We have developed a new zero bias millimeter wave diode based on quantum tunneling in an InAs/AlSb/GaSb nanostructure. It is ideal for square law radiometry and passive millimeter wave imaging. Excellent sensitivity has been demonstrated at present up to 110 GHz, with higher bandwidth predicted for smaller area diodes

Chow, D. H.

Croke, E. T.

Dunlap, H. L.

Holabird, K. S.

Morgan, M. A.

Schulman, J. N.

Weinreb, S.

Caltech Authors - Main

Quantum Tunneling Sb-Heterostructure Millimeter-Wave Diodes 
J.  N. Schulman, E, T. Croke, D. H. Chow, H. L. Dunlap, K. S. Holabird 
HRL Laboratories, Malibu, CA 
M. A. Morgan, S. Weinreb 
California Institute of Technology, Pasadena, CA 
ABSTRACT 
We have developed a new zero bias millimeter wave 
diode based on quantum tunneling in an InAslAlSblGaSb 
nanostructure. It is ideal for square law radiometry and 
passive millimeter wave imaging. Excellent sensitivity has 
been demonstrated at present up to 110 GHz, with higher 
bandwidth predicted for smaller area diodes. 
n 
AlSb 
EF 1 p-GaAlSb 
VP- .  YI -1nAs 
--J 
Figure 1. Schematic band edge diagram of InAs/AISb/GaAISb 
heterojunction in forward bias. 
INTRODUCTION 
We have recently proposed a new type of millimeter 
wave diode based on epitaxial layers of the InAs/AlSb/GaSb 
family of nearly-lattice matched IIl-V semiconductors [ 11. 
The principal of operation is similar to that of an Esaki 
backward diode in that the controlling current mechanism is 
interband tunneling between adjacent energetically offset 
semiconductor regions. The Type I1 band gap line-up 
between InAs and GaSb, where the conduction band 
minimum of InAs lies energetically below the valence band 
maximum of GaSb, creates a natural asymmetry in the 
current flow with bias direction, Fig. 1. Forward bias current 
is low because the conduction band electrons from the InAs 
are blocked by the GaAlSb band gap. Backward bias current 
is large due to GaAlSb valence electrons flowing freely into 
the InAs conduction band. The resulting curvature in the 1 0  
characteristic is ideal for zero bias direct detection, or mixing 
with low local oscillator power [2]. Previously reported DC 
0-7803-7050-3/0 1/$10.00 0200 1 IEEE 
characteristics were promising, especially the high curvature 
coefficient and relatively low junction resistance, but the 
high frequency performance was yet to be ascertained. In this 
paper we present the results of S-parameter and voltage 
sensitivity measurements from 1 to 110 GHz. They show 
that diodes with 2x2 pm2 area have excellent response at 110 
GHz, and higher frequencies should be achievable with 
smaller areas. 
S A M P L E S  
The basic structure of the diodes was described 
previously [l]. There is great flexibility in the design 
parameters available, and we have only begun to explore the 
design space. The device can be grown on lattice matched or 
mismatched substrates including GaAs and InP. In this it is 
similar to resonant interband tunneling devices that we have 
grown and that have been monolithically integrated with InP- 
based high electron mobility transistors [3]. At this time we 
have standardized on a family of structures that have the layer 
design, doping, alloy concentrations, and layer widths of 
sample SB2407 [l]. SB2407 had a nominal AlSb barrier 
thickness of 20A. We have grown a series of similar samples 
but with the AlSb thickness varied from 20 to 50A. The 
main effect of the barrier thickness is in controlling the 
magnitude of the tunneling current, as seen in the video 
resistance Rv. We have plotted Rv, as determined from the 
slope of the DC 101’) curve at zero bias, versus AlSb barrier 
thickness in Fig. 2. In the simplest model ignoring series 
resistance, siv should exponentially increase as the thickness 
increases. Figure 2 confirms this and also illustrates the 
reproducibility and control of the MBE process for this 
purpose. The data points are all reasonably close to the fit 
represented by the solid line, even though the samples were 
grown over a several month period and in-situ monitoring 
and feedback techniques were not employed. This is of great 
benefit for imaging array applications where reproducibility 
and uniformity can greatly simplify design and reduce cost. 
