Diode fundamentals.

If a junction formed between a sample of p type and one of n type semiconductor, this combination possesses the properties of a rectifier (diode).
At this junction, one side has a high concentration of holes whereas the other side has high concentration of electronics due to this movement of charge carriers takes and this phenomenon is called diffusion.
The potential barrier (depletion region) is negative on p side and positive on n side which restricts the recombination of electrons with holes.
Within this narrow space charge layer there are no mobile carriers (no holes and no electrons).
If we want charge carriers to cross the junction, then an external voltage of appropriate polarity has to be applied in order to overcome the opposition of the barrier potential known as cut-in voltage.
In semiconductor both diffusion and conduction currents exist simultaneously.
The reverse saturation current depends on temperature and independent of magnitude of reverse bias.
Doping inversely proportional to the square root of the potential width.
Depletion region opposes flow of majority carrier and assist the flow of minority carriers across the junction (drift currents).
Charge density is depends on amount of doping.
Pn junction device s bipolar device.
Diode characteristics:
Volt equivalent of temperature is VT = T/11600 = 26 mV.
At high currents, the diode behaves more like a resistor than diode, and the current increases linearly rather than exponentially with applied voltage.
The electron in germanium 3 times faster than silicon, for high frequency applications germanium is suitable.
The value of reverse saturation current is about double in magnitude for every 10o C rise in temperature.
The generation and recombination process occur simultaneously. But generation rate is higher than the rate of recombination in extrinsic semiconductor.
In intrinsic semiconductor both rates same.
Static resistance R of a diode is defined as the ratio V/I of the voltage to the current. It is not useful parameter because of its wide variation due to V and I.
Dynamic resistance of a diode is defined as the slope of the V-I characteristics of the diode.
Dynamic resistance inversely proportional to the forward current.
Capacitive effect at junction due to uncovered charge with applied voltage is called transition or space charge capacitance, CT.
The CT is not constant, but depends upon the magnitude of reverse voltage.
The rate of change of injected charge with voltage called diffusion or storage capacitance CD in forward bias.
CD is much larger than CT in forward bias.
The CD is proportional to the current.