Although PIN diodes have low capacitance, it's still too high for millimetre wave applications. Unfortunately, there are few options to improve the situation. A PIN diode consists of three layers, the P-type anode, a non-doped intrinsic layer ‘I’, and the N-type cathode. The junction area and the thickness of the ‘I’ layer determine the device capacitance and series resistance. Reducing the junction area reduces the capacitance but at the cost of increasing the series resistance.
Make PIN diodes from aluminiumA solution is changing the material used for constructing the PIN diodes. But which material? Initially such devices were made from silicon (Si). Replacing Si by GaAs (gallium arsenide) is a step in the right direction, and now MACOM has added aluminium to the mix to create AlGaAs PIN diodes.
The aluminium functions as a p-type dopant in the anode layer of the diode, effectively increasing the junction’s band gap. Because this increases the amount of free charge carriers in the ‘I’ layer when forward biasing the diode, a lower series resistance is the result. Consequently, reducing the junction area lowers the diode’s capacitance while maintaining a series resistance similar to GaAs devices.