This paper reports on the recent development of monolithic frequency tripler array employing a back-to-back configuration of varactor diodes. Even harmonic idler

circuits are unnecessary in this design. Furthermore, no external dc bias is required. The arrangement results in highly efficient, easily-fabricated and inexpensive frequency triplers

Hwu, R. J.

Luhmann, N. C., Jr.

Rutledge, D. B.

Sadwick, L. P.

Sokolich, M.

English

Caltech Authors - Main

T4.3 
Highly Efficient Frequency Triplers in the Ifillheter Wave Region- 
Incorporating A Back-to-Back Configuration of Two Varactor Diodes 
p. J .  Hvu, L. P. Sadvick, and N. C. Tllhuum, Jr. 
University of California, Los Angeles 
Department of Electrical Engineering 
Boelter Hall 7535 
Los Angeles, California 90024 
Telephone: (213) 825-9737 
D. B. Rutledge 
California Institute of Technology 
Pasadena, California 
H. Sokolich 
Hughes Aircraft Company 
Torrance, California 
ABSTRACT 
This paper reports on the recent development of monolithic frequency tripler array 
employing a back-to-back configuration of varactor diodes. Even harmonic idler 
circuits are unnecessary in this design. Furthermore, no external dc bias is 
required. The arrangement results in highly efficient, easily-fabricated and 
inexpensive frequency triplers. 
INTRODUmION 
Conventional sources of millimeter wavelength power for heterodyne receiver 
local oscillator applications are expensive and short-lived klystrons or Gunn 
devices which provide limited power at frequencies beyond - 90-120 GHz. An 
alternate approach is to use efficient, broad band frequency multipliers in 
conjunction with more reliable, lower frequency oscillators to provide power [l-31. 
This paper reports the development of highly efficient frequency triplers which 
employ a back-to-back configuration of two varactor diodes. By monolithically 
fabricating large arrays of diode pairs, watt-level output powers may be obtained. 
VARACXOR DIODE FREQUWCY TRIPLER 
The varactor diodes we use incorporate an ultrathin electron-blocking barrier 
layer [4]. This blocking layer can be formed by an insulator, a semiconductor with 
a very wide band gap, or a Mott barrier. Therefore, the device can be switched 
rapidly between two capacitance states which correspond to accumulation of 
electrons at the barrier and depletion of the epilayer by the applied bias. This 
results in a highly nonlinear capacitance-voltage characteristic that is needed for 
efficient harmonic generation. 
Due to the blocking barrier of the structure, two diodes can be operated back- 
to-back generating a sharp symmetric spike in the capacitance-voltage curve, which 
eliminates even harmonics and thus favors tripler operation. The symmetric 
capacitance-voltage curve experimentally observed from two diodes connected back- 
to-back demonstrates the concept of the back-to-back operation. The height and 
width of this capacitance-voltage curve can be adjusted by doping control alone. 
This arrangement needs no external ohmic contact and no even harmonic idler 
circuits resulting in highly efficient, easily fabricated and inexpensive frequency 
triplers. 
100 
In the past, we have proven the feasibility of near-millimeter wave operation 
with a silicon MOS diode used in a waveguide/whisker configuration as a frequency 
doubler [4]. This diode (Fig. 1) had an intrinsic cut-off frequency of 320 GHz. 
The capacitance-voltage characteristic is shown in Fig. 2 .  A maximum efficiency of 
15% was experimentally obtained at 95 GHz which is in good agreement with the 
theor6tical predictions [ 5 , 6 ] .  
Si02 Siiicon - 
0 . .  Ohmic 
0 . 0  Contact 
0 .  
Fig. 1 Structure of the Si MOS diode. 
Experiment 
Space Charge Theory 
[Howson. Owen, 
Wright 1968l 
tc,: + 1 
I I m I I c 
Depletion - 2  -l v z s  ' 1 2 Accumul. 
Fig. 2 Capacitance-voltage characteristic of the Si MOS diode. 
Recently, a novel concept which allows the construction of a Barrier-Intrinsic- 
$ (BIN) diode on 111-V compound semiconductor material entirely by selective 
doping during MBE growth (Fig. 3 ) has been described by us [4]. A Mott-barrier is 
