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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 Fmin on device parameters can be obtained from Fukui's equation [76]. Fukui showed that
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 ID as CGS rises more rapidly than . At very low currents gm tends to zero whereas CGS becomes dominated by parasitic capacitances. Thus, at very low currents Fmin is expected to rise again giving a minimum value of Fmin at low drain currents. This shows that high gm has to be obtained with a low ID, in other words, ID should increase very rapidly near VT.

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 VT, 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).

Figure 6.5 Conduction band diagram and the electron distribution in the channel of a HEMT biased near the threshold voltage VT.

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

next up previous contents
Next: 6.1.1 DC Characteristics Up: 6 Applications Previous: 6 Applications

Helmut Brech