Noise Characteristics of Heterojunction Bipolar Transistors (HBTs)

Graduate Student: Marcel Tutt*, Saeed Mohammadi
Professor: D. Pavlidis
U.S.Army Research Office DAAL03-92-G-0109, Texas Instruments P-109211663

Heterojunction Bipolar Transistors (HBTs) have demonstrated very good microwave and millimeterwave performance in analog and digital circuit applications. Baseband noise (commonly referred to as 1/f noise) is a very important quantity in many applications because it will be upconverted. The result is AM noise and PM noise. Hence, knowledge of this fundamental phenomenon is required in order to be able to design devices and circuits which can minimize this quantity and its affects.

The short circuit noise current model has been determined to be the most appropriate two-port model for describing the terminal noise characteristics of HBTs. In addition, two different measurement techniques have been evaluated. One is based on the noise figure technique and is considered to be an indirect measurement technique. The second is a direct measurement technique. The second technique has been adopted because it is faster and still provides the noise magnitude at each terminal.

Studies of AlGaAs/GaAs HBTs have determined several key points. First, the collector noise spectra is always greater than the base noise spectra. Second, the spectra are often distorted from ideal 1/f type of behavior. Distortion is observed in both collector and base spectra. This distortion is due to the presence of one or possibly two Lorentz components. One Lorentz component has a corner frequency between 10kHz and 20kHz. The second has a corner frequency beyond 100kHz. Third, the spectra are strongly dependent on the bias current. In contrast, they are only weakly dependent on the collector voltage. The current dependence results appear to support recombination mechanisms. Fourth, temperature dependent studies of the spectra have been used to determine trap activation energies, Ea. Two different traps have been identified, one with Ea=0.16eV, the other with Ea=0.58eV.

Similar studies have been carried out on InP/InGaAs HBTs. The results are substantially different. First, the spectra show little deviation from a 1/f type of behavior. Bias dependent studies have shown that the collector spectra has two very different bias dependences. At low bias an Ic^1.7 dependence is obtained. At high bias an Ic^0.7 dependence is obtained. This indicates that the noise mechanisms are changing as the bias is changed. Temperature dependent studies revealed traps with Ea=0.07-0.10eV.

The impact of geometry and technology on 1/f characteristics is currently investigated. Tradeoffs in low frequency noise and microwave characteristics are estimated. The possibility of employing low frequency noise characteristics as a means of predicting the reliability of HBTs is also under study.

Equivalent input base noise current spectral density for self-aligned (SA), self-aligned with ledge (SAL), and nonself-aligned (NSAL) AlGaAs/GaAs HBTs at Ic=10 mA, Vce=4V. The noise spectral density has an overall 1/f lineshape. The noise is reduced in SAL and NSAL HBTs compared with SA devices due to the ledge incorporation.

fT, fMAX as a function of collector current for SA, SAL, NSAL AlGaAs/GaAs HBTs, The fMAX of NSAL HBTs is lower than that of SA and SAL due to increased base resistance of the former. Both SAL and SA devices have comparable fT and fMAX. Comparing these data to those of low frequency noise, one sees that the use of ledge improves the base spectral density but degrades the maximum available gain.