www.elsevier.com/locate/sse

Solid-State Electronics 49 (2005) 915–922

Experimental investigation of the kink effect and the lowfrequency noise properties in pseudomorphic HEMT�s

F. Principato a,*, A. Caddemi b, G. Ferrante a

a Dipartimento di Fisica e Tecnologie Relative and INFMUnita di Palermo, Universita di Palermo, Viale delle Scienze, Edificio 18, 90128 Palermo, Italyb Dipartimento di Fisica della Materia e Tecnologie Fisiche Avanzate and INFM Unita di Messina, Universita di Messina-Contrada Sperone,

31-S. Agata, 98166 Messina, Italy

Received 14 July 2004; received in revised form 20 March 2005; accepted 21 March 2005

The review of this paper was arranged by Prof. S. Cristoloveanu

Abstract

The kink effect in low-noise pseudomorphic (AlGaAs/InGaAs) HEMT�s has been examined in detail by investigating the steady-

state and pulsed I–V characteristics, the behavior of the output conductance dispersion and the performance of the gate leakage

current to understand its origin. No clear evidence of impact ionization occurrence in the InGaAs channel at kink bias conditions

(VDS,kink = 1.5 V) has been found, thus suggesting that the predominant mechanism should be attributed to trap-related phenom-

ena. A significant rise of the gate current has been found at very high drain voltages (far from VDS,kink) associated with low drain

current values which is probably due to impact ionization onset within the channel. In addition, the low frequency gate and drain

noise has been measured at bias conditions below and above the kink to analyze its correlation with the processes involving discrete

energy trap states. A Lorentzian noise contribution has been found in the drain noise spectrum traced at kink bias VDS,kink with a

corner frequency corresponding to that exhibited by the output conductance dispersion curve at the same bias. Thus, the devices

under test classified as short-channel low-voltage GaAsFET�s exhibit a marked kink effect whose origin is likely to be due trapping

mechanisms.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Kink; PHEMT; LF noise; Frequency dispersion

1. Introduction

The kink effect (also known as soft breakdown) is a

very common feature of the I–V characteristics of either

AlGaAs/InGaAs/GaAs and InAlAs/InGaAs/InP sub-

micron gate HEMT�s consisting of a sudden current rise

even at low drain voltage values.

Firstly, the main underlying mechanisms were attrib-uted to charge release from traps associated to deep lev-

els (DX centers) in the donor or buffer layers after hot

0038-1101/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.sse.2005.03.021

* Corresponding author. Tel.: +39 91 661 5027; fax: +39 91 661 5063.

E-mail address: fabiopr@unipa.it (F. Principato).

electron injection [1,2]. More recently, accurate experi-

mental investigation of impact ionization effects have

pointed out a direct correlation with anomalous conduc-

tion aspects such as the kink in the output current and

the bell-shaped curve of the gate leakage current, espe-

cially in InP-based structures [3].

The impact ionization hypothesis may also explain

the light emission observed when the FET is biased atthe kink voltage and another kink-related effect which

consists of a reduction of the source resistance value

[4,5].

A comprehensive physical and circuit model for the

kink in InAlAs/InGaAs HEMT�s calls for field-generated

1x10-8

1x10-7

1x10-6

]| 20

30

]

916 F. Principato et al. / Solid-State Electronics 49 (2005) 915–922

hole accumulation under the source end of the gate and a

related channel potential shift that varies the threshold

voltage of the device. Therefore, the kink current would

be caused by either the additional drive and the charges

directly generated by the impact ionization process [6].

In the present paper, we report the results of ourinvestigation on packaged AlGaAs/InGaAs low-noise

pseudomorphic HEMT�s exhibiting a strongly tempera-

ture-dependent kink [7]. We have carefully examined the

device performance at room temperature by investigat-

ing the steady-state and pulsed I–V characteristics, the

transconductance and output conductance dispersion

behavior and the gate leakage current performance to

understand its origin. In addition, we have measuredthe low frequency gate and drain noise over the bias

conditions below and above VDS,kink to analyze the rela-

tionship with the mechanisms generating the kink since

the occurrence of trap-assisted generation-recombina-

tion processes is well tracked by LF noise spectra versus

bias [8].

