Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicable.

Semiconductor devices such as AlGaN/GaN high-electron-mobility transistors (HEMTs) have attracted a great deal of attention for applications reaching through W-band frequencies. Such devices feature a high two-dimensional electron gas (2DEG) structure, high saturation drift velocity, high breakdown voltage, and good high-temperature resistance for use in high-frequency amplifier circuits. Millimeter-wave HEMTs have become a focal point for international research in recent years.1

Based on this background, a 2 × 50 μm AlGaN/GaN HEMT with 0.1-μm gate length and 2 μm between the source and drain is 2 μm was designed and simulated with the aid of the Technology Computer Aided Design (TCAD) computer-aided-engineering (CAE) software from Silvaco. The device was taped out using actual semiconductor processing technology, and the DC and signal characteristics were measured. A novel symbolically defined device (SDD) large-signal model was created which obtains excellent DC and S-parameter results. With this model, device large-signal characteristics can be known through source and load-pull simulations. The positive results for the model indicate that it is an effective tool for designing power amplifiers at W-band frequencies.

Cross-sectional HEMT structure

Figure 1 shows the cross-sectional structure of a double-finger AlGaN/GaN HEMT. The gate-length is 0.1 μm and the gate cap is 0.35 μm long. The silicon-nitride (SiN) passivation layer helps reduce surface defects and isolation boost electrode isolation. A single-finger gate width of 50 μm was used based on the principle that the phase difference be no more than π/16 at W-band frequencies. The distance between the source and drain is 2 μm and the lateral structure of the HEMT is symmetric.2 This contributes to lower channel series resistance and improved high-frequency characteristics.

The device heterostructure consists of a 1.5-μm-thick GaN buffer layer and a 23-nm-thick AlGaN barrier layer with 24% Al content. To enhance 2DEG channel characteristics, the AlN layer is inserted between the barrier layer and the 1-nm-thick buffer layer. All the vertical layers are unintentionally doped. The 2DEG mobility and sheet carrier concentration are 2000 cm2/V·s and 1.1 × 1013 cm-2, respectively. A silicon-carbide (SiC) substrate was chosen for its excellent thermal conductivity.

The 2DEG of AlGaN/GaN HEMTs is determined by polarization, which is described by Eq. 1:

Eq. 1

where Psp represents spontaneous polarization for the GaN materials and Ppi is the piezoelectric polarization between AlGaN and GaN materials which is given by Eq. 2:

Eq. 2

where α, C, and E represent lattice constants, elastic constants, and piezoelectric constants, respectively; their values can be obtained according to the material statement so that polarization can be calculated.3

The channel, regarded as a thin layer, is formed in the buffer layer approaching to surface of the barrier layer because of polarization effect. Empirical fitting function can be used to describe mobility models of the GaN channel.4

Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicable.