Diode characteristics:Static Resistance, Dynamic Resistance, Average AC Resistance, Transition Capacitance, Diffusion Capacitance, Storage Time, Transition Time, Reverse Recovery Time
DC Resistance or Static Resistance
Static or DC resistance of a PN junction diode
characterizes the diode's resistive nature when a DC source is associated with
it. In the event that an external DC voltage is applied to the circuit in which
the semiconductor diode is a piece of it, results in a Q-point or operating
point on the PN junction diode characteristic curve that does not modify with
time. The static resistance at the knee of the curve and beneath of it will be
much more noteworthy than the resistance values of the vertical rise segment of
the characteristic curve. Minimal is the current passing through a diode most
extreme is the level of DC resistance.
RDC = VDC/ IDC
AC or Dynamic Resistance
Dynamic resistance is obtained from Shockley's Diode
Equation. It characterizes the diode resistive nature when an AC source which relies
upon on the DC polarization of the PN intersection diode is associated with it.
On the off chance that an external sinusoidal signal is given to the circuit
comprising of a diode, the modifying input will move the immediate Q – point
somewhat from the current position in the qualities and hence it characterizes
a definite change in voltage and current. At the point when no external ac sign
is connected, the operating point will be the quiescent point that is controlled
by the connected DC signal levels. The AC resistance of diode is expanded by
bringing down the quiescent point of operation. So, it is proportional to slope
of voltage – current of the PN junction diode.
rd = ΔVd / ΔId
Average AC Resistance
On the off chance that the input signal is sufficiently
enough to deliver a substantial swing, then resistance identified with the
diode for this region is known as AC average resistance. It is dictated by the
straight line that is drawn connecting the crossing point of the base and most
extreme values of externally applied input voltage.
R avg = ( Δvd/ Δid ) point to point
Transistion Capacitance
Transistion capacitance can likewise be termed as
depletion layer capacitance or space charge capacitance. It is basically seen
in an reverse bias configuration where P type and N type areas has lower
resistances and depletion layer acting as a dielectric medium. This sort of
capacitance is because of the varieties in the external applied voltage where
the fixed charges get differ at the edges of the layer of the depletion region.
It relies on the dielectric constant and width of depletion layer. On the off
chance that the depletion layer width builds the transition capacitance
diminishes.
CT = εs/ w = √{[qεs/ 2(ϕi – Vd)] [NaNd/ (Na + Nd)]}
Diffusion Capacitance
Diffusion or Storage capacitance primarily observed in
forward biased configuration, it is the capacitance created by the
transportation of charge bearer’s b/w the two terminals of a diode that is from
anode to cathode in the forward biased configuration of PN junction diode. In
the event that the electric current is permitted to pass through the
semiconductor gadget, there will be some charge made over the device sooner or
later of time. In the event that if the connected external voltage and current
changes to a differing quality, there will be an alternate measure of charge
made in the transit.
The degree of transiting charge made to the differential
change in voltage will be diffusion capacitance. In the event that the level of
current is increased then diffusion capacitance levels automatically
increments. Increased levels in current
will bring about decreased levels of related resistance furthermore the time constant
that is imperative in fast applications. The diffusion capacitance worth is
much more prominent than the value of transition capacitance and it is
specifically corresponding to the value of direct current.
Cdiff = dQ/dV = [dI(V)/dV]ΓF
Storage Time
PN junction diode
acts as perfect conductor in forward bias mode and acts as insulator in reverse
bias mode. Amid the exchanging time from forward bias to reverse bias the
current stream switches and stays steady at the same level. This time term over
which the current inverts and keeps up steady level is known as storage time
(Ts).
The time taken by the electrons to move from P type once
again to N type & holes to move from N type over to P type is known as
storage time. This value can be controlled by the geometry of the PN junction
of the diode. Amid this storage time the diode acts as a short circuit.
Transition Time
The time for the current to reduce to reverse leakage
current value after it stays at a steady level is known as a transition time.
It is meant as the transition time value is controlled by the geometry of the
PN intersection and centralization of doping levels of the P type and N type.
Reverse Recovery Time
The aggregate of storage time &transition time is
termed as reverse recovery time. That is the time taken by the diode to raise
the connected current signal to 10% of the steady state value from reverse
leakage current. The reverse recovery time esteem for PN junction diode is
generally of the request of μs. Its value for a generally utilized diode
rectifier 1n4148 is normally 4 ns and for general purpose rectifier diode it is
2 micro seconds. The quick switching speeds can be accomplished by the high
estimation of reverse leakage current & high forward voltage drops. It is
meant by Trr.
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