Indium Phosphide (InP) HEMTs

Taking advantage of the fact that the 2DEG offers exceptional high carrier mobilities compared to bulk material, a typical InP-HEMT has the following layer structure:

• Silicon δ-doping layer: Highly doped layer with only few atomic layers thickness. Located between the Schottky-Barrier and Spacer layer. Acting as a donor of charge carriers, i.e. it provides electrons to the channel. Since electrons tend to occupy the lowest allowed energy state, they drain into the potential well and form the confined 2DEG in the channel. A high δ-doping level provides high electron densities in the channel and therefore results in high transconductances, current densities and
  cut-off frequencies.
• The Spacer layer assures the separation between the electrons and their positively charged Si-donors, reducing impurity scattering and hence enhancing electron mobility.
• Cap: A highly n-doped cap layer helps minimize the contact resistance of the source and drain contacts. The cap also provides protection from oxidation for the sensitive InAlAs layer beneath.
• Schottky-Barrier: The Schottky-Barrier layer, in contrast to the Ohmic source and drain contacts, provides a so-called Schottky contact between gate-metal and semiconductor material with a rectifying characteristic. It prevents large currents from flowing trough the gate and limits tunneling to the channel.

Channel properties have a major impact on the device performance. This is why InGaAs, with its excellent electron mobility properties at room and cryogenic temperatures, is the material of choice.

The special T-shape of the gate helps minimize the gate resistance by enlarging the cross section while maintaining a small foot-print and thus a small gate length.