Understanding of the excess channel noise in In Al As ∕ In Ga As ∕ In P high electronmobility transistors in impact ionization regimeHong Wang, Yuwei Liu, Rong Zeng, and Chee Leong Tan Citation: Applied Physics Letters 90, 103503 (2007); doi: 10.1063/1.2711376 View online: http://dx.doi.org/10.1063/1.2711376 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/90/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Impact ionization in N-polar AlGaN/GaN high electron mobility transistors Appl. Phys. Lett. 105, 063506 (2014); 10.1063/1.4892449 Improved microwave and noise performance of InAlAs/InGaAs metamorphic high-electron-mobility transistor witha liquid phase oxidized InGaAs gate without gate recess Appl. Phys. Lett. 96, 203506 (2010); 10.1063/1.3430569 Improved breakdown voltage and impact ionization in In Al As ∕ In Ga As metamorphic high-electron-mobilitytransistor with a liquid phase oxidized InGaAs gate Appl. Phys. Lett. 87, 263501 (2005); 10.1063/1.2151252 Excess low-frequency noise in AlGaN/GaN-based high-electron-mobility transistors Appl. Phys. Lett. 80, 2126 (2002); 10.1063/1.1463202 Electroluminescence of composite channel InAlAs/InGaAs/InP/InAlAs high electron mobility transistor J. Appl. Phys. 87, 2548 (2000); 10.1063/1.372217

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Understanding of the excess channel noise in InAlAs/ InGaAs/ InPhigh electron mobility transistors in impact ionization regime

Hong Wang,a� Yuwei Liu, Rong Zeng, and Chee Leong TanMicroelectronics Centre, School of Electrical and Electronics Engineering,Nanyang Technological University, Singapore 639798, Republic of Singapore

�Received 22 December 2006; accepted 29 January 2007; published online 6 March 2007�

The characterization and analysis of high frequency noise of InP-based high electron mobilitytransistors in the impact ionization regime reveal that different physical mechanisms are responsiblefor the excess channel noise at different frequency ranges. The excess noise at frequencies below10 GHz is mainly due to the hole recombination in the channel. However, excess channel noise athigher frequencies could be dominated by the fluctuation of impact ionization which has a muchsmaller time constant and therefore a much higher cutoff frequency. © 2007 American Institute ofPhysics. �DOI: 10.1063/1.2711376�

The presence of an excess noise related to impact ion-ization in InP-based high electron mobility transistors�HEMTs� has increasingly attracted attention.1–3 It has beenfound that the enhancement of device noise in the impactionization regime may substantially affect the device lownoise application even at microwave frequencies. The largenoise observed in the impact ionization regime was initiallyascribed to a high gate-drain leakage or measurement errors.4

Rouquette et al.5 and Murti et al.1 lately showed that a timeconstant as small as 25 ps could be determined by fitting thenoise spectral density to the form of Lorentzian function,which suggests the possibility of the excess noise in the im-pact ionization regime being induced by the fluctuation ofimpact ionization events. On the other hand, a Monte Carlosimulation3 has shown that the impact ionization and its re-lated kink effect provoke an enhancement of the channelnoise with a characteristic cutoff frequency related to thehole recombination. The significant contribution of the gen-eration of holes by impact ionization and their further recom-bination, which lead to fluctuations in the charge of the holepileup and thus the excess noise, was strongly implied. How-ever, there is no experimental data to validate the contribu-tion of hole recombination to the excess noise. In this letter,we investigate the microwave noise of InP-based HEMTs togain a fundamental understanding on the physical origin ofthe excess noise in the impact ionization regime, which hasso far not received much attention. The results strongly sug-gest that the hole recombination plays an important role inthe enhancement of channel noise in gigahertz range, whilethe contribution of the fluctuation of impact ionization to theexcess noise is substantial at relatively higher frequencies.

