High Electron Mobility Transistors (HEMT)

The High Electron Mobility Transistor (HEMT) is a heterostructure field-effect transistor (FET).

Its principle is based on a heterojunction which consists of at least two different semiconducting materials brought into intimate contact. Because of the different band gaps and their relative alignment to each other, band discontinuities occur at the interface between the two semiconducting materials.

Diagram of the band structures of two InAlAs and InGaAs at the equilibrium.

These discontinuities are referred to as the conduction and valence band offsets ΔEc and ΔEv. By choosing proper materials and compositions thereof, the conduction band offset can form a triangular shaped potential well confining electrons in the horizontal direction. Within the well the electrons can only move in a two-dimensional plane parallel to the heterointerface and are therefore referred to as a two-dimentional electron gas (2DEG).

To determine the exact shape of the conduction and valence bands, the Schrödinger and Poisson equations must be solved self-consistently.

Semiconductors in contact at the equilibrium. A 2DEG is formed at the interface.

Indium Phosphide (InP) HEMT

Taking advantage of the fact that the 2DEG offers exceptional high carrier mobilities compared to bulk material, a typical InPHEMT 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 arriers, 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.

-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.

-The Schottky-Barrier layer, in contrast to the Ohmic source and drain contacts, provides a so-called Shottky 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.

Scanning Electron Micrograph of the cross-section of one of our HEMTs.

Applications

InP-HEMTs show excellent noise and gain performances at microwave frequencies. At cryogenic temperatures, these properties improve further. This predestines InP-HEMTs for receiver systems in and, which have the most stringent requirements for low noise and high sensitivity. Together with (Institut de Radio Astronomie Millimetrique), the Space Observatory, and (European Space Agency), the radio astronomy deep space communications IRAM Herschel ESA IFH/ETH has contributed to several projects involving cryogenically (~10K) cooled two- and three-stage low noise amplifiers (LNA). ETH "in-house" developed and processed HEMTs are being deployed in such missions.

Gallium Nitride (GaN) HEMT

The second species of HEMTs in our group is based on GaN/AlGaN heterojunctions. Instead of using InP substrates the substrates are based on Sapphire (Al2O3 ) or Silicon Carbide (SiC). These semiconductors are both wide bandgap materials (3.4 eV and 3.3 eV compared to 1.3 eV for InP) and therefore have high electric breakdown fields, which enables applications at high supply voltages. Furthermore, this allows the material to withstand high operating temperatures and provides improved radiation hardness.

To achieve high currents and high frequency operation, high carrier mobilities and high saturation velocities are desirable. Typically, wide band gap semiconductors attain only relatively low mobilities but high saturation velocity values. Compared to the InP-HEMT structure the main differences are:

1) No doping in the AlGaN barrier layer is required. Built-in polarisation fields, due to spontaneous polarization and piezo-polarization help induce the 2DEG.

2) Higher 2DEG concentrations are achievable (above 10¹³/cm²) due to the very large conduction band discontinuity.

Applications

The direct bandgap of GaN and its alloys enables the material to be used for both optical and electronic applications. At 300 Kelvin the bandgap of GaN is 3.44 eV, which corresponds to a wavelength in the near ultra violet region of the optical spectrum. This enables the fabrication of high-power optical devices as LEDs and Lasers.

With respect to electronics, GaN is an excellent option for high-power/high-temperature microwave applications because of its high electric breakdown field and high electron saturation velocity (~1.5 x 10^7 cm/s). The former is a result of the wide bandgap (3.44 eV at room temperature) and enables the application of high supply voltages, which is one of the two requirements for high-power device performance. In addition, the wide bandgap allows the material to withstand high operating temperatures

(300°C - 500°C) enabling applications in many commercial areas not covered by other materials.

Freddy Vallenilla, EES SECC 2