-1 -0.8 -0.6 -0.4 -0.2 01x10-11

1x10-10

1x10-9

|I GS[

A

0

10

I DS[

mA

VDS

VGS[V]

2. DC characterization

The devices employed in this study are commercial

AlGaAs/InGaAs/GaAs Schottky gate FET�s (MGF

4919, by Mitsubishi) enclosed in a ceramic short lead

package. Their precise structure is unknown, but from

the results of their electrical DC and microwave perfor-

mance we reasonably believe to deal with 0.25 lm-gatelength and 200 lm-gate width devices [9].

0 0.5 1 1.5 2 2.5 3 3.5

0

10

20

30

40

I DS[

mA]

VDS[V]

Fig. 1. IDS–VDS characteristics exhibiting a marked kink at 1.5 V.

VGS ranges from �0.4 V to 0.0 V with steps of 0.05 V.

The samples have been tested by using an air-coaxial

metallic test fixture (MT-950, by Maury Microwave),

which is also suitable for low-temperature measure-

ments in a cryogenic chamber [7].

We first analyzed the DC performance by tracing the

steady-state IDS–VDS characteristics shown in Fig. 1 andthe transfer curves IDS–VGS reported in Fig. 2. A

marked kink in the drain current is recognizable at the

drain-source voltage VDS,kink = 1.5 V. The kink effect

in turns produces a strong variation of the output con-

ductance gds vs. VDS, as shown in Fig. 3 at different val-

ues of the gate–source voltage VGS.

Fig. 2. IDS–VGS and IGS–VGS characteristics at low drain voltage

conditions 0 6 VDS 6 2.0 V (0.25 V step).

0 0.5 1 1.5 2 2.5 3 3.5

0

5

10

15

20

25

g DS[

mA]

VDS[V]

Fig. 3. Output conductance gds curves derived from the I–V charac-

teristics of Fig. 1.

F. Principato et al. / Solid-State Electronics 49 (2005) 915–922 917

The question arises as far as the physical origin of this

kink behavior is concerned. According to the hypothesis

of the onset of an impact ionization process sustaining

the drain current rising, we would expect concurrently

a prominent rise in the gate current due to the impact

ionization-generated holes which are collected at thegate electrode. The link between a kink effect and the

impact ionization mechanism is well tracked by a bell-

shaped curve of the gate leakage current [3,4,10].

We have thus measured the input characteristics IGS–

VGS at different drain voltage values below and after

VDS,kink (0 6 VDS 6 2.0 V, 0.25 V step). These devices

feature very low values of the gate leakage current (in

the range of 10�10 to 10�7 A), but as shown in Fig. 2no current rise has been found to occur over the VDS

and VGS voltage ranges investigated.

Therefore, the absence of any variation in the gate

current should rule out the onset of an impact ionisation

process in the channel as the predominant cause of the

observed kink.

We extended the analysis of the gate current behaviour

up to high drain voltage values (i.e. in the pre-breakdownregion) by concurrently reducing the gate-source voltage

to comply with the maximum voltage ratings of the de-

vices under test. We then examined the input characteris-

tics in the �0.4 V 6 VGS 6 0 V, 2.0 V 6 VDS 6 3.75 V

source and drain voltage ranges. The relevant curves are

reported in Fig. 4 where a significant rise of IGS can

now be seen asVDS is increased beyond 3.0 V, which sup-

ports the hypothesis of an impact ionisation regime. This,however, does not cause any sudden variation in the out-

put characteristics.

-0.4 -0.3 -0.2 -0.1 0

0x100

2x10-6

4x10-6

6x10-6

VDS

|I GS[

A]|

VGS[V]

Fig. 4. IGS–VGS characteristics at high drain voltage conditions

2.0 V 6 VDS 6 3.75 V (0.25 V step).

The shape of the gate current curve, where the peak

occurs, is not smooth and the corresponding drain cur-

rent values are moderately low (IDS 6 5.0 mA). As VGS

moves toward less negative values, the gate current

decreases and again exhibits a monotonic behaviour

thus indicating that its rise is driven by a field-inducedprocess.

