Noise Characteristics of InAlAs/InGaAs Heterostructure Devices

Graduate Student: Marcel Tutt*, Kevin Hein*
D. Pavlidis
U.S.Army Research Office DAAL03-92-G-0109

Double-doped double-heterojunction HEMTs have demonstrated excellent small signal performance demonstrated by measured fT and fMAX values of 150GHz of 310GHz, respectively. The high fMAX is a result of improved output conductance. The current drive capability is also greatly enhanced due to doping above and below the channel. The microwave noise characteristics of these devices are not well known at this time. The purpose of this work is to characterize and understand the microwave noise characteristics of these devices.

The Fmin of double-doped double-heterojunction HEMTs has been measured and found to be as low as 0.3dB at 10GHz. This is a very good result which is in fact comparable to that of more conventional structures. The corresponding Fukui factor was about 2. This is comparable to values obtained for other HEMT structures. There are two factors which contribute to this excellent Fmin. These are the device gain and the intrinsic device noise. The device gain can be readily measured different ways. The intrinsic noise is far more difficult to obtain.

The extrinsic drain noise was measured using a fully calibrated on-wafer noise measurement system and a cold noise algorithm. The intrinsic noise was estimated using a modified Pospieszalski noise model. This was possible due to the fact that the measurement frequency was low enough to ensure that the gate noise was essentially thermal noise. Baseband noise measurements indicated that the 1/f noise would not contribute to the measured noise.

The results of this work indicate that the intrinsic noise of these devices is in fact greater (as much as 10dB greater) than that for simpler devices such as 1.0 micron ion-implanted GaAs MESFETs. This has been explained using a simple noise model which indicates that this is due to the shorter channel length and the very high effective carrier density. This indicates that these are not truly low noise devices. The reason for the lower Fmin is now understood to be due to the intrinsic device gain increasing more than the intrinsic noise.

The noise characteristics of novel HEMT designs using for example Quasi-1D channels is also investigated and correlation to technology and pitch design are being made.

Intrinsic drain noise Idsn of InAlAs/InGaAs HEMTs and MESFETs for comparison. I^2dsn is larger for HEMTs, contradicting the Fmin results where the HEMTs is better than the MESFET. The analysis shows that the I^2dsn features can be explained by considering the larger carrier density and smaller channel length of the HEMTs.

Minimum noise figure (Fmin) and associated gain (G) of InAlAs/InGaAs HETMs fabricated at UofM and GaAs MESFETs for comparison purposes. The HEMTs show very low Fmin (0.3-0.4 dB) and high gain while displaying a broad noise minimum and thus reduced sensitivity. A large Kf~2.07 is found for the Fukui expression and data analysis shows taht Kf is not necessarily a good noise figure of merit.

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