formed by a thin intrinsic layer sandwiched between the top metal contact and a 
charge sheet created by selective doping. CaAs and InP are superior to Si (as used 
in the earlier studies) due to the higher mobility and maximum velocity which will 
reduce the transit time and render negligible the parasitic resistance of the back 
contact. 
The advantages of the BIN diode over the Schottky diode are seen in (1) the 
stronger nonlinearity of the C-V curve, which generates harmonics more efficiently 
(especially the triple of the pump frequency) and (2) the ability to reach a high 
capacitance state before forward conduction sets in, making possible the capacitive 
tripler. As the steepness of the transition between the capacitance states 
increases with lower temperature, cooling will improve the efficiency of the 
frequency multiplication. 
101 
~~ + 
Schottky 
. Metai i 
I++++ ++ + i k:ztB Ohmic 
Contact ++++ t i -+ ++ ++$++ + 
Fig. 3 Structure of the GaAs BIN diode. 
The initial proof-of-principle BIN diode structure was grown on GaAs substrate 
with a 1500 A epitaxial layer from a conservative fabrication design. This gives 
an intrinsic cut-off frequency of 640 GHz. The experimentally measured C-V curve 
is shown in Fig. 4. Projected improvement of the device performance can be 
obtained with realizable diode parameters by advanced desip and improved 
fabrication technique. For example, a GaAs BIN diode structure with a 1000 A thick 
epitaxial layer has an intrinsic cut-off frequency of 960 GHz. 
2000 
1000 
N+ - 2 x 1018 cm-3 
f,i = 640 GHz .. 
I I I I I I I I I 1 
-2 0 2 
VOLTAGE (V) 
Fig. 4 Capacitance-voltage characteristic of the GaAs BIN diode. 
The metal grid we have designed for the BIN diode tripler consists of a columnar 
mesh of aluminum strips with Schottky electrodes on each end as shown in Fig. 5. 
The period of the grid is chosen to be about half the dielectric wavelength to 
avoid exciting substrate modes. The two neighboring Schottky electrodes are 
designed to provide the back-to-back configuration for two BIN diodes. The design 
requires only one metal pattern which greatly facilitates the fabrication. The 
symmetric capacitance-voltage and conductance-voltage curves from two back-to-back 
connected GaAs BIN diodes illustrated in Fig. 6 demonstrate the concept of the 
back-to-back operation. 
In this work, we have extensively used a nonlinear analysis program [7] in order 
to investigate and optimize the efficient tripling operation from two BIN diodes 
102 
under the back-to-back configuration. It should be mentioned that an interpolation 
sub-program has been developed and employed in the nonlinear analysis program in 
order to use the experimentally measured C-V data. The measured series resistance, 
Rs, is also used for the following computatLon cases. 
Due to the symmetric capacitance-voltage characteristic of two back-to-back 
connected GaAs BIN diodes, even harmonic currents cancel, so that even harmonic 
idler circuits are unnecessary. This- can also .be-shown from the large signal 
analysis results in Fig. 7, which shows that the efficiency is insensitive to 
second harmonic impedances. 
It should be mentioned that the input power is 10 mW for the above analysis 
studies. In addition, Fig. 8 shows the tripling efficiency versus the input power 
level at various input frequencies. The input and output tuning was optimized at 
each frequency. As shown in Fig. 8, the efficiency is highest with low input power 
levels (5-10 mu). Over the output frequency range of 100-150 GHz, an efficiency 
greater than 15% is predicted for the GaAs BIN array which has recently been 
fabricated. The highest predicted efficiency is 24% at 100 GHz obtained with an 
input power of 9.0 mW, which shows excellent agreement with the analytical 
predictions ( 5 . 6 1 .  
- BIN diode 
5OOpm 
\ 
\ 
' 
20pm - 
500pm I 
c 
Fig. 5 Design of metal grid for the BIN diode tripler array. 
103 
1000 
500 
0 
-2 0 
VOLTAGE(V) 
2 
Fig. 6 Symmetrical capacitance-voltage characteristic of two GaAs B I N  diodes with a 