At this point of our experimental tests performed on

the MGF 4919 HEMT�s, we attribute the drain current

kink to the role played by deep-level trap (DX center)

processes occurring in the AlGaAs donor layer. The car-

riers trapped after hot electron injection, could have

been released by a deep-level impact ionisation which

would not imply hole generation, i.e. gate currentvariations.

The kink effect is enhanced as VGS increases since the

trapping and de-trapping mechanisms are controlled by

a lower gate-to-drain potential. In addition, the reduc-

tion of IDS and the more pronounced kink seen in the

I–V characteristics of these devices down to 50 K

may also be explained by trap-assisted processes in the

AlGaAs layer [7].

3. Sinusoidal and transient excitation measurements

Being related to trapping mechanisms, the previous

described current anomaly is expected to be inherently

a low-frequency process. The dispersion behavior of

the kink can be analyzed in a variety of ways to fullyunderstand its behavior [2,11].

In our work, the dispersion of the output conduc-

tance gds has been analyzed by measurements performed

at different values of VDS below and above VDS,kink. To

accomplish that, the drain terminal is connected trough

a resistor R to the output of an amplifier which sums a

sinusoidal signal to a constant voltage giving the drain

bias. By measuring with the digital scope the sinusoidalsignal vR across the resistor and the drain voltage vDS

the output conductance is obtained as gds = vDSR/vR.

The sinusoidal signal amplitude is 100 mV and the fre-

quency ranges from 1 Hz up to 100 kHz. The dispersion

of the output conductance at fixed VGS = �0.1 V as a

function of the drain voltage is shown in Fig. 5. The

experimental results show a smooth trend of the gdscurves at high frequencies, all converging to a final valueof about 7.0 mS. A significant difference between low

frequency and high frequency values is exhibited by

the curve traced at VDS,kink with a transition frequency

around 100 Hz. Previous investigation of frequency dis-

persion of the output conductance in InP-based

HEMT�s led to different results with stronger anomalies

in its behavior vs. frequency and bias [2,11]. In Fig. 6 we

reported the frequency dispersion of gds at VDS,kink as afunction of the drain bias current (i.e. at different VGS).

The dispersion diminishes as the drain current decreases

1x100 1x101 1x102 1x103 1x104 1x105

0

2

4

6

8

10

12

14

g ds[m

S]

VDS2.00 V1.75 V1.50 V1.25 V1.00 V0.75 V

f [Hz]

Fig. 5. Frequency dispersion of gds at VGS = �0.1 V for different

values of the drain–source voltage.

1x100 1x101 1x102 1x103 1x104 1x105

f [Hz]

0

2

4

6

8

10

12

14 IDS

2 mA1 mA

5 mA10 mA20 mA

500 µA

g ds[m

S]

Fig. 6. Frequency dispersion of gds at VDS,kink for different values of

the drain current.

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25

0

5

10

15

20

25

30

35

I DS[

mA]

100 ms

10 ms

1 ms

100 µs

DC

VGS=0.0V

VDS[V]

Fig. 7. Pulsed I–V measurements of the output characteristics with

pulse duration varied in the range 100 ls–100 ms with 10% duty cycle.

Also shown is the DC I–V curve.

918 F. Principato et al. / Solid-State Electronics 49 (2005) 915–922

and completely disappears at IDS � 1.0 mA (VGS ��0.34 V). This result corresponds to that found for the

DC output conductance of this device where the peak

at VDS,kink disappears at the same value of IDS(VGS).

By comparing these results with those obtained in [2]

where the dispersion of the output conductance occurs

even at drain bias different from the VDS,kink, we here

note that the dispersion takes place only at VDS,kink witha characteristic transition frequency. Again, this may

suggest that in our device few trapping mechanisms

are involved in the kink effect.

As a second step, we analyzed the output characteris-

tics by pulsed I–V measurements. In such tests a DC

power supply is connected to the gate and a pulse volt-

age generator with varying amplitude is used as thedrain stimulus. The pulse generator is connected to the

drain of the device under test through a load resistor.