Note that the fit extrapolates to less than 200 SZ.pm2 for 
LB=l  OA. 
The voltage sensitivity is the DC voltage out from the 
diode per power reaching it. It is proportional to the 
curvature coefficient, y = dZI/dVZ/(dUdV), which should be 
as large as possible. We have found that y is only weakly 
dependent on bamer thickness, but that there is a noticeable 
dependence on growth order as follows. The samples 
numbered below (SB)2700 in Fig. 2 were grown in the order 
Substrate/InAs/AlSb/GaAlSb/GaSb/lnAs [I]. The curvature 
coefficients were 16.2, 17.8, and 19.6Nolt for the three 20A 
35.1 .I IEDM 0 1-765 
barrier thickness cases, and 20.7 and 21.7Nolt for the 25A 
and 30A cases, respectively. The layers for the four samples 
2717, 2774, 2775, and 2776 were all grown in the reverse 
order. Their curvature coeEcients were significantly larger, 
27.5 and 27.4Nolt for the two 30A bamer samples, and 
28.0 and 29.3fVolt for the 40A and 50A samples. We do not 
have an explanation for this effect, but it is well known that 
the growth order as well as the precise chemical nature of the 
non-common anion interface affect several of the electrical 
and optical properties [4]. 
105 
1 04 
I 03 
R" 
$2 
1 02 
10 20 30 40 50 
AlSb L, A 
Figure 2. Diode video resistance, dV/dI, determined from the slope of the 
DC I(V) curves at zero bias versus AlSb barrier thickness. 
MEASUREMENTS 
We present the high frequency measurement results in 
detail for one particular diode, SB2775, for four device cross- 
sectional areas in Table 1. The diodes were tested below 50 
GHz with a vector network analyzer and probe station. The 
power delivered to the chip was between 0.3 and 1.2 pW. 
For higher frequency impedance measurements an Oleson 
WR-8 VNA extension was used with the network analyzer. 
To avoid spurious tones a backward wave oscillator was 
employed for the higher frequency sensitivity data. 
TABLE I 
Area C,ff R , n  R , n  C,fF 2,Q AVW R.,,VW 
S-PARAMETER FIT, DIODE IMPEDANCE AND SENSITIVITY 
pm'(#) (fir) (fit) (fio (fit) YSGHZ 95Gm 95GHz 
2xZ(a) 6 63 9567 I I  26.2-j92.4 2768 7576 
2x2@) 6 69 I3204 11 30.2-j91.0 2660 6483 
3x4(a) 17 I I  2249 39 8.28-j23.2 750 1781 
3x4@) 17 I I  2284 37 8.14-j23.1 837 2013 
4x4(a) 22 9 IS2 57 6.19-j13.6 336 907 
4x4@) 22 10 1 6 0  57 6.57-j13.0 385 986 
5xXa) 31 7 8C8 103 4.62-j6.m 101 331 
5x5@) 31 7 781 97 4.83-j5.98 106 333 
The diode small-signal equivalent circuit used for this 
analysis is shown in the inset to Figure 3. The package 
capacitance, C,, was estimated numerically from the diode 
geometry. A nominal value of Lp=20 pH was determined for 
the lead inductance as a best fit to the data. RI was calculated 
as the ratio of the open-circuit voltage and short-circuit 
current produced in the diode under RF drive. This method 
was considered to be more accurate than extraction from the 
S-parameter measurements, given the typically large values 
of RI compared to 50 52. The geometric trend in RJ revealed 
that the smallest diodes, drawn as 2x2 pm2, came out 
smaller after fabrication. The data suggest that they are 
closer to about 3 pm2 in size and are less uniform. 
I I i 1 
2000 
1000 
500 
PV 
VNV 
200 
100 ?.----..'. 
1 2 6 10 20 50 ,100 
f, GHz 
Figure 3. Measured unmatched voltage sensitivity of SB2775 versus 
frequency. Inset is diode circuit model. 
The remaining two parameters, Rs and CJ, were 
calculated for each frequency from the data. The calculation 
was most reliable at the higher frequencies where the 
reactance of CJ was not too large. The final arameter values 
are those obtained at 95 GHz for the 2x2 pm diodes, and the 
average of those obtained at 30,40, and 95 GHz €or the other 
diodes. The measured voltage sensitivity, I)v, was determined 
as the open-circuit DC voltage divided by the incident RF 
power. 
The results of the diode model parameter fitting are 
listed in Table 1, with the 95 GHz diode impedances and 
sensitivities. We compared the measured sensitivity to a 
simple model as follows. The optimal, perfectly matched 
sensitivity and the measured sensitivity are related 
. The matched sensitivity into an open 
load is given by [2] 
P 
by a = BV.opr (1 -Is, I I ' )  
S11 can also be written in terms of a lengthy 
