next up previous contents
Next: Comparison of HEMT A and HEMT B Up: 6.1 Low Noise HEMT Previous: 6.1 Low Noise HEMT

6.1.1 DC Characteristics

The investigations will be based on three manufactured SH­PHEMTs without a double recess, i. e. LDR = 0. The shape of the T­gates of the devices are not know quantitatively, hence a simplified cross section is used in the simulations as shown in  Figure 6.6.

Figure 6.6 Schematic cross section of a low noise SH­HEMT with homogeneously doped supply layer. The parameters investigated in this section are dGC, LG, LR, and er.

The epitaxial structure common to all devices from bottom to top consists of a GaAs buffer on a S. I. GaAs substrate followed by a 12 nm In0.2Ga0.8As channel layer, a 3 nm undoped Al0.23Ga0.77As spacer layer, a 15 nm Al0.23Ga0.77As layer with an active doping of about 3.5 1018cm-3 and a 7 nm undoped Al0.23Ga0.77As Schottky barrier layer. The top layer is formed by a highly doped GaAs cap to facilitate the formation of the source and drain ohmic contacts. The different geometries of the three investigated SH­PHEMTs A, B, and C are given in  Table 6.1.

Table 6.1 Geometry parameters of the simulated low noise HEMTs
LG [nm]
LR [nm]
dGC [nm]
25 (23.3)

In Section 5.3.1 a qualitative discussion of the most important parameters and their impact on the transconductance was given. Based on these considerations only a small difference of gm max must be expected between devices A and B as they differ only in LG. Only for device C a slightly higher value for gm max is expected due to a significantly shorter LR which reduces the series resistance RS. The gate length variations are not likely to have a large impact on gm max but will have other consequences. The short LG of HEMTs A and C will lead to a small CGS and, thus, to a higher fT compared to HEMT B. However, further consequences of the parameter variations in  Table6.1 cannot be easily estimated quantitatively. Such consequences are the increase of the output conductance g0 (and the decrease of the voltage gain gm/g0) that is expected with a decrease of LG or LR. A small LR will cause a small RS, but unfortunately, a large CGD undesirable for high fT and high power-gain cutoff frequency fmax. Numerical simulations are requested for the calculation of these effects.

All DC and RF measurements were performed on HEMTs with a gate width of 4 x 40 = 160 µm. For the simulation another fitting procedure had to be performed as the epitaxial structure is substantially different from the structure of HEMTref.

First, the simulation of HEMT A was fitted to the measurements as described in Section 5.3.2. A HD model was used only in the channel, whereas DD was applied on all other semiconductor layers. For the low field mobility µ in the InGaAs channel, the value obtained from Hall measurements of an equivalent layer structure was adopted.

The saturation velocity assumed for GaAs is unrealistically low. This was deliberately chosen to compensate the overestimation of the buffer current for the reason described in Section 5.2. Other main fitting parameters are dGC, the concentration of active dopant atoms and a constant interface charge density between the passivation and the semiconductor [77]. The data given in  Table 6.2 and Table 6.3  lead to the best simultaneous fit to the DC measurements of threshold voltage VT, drain current ID and transconductance gm and are well within their respective ranges of uncertainty.

Table 6.2 Parameters used for the simulation of the low noise HEMTs
Effective tunnel length
Insulator permittivity
Interface charge density (insulator/semiconductor)
7 nm
2.3*1012 cm-2
Table 6.3 Transport parameters and doping
Al0.23Ga0.77As supply
GaAs cap
µ [cm2/Vs]
vsat [107cm/s]
tw [ps]
ND [1018cm-3]

Interestingly enough, significantly better transport properties (vsat and b) in the channel had to be used for the simulation compared to HEMTref. This assumption goes along with a very high activation (80 %) of dopands assumed in the supply. As shown in the fitting procedure of Section 5.3.2 the uncertainty of the velocity in the channel is very high, but the differences in the simulation are large enough to assume that the use of a GaAs barrier under the channel improved the transport properties compared to HEMTref.

next up previous contents
Next: Comparison of HEMT A and HEMT B Up: 6.1 Low Noise HEMT Previous: 6.1 Low Noise HEMT

Helmut Brech