The drain current is determined by measuring the pulsed

drain-source voltage and the voltage at load resistor by

using a digital oscilloscope. The pulse duration varies in

the 100 ls–100 ms range with a variable duty cycle. In

the relevant curves plotted in Fig. 7 are the results

obtained at VGS = 0 V are compared with the staticI–V curve. It is clearly seen the difference between the

DC and the pulsed curves, and that the kink disappears

for pulse duration shorter than 10 ms. The duty cycle in

this case is 10% and we observed a slight change in the

I–V curve by using lower duty cycle. The disappearence

of the kink at pulse duration lower than 10 ms is consis-

tent with the previous observation on the transition fre-

quency of the gds dispersion around 100 Hz. Moreover,we note that the pulsed IDS value exceeds the DC value

and this effect is more pronounced below the VDS,kink.

The frequency dispersion of the device transconduc-

tance gm was also monitored. Its frequency-dependent

value was extracted by applying a 50 mV sinusoidal sig-

nal vgs superimposed to the offset bias voltage and by

measuring the variable voltage signal vR across the resis-

tor R placed in series with the drain. The transconduc-tance is obtained as gm = (1/R + gds)vR/vgs, where gds is

1x10-19

1x10-18

1x10-17

F. Principato et al. / Solid-State Electronics 49 (2005) 915–922 919

the measured output conductance. The frequency range

extends from 1 Hz up to 100 KHz. The values of the

transconductance gm were measured at the same bias

of the output conductance and they do not exhibits

any significant frequency dispersion even at VDS,kink.

1x10-9 1x10-8 1x10-7 1x10-6 1x10-5

1x10-26

1x10-25

1x10-24

1x10-23

1x10-22

1x10-21

1x10-20

VDS

1.0 V1.5 V2.0 V

IG2

IG[A]

S IG[A

2 /H

z]

Fig. 8. Gate current noise spectral density at 1 Hz as a function of the

gate current for different drain biases.

4. Low frequency noise measurements

Noise measurements were performed with the device

placed in common source configuration. The noise sig-

nals of the gate and drain are both measured indepen-

dently and simultaneously. The device is biased by

means of batteries and wire-wound resistors. The gatecurrent is amplified by a low-noise current amplifier

(SR570, by Stanford Research System), which also pro-

vides the bias of the gate-source voltage. The amplifier

operates with 0.01 Hz cut-off frequency of the high-pass

filter and the frequency range, which depends on the

used sensitivity of the amplifier, varies from 15 Hz to

2 kHz. The drain noise current is measured by amplify-

ing the voltage across the drain load resistance RL byusing a low-noise voltage amplifier (SR560, by Stanford

Research System) with variable gain and 0.1 Hz–

1.0 MHz frequency bandwidth. Both amplifiers are sup-

plied with batteries and the whole system is enclosed in a

shielded box. The two amplified noise signals are mea-

sured with a computer based instrument operating in

FFT analyser mode (DSA4551, by National Instru-

ments). The auto power spectrum of each channel andthe coherence between them is monitored under LAB-

VIEW software control. All the spectra are obtained

by averaging over 100 measurements.

Noise measurements of the gate current IG have been

performed at different reversed gate-source voltages and

at constant VDS below and above the kink. The investi-

gated frequency range 1 Hz–500 Hz, which is imposed

by the highest sensitivity of the current amplifier used.The current noise spectrum SIG(f) exhibits a 1/f c law,

with c ffi 1, for all the investigated values of the voltage

drain bias VDS = 1.0, 1.5 and 2.0 V. White noise due to

the thermal noise of the leakage conductance and due to

shot noise 2eIG was not been observed in the investi-

gated frequency range. In Fig. 8 the spectral density

SIG is shown, evaluated at 1 Hz, plotted against IG cur-

rent. At lower values of the gate current, which areobtained only at VDS = 1.0 V, the gate current noise var-

ies as I3G, whereas for higher values of the gate current

the variation is approximately as I2G. The power spectraare rather insensitive to the value of the VDS. This

