In this experiment, the relationship between breakdown voltage (VBD) and temperature was investigated for a signal diode and an unknown diode. The temperature was varied using a Vötsch VT4002 climate chamber while a variable current was supplied from the Keithley 2600 Series Sourcemeter . These parameters were controlled using an existing python script which was further developed to include climate chamber control. Some helpful information on how to get started with python can be found here. The python script used to perform this measurement will be provided near the bottom of this page.
Breakdown in a diode occurs when a sufficiently large voltage is applied in reverse-bias, such that the device begins to conduct in reverse. Breakdown voltage is the minimum applied voltage that this occurs at. Diode breakdown is characterised by an exponential increase in current due to the avalanche effect, where accelerated charge carriers have sufficient energy to form further charge carriers from impacts with bound electrons. In the literature, a decrease in the breakdown voltage refers to its value moving towards 0 V, as this means breakdown can be achieved at lower reverse voltages.
Breakdown is understood to have a positive temperature coefficient, such that the breakdown voltage increases with increasing temperature. (Sze, 2006) This is due to avalanche breakdown being a band-to-band excitation process and the bandgap energy of silicon reducing with increasing temperature.
The figure above depicts the expected breakdown voltage dependency on temperature for varying doping concentration and both abrupt and linearly graded junctions, as per (Sze, 2006)
The signal diode was measured in 10 °C steps between the temperatures of 10 to 50 °C. The Zener diode was measured in 5 °C steps between the temperatures of 10 to 35 °C.
As chamber temperatures tend to fluctuate upon reaching specified temperatures, the chamber remains at the specified temperature for 60 s before measuring. After this, the voltage sweep is applied. The signal diode has a VBD = -85.8 V at 10 °C . It increases slightly at 20°C and then sharply decreases to a value of -83.0V at 50 °C
The Zener diode shows a near constant VBD = -3.85 V , between 10 and 25 °C, with flucuations indistinguishable from noise. At 30 °C, a pronounced decrease to -3.8 V can be seen. It is possible that due to different Zener properties in reverse bias, the cutoff current was set at a too low value to see conclusive results.
The resulting breakdown voltage - temperature relationship matches theoretical predictions; in that it follows a positive temperature coefficient. A signal diode is made for use in forward bias, so information on its breakdown voltage is sparse, however datasheets from the 1N4148/1N4448 signal diode show the Working Peak Reverse Voltage to be 75 V at 25 °C, which is in an appreciable range of what has been measured in this experiment. Further measurement of the Zener diode is needed to verify the abrupt change in breakdown voltage between 25 and 30 °C. It is likely however that the breakdown voltage was simply not reached with this diode as Zener diode typically can withstand much higher reverse voltages before breakdown occurs.
Ideas for improvements and future experiments