Semiconductor Electronics/Diode/Diode Maths

Experimentation method

Above shown is a apparatus to measure the characteristics of a diode. This apparatus has got a regulated voltage supply V_s. A potentiometer POT is used to get a variable voltage supply from 0V to V_s. A Voltmeter is connected parallel to the diode to measure voltage drop across the diode, an Ammeter is connected in series with the diode to measure the current flowing via the diode.

The voltage must be varied from 0V to V_s and the corresponding Voltmeter and Ammeter readings must be noted in a tabular format

Diode I-V Characteristic Curve

Figure : I–V characteristics of a P-N junction diode (not to scale).

$I=I_{\mathrm {S} }\left(e^{V_{\mathrm {D} }/(nV_{\mathrm {T} })}-1\right),\,$

where

I is the diode current,

I_S is a scale factor called the saturation current,

V_D is the voltage across the diode,

V_T is the thermal voltage,

and n is the emission coefficient, also known as the ideality factor.

The saturation current I_S is typically very small, so the diode current is often approximated as

$I=I_{\mathrm {S} }e^{V_{\mathrm {D} }/(nV_{\mathrm {T} })}$

The emission coefficient n varies from about 1 to 2 depending on the fabrication process and semiconductor material and in many cases is assumed to be approximately equal to 1 (thus omitted). The thermal voltage V_T is approximately 25.7 mV at room temperature (25 °C or 298 K) and is a known constant. It is defined by:

$V_{\mathrm {T} }={\frac {kT}{q}},$

where

q is the electrical charge of the electron (the elementary charge), often called e

k is Boltzmann's constant,

T is the absolute temperature of the p-n junction.