combination of the model parameters. Figure 3 plots the 
unmatched sensitivity versus frequency data with the model 
curve obtained by substituting for Pv,,,,~ and SI I .  Note that 
the sensitivity values themselves are not used in the 
parameter fitting. Excellent agreement between measurement 
and simulation is obtained for the diodes with areas above 
766-IEDM 01 35.1.2 
2x2 ym'. The simulation for the 2x2 ym2 diodes rolls off 
slightly faster with frequency than the measured values 
indicate. Better agreement at the higher frequencies may 
depend on more accurate knowledge of the nature of the 
diode contacts. There seems to be an instrumental 
discrepancy at 30 GHz €or the two smaller diodes. A check of 
the sensitivity versus input power level shows that the 2x2 
pm2 diode obeys the square law dependence up to about - 15 
dBm. 
OPTIMAL SENSITIVITY MEASUREMENTS 
At low frequency, and ignoring RS relative to RI, Eq. 1 
reduces to b,opt = RJ y 1 2. At high frequency the frequency 
term in the denominator dominates and P v , ~ ~ ~  = y /8 
(n f CJ)'RS. The frequency dividing these two regimes is 
f, = 1/2RJR,R,c?. The limits show that RJ is critical 
for obtaining large pv,opt in the low frequency regime, whereas 
RS and CJ are more important for high frequencies. Since CJ 
should be proportional to area, and RS should be proportional 
to inverse area or less, it is clear that decreasing the diode 
area will lead to further enhancement of the frequency 
response. 
S I I  has the simple low frequency limit S I I =  
(RJ+R~-~OQ)/(R~+R~+~OQ). Substituting this in b and 
taking the further limits that usually RJ>>~OSZ and RI>> RS 
produces the very simple low frequency estimate pV = 100 x 
y , in Volts/Watts. The low frequency data points of Fig. 3 
are consistent with the value of y=28.O/Volt for SB2775. 
This gives additional confidence that the low frequency 
estimate for the optimal sensitivity, PV, , ,~~  =RJ y /2, is 
applicable for f<fD. For example, an RJ of 10,OoOQ would 
yield an optimal sensitivity of about 10,000~28/2= 140,000 
Table 1, last column, gives an estimate for the optimal 
sensitivity at 95 GHz. This is well beyond fD which ranges 
from 15 to 25 GHz. Thus the optimal sensitivity is 
decreasing approximately as l/?. The p ~ , ~ ~ ~  estimate is 
obtained from the measured sensitivity value in the adjacent 
column enhanced by the directly measured reflection 
coefficient. SI  I is related to ZD by ~ l l ~ ( ~ D - ~ ~ ~ ) / ( ~ D + ~ ~ ~ ) .  
The 2x2 ymz diodes are seen to have optimal sensitivities of 
about 7000 VJW. 
vm. 
CONCLUSIONS 
The high sensitivity, low cost, and reproducibility of 
this new diode indicate that it will find a variety of usages 
where zero bias direct detection is required. The flexibility of 
independent barrier width and diode area control of RJ 
provides the ability to optimize among several possible 
desired properties, including low or high frequency 
sensitivity, dynamic range, square law linearity, noise, 
bandwidth, temperature sensitivity, etc. We would like to 
acknowledge essential technical discussions with John 
Lovberg of Trex Enterprises. 
REFERENCES 
(1) J. N. Schulman and D. H. Chow, "Sb-heterostructure interband 
backward diodes," IEEE Electron Device Letters 21, pp. 353-355, July 
2000. 
(2) 1. Bahl and P. Bhartia, Microwave Solid State Circuit Design, John 
Wiley and Sons, New York, 1988, ch. 1 1 .  
(3) P. Fay, Lu Jiang, Y. Xu, G. H. Bemstein, D. H. Chow, J. N .  Schulman, 
H. L. Dunlap, and H. J. De Los Santos, "Fabrication of monolithically 
integrated InAIAs/InGaAs/InP HEMTs and InAs/AlSb/GaSb resonant 
interband tunneling diodes", IEEE Trans. Electron Devices 48, pp. 
1282-1284, June 2001. 
(4) J. Steinshnider, M. Weimer, R. Kaspi, and G. W. Turner, "Visualizing 
interfacial structure at non-common-atom heterojunctions with cross- 
sectional scanning tunneling microscopy", Phys. Rev. Lett. 85, pp. 
2953-2956, October 2000. Also, see references 6-14 therein. 
35.1.3 IEDM 01-767 


Quantum tunneling Sb-heterostructure millimeter-wave diodes

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Quantum tunneling Sb-heterostructure millimeter-wave diodes

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