So far in our discussion on diodes, Si was the semiconductor material chosen as the base material. But what difference does it make if we use diodes whose base material is Germanium or Gallium Arsenide? In this section we shall try to find the answer to this question by comparing their characteristics and their response time.
The graph shown below compares the characteristics of Silicon, Germanium and Gallium Arsenide diodes.
Si, Ge and GaAs diodes are compared based on the following points.
- Forward characteristics
- Reverse characteristics
Forward characteristics : (Refer to the graph shown above). It is quite clear from the graph that the general shape of the curve for all three type of semiconductor diodes is same. However the threshold voltage of all the three diodes is different. The threshold voltage for Ge, Si, and GaAs diodes is 0.3, 0.7 and 1.2 V respectively. Thus if a Si diode is connected in a circuit in forward bias configuration, the voltage drop across the diode would be 0.7 V (assuming correct operation of diode). If Ge diode is used, then the voltage drop would be about 0.3 V and for GaAs diodes the voltage drop would be about 1.2 V.
Reverse characteristics : (Refer to the graph shown above). The characteristics curve of Ge, Si and GaAs diodes looks quite similar. However they are not same. The reverse saturation current of GaAs is least as compared to Si and Ge. The reverse saturation current of Si is more than that of Ge. Typical values of reverse saturation current are 1 pA, 10 pA and 1µA for GaAs, Si and Ge diodes respectively.
It is also clear from the above graph that the reverse breakdown voltage of all the three types of diodes are also different. The reverse breakdown voltage of GaAs is highest while that of Ge is lowest. Typical value of reverse breakdown voltage for Ge is less than 100 V, with maximum around 400 V. The reverse breakdown voltage of Si and GaAs diodes lies between 50 V and 1 kV.