Semiconductor Notes

Applying an electrical potential difference causes outermost electrons to detach and move towards the anode (positive terminal).
This creates a flow of negative charge towards the anode.
Electron departure ionizes the atom, leaving a 'hole'.
Neighboring electrons fill these 'holes', effectively moving 'holes' towards the cathode (negative terminal) which simulates positive charge flow.
Electric current consists of negative electrons flowing one way and positive 'holes' flowing the opposite way.

Intrinsic Semiconductor: Pure silicon or germanium with no impurities.
Extrinsic Semiconductor: Impurities added to modify conduction characteristics; this process is called doping.

Doping with Group 5 elements (e.g., antimony, arsenic, phosphorus) adds excess electrons.
Each doping atom introduces one extra electron for current flow.
Excess of negatively charged electrons makes it an n-type semiconductor.

Doping with Group 3 elements (e.g., boron, aluminum, gallium) creates electron shortages, i.e., 'holes'.
Each doping atom results in one 'hole'.
Extra 'holes' act as positive charge carriers, creating a p-type semiconductor.

A crystal with p-type material at one end and n-type at the other forms a p-n junction.

Electrons diffuse from the n-type to the p-type material near the junction.
These electrons combine with holes in the p-type material.
The p-type region gains a net negative charge, while the n-type region becomes positively charged.
The area between these charges is depleted of majority carriers, forming a non-conductive depletion region.
This acts like an insulator separating the p- and n-doped regions.

The separation of charges at the p-n junction creates a potential barrier.
An external voltage must overcome this barrier for conduction.
The barrier forms during manufacturing and depends on the materials used.
Silicon junctions have a higher potential barrier (approximately 0.6V) than germanium junctions (approximately 0.2V).
High currents in silicon achieved with ~0.7V applied voltage.

Negative terminal supplies electrons to the n-type material; electrons diffuse towards the junction.
The positive terminal removes electrons from p-type material, creating holes that diffuse towards the junction.
If the battery voltage exceeds the junction potential (0.6V for Si), electrons and holes combine (annihilate) at the junction.
This allows more carriers to flow towards the junction.
Currents of n-type electrons and p-type holes flow towards the junction.
Recombination at the junction allows current to flow through the diode.
This setup is called forward bias.

Reversing battery polarity attracts majority carriers (electrons and holes) away from the junction.
The positive terminal attracts n-type electrons, and the negative terminal attracts p-type holes.
This widens the non-conducting depletion region, stopping conduction.
This is called reverse bias.

The diode allows electron current to flow in only one direction (against the arrow in the symbol).
Symbol: The arrow points in the direction of conventional current flow.
The cathode (bar) of the diode symbol represents the n-type semiconductor.
The anode (arrow) represents the p-type semiconductor.

In forward bias, current increases slightly with voltage up to ~0.6V for silicon.
Beyond 0.6V, current increases significantly.
Exceeding 0.7V can damage the diode due to high current.
Forward voltage (V+) is a characteristic of the semiconductor: 0.6-0.7V for silicon, 0.2V for germanium, a few volts for LEDs.
Forward current ranges from milliamps to thousands of amperes depending on the diode type.

In reverse bias, only a small leakage current (up to 1 $μA$ for silicon signal diodes) flows.
This current remains low until breakdown.
At breakdown, current increases rapidly, potentially destroying the diode.
Diodes are selected with a higher reverse voltage rating than applied voltage to prevent breakdown.
Silicon diodes have breakdown ratings from 50V to over 800V.
Diodes with lower voltage ratings (a few volts) are used as voltage standards.