InGaAs-HFET

Microelectronic Engineering 19 (1992) 401-404 Elsevier

401

Analysis of gate leakage on MOVPE grown InAIAs/InGaAs-HFET F. Buchali, C. Heedt, W. Prost, I. Gyuroa, H. Meschede, F.J. Tegude Duisburg University, SFB 254, Kommandantenstr 60, D-4100 Duisburg, Germany ~Alcatel-SEL Research Center, D-7000 Stuttgart, Germany

Abstract The leakage of reverse biased Schottky gates on lattice matched InAlAs/InGaAs HFET grown by MOVPE on s.i. InP subtsrates is adressed. The contribution of (i) (thermionic-) field emission across the Schottky barrier, (ii) generation-recombination in ~ e space charge region and (iii) impact ionization with subsequent hole tunneling are identified by means of their temperature dependence. Taking the overall importance of the gate to channel potential into account we will show that the leakage in biased HFET with a doping level ND< 4"10~8cm 3 is dominated by impact ionization.

1. INTRODUCTION High sensitivity optical receiver circuits require amplifying devices with a low leakage current. lnGaAs JFET providing a high tunneling barrier due to the p-n junction are intensively studied for this application however exhibiting a drastic excess of gate leakage. The origin is controversely attributed to impact ionization ~ or band to band tunneling at the p-n junction 2. More recently InAIAs/InGaAs HFET on InP with a simple Schottky gate technology are considered 3. The leakage of this device is attributed to tunneling3 due to a highly doped donor layer, parasitic InGaAsSchottky metal cross over 4, and impact ionization in case of a minor hole barrier 5. In this paper we identify the contribution tunneling, generation-recombination and impact ionization. We will show the impact of the size of the device, the doping level and most important, the bias of the device on the leakage of MOVPE grown lattice matched InAIAs/InGaAs HFET.

2. FABRICATION InAIAs/InGaAs-HFET were grown by LP-MOVPE using trimethylgallium (TMG), trimethylaluminium (TMA), trimethylindium (TMI), 100% arsine and 100% phosphine, respectively. The layer structure is shown in fig. 1. The ohmic contacts were fabricated evaporating a Ge/Pt/Au metallization with subsequent alloying in a RTA at 430°C for 12s. The contact resistance is 0.22ftmm. The Schottky gates are fabricated using enhanced optical lithography and lift off technology. The gate length are down to 0.4#m. The gate recess was performed using a phosphoric etch with a etching rate of 5A/s and a Ti/Pt/Au metallization was evaporated for the Schottky contacts (cbB=0.7eV). The devices exhibit a transconductance of 400mS/mm, a drain saturation current of 500mA/mm, a transit frequency of 60GHz and a fMA~ of 107GHz. The l(V)-characteristic (cf. fig. 2) shows an influence of the Kink-effect. 0167-9317/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved.

402

F. Buchali et al. / Gate leakage on MOVPE grown lnAIAs/lnGaAs-HFET 15

InGaAs cap layer 20 nm InAIAs barrier layer

i

• ! ..... [ LG'll~m

i

i

q ]

VGS " O V •

12 nm InAIAs doping layer 3.5"10 ts cni3 InAIAs spacer I n G a A s channel InAIAs buffer l n P buffer

I

:

-1.5 -2

0

s.i. l n P substrate

0

0.5

1

v~r¢

1.5

Fig. 2. l-V characteristic of a HFET

Fig. 1. HFET layer sequence 3. T H E O R Y

The current voltage characteristic of Schottky gates have shown different dominating leakage mechanisms. We will identify these processes by their different l(V)- and I(T)-characteristics. For tunneling the leakage current is given by the Padovani and Stratton modeP It = /s exp q___V

(l)

with E o = Eoo coth E°° kT

Eo

where Eoo is the tunneling parameter. At low temperatures the current is determined by field emission (FE) and remains temperature independent, at higher temperatures there is an current increase due to thermionic field emmission (TFE) determined by the temperature dependence of eq. (1). The generation recombination current is due to the generation of electron-hole pairs via deep levels inside the space charge region in the volume, at the surface and at the heterointerface. Considering the contribution of the volume, the gr-current is given by 7 w

I

= Aq

futh(x)

with U,h(,

o

exp

AE 7

(2)

kT

where Uth is the net recombination rate and w the width of the space charge region and AEv is the activation energy of the gr-center referred to the band edge corresponding to the dominating emission process. The current-voltage characteristic is given by I ~V'V for homogenious materials. The net recombination rate depends on the emission time of electrons or holes of the most efficient gr-center. The temperature dependence of the gr-current is mainly given by that of the net recombination rate. At the drain end under the gate the electric field is sufficient for impact ionization in the channel, The electrons in the channel generate electron-hole pairs. Whereas the electrons flow directly to the drain, the holes flow to the gate and cause an additional leakage current. This current is given by Ii ~ MID

with8 M =

1 [1 - ( ~ ) " ]

(3)

where VB is the breakdown voltage in the channel, lmpact ionization occurs only in the high field

403

F. Buchali et al. / Gate leakage on MOVPE grown InAIAs/InGaAs-HFET

region of the channel. Due to gain saturation the multiplication factor is limited by the resistance R , of that part of the channel, where no impact ionization occurs. Contrary to Ohnaka et al 1 the multiplication is given by the second expression of eq. (3). The temperature dependence of the multiplication is mainly determined by that of the breakdown voltage, which decreases with temperature (cf. eq. 3). From our model we have an increasing multiplication with temperature leading to an increasing leakage.

