Lecture -2: Diode

Temperature Effects:

Temperature plays an important role in determining the characteristic of diodes. As temperature increases, the turn-on voltage, vON, decreases. Alternatively, a decrease in temperature results in an increase in vON. This is illustrated in fig. 2, where VON varies linearly with temperature which is evidenced by the evenly spaced curves for increasing temperature in 25 °C increments.

The temperature relationship is described by equation

VON(TNew ) VON(Troom) = kT(TNew T room)            (E-10)

Fig. 2 - Dependence of iD on temperature versus vD for real diode (kT = -2.0 mV /°C)

where,

           Troom= room temperature, or 25°C.
            TNew= new temperature of diode in °C.
VON(Troom ) = diode voltage at room temperature.
 VON (TNew) = diode voltage at new temperature.
                kT = temperature coefficient in V/°C.

Although kT varies with changing operating parameters, standard engineering practice permits approximation as a constant. Values of kT for the various types of diodes at room temperature are given as follows:

kT= -2.5 mV/°C for germanium diodes
kT = -2.0 mV/°C for silicon diodes

The reverse saturation current, IO also depends on temperature. At room temperature, it increases approximately 16% per °C for silicon and 10% per °C for germanium diodes. In other words, IO approximately doubles for every 5 °C increase in temperature for silicon, and for every 7 °C for germanium. The expression for the reverse saturation current as a function of temperature can be approximated as

     (E-11)

where Ki= 0.15/°C ( for silicon) and T1 and T2 are two arbitrary temperatures.

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