The devices used in this study are InAlAs/ InGaAsHEMTs with a strained In0.6Ga0.4As channel on semi-insulating InP substrate grown by molecular beam epitaxy.The detailed layer structure consists of, from top to bottom, a10 nm InGaAs cap layer, 20 nm InAlAs barrier, 5�1012 cm−2 upper Si �-doping layer, 5 nm InAlAs spacer,15 nm undoped InGaAs channel with 60% of indium con-tent, and 250 nm InAlAs buffer on InP substrate. The deviceswere passivated by 120 nm of SiN deposited by plasma en-

hanced chemical vapor deposition. Two-finger devices with agate length �Lg� and width �Wg� of 0.25 and 40 �m, respec-tively, were used for the high frequency noise characteriza-tion. The transistors demonstrated typical drain-source�Id-Vd�–static output characteristics with a noticeable kinkeffect in the Vd range of 0.75–1.5 V. The device thresholdvoltage �Vth� is around 0.75 V and the peak transconductanceis around 600 mS/mm. The maximum fT is about 105 GHz.The noise characterization was carried out using an ATNNP5 noise and S-parameter measurement system in the fre-quency range of 2–20 GHz.

As shown in Fig. 1, the electron impact ionization in theInGaAs channel induces a significant increase of the reversegate current with respect to the Schottky diode gate leakagecurrent resulting in a typical bell-shape curve. The bell-shaped curves start to emerge at Vd�0.9 V with the peakshowing at Vg around −0.6 V. The evolution of high fre-quency noise in the impact ionization regime has been sys-tematically characterized by varying Vg �at a constant Vd�and Vd �at a constant Vg� in the “bell-shape” region. Figure 2shows the minimum noise figure �NFmin� as a function offrequency at different Vd. The bias condition of Vg=−0.6 V

a�Author to whom correspondence should be addressed. Fax: �65�6793 3318.Electronic mail: ewanghong@ntu.edu.sg

FIG. 1. Typical behavior of the increase in reverse gate current induced bychannel electron impact ionization resulting in a bell-shaped gate current.

APPLIED PHYSICS LETTERS 90, 103503 �2007�

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shown in this plot is to ensure that the devices were biased inthe high impact ionization region during the noise measure-ments. At Vd�1 V, NFmin is insensitive to Vd and increasesmonotonically with the increase in frequency. However, asthe drain voltage is further increased from 1 to 2.5 V, a dras-tic increase in NFmin is measured. This increase correspondsto the triggering of the impact ionization mechanism in thechannel �Fig. 1�. One salient feature to be highlighted is that,at Vd higher than 1.5 V, the NFmin versus frequency curvesexhibit a reverse trend showing a higher NFmin value at lowerfrequencies. This has been reported by different researchgroups and is considered to be an important feature of thenoise behavior due to the presence of impact ionization inthe channel.1,5 However, a fundamental explanation on thisanomalous enhancement of NFmin in the gigahertz range hasnot been provided so far.

The output noise of a HEMT is an overall contributionfrom the following two main noise sources: the channel

noise id2 and gate current noise ig

2. The channel noise is com-posed of both the thermal noise and the flicker noise �can beignored at high frequency�, and the gate current noise thatincludes an induced gate noise and shot noise due to the gateleakage current. To gain a better insight into the physicalorigin of the excess microwave noise in the impact ionizationregime, the experimental results were modeled by using anapproach given in Ref. 6 to determine both the channel noiseand the gate current noise. Fundamentally, there is no differ-ence between the noise models for Si metal-oxide-semiconductor field-effect transistors and metal-semi-conductor field-effect transistor or HEMTs �Refs. 7 and 8�except for the negligible substrate effect and much smallergate resistance observed in our InP HEMTs. Figure 3�a�shows the extracted channel �drain� noise versus Vg as afunction of Vd at 10 GHz. It is obvious that the extracted

channel noise id2 presents a distinct bell-shaped behavior, sug-

gesting a close correlation between the channel noise and thecarrier impact ionization and its related effects. The presenceof the bell-shaped channel noise behavior at Vd higher than1 V is consistent with the onset of the bell-shaped gate cur-rent shown in Fig. 1. The bell-shaped channel noise can beinterpreted as a superposition of a conventional channel ther-mal noise component9 and an impact ionization related noise

component �idimp2 . The extracted Vg-dependent gate current

noises ig2 at different Vd are shown in Fig. 3�b�. Compared to

that of a channel noise, no clear bell-shaped behavior wasobserved except for the presence of a small shoulder in therange of Vg from −0.4 to −0.2 V at Vd=1.5 and 2 V. The

drastic increase in ig2 at Vg�0 V could originate from an

increase in the gate shot noise due to the large gate current.The contribution of the gate shot noise to the overall gatenoise in the reverse gate bias region is insignificant. For ex-ample, in the bell-shaped gate current region, the peak leak-age of �10 �A at Vd=2 V only gives a gate shot noise�2qIG� of 3.2�10−24 A2/Hz which is about ten times lower

than the extracted ig2. Hence, the major part ig

2 in the impactionization regime should be the induced gate noise, i.e., thenoise coupled to the gate through the gate capacitance fromthe noise generated in the channel.