4. EXPERIMENTAL RESULTS Experimental investigations were carried out at HFET with different gate length and width under different conditions. The large area devices, devices with thin barrier layers and highly doped devices with ND> 4"1018cm3 exhibit an I(V)-characteristic as shown in fig. 3. The current increases exponential with voltage. The temperature dependence of this component is shown in fig. 4 for constant bias. At low temperatures the current is temperature independent due to dominating field emission. Above 100K thermionic field emission dominates, the current increases with temperature according eq. (1). 1o

60

!

|

10 °

l 0 "2

0.5

1

0

Fig. 3.

Leakage current characteristic of a Fig. 4. large area Schottky contact A typical I(V)-characteristic of a small area device similar to I~v/V valid for generation-recombination dependence of this current is according to eq. (2) AEr=0.25eV for the dominating gr-center. "

i

"

i

8 - G pan f L =1

"

. . . . . . . . .

1.5

-VGs/mA

10[

!

!

40 ~ [A=4*10"4cm2

0

!

'

10

20

30

40

10oofr/KI Arrheniusplot for the leakage current of fig. 3. is shown in fig. 5. The characteristic is current mechanisms. The temperature which yields an activation energy of

lO

......... i

. . . . . . . . . . . . . . . . . . . . . . . . . . .

~ E T-0.25eV

. . . . . . . . . . . . . . . .

It

0

0.5

1

1.5

2

3.3

-VGs/V

Fig. 5.

Leakage current characteristic with dominating gr-components

Fig. 6.

3..5

3.7 .9 leeotr/ga

4.1

4.3

Arrheniusplot determined from the temperature dependence of the gr-current.

404

F. Buchali et al. / Gate leakage on MOVPE grown InAIAs/InGaAs-HFET

At a HFET with applied drain to source bias we have found a dependence of the gate leakage on the gate-source and drain-source voltage shown in fig. 7. The additional gate leakage increases with drain current and decreases drastcally at pinch off (cf. I(V) characteristic in fig. 2) even with increasing gate - source and gate - drain bias. Due to the leakage decrease with increasing gate bias tunneling is excluded as leakge source. The temperature dependence of the gate leakage - drain current ratio is shown in fig. 8, the variation of the drain current with temperature is very low. We have found a nearly linear increase of this ratio with temperature, which is in agreement with our model. 1.5

I

!

I

I

0.st

I

V G S - -2.5V 1

W ffi201un

•/dr

l Vns- 2V

/

'T, 0.1.5

0.3 I.

o

o

1

2

-Vcs/V Fig. 7.

0.21 3

24O

Gate leakage versus gate source voltage Fig. 8. for the biased HFET.

i

I

I

I

280

i

I

300

I

320

T~

Temperature dependence of the leakage due to impact ionization.

5. S U M M A R Y Using temperature dependent l(V)-measurements we have identified the main Schottky gate leakage mechanisms. Tunneling occurs in large area devices due to inhomogenities and in high doped devices and with thin barrier layers due to the thin tunneling barrier. The generationrecombination current, caused by a gr-center at AET=0.25eV, should be the lower limit of all leakage and needs no further consideration. But the impact ionization causes a leakage current, which is due to the low breakdown voltage of InGaAs. The amount depends on the transistor bias and on the device- and layer structure. The temperature dependence of this component was investigated for the first time in detail and a linear increase of leakage with temperature according to our model was found. It should be lowered by modyfied layer structures using channel layers with higher breakdown voltages like strained lnGaAs channels or lnP-channels. 6. R E F E R E N C E S

2 3 4 5 6 7

K. Ohnaka, J. S,hibata, J.Appl.Phys., No. 63 (1988) 4714. Y.K. Chung, D.C. Lo, S.R. Forrest, Proc. InP & Related Materials, Newort, 1991, 393. J.H. Reemtsma, W. Kuebart, H. Grosskopf, U. Koerner, D. Kaiser, I. Gyuro, Proc. lnP & Related Materials, Newort, 1991, 585. S.R. Bahl, J.A. del Alamo, lEEE Electr. Dev. Lett., No. 4 (1992) . D.J. Newson, R.P. Merrett, B.K. Ridley, B.K. Ridley, Eletr. Lett., No. 27 (1991) 1592. F.A. Padovani, R. Stratton, Sol.State Electr., 9 (1966) 695. F. Buchali, R. Behrendt, G. Heymann, Electron. Lett., No. 27 (1991) 235. S. M. Sze, Physics of semiconductor devices, John Willey & Sons, New York, 1981