behavior is observed in similar devices and the quadratic

dependence of the noise gate current is typical for 1/f

noise in Schottky junction [12,13]. By fitting the gate

current noise spectral density for gate current above50 nA with the empirical relation SIG ¼ A� I2G, we

found an average value of 4.5 · 10�7 Hz�1 over the dif-

ferent VDS which is comparable to that reported for InP

based HEMT [14]. Thus the main contribution to the

noise spectrum of the gate current is certainly located

in the Schottky barrier and seems to be not related with

the mechanisms responsible of the kink.The output voltage noise is measured at constant

drain voltage with the gate–source junction reverse

biased and the drain current varying from 10 lA up to

20 mA. The higher bound of the frequency range is

90 kHz. We investigated the drain current noise SID

which is related to the measured output drain voltage

noise SV0 by the following relation

SIDðf Þ ¼ SV0ðf ÞGL þ gdsðf Þ

A

� �2

ð1Þ

where A is the gain of the amplifier and GL is the

dynamic load conductance. In the above relation the

output conductance gds(f ) is a function of the frequency

as determined by the direct measurements previously

described. Thus the estimated drain current noise is

not affected by the circuit effect of the output conduc-tance dispersion. Generally, we found two different spec-

tra for the drain current noise according to drain voltage

below or above the kink. At VDS = 1.0 V (pre-kink) the

power spectra SID are 1/f c-like (with c close to 1) with

superimposed Lorentzian components at high bias cur-

rent drain (>1.0 mA) with the corner frequency greater

than 10 kHz. No white noise component is observed.

The intensity of the spectral density SID at 1 Hz versusthe drain current shows a quadratic dependence at low

drain currents, which tends to level off at higher bias.

This dependence is different from that reported in [15]

although there is no general model to explain the drain

920 F. Principato et al. / Solid-State Electronics 49 (2005) 915–922

current 1/f noise in the saturation regime. The noise

spectral density SID measured at the drain bias of

1.5 V (VDS,kink) and 2.0 V has a different shape with re-

spect to the pre-kink regime showing the appearance of

Lorentzian components. At VDS,kink the Lorentzian

component is superimposed on the 1/f c-like spectrum,with c ffi 0.5, and the corner frequency increases with

the drain current. We report in Fig. 9 the SID · f product

versus the measured frequency f at different values of the

drain current, in order to improve the determination of

the corner frequency fc. The Lorentzian component is

present over all bias conditions and its corner frequency

fc increases from 4 Hz at IDS = 10 lA (VGS = �0.54 V)

up to 75 Hz at IDS = 20 mA (VGS = �0.06 V). The highfrequency peak at low drain current is the cut-off due to

the high RC constant of the measurement set-up. The

presence of a Lorentzian spectrum is due to genera-

tion–recombination (G–R) noise generated by traps lo-

cated in the device with activation energy that can be

evaluated by means of temperature dependent measure-

ments [16,17]. The corner frequency fc of the G–R noise

is comparable to the transition frequency of the outputconductance gds found at the drain bias associated to

the kink, although its value is not exactly defined. Thus

it is likely that the traps responsible for the G–R noise

are also involved in the mechanisms that generate the

1x100 1x101 1x102

1x10-18

1x10-17

1x10-16

1x10-15

1x10-14

1x10-13

1x10-12

f [

f SID

[A2 ]

Fig. 9. Current drain dependent SID · f versus fre

kink. As reported in [16], the values of the activation

energies associated with the traps for InP HEMT�s as

determined from drain current noise measurements

compared well to those related to output resistance dis-

persion measurements observed in [2]. As evidenced by

the plots of the Fig. 9, a Lorentzian spectrum occurseven at bias currents lower than 2.0 mA, where the effect

of the output conductance disappears. Moreover, the

shift of the corner frequency approaches approximately

the value of the transition frequency of the gdsdispersion.

