There are two main errors that one should be aware of when using p-n junction se
ID: 2084633 • Letter: T
Question
There are two main errors that one should be aware of when using p-n junction sensors. One is the self-heat of the junction, the other is introduced by variations in the current through the diode. Consider a germanium diode with a known forward voltage drop of 0.35 V at a current of 5 mA and an ambient temperature of 25 degree C. The self-heat of the sensor is given in the device data sheet as 1.3 mW/degree C. a. Calculate the sensitivity of the sensor and the expected voltage reading at 50 degree C neglecting effects of self-heat. b. Suppose that the current in the diode varies by plusminus 10% due to variations in the power supply. Calculate the error in the measured temperature due to this variation as a percentage. Evaluate it at the values given above (V_f = 0.35 V, T = 25 degree C, I = 5 mA). c. Calculate the error in the measured temperature due to self-heat at a junction current of 5 mA and an ambient temperature of 50 degree C. d. Discuss these errors, their relative importance, and means of reducing them.Explanation / Answer
a) The current equation of a PN junction diode can be written as;
I = I0(exp(V/nVT)-1)
where I0 = reverse saturation current; then the sensitivity of the sensor can be defined as the ratio of I/I0
Sensitivity = exp(0.35/25m)-1 = 37.
As we know that with incraesing T, minority carriers will be increased, so I0 and I will be increased for given V.
The dv/dt = -2.5 mv/0C at room temperature;
then we can write that;
V(500C) - V(250C) = (T2 - T1)*(dv/dt)
V(500C) = 0.2875V
b) Current changes by 10%.
We can write that;
V(T2) - V(T1) = (T2 - T1)*(dv/dt)
dt/(T2 - T1) = dv/(V(T2) - V(T1) ) = dv/(25ln(I2/I1)) = dv/25
Therefore the % of changes in temperature reading will be 100/25 = 4%.
c)The general current equation of PN junction diode can be written as;
I = I0(exp(V/nVT)-1)
Here T = 273+50 = 323 K
then VT = 323/11600 = 28mV
Hence the error in voltage for measuring the temperature will be 3mV.
d) Error in PN junction diode is mainly occurs due to the leakage current flowing through the junction. Due to heating effect the leakage current will be increased. To reduce the leakage current we need to reduce temperature, reduce doping concentration and increased the physical size of the junction.