Semiconductors are materials whose electrical conductivity can be controlled, making them the foundation of modern electronics. Pure silicon conducts only weakly at room temperature, but adding tiny amounts of impurity atoms can dramatically change how charge moves through it. This process, called doping, lets engineers create regions with extra mobile electrons or extra mobile holes.
The ability to shape these regions is what makes diodes, transistors, solar cells, and integrated circuits possible.
In n-type material, donor atoms add electrons that can move through the crystal, while in p-type material, acceptor atoms create holes that act like positive charge carriers. When p-type and n-type regions touch, electrons and holes diffuse across the boundary and recombine, leaving behind fixed charged ions. This forms a depletion region with an internal electric field that opposes further diffusion.
By applying voltage, engineers can shrink or widen this region, controlling current flow in devices such as PN junction diodes and transistor junctions.
Understanding Engineering: Semiconductors and Doping
The behavior of a semiconductor comes from its energy bands. In a solid, electrons are allowed to occupy certain energy ranges. The valence band holds electrons involved in bonds between atoms.
The conduction band contains electrons free to move through the material. A gap separates these bands. Silicon has a gap small enough that heat can lift some electrons into the conduction band.
Each lifted electron leaves an empty bond state behind. That empty state can move when nearby electrons fill it, so it behaves as a mobile positive carrier. This band picture explains why silicon responds strongly to heat, light, and added atoms.
Dopant atoms fit into the silicon crystal at selected sites. Their extra or missing bonding electron is only weakly attached, so ordinary thermal energy can free a carrier. The amount of dopant matters greatly.
A small change in concentration can change resistance by many times. Engineers choose concentrations carefully because high conductivity is not the only goal. Heavy doping can reduce carrier mobility because moving carriers collide more often with charged impurity atoms.
It can also make a junction behave differently under high electric fields. Device design is therefore a tradeoff between low resistance, switching speed, voltage strength, and unwanted leakage current.
Doping is placed where it is needed through fabrication steps on a silicon wafer. A protective layer covers areas that must stay unchanged. Open areas receive dopant atoms by diffusion at high temperature or by ion implantation, where ions are accelerated into the surface.
Heating afterward repairs crystal damage and lets the dopants settle into useful positions. The depth and concentration profile are important. A shallow region can help a transistor switch quickly.
A deeper region can spread current and reduce resistance. Modern chips contain many carefully aligned doped regions, separated by insulating layers and connected by metal tracks.
A junction does more than allow or block current. Its electric field separates charge, which is useful in solar cells and light sensors. Incoming light can create electron and hole pairs near the junction.
The field pulls the two carriers in opposite directions, producing a measurable current. In transistors, a small voltage changes the charge distribution near a junction or surface. That change controls a much larger current in another path.
When studying these devices, track three ideas at every stage. Identify the carrier that moves, identify the direction of the electric field, and distinguish mobile carriers from fixed ions in the crystal.
Temperature deserves attention too. Higher temperature creates more carriers but can lower mobility, so real devices do not always behave like simple room temperature examples.
Key Facts
- Intrinsic silicon has equal electron and hole concentrations: n = p = ni.
- n-type doping adds donor atoms, increasing electron concentration so n >> p.
- p-type doping adds acceptor atoms, increasing hole concentration so p >> n.
- Mass action law at thermal equilibrium: np = ni^2.
- A PN junction forms a depletion region with fixed ions and an internal electric field.
- Diode current is modeled by I = Is(e^(qV/kT) - 1), where forward bias increases current strongly.
Vocabulary
- Semiconductor
- A material with electrical conductivity between that of a conductor and an insulator, whose conductivity can be controlled by doping, temperature, or light.
- Doping
- The process of adding small amounts of impurity atoms to a semiconductor to change its charge carrier concentration.
- Electron
- A negatively charged mobile particle that can carry current through the conduction band of a semiconductor.
- Hole
- A mobile absence of an electron in the valence band that behaves like a positive charge carrier.
- Depletion region
- The region near a PN junction where mobile electrons and holes have recombined, leaving fixed ions and an internal electric field.
Common Mistakes to Avoid
- Thinking n-type material is negatively charged overall, which is wrong because donor ions and mobile electrons balance so the bulk material is usually neutral.
- Thinking p-type material contains no electrons, which is wrong because it still has electrons, but holes are the majority carriers.
- Forgetting that the depletion region contains fixed ions rather than many mobile carriers, which is wrong because electrons and holes have mostly recombined near the junction.
- Reversing forward and reverse bias, which is wrong because forward bias reduces the junction barrier and allows current, while reverse bias increases the barrier and mostly blocks current.
Practice Questions
- 1 An intrinsic silicon sample has ni = 1.0 x 10^10 cm^-3 at room temperature. If it is doped n-type so that n = 1.0 x 10^16 cm^-3, use np = ni^2 to find the hole concentration p.
- 2 A diode has Is = 1.0 x 10^-12 A and is forward biased at V = 0.60 V. Using kT/q = 0.026 V and I = Is(e^(V/0.026) - 1), estimate the diode current.
- 3 Explain why a PN junction conducts much more easily under forward bias than under reverse bias, using the ideas of the depletion region and internal electric field.