The transformed values of the current noise spectrum

SID · f at fixed VDS = 2.0 V, i.e. above the kink, are

shown in Fig. 10. The Lorentzian peak occurs at draincurrents lower than 5.0 mA and disappears at higher

values of the drain bias current. We note that the corner

frequencies at VDS = 2.0 V are higher than those

observed at VDS,kink and the difference increases as the

drain current is higher. The presence of the kink in

MESFET�s has been associated to the presence of G–

R noise traps for all values of VDS below and above

the kink, and the corner frequency has found to dependon the values of VDS only above the kink [8]. This

behavior is consistent with an impact ionisation mecha-

nism responsible of both G–R noise and kink effect. This

mechanism seems to be not valid in our case where the

1x103 1x104 1x105

500 µA

200 µA

100 µA

50 µA

20 µA

10 µA

10 mA

IDS=20 mA

5 mA

2 mA

1 mA

Hz]

quency plots measured at the kink VDS,kink.

IDS=20 mA

1x100 1x101 1x102 1x103 1x104 1x105

1x10-18

1x10-17

1x10-16

1x10-15

1x10-14

1x10-13

1x10-12

f [Hz]

f SID

[A2 ]

500 µA

200 µA

100 µA

50 µA

20 µA

10 µA

5 mA

10 mA

2 mA

1 mA

Fig. 10. Current drain dependent SID · f versus frequency plots measured at VDS = 2.0 V.

F. Principato et al. / Solid-State Electronics 49 (2005) 915–922 921

effects of traps are not present at high drain current for

drain voltage different from VDS,kink. In the device undertest the presence of G–R noise only at the VDS,kink where

the output conductance variation is observed should

therefore involve the presence of traps. Further analysis

is needed to clarify this point, e.g. by means of temper-

ature dependent output conductance and noise measure-

ments in order to investigate in more details the

characteristics of these traps.

Finally, we have also performed simultaneous gateand drain noise measurements for all considered bias

conditions. We have measured the coherence function

C(f) between output drain voltage and gate current

defined as

Cðf Þ ¼ jSIGVDj2

SIGðf ÞSVDðf Þð2Þ

where SIG,VD is the cross-spectrum between gate current

and drain voltage. The frequency range was determined

by the current noise amplifier and is 1 Hz–500 Hz where

no white noise is observed. We observe a null correlation

between gate and drain noise for all bias condition even

at VDS,kink. A non null coherence between gate anddrain noise is reported at the kink bias in stressed

PHEMT devices, although the authors found a more

marked effect of the kink on the noise [18]. However,

in our case, the absence of coherence between gate and

drain noise result is consistent with previous results

showing the lack of any change in DC gate current

and gate current noise.

5. Conclusions

We have investigated the kink effect in low-noise

pseudomorphic (AlGaAs/InGaAs) HEMT�s by analyz-

ing the steady-state and pulsed I–V characteristics and

low frequency noise properties. Since the structure ofthe device is unknown, we have not proposed a model

to explain the physical mechanisms of the kink but have

compared our results with the data and the models

reported in the literature for the kink effect occurring

in similar devices.

In our investigation, no bell-shape in the gate current

characteristics has been found to support the hypothesis

of impact ionization mechanisms. The gate current noiseis totally uncorrelated with the drain current noise and

does not show any visible change near the kink. We

observed an appreciable dispersion of the output con-

ductance only at VDS,kink. The relevant transition fre-

quency is consistent with the inverse of the pulse

duration at which the kink disappears in the I–V pulsed

curves. The low frequency drain current noise evidences

G–R components only at drain bias greater than theVDS,kink and the values of the corner frequency of the

Lorentzian found at VDS,kink are comparable with

922 F. Principato et al. / Solid-State Electronics 49 (2005) 915–922

the transition frequency of the output conductance dis-

persion. Thus, we may conclude that the kink occuring

in the MGF 4919 PHEMT device is not caused by an

impact ionization based phenomena but is likely to be

generated by generation-recombination processes.

Acknowledgement

This work is supported by Italian Ministry of Univer-

sity and Research (MIUR) and by the National Institute

for the Physics of Matter (I.N.F.M.). Additional sup-

port from the C.N.R. MADESS II Project is also

acknowledged.

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