6.1 Low Noise HEMT

The first HEMTs were developed mostly for low noise applications like receivers [73, 74, 75]. The noise of an amplifier is characterized by the noise figure F which is defined as the signal-to-noise ratio at the amplifier output divided by the signal to noise ratio at the input. Noise figures of a two stage amplifier as low as 1.7 dB with an associated gain of 17 dB at 32 GHz were obtained [73].

The principal source of noise added by the transistor at high frequencies are related to power dissipation in the resistances of the device. At lower frequencies generation/recombination processes become dominant. General dependencies of F_min on device parameters can be obtained from Fukui's equation [76]. Fukui showed that
 

(61)

obtained from noise calculations represent the behavior of GaAs MESFETs very well for the factor . The equation can be also used for a qualitative discussion of the noise of HEMTs. (61) predicts a noise figure increasing with I_D as C_GS rises more rapidly than . At very low currents g_m tends to zero whereas C_GS becomes dominated by parasitic capacitances. Thus, at very low currents F_min is expected to rise again giving a minimum value of F_min at low drain currents. This shows that high g_m has to be obtained with a low I_D, in other words, I_D should increase very rapidly near V_T.

From a microscopic point of view important noise sources are scattering mechanisms such as alloy fluctuations and interface roughness on heterojunctions. These will have an impact on the device parameters such as resistances, thus they are indirectly included in (61). The conduction band diagram and the electron distribution at the low noise bias point, i. e. near V_T, is shown schematically in  Figure 6.5. As depicted in the diagram the maximum of the electron distribution is near the lower heterojunction interface of the channel. Therefore, this interface is considered to be very important.

During the growing sequence of a typically pseudomorphic HEMT the surface morphology of AlGaAs is inferior to pure GaAs which increases the roughness of the heterojunction interface and thus the electron scattering. To reduce scattering and thus noise, pure GaAs is used below the channel. Such HEMTs without an AlGaAs barrier below the channel are referred to as single heterojunction HEMTs (SH­HEMTs).
 

Besides F_min good RF performance of low noise HEMTs is required. As discussed in Section 5.2 the transconductance g_m near V_T is not simulated very well, thus the determination of F_min using (61) by simulation will not be further investigated directly. But (61) reveals that a large ratio g_m/C_GS is necessary for a low F_min which are similar requirements as for high f_T . It can be expected that devices with a high maximum f_T will also exhibit low F_min. Therefore, low noise HEMTs will be investigated with respect to their RF performance at V_DS = 2.0 V and g_{m max}.
 

• 6.1.1 DC Characteristics
• 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