Why is a germanium diode inferior to a silicon one in high t

Why is a germanium diode inferior to a silicon one in high temperature applications?

Why is a silicon diode inferior to a germanium one in low temperature applications?

Which of the two semiconductors (silicon and germanium) is more suitable for high voltage applications and why?

Solution

The Silicon diodes have higher peak inverse voltage (PIV) and current ratings and wider    temperature ranges than germanium diode. Silicon can be operated at temperatures up to about 75-80C for germanium.

Germanium is more temperature dependent than silicon because it is observed that reverse saturation current Io doubles for every 10 degree Celsius rise for Ge and every 6 degree Celsius rise for Si.

There are several reasons Silicon has become the preferred semiconductor in the present, over Germanium.

First reason is that Si forms on its surface very easily a thin layer of SiO2 which is a very good insulator and which technologically can be very easily processed. This layer of oxide is very useful to form the gates of MOSFET transistors (it is situated between the gate and the channel). Also this layer is very useful when one wants to form the junctions in a transistor because it acts as a masking layer that prevents the diffusion of the dopants in the regions it protects. In contrast Ge does not form this oxide layer on its surface so easily and the technology to obtain the germanium devices is more complicated.

Second reason is that Si has a larger band-gap (0.7 eV) than Ge (0.2 eV) and because of this, the phenomenon of thermal pair generation is smaller in Si than in Ge. This means that at the same temperature the noise of the Si devices is smaller than the noise of Ge devices and also that the reverse current of a p-n junction is smaller for Si than for Ge.

Third reason is that Si is widely encountered in nature in the form of sand, from which it is extracted by reduction with carbon. In contrast Ge is not so easily found in nature, and when found is encountered only in chemical compounds that it forms because of its high reactivity.

However, Ge has one major advantage over Si. Ge has higher electron and hole mobility and because of this Ge devices can function up to a higher frequency than Si devices.