6.1.2 RF Characteristics

One of the most important figure of merit of a HEMT is the current-gain cutoff frequency f_T given by (10) and f_max given by (11) as described in Chapter 3.

For the case of HEMT A,  Figure 6.13 shows a comparison of the functions f_T (V_GS) determined in three different ways: i) measured (i. e., obtained by S­parameter measurements), ii) calculated with (10) where g_mi, C_GSi, and C_GDi were obtained from extractions using the small-signal equivalent network shown in Figure 3.1, and iii) obtained from simulations also using (10) with the approximation for the extrinsic values described in Chapter 3.
 

The last two cases offer the possibility to determine the device performance excluding the contacting network which is necessary for circuit design.

The maximum f_T of the simulation was fitted to f_{T max} calculated with extracted small signal parameters by adjusting the cross section of the T­gate. The smaller simulated f_T's below and above g_{m max} reflect the underestimated electron velocity due to the DD model in the GaAs and AlGaAs layers. The measured maximum f_T is about 13 GHz lower than the calculated ones mainly due to the presence of the parasitic elements of the small signal equivalent shown in Figure 3.1. The values of the small-signal circuit elements for HEMT A obtained by parameter extraction, which are considered to be not bias dependent, are given in  Table 6.4.
 

In the following we want to examine the influence of the geometrical parameters L_G and L_R on the magnitude of f_T and f_max simulated according to (10) and (11), respectively. The capacitances C_GS and C_GD entering the equations are considerably influenced not only by L_G and L_R but also by the shape of the gate metal cross section (imagine, for instance, the case of a T-gate) and the dielectric constant e_r of the passivation material that fills the space between metal structure and semiconductor surface. The simulator is able to take all these effects of surface topology fully into account. On the other hand, only in simulation it is possible to analyze the hypothetical case that the gate does not interact capacitively with the surrounding semiconductor surfaces or with the neighboring ohmic contacts. This can be achieved by choosing e_r = 0 and leads to the determination of the theoretical maximum of f_T for given L_G and L_R.
 

• 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