Semiconductor Diodes in Static Relays:
Semiconductor Diodes in Static Relays – The pn junction has rectifying characteristics as shown in Fig. (10.1). If a source of emf is connected to the pn junction in the polarity shown in Fig. (10.1a), then the spare holes in the p-type region are drawn easily to the negative pole and the electrons are drawn from the n-type to the source positive.
If the source potential is reversed then the holes are repelled from the positive pole and likewise the electrons from the negative pole. Consequently, little current flows until such time as the electric stress is so high that a process similar to an electric discharge occurs.
The approximate static behavior of either the silicon or germanium pn-junction diode is given by
where
Is = reverse saturation current, amperes
V = applied voltage in volts
q = charge on electron (1.59 x 10-19 C)
K = Boltzmann’s constant (1.37 X 10-23 W-S/°K)
T= Absolute temperature, degrees Kelvin
At the normal room temperature of about 300°K
In the forward direction, when V becomes larger than a few tenths of a volt, the forward current becomes
Correspondingly, in the reverse direction the current expression becomes
In the reverse direction the silicon diode has a very small reverse current which increases slowly as V is made more negative until a critical or breakdown voltage is reached. The voltage at which this breakdown occurs is controllable in the manufacture of diode, and a variety of diodes are commercially available with breakdown voltages from about 2 V to 2,000 V.
In a rectifier circuit the reverse breakdown is undesirable, and the diode is chosen to have a higher breakdown voltage than any voltage likely to appear across it. In other applications the breakdown is actually used, and the diode is chosen for a specific voltage. Diodes so used are variously called zener, avalanche, regulator, reference or just breakdown diodes.
The practical Semiconductor Diodes in Static Relays fall short of the ideal in three respects.
- They do not provide a perfect short circuit in the forward (Region II) direction.
- They do not provide a perfect open circuit in the reverse (Region I) direction.
- They possess inertial effects, i.e. the voltage or current cannot change instantaneously from one value to another; for instance, certain of the semiconductor devices are relatively slow to switch from on to off (Region II to Region I).