back-to-back configuration. 
Using the quasi-optical diode-grid tripler configuration shown in Fig. 9 for a 
proof-of-principle test, a watt output power at an output frequency of 100 GHz with 
a tripling efficiency 8.5% has been experimentally obtained with approximately 4 mW 
incident power on each diode. This experimental measurement is in good agreement 
with the large-signal nonlinear circuit analysis prediction (see Fig. 8). It 
should be mentioned that this preliminary result was performed in the low pumping 
power region. However, much higher efficiency (- 24%) is predicted for higher 
input power level (- 9 mW). 
In addition, to direct the concept of the monolithic diode-grid frequency 
tripler array with a back-to-back configuration into advanced solid state structure 
such as heterojunction device, the capacitance-voltage relation of the 
heterojunction device has been studied in detail. To support this work we have 
developed the cabability to predict the capacitance-voltage characteristics which 
have been incorporated into the large-signal nonlinear circuit analysis program for 
efficiency studies and optimization. Here, it should be noted that the narrow 
width of the capacitance-voltage depedence for the AlGaAs/GaAs heterojunction 
should provide for highly efficient frequency multiplication even at low input 
power. 
SUlMARY AND CONCUISION 
A watt output power at an output frequency of 100 GHz has been achieved for the 
grid fabricated in the proof -of -principle study of the tripling operation concept 
employing the back-to-back configuration. The device performance is limited by the 
parameters of the fabricated diodes. Significant improvement can be obtained with 
realizable diode parameters and optimized pumping conditions. In addition, the 
104 
application of advanced heterojunction devices as highly efficient frequency 
multipliers is also under active investigation. 
30 
20 
I I I 1 I 
- 
10 
n l  I I I I I v 
100 101 102 103 104 105 io6 
SECOND HARMONIC IMPEDANCE (0) 
Fig. 7 Tripling efficiency versus second harmonic resistance and reactance. 
[l]. D. B. Rutledge and S. E. Schwarz, "Planar Multimode Detector Arrays for 
Infrared and Millimeter-Wave Applications," IEEE J. Quantum Electro. OE-17, pp. 
121. C. F. Jou, W. W. Lam, H. Chen, K. Stolt, N. C. Luhmann. Jr., and D. B. 
407-414, 1981. 
Rutledge, "Millimeter-Wave Monolithic Schottky Diode-Grid Frequency Doublers, " IEEE 
Trans. on Microwave Theory and Tech., m - 3 6 ,  No. 11, pp. 1507-1516, 1988. 
(31. W. W. Lam, C. F. Jou, H. Chen, K. Stolt, N. C. Luhmann, Jr.. and D. B. 
Rutledge, "Millimeter-Wave Monolithic Schottky Diode-Grid Phase Shifters." IEEE 
Trans. on Microwave Theory and Tech., mT-36, No. 5, pp. 902-907, 1987. 
[4]. R. J. Hwu, C. F. Jou, W. W. Lam, U. Lieneweg, N. C. Luhmann, and D. B. 
Rutledge, "Watt-Level Millimeter-Wave Monolithic Diode-Grid Frequency Multipliers," 
1988 IEEE MTT-S Digest, pp. 533-536, 1988. 
[5]. R. P. Rafuse and D. H. Steinbrecher, "Harmonic Multiplication with Punch- 
Through Varactors," Intl. Solid-state Circuit Conf., Digest of Technical Papers, 
[6]. K. Schunemann and B. Schiek, "Optimal Efficiency of Charge-Storage Multipliers 
[7]. H. Siegel, A. R. Kerr, and W. Hwang, "Topics in the Optimization of MM Wave 
Mixers," NASA Technical Paper 2287, 1987. 
pp. 68-69, 1966. 
11," A.E.U. 2, pp. 293-302, 1968. 
105 
25 
s 
Y 
I- 
O 
0.0 5.0 10.0 15.0 20.0 
INPUT POWER (mW) 
Fig. 8 Tripling efficiency versus input power at various input frequencies. 
,DIODE GRID 
DIFFRACTION 
GRATING TUNING SLABS 
TUNING SLABS 
INPUT FUNDAMENTAL 
Fig. 9 Configuration of the quasi-optical diode-grid tripler. 
This work has been supported by the California MICRO program. 
106 


Highly efficient frequency triplers in the millimeter wave region incorporating a back-to-back configuration of two varactor diodes

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Highly efficient frequency triplers in the millimeter wave region incorporating a back-to-back configuration of two varactor diodes

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