01 Semiconductors Flashcards
Define “Conductor”
Allows the flow of electrical current
Define “Free Electrons”
Electrons in the highest (valence shell)
How many electrons in a conductor valence shell
1 valence electron
How many electrons in an insulator valence shell
8 valence electrons
How many valence electrons in a semi-conductor
4
Define Valence Saturation
When the valence shell is full of electrons
Describe the effect temperature has on creation of holes and free electrons
As the temperature increases, an electron gets bumped to a higher orbit, leading to a free electron and a hole in the previous shell.
Define Recombination
When an electron merges with a hole.
What is the Lifetime of a free electron
The time between the creation and disappearance of a free electron.
Define Extrinsic Semi-Conductor
Pure semiconductor, only contains atoms of that material.
Has an equal number of hole and free electron charge carriers.
Define Extrinsic Semi-Conductor
Doped with other atoms.
Describe the atomic make up and resultant majority carrier resulting from n-type doping.
Doped with pentavalent atom.
Majority carriers are free electrons.
Describe the atomic makeup and resulting majority carrier resulting from p-type doping.
Doped with trivalent atoms.
Holes are majority carriers.
Define a pn junction
A p-type semiconductor interfaced with an n-type.
What is the net charge of a pn-junction
Net neutral charge
Define Depletion Layer
Free electrons cross into the p region where they have a short lifetime and recombine with holes, creating dipole pair atoms. The dipoles form at the interface of the pn-junction, creating a region void of free carriers.
As dipole pairs form at the dpletion layer, they each have an electric field which repulses further free carriers from entering the p region and creating more dipoles. Eventually this reaches equilibrium and there is a stable depletion layer
Define Barrier Potential
Voltage potential across the depletion layer, created by the electric field between the ions.
The voltage requierd to push free carriers across the depletion layer.
Define forward bias
Positive source connected to the p-type material, negative source connected to the n-type material.
The source voltage pushed free electrons towards the depletion layer, against the electric field.
Current flows easily through a forward-biased diode.
Define reverse bias
The negative terminal is connected to the p-type material, the positive terminal is connected to the n-type material.
Voltage applied to a reverse bias is called “reverse voltage”
This pulls the atoms away from the depletion layer, widening it, creating a larger barrier potential. The depleiton layer stops widening when the barrier potential equals the applied reverse voltage.
Does a current still flow after a reverse bias has reached equilibrium?
Yes, through minority carrer current and surface-leakage current.
Describe break down voltage?
When large reverse voltage is applied, the minority carriers are accelerated and knock electrons out of their valence shells, creating more free electons. This cascades until a large current is able to pass through the depletion layer.
Describe the Conduction Band
When thermal energy moves electrons into the conduction band and their holes are left in the valence band. The conduction band is where free electrons are.
Describe the difference between n and p-type energy bands.
n-type: majority carriers are free electrons in the conduction band, minority carriers are holes in the valence band.
p=type: majority carriers are holds in the valence band, minority carriers are free electrons in the valence band.
Describe the effect of temperature on the barrier potential.
As the junction temperature increases, it creates more free electrons and holes in the droped regions, which diffuse into the depletion layer, shrinking it and reducing the barrier potential.
Describe transient current
When a reverse voltage is applied and the depletion layer widens, it takes time to adjust to its new width, during which time transient current flows through the circuit.
Describe RC time constant
the time before the transient current drops to 0 after a reverse voltage is removed.
