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 Vs. A potentiometer POT is used to get a variable voltage supply from 0V to Vs. 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 Vs and the corresponding Voltmeter and Ammeter readings must be noted in a tabular format

Diode I-V Characteristic Curve

 * [[Image:Rectifier vi curve.GIF|frame|200px|Figure : I–V characteristics of a P-N junction diode (not to scale).]]


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

where
 * I is the diode current,
 * IS is a scale factor called the saturation current,
 * VD is the voltage across the diode,
 * VT is the thermal voltage,
 * and n is the emission coefficient, also known as the ideality factor.

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


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

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 VT is approximately 25.7 mV at room temperature (25 °C or 298 K) and is a known constant. It is defined by:


 * $$V_\mathrm{T} = \frac{k T}{q},$$

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.