Semiconductor Devices – The Semiconductor Diode

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In the previous post on the P-N junction, we looked at the formation of a P-N junction. The resulting device is called a diode.

The semiconductor diode is closest to the ideal diode. To study electronic devices made up of the P-N junctions, we look into their VI characteristics.

The VI characteristics of a device is simply a plot of the V vs I curve for the device. For instance, the resistor follows ohm’s law & hence, we obtain a liner VI characteristic for resistance.

The curve below shows the VI characteristics of an ideal & a real diode…

diode_charactersictics

The curve in the first quadrant represents the diode in its forward bias. The diode starts conducting at the voltage Vc called the Cut In Voltage. This voltage is about 0.6 V for Silicon & 0.2 V for Germanium.

In case of the reverse bias, a small current flows. This current, independent of the applied voltage & present only due to the minority charge carries, is called the reverse saturation current.

In reverse bias, the external applied voltage breaks the covalent bonds in the junction region. This leads to the breakdown of the junction at a specific voltage called the breakdown voltage(V b).

Notice that we only consider Vb in the real case. Hence, ideally, there should be almost no current in the reverse bias state, & hence, no breakdown of the junction.

The breakdown simply means that the diode now allows all the current to flow through it. It is just a malfunctioning of the diode & not the damage of the diode. The diode is otherwise supposed to allow current through just one direction & stop all the current in the other.

Further, two kinds of resistances exist in the diode corresponding to the direct & alternating currents respectively.

diode_resistance

The diode can also behave as a capacitor, and even as a variable capacitor. There are two different ways of looking at a diode as a capacitor.

diode_capacitance

The current that flows through the diode depends upon the applied voltage & the temperature(& hence, the voltage due to the temperature). It is given by…

where I0 is the reverse saturation current, e is the electronic charge, k is the Boltzmann constant, V the applied voltage & T is the temperature. The term kT/e is also called the voltage equivalent temperature. Also, η is a constant = 1 (for silicon)
& 2 (for germanium).

All other diodes like the Zener Diode, LED, etc are derived from the basic semiconductor diode with a few changes in their design & functions.

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