Semiconductor Doping · aiwiki.page

Semiconductor doping is the deliberate introduction of selected impurity atoms into a semiconductor to control its electrical properties, principally the concentrations of mobile electrons and holes. The impurity species are called dopants. By controlling their identity, concentration, and spatial distribution, manufacturers create regions with different conductivity types and form the junctions required for electronic and optoelectronic devices. A semiconductor whose carrier population is governed primarily by dopants is called an extrinsic semiconductor, in contrast to an intrinsic material governed by thermally generated electron–hole pairs. (ocw.mit.edu)

Physical basis

In a crystalline semiconductor, electronic states form energy bands. The valence band and conduction band are separated by a band gap. Appropriate impurity atoms introduce additional electronic states within this gap, allowing mobile carriers to be generated with less energy than is needed to excite an electron across the full gap. Shallow donor states lie near the conduction-band edge; shallow acceptor states lie near the valence-band edge. (ocw.mit.edu)

Silicon illustrates the mechanism particularly clearly. Each silicon atom has four valence electrons and forms covalent bonds with neighboring atoms. A dopant that replaces a silicon atom changes the local electron count and bonding environment. The resulting electrical behavior depends on the dopant’s electronic states, not simply on its presence as a chemical impurity. (ocw.mit.edu)

Two distinctions are important:

Electrical activation concerns whether an incorporated dopant occupies a configuration in which it can function as a donor or acceptor.
Dopant ionization concerns whether an electrically active dopant has released or accepted an electron under the prevailing conditions.

An impurity can therefore be chemically present without contributing the expected mobile carrier. Post-implantation processing commonly aims to place dopants on suitable substitutional sites and repair lattice damage. (ocw.mit.edu)

Donors, acceptors, and conductivity types

Donor doping produces n-type material, in which electrons are the majority carriers. In silicon, group-15 elements such as phosphorus and arsenic have five valence electrons. Four participate in bonding, while the additional electron can be released into conducting states. The ionized donor remains as a positively charged, comparatively immobile impurity site. (ocw.mit.edu)

Acceptor doping produces p-type material, in which holes are the majority carriers. Boron, a group-13 element, has three valence electrons. When incorporated substitutionally into silicon, it can accept a bonding electron, leaving a mobile hole in the valence-band system. The ionized acceptor is negatively charged. (ocw.mit.edu)

The labels n-type and p-type identify the predominant mobile carrier, not the net electric charge of the material. Away from junctions and surfaces, mobile carriers and ionized impurities generally balance to give an approximately neutral bulk region. Both carrier types remain present: the less abundant species is the minority carrier. (ocw.mit.edu)

Carrier concentration and compensation

Let $n$ and $p$ denote electron and hole concentrations, and let $N_D^+$ and $N_A^-$ denote ionized donor and acceptor concentrations. In a neutral bulk region,

n+N_A^-=p+N_D^+.

Under thermal equilibrium and nondegenerate carrier statistics, the mass-action relation is

np=n_i^2,

where $n_i$ is the intrinsic carrier concentration at the specified temperature. Together, these relations determine the equilibrium carrier populations. (ocw.mit.edu)

With approximately complete ionization and donor concentration much greater than both acceptor concentration and $n_i$ ,

n\approx N_D-N_A, \qquad p\approx\frac{n_i^2}{n}.

For acceptor-dominated material, the corresponding approximation is $p\approx N_A-N_D$ . These expressions concern electrically active dopants, rather than every impurity atom detected chemically. (ocw.mit.edu)

When donors and acceptors coexist, their effects partly cancel; this is called compensation. Adding acceptors to n-type material can reduce its electron concentration and, if the acceptors predominate, convert it to p-type material. Compensation does not remove the impurity atoms: a nearly compensated sample can contain many charged scattering centers despite having a relatively small net carrier concentration. (ocw.mit.edu)

Electrical transport

In the low-field regime, the conductivity is

\sigma=q(n\mu_n+p\mu_p),

where $q$ is the elementary charge and $\mu_n$ and $\mu_p$ are electron and hole mobilities. Mobility describes the carrier drift response to an applied electric field. (ocw.mit.edu)

Doping increases the available carrier population, but it also introduces impurity scattering, which can reduce mobility. Consequently, conductivity does not generally increase in direct proportion to dopant concentration. Temperature, lattice vibrations, and implantation-induced defects also influence transport. Electrical resistivity measurements alone cannot uniquely distinguish carrier concentration from mobility; measurements such as the Hall effect provide additional information. (ocw.mit.edu)

Fabrication methods

Doping processes must control both the amount of dopant and its three-dimensional distribution. Photolithography and masking define selected regions, while implantation, diffusion, or growth processes establish depth profiles. (ocw.mit.edu)

