6.1.1 DC Characteristics

The investigations will be based on three manufactured SH­PHEMTs without a double recess, i. e. L_DR = 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.
 

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 In_0.2Ga_0.8As channel layer, a 3 nm undoped Al_0.23Ga_0.77As spacer layer, a 15 nm Al_0.23Ga_0.77As layer with an active doping of about 3.5 10¹⁸cm^-3 and a 7 nm undoped Al_0.23Ga_0.77As 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.
 
 

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 g_{m max} must be expected between devices A and B as they differ only in L_G. Only for device C a slightly higher value for g_{m max} is expected due to a significantly shorter L_R which reduces the series resistance R_S. The gate length variations are not likely to have a large impact on g_{m max} but will have other consequences. The short L_G of HEMTs A and C will lead to a small C_GS and, thus, to a higher f_T 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 g_m/g₀) that is expected with a decrease of L_G or L_R. A small L_R will cause a small R_S, but unfortunately, a large C_GD undesirable for high f_T and high power-gain cutoff frequency f_max. 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 HEMT_ref.

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 d_GC, 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 V_T, drain current I_D and transconductance g_m and are well within their respective ranges of uncertainty.
 

Interestingly enough, significantly better transport properties (v_sat and b) in the channel had to be used for the simulation compared to HEMT_ref. 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 HEMT_ref.
 

• 6.1.1.1 Comparison of HEMT A and HEMT B
• 6.1.1.2 Comparison of HEMT A and HEMT C
• 6.1.2 RF Characteristics
• 6.1.2.1 Reduction of the Gate Length
• 6.1.2.2 Contributions to the Gate Capacitance
• 6.1.2.3 Reduction of the Recess Length
• 6.2 Power HEMT for 0.9/1.9 GHz and 40 GHz Applications
• 6.2.1 DC Characteristics
• 6.2.2 Dependence of Recess Geometry on the E-Field Distribution
• 6.2.3 RF Characteristics
• 6.2.3.1 Reduction of the Gate Length
• 6.2.3.2 Contributions to the Gate Capacitance
• 6.2.3.3 Dependence of C_G on Passivation Thickness
• 6.2.3.4 Dependence of C_G on T-gate Cross Section
• 6.3 Millimeter Wave HEMTs
• 6.3.1 DC Characteristics
• 6.3.1.1 Dependence on the Gate-to-Channel Separation
• 6.3.1.2 Dependence on the Gate Length
• 6.3.2 RF Performance
• 6.3.2.1 Dependence on the Gate Length
• 6.3.2.2 Contributions to the Gate Capacitance
• 6.3.2.3 Dependence of f_T on d_GC
• 6.3.2.4 Dependence of f_T and f_max on L_R
• 6.3.2.5 Dependence on the Passivation Thickness
• 6.3.3 Optimization of Millimeter Wave HEMTs
• 6.4 Comparison between Low Noise, Power, and Millimeter Wave HEMTs
• 6.5 Influence of Backside Doping on the C_G(V_GS) Characteristics