Class 12 Physics Semiconductor Electronics Characteristics of a p-n junction diode

Characteristics of a p-n junction diode

There are two types of characteristics – Forward characteristics and Reverse characteristics Forward characteristics

The graphical relationship between the forward bias voltage applied and the forward current through the p-n junction is called forward characteristics

• We use a milliammeter in forward bias as the expected current is large
• The current increases very slowly, almost negligibly, till the voltage across the diode crosses certain value
• After this voltage, the current increases exponentially, even for a very small increase in the diode bias voltage. This voltage is called the threshold voltage or cut-in voltage which is 0.2V for germanium diode and 0.7 for silicon diode

Reverse characteristics

The graphical relationship between the reverse bias voltage applied and the reverse current through the p-n junction is called reverse characteristics

• In reverse bias, we use micro ammeter, as the current is very small
• The current almost remains constant even if voltage is increased and it is called reverse saturation current

ProblemThe number of silicon atoms per m3 is 5 * 1028.  This is doped simultaneously with 5 * 1022 atoms per m3 of Arsenic and 5 * 1020 per m3 atoms of Indium.  Calculate the number of electrons and holes. Given that ni = 1.5 * 1016 m-3 . Is the material n-type or p-type ?

Solution  : We know that ni= ne nh

Given ni = 1.5 * 1016 m-3

Then, ne = 5 * 1022  - 5 * 1020 = (5 – 0.05) * 1022 = 4.95 * 1022 m-3

Hence, nh = ni2 / ne = (1.5 * 1016)2 / 4.95 * 1022 = 4.54 * 109 m-3

Comparing ne and nh , we find ne > nh and so the material is n-type semiconductor

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