Thermal diffusion

In diffusion doping, dopants introduced at or near a semiconductor surface migrate into the crystal during heating. Their concentration profile depends on the diffusion coefficient, temperature, duration, and source boundary conditions. An ideal constant-surface-concentration process produces a complementary-error-function profile; a limited-source process has a different distribution. Diffusion is also important after other doping methods because later thermal steps can redistribute previously introduced impurities. (ocw.mit.edu)

Ion implantation

Ion implantation accelerates dopant ions into the semiconductor. Implant dose controls the number introduced per unit area, while ion species, energy, and incidence conditions influence penetration depth and profile shape. Implantation can therefore control dose and depth separately, although the resulting distribution has a finite spread rather than a sharply defined stopping plane. (ocw.mit.edu)

Collisions damage the lattice and can produce an amorphous layer at sufficiently high doses. Subsequent annealing repairs damage and activates dopants, but also risks unwanted redistribution. Process design must balance activation and crystal recovery against the need to preserve shallow or abrupt profiles. (ocw.mit.edu)

Doping during crystal growth

Dopants can also be incorporated while a semiconductor layer grows. In epitaxial growth, controlled incorporation enables doped layers and junction structures without first implanting a completed film. Dopant segregation and unintended transfer from a substrate—called autodoping—can nevertheless broaden profiles or introduce an unwanted background concentration. (repository.lib.ncsu.edu)

An especially confined distribution is a delta-doped layer, in which dopants occupy a very thin region. Phosphorus delta layers in silicon have been fabricated using atomic-scale surface patterning followed by crystalline overgrowth; maintaining confinement requires control of segregation, diffusion, and defects during encapsulation. (nist.gov)

Device applications and historical development

Adjacent p-type and n-type regions form a p–n junction. Carrier redistribution leaves a depletion region containing ionized dopants and establishes an internal electric field. Spatially patterned doping thus creates active device structures, rather than merely changing a material’s overall conductivity. (ocw.mit.edu)

In metal–oxide–semiconductor field-effect transistors, doped source and drain regions connect to a channel controlled by the gate. Substrate doping and counterdoping help establish the required device polarity and structure. Combining complementary transistor types underlies CMOS circuits and their use in integrated circuits. Doped semiconductor junctions also form essential structures in solar cells, light-emitting diodes, and photosensors. (ocw.mit.edu)

The development of semiconductor electronics made controlled impurity incorporation a central manufacturing capability. The first point-contact transistor, demonstrated at Bell Laboratories in 1947, used germanium and stimulated extensive research into solid-state devices. Subsequent junction and integrated-circuit technologies required reproducible control of conductivity type and dopant profiles. (educationalgames.nobelprize.org)

Characterization and scaling limits

Doping characterization must distinguish chemical concentration, electrical activation, mobile-carrier concentration, and spatial distribution. These quantities are related but not interchangeable. (nist.gov)

Secondary-ion mass spectrometry (SIMS) measures chemical concentration as a function of depth, but does not by itself establish electrical activation.
Hall-effect measurements determine carrier density and mobility within an appropriate transport model.
Capacitance-based profiling relates electrical response to dopant concentration.
Atom-probe and scanning-probe methods investigate nanoscale distributions and individual dopant arrangements. (nist.gov)

At small device dimensions, specifying an average concentration becomes insufficient: the number and exact positions of individual dopant atoms can affect device behavior. Random dopant distributions produce variations between nominally identical devices, including differences in transistor threshold voltage. Atomically precise placement is therefore investigated both to reduce variability and to construct dopant-based quantum-computing structures. Even after precise placement, subsequent processing must preserve the intended positions and electrical activity. (nist.gov)

References

Semiconductors — MIT OpenCourseWareocw.mit.edu
Archived Lecture Notes #3: Bonding in Metals, Semiconductors and Insulators — Band Structureocw.mit.edu
012 Microelectronic Devices and Circuits, Lecture 2ocw.mit.edu
012 Microelectronic Devices and Circuits, Lecture 3ocw.mit.edu
012 Microelectronic Devices and Circuits, Lecture 4ocw.mit.edu
155J/6.152J Lecture 2: Device and Process Overviewocw.mit.edu
Lecture Notes — Microelectronic Devices and Circuitsocw.mit.edu
Micro/Nano Processing Technology, Lecture 6ocw.mit.edu
Physics of Microfabrication: Front End Processing, Lecture 13 Transcriptocw.mit.edu
Physics of Microfabrication: Front End Processing, Lecture 17 Transcriptocw.mit.edu
Formation of N+P Junctions Using In-situ Phosphorus Doped Selective Si1-xGex Alloys for CMOS Technology Nodes Beyond 50nmrepository.lib.ncsu.edu
The Transistor — Historyeducationalgames.nobelprize.org