To further understand the physical origin behind the ad-ditional channel noise component associated with the impact

ionization, �id2 at different frequencies were extracted by

subtracting the background channel thermal noise from thetotal channel noise, as illustrated in Fig. 3�a�. Figure 4 shows

the �id2 as a function of frequency from 2 to 20 GHz. If the

fluctuation of the carrier density in the channel region is dueto impact ionization and its related effects, such as hole re-combination and pileup that are responsible for the excesschannel noise in the impact ionization regime, the spectralnoise density could be predicted by using van der Ziel’schannel current fluctuation model10 in the form of

Sid=

A�

1 + �2�f�2�2 , �1�

where � is the characteristic time constant associated with themechanism causing the carrier fluctuation. As we can see in

FIG. 2. NFmin vs frequency as a function of Vd measured in the bell-shaperegion.

FIG. 3. Extracted �a� channel noise id2 and �b� gate noise ig

2 vs Vg as afunction of Vd at 10 GHz.

103503-2 Wang et al. Appl. Phys. Lett. 90, 103503 �2007�

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Fig. 4, the �id2 spectrum could not be fitted using a single

time constant, implying the presence of more than onemechanism contributing to the excess channel noise in theimpact ionization regime. The experimental data can be wellfitted by using two noise components �Sid�1

and Sid�2in Fig.

4� with time constants of �1=1 ns and �2=9 ps, respectively.Notably, the time constant of 1 ns is consistent with the re-ported hole recombination lifetime in InP-based HEMTs,11,12

corroborating the impact-ionization-induced hole recombina-tion mechanism. The mechanism responsible for the Sid�2

could be due to the fluctuation of the impact ionization itself.Considering the impact ionization time constant, which isrepresentative of the time for avalanche buildup, the timeconstant can be calculated by13

� = NkW

vmM0, �2�

where k is the effective ionization rate, N is a constant de-pending on k �between 1/3 to 2�, W is the avalanche gain,and vm is mean velocity of the carrier in the InGaAs channel.To a first order approximation, using W�Lg=0.25 �m,k�1, vm�1.5�107 cm/s,14 and M0 less than 2 consideringthe weak avalanche multiplication for Vd=1.2 V, we wouldexpect an impact ionization time constant between 2.2 and6.7 ps, which shows a good agreement with �2 of 9 ps. If weconsider that the channel thermal noise without impact ion-

ization is in the order of 1�10−22 A2/Hz �i.e., the value of id2

at Vd=0.5 V in Fig. 3�a��, it can be seen from Fig. 4 that thehole recombination may result in an excess channel noise atfrequencies below 8 GHz. The excess noise due to carrierfluctuation induced by impact ionization shows a muchhigher cutoff frequency ��40 GHz�. These results stronglysuggest that the hole recombination plays an important rolein the enhancement of channel noise in gigahertz range,while the contribution of the fluctuation of impact ionizationto the excess noise is substantial at relatively higher frequen-cies. In addition to providing a new insight into the excesschannel noise of InP HEMTs operating in the impact ioniza-tion regime, the findings here also provide important infor-mation for the understanding of noise behavior in HEMTsusing a narrow band gap material, such as InAs and InSb,where the impact ionization effect is significant.

In conclusion, the channel noise of InP-based HEMTshas been characterized and analyzed to gain a fundamentalunderstanding of the physical origin of the excess noise inthe impact ionization regime. We find that the excess noise ingigahertz range is mainly accounted for by the mechanismthat is related to the hole recombination in the channel, whilethe excess noise with a much higher characteristic cutofffrequency could be attributed to the fluctuation of impactionization which has a small �picosecond� time constant.

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FIG. 4. Excess channel noise component induced by impact ionization ��id2�

as a function of frequency. The experimental data are fitted by van der Ziel’schannel carrier fluctuation model using two noise components �Sid�1 andSid�2� with time constants of �1=1 ns and �2=9 ps, respectively.

103503-3 Wang et al. Appl. Phys. Lett. 90, 103503 �2007�

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