). Note that pn=ni2 (we’ll use this in a couple of minutes). Finally, although the EHP generation is temperature dependent, free carrier generation through external excitation alone is a recipe for disaster!!!

To create practical, useful semiconductor devices, some really clever folks came up with a method of modifying the intrinsic crystal lattice. This process, called doping, involves introducing a specific number of atoms with a different valence number than the host semiconductor into the crystalline lattice. Pretty cool, huh? When the semiconductor crystal is doped such that its intrinsic nature is modified, it is termed extrinsic. In an extrinsic semiconductor, the equilibrium number of free electrons and holes are no longer equal since a tool other than EHP generation is used to create free carriers. Depending on the amount and type of impurity (or impurities)

introduced, many electrical and optical properties of the semiconductor material may be modified or controlled to optimize desired behaviors or characteristics.

We’re going to concentrate on silicon here and the creation of the two basic types of extrinsic (or doped) silicon, n-type and p-type - as illustrated in Figure 3.10 of your text (with a modified version of each type in the appropriate discussion below). Keep in mind that silicon is valence IV (four)… n-type Silicon When a silicon atom in the crystal lattice is replaced with a valence V (five) atom (such as phosphorous, arsenic or antimony) the four valence electrons of silicon are satisfied and there is an electron “left over.” The left over electron requires very little thermal energy to release it from its parent atom and easily becomes a free carrier. Introducing group five impurities into the silicon lattice is known as n-type doping since the concentration of free electrons (remember the symbol for electrons is n) is greater than that of the intrinsic material. The elements used for n-type doping are known as donors since they “donate” electrons to the silicon host and their concentration in the host material is designated by ND. When the donor electrons move from their parent atoms, they leave behind a positively charged ion. These ions are fixed in the semiconductor lattice so they cannot contribute to current but, have no fear, they play an important role when we start combining materials to create devices! p-type Silicon Not surprisingly, there is a similar discussion for the case where a silicon atom in the host lattice is replaced with a valence III (three) atom, most commonly boron. For this case, as illustrated below, there is one too few electrons in the impurity atom to satisfy the four valence electrons of silicon. This results in an unsatisfied covalent bond, or looking at it another way, a free hole. Introducing group

three atoms into the silicon lattice is known as p-type doping since the concentration of free holes (remember the symbol for holes is p) is greater than that of the intrinsic material. The elements used for p-type doping are known as acceptors since they “accept” any free electrons that may roam by to satisfy the fourth covalent bond. NA designates the concentration of acceptors in the host material. When the acceptor atom “picks up its fourth,” it becomes a negatively charged ion. Analogous to the discussion of the donor ions above, these are immobile charged atoms whose effect we will see later.

Carrier Concentrations and Excess Carriers It is important to note that, even when a semiconductor material is doped with impurities, it remains electrically neutral. This means that overall (macroscopically), the number of positive charges is equal to the number of negative charges. Conventional doping levels do not change the chemical or mechanical properties of a semiconductor, but do make a world of difference in the electrical properties!

For our purposes, we are going to assume that at room temperature all impurity atoms are ionized – all donors have contributed a free electron and all acceptors have contributed a free hole (captured an electron). Remember that, for an intrinsic material, all free carriers are due to electron-hole pair generation and that the number of free electrons is always equal to the number of free holes (and is equal to the intrinsic carrier concentration ni).

Well… this doesn’t hold true for extrinsic semiconductors. When one type of impurity has been intentionally introduced into the host material, it is seen to dominate and we speak of majority carriers and minority carriers.

Specifically, ¾ for n-type semiconductors: ND>> NA, ND >> ni, electrons (n) are the majority carriers and holes (p) are the minority carriers. ¾ for p-type semiconductors: NA >> ND, NA >> ni, holes (p) are the majority carriers and electrons (n) are the minority carriers.

Under thermal equilibrium conditions, the pn product remains constant, or p0n0=ni2, where the subscript indicates equilibrium and ni is still the intrinsic carrier concentration. For practical doping levels at room temperature, a valid approximation is that the number of free majority carriers is equal to the doping level and the minority carriers can be found from the above equation.

Specifically, for

n-type: nn0•ND and pn0•ni2/ND p-type: pp0•NA and np0•ni2/NA Again, keep in mind that the material – n-type or p-type - remains electrically neutral. Although free charges are moving in the material, creating ionized impurities in extrinsic material, the free carriers are neutralized by the bound charges associated with the ionized impurities.

OK, take a deep breath. The next thing we’re going to do is talk about how charges move in the material, then take the two types of extrinsic materials (n-type and p-type), put them together and actually create something!