Characteristics:
The current that flows through a diode is given by the equation:
where ID - diode current. (Positive for forward and negative for reverse)
IS - constant reverse saturation current
V - applied voltage. (Positive for forward and negative for reverse)
- factor dependent upon the nature of semiconductor. (1 for
germanium and 2 for silicon)
VT - volt equivalent of temperature which is given by T/11600. (T is
Temperature in Kelvin)
When a forward voltage is applied at the terminals of a diode, the diode begins to conduct. During conduction, the cut in or threshold voltage exceeds the applied forward voltage. The threshold voltage for a germanium diode is 0.3V and for silicon diode is 0.7V. The forward current (miliampere range) initially increases linearly and then increases exponentially for high currents.
When a a reverse voltage is applied, a reverse saturation current flows through the diode. The diode continues to be in the non conducting state until the reverse voltage drops below the zener voltage. As the reverse voltage approximates the peak inverse voltage a breakdown called as the ’Avalanche breakdown’ occurs. During the breakdown, the minority charge carriers ionize the stable atoms which are followed by a chain ionization to generate a large number of free charge carriers. Thus the diode becomes short circuited and gets damaged.
Note: When diodes are connected in series their equivalent peak inverse voltage is increased while in parallel connection the current carrying capacity is increased.
As the temperature increases, the electron pairs generated thermally also increases thereby increasing the conductivity in both directions. The reverse saturation current also increases with the increase in temperature. The change is 11% per °C for a germanium diode and 8% per °C for a silicon diode. On the other hand the diode current is doubled for every 10°C rise. With increase in voltage, the firing voltage in forward characteristics is reduced while peak reverse voltage is increased.
Note: The peak inverse voltage can be reduced by increasing the doping level. The same concept is used to design zener diodes.
Diode resistances: The resistance associated with the diode can be evaluated in three fashions and the three types of resistances associate with a diode accordingly.
· DC or Static resistance: It is the ratio of diode voltage to the diode current at any point of its characteristic curves. It is defined at a point on the characteristic curves.
· AC or dynamic resistance: It is the ratio of change in diode voltage to the change in diode current. It is defined at a point on the characteristic curves over a tangent.
· Average AC resistance: It is the ratio of change in diode voltage to the change in diode current over a straight line joining two limits of operation.
Diode capacitances: The diode exhibits two types of capacitances transition capacitance and diffusion capacitance.
. Transition capacitance: The capacitance which appears between positive ion layer in n-region and negative ion layer in p-region.
· Diffusion capacitance: This capacitance originates due to diffusion of charge carriers in the opposite regions.
The transition capacitance is very small as compared to the diffusion capacitance.
In reverse bias transition, the capacitance is the dominant and is given by:
where CT - transition capacitance
A - diode cross sectional area
W - depletion region width
In forward bias, the diffusion capacitance is the dominant and is given by:
where CD - diffusion capacitance
dQ - change in charge stored in depletion region
V - change in applied voltage
- time interval for change in voltage
g - diode conductance
r - diode resistance
The diffusion capacitance at low frequencies is given by the formula:
The diffusion capacitance at high frequencies is inversely proportional to the frequency and is given by the formula:
Note: The variation of diffusion capacitance with applied voltage is used in the design of varactor.
Diode switching time: In AC applications, when diode is instantaneously switched from a conduction state to a non conduction state it needs some time to return to non conduction state and behaves short circuited for a little time period in reverse direction. This occurs because when the diode biasing is suddenly changed, the majority charge carriers migrated to other region is the minority charge carriers in the region. Specifically, holes are the minority carriers migrated from p-type to n-type in reverse bias. . These holes require some time to return back to state of non conduction which is called as the ‘Reverse recovery time’. Reverse recovery time is the sum of storage time and the transition time.
· Storage time: The time period for which diode remains in conduction state even in the reverse direction.
· Transition time: The time elapsed in returning back to state of non conduction.
It is desirable those diodes has minimum switching or reverse recovery time trr. Switching time of diodes is of the order of few nanoseconds to 1 microsecond. Now fast switching diodes with switching time up to few picoseconds are also available.
Identification:
A diode is marked with a bar which indicates the cathode terminal of a diode which is as shown in the figure below:
Note: Various small signal diodes like IN4148, 0A90 and rectifying diodes like IN4001-4007,IN5400-5408,BY125-127 are available with different current, reverse saturation current and peak inverse voltage rating.
source: engineersgarage.com
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