Aluminium makes gallium arsenide PIN diodes even better

November 27, 2017 | 11:10
PIN diodes make for excellent switches and attenuators in RF circuits. Acting as charge-controlled variable RF resistors, they outperform FETs in most cases, exhibiting low insertion loss, high isolation, besides excellent power handling and linearity. The impedance of a PIN diode can go from nearly open-circuit to nearly short-circuit.

 
AlGaAs PIN diodes from MACOM
MA4AGBL912 AlGaAs PIN diode from MACOM (Source: MACOM)

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 aluminium

A 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.
 
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