IEEE Copyright

Solid State Electronics Laboratory
Department of Electrical Engineering and Computer Science
The University of Michigan,
Ann Arbor, MI 48109-2122

Abstract: InGaAs/InP PIN diodes were fabricated on epitaxial layers grown by MOCVD. DC and microwave characterization of the PIN diodes demonstrated low turn-on voltage (0.46 V), low insertion loss (< 1.2 dB up to 38 GHz), and high switching cutoff frequency (17 THz), as necessary for microwave and millimeter-wave switching and limiting applications.

I. INTRODUCTION:

PIN diodes are key elements for microwave switching and optical signal detection. Most results reported today refer to microwave PIN switches fabricated on GaAs[1,2] and optical PIN detectors[3] using InP-based materials. The use of InP-based PINs for microwave applications has drawn less attention, but offers unique and very attractive features, namely low insertion loss, due to the high mobility of InGaAs layers, reduced ohmic contact resistance on such materials, and low turn-on voltage, as necessary for limiting applications of low-power InP-based electronics. Moreover, InP- based PIN diodes can be used for switching functions and are compatible with InP HBT and HEMT millimeter wave technology. This paper addresses these features by evaluating the discrete characteristics of InP-based PIN diodes and examining their use as microwave switches.

II. EXPERIMENTAL RESULTS AND DISCUSSION:

The diodes were fabricated on PIN layers grown by our-in-house MOCVD facility. Starting from the substrate a 1 µm thick n+-InGaAs layer (2x1019 cm-3) was grown followed by 1 µm of NID-InGaAs (~ 1x1015 cm-3) and 0.15 µm of p+-InGaAs (2x1019 cm-3). TMIn, TMGa and 100% AsH3 were used for growth. n- and p-doping were introduced using DEZn and Si2H6. All layers were grown at 60 torr and susceptor was rotated at 100 rpm, which leads to good thickness and compositional uniformity of InGaAs. n- and NID InGaAs were grown at 570oC while p-InGaAs was grown at rather lower temperature (530oC) to enhance maximum Zn incorporation and to reduce Zn diffusion into the underlaying NID layer. The diodes were defined by 5 and 8 µm circles on the p-layer and wet etching was used to form the mesas. Air bridge technology was used to contact the top p-layer. An SEM photograph of a typical PIN diode is shown in Fig. 1. In addition to discrete diodes, a single pole single throw (SPST) switch was fabricated and studied.

The impact of i-layer thickness on OFF-state capacitance (Cd) was studied theoretically and experimentally. The 8 µm diameter diodes with 0.3 µm thick i-layer resulted in Cd values of 30 fF. By increasing the i-layer thickness up to 1 µm and reducing the diode diameter to 5 µm, Cd was reduced to 2 fF. The latter design was chosen for switching studies since it offers small displacement current, thus minimizing the losses, as well as, higher operating capability due to its smaller RC time constant. Further increase in the thickness of the i-layer was found to be unnecessary due to the diode parasitics becoming of the same order as the i-layer capacitance.

DC characterization of the PIN diodes (see Fig. 2) revealed a turn-on voltage of 0.46 V at I=10 µA. This value is smaller than that obtained using GaAs PINs where the threshold voltage is typically 0.7 to 0.8 V. The reverse characteristics of the diodes showed a breakdown voltage of 23.5 V at 10 µA current.

The diodes were characterized at high frequency by on-wafer probing. Equivalent circuits were extracted for the ON- and OFF-states and are shown in Fig. 3. Ls, Rs, Cp represent here the diode parasitics, while Rd and Cd represent i-layer characteristics. By changing the 5 µm diode state from ON (I=30 mA) to OFF (V=-10 V) the i-layer characteristics were found to vary from capacitive (Cd=2 fF) to resistive (Rd=4 Ω). The access resistance Rs was 0.5 Ω and the air bridge inductance was 21 pH in both states. The parasitic capacitance of the diode was estimated to be 4 fF.

Based on the above equivalent circuit one can estimate useful parameters of the InP-based PIN diodes indicating their high frequency potential. The switching cutoff frequency of the PINs was estimated from

where (Rs+Rd) is series resistance in the low impedance state and Cd is capacitance in the high impedance state. fcs was found to be of the order of 17 THz for the 5 µm diameter diode. Considering that PIN diodes are used at 1/100 to 1/50 of their switching cutoff frequency one can estimate that the InGaAs/InP PIN diodes of this work can be used up to 170-340 GHz range. This indicates millimeter-wave capability and improved high frequency operation compared with estimated values 3.6 and 1.6 THz for high performance 20 and 9 µm diameter GaAs PINs respectively[1,2].

The quality factor of the PIN diodes is given by

and is related to its impedance ZON =(R1 + j X1) and ZOFF=(R2 + j X2) in the ON and OFF states respectively. Evaluating the quality factor of the InP based PIN diodes at 35 GHz one find a value of 1500 which again suggests the high superiority of the InGaAs/InP approach compared with other material systems and designs.

Since PIN diodes are used for switching and limiting applications, it is highly important to evaluate their large-signal characteristics. The large-signal microwave input impedance of the diodes was measured at different power levels in order to evaluate their power handling capabilities. An on-wafer testing system with electro-mechanical tuners was used for this purpose. The diode impedance was found as the complex conjugate of the `pull' tuner impedance resulting in minimum power reflected from the diode. In this fashion, the large-signal impedances at 10 GHz were determined for different power levels and are shown in Fig. 4. PIN diodes switched into ON-state demonstrated no significant variations up to 17 dBm input power delivered to the device. On the other hand, the large signal impedance of the `Zero-bias' OFF-state changed from a capacitive to the self-biased inductive ON-state when the input power was increased.

An SPST switch was finally realized by implementing the above described diode in a 50 Ohm coplanar waveguide which was integrated on the same SI InP substrate. The diodes were switched from ON- to OFF-state and their isolation and insertion loss characteristics were measured and shown in Fig. 5. When the switch is turned on, the insertion loss was less than 1.2 dB and the return loss was better than 16 dB for frequencies up to 38 GHz. When the switch was turned off the return loss was less than 0.6 dB and the isolation was larger than 17 dB up to 37 GHz.

III. CONCLUSIONS:

In conclusion we have grown and fabricated 5 µm diameter InGaAs PIN diodes with low turn-on voltage of 0.46 V, high breakdown voltage of 23.5 V, and low insertion loss of 1.2 dB as demonstrated by SPST switch performance evaluation up to 38 GHz. A high switching cutoff frequency of 17 THz and a high quality factor of 1500 at 35 GHz were evaluated for these devices. These excellent high frequency characteristics, compared with GaAs devices, are due to the high electron mobility and low ohmic contact resistance of the MOCVD grown InGaAs epitaxial layers.

ACKNOWLEDGMENT: The authors are grateful to Mr. Steve Robertson for the assistance in high frequency characterization.

Reference:

[1] J.Putnam, M.Fukuda,P.Staecker,Y.Yun, "A 94 GHz Monolithic Switch with a Vertical PIN Diode Structure", GaAs IC Symposium, pp. 333-336, 1994.

[2] K.W.Kobayahi, L.Tran, A.K.Oki, D.C.Streit, "A 50MHz-30GHz Broadband Co-Planar Waveguide SPDT PIN Diode Switch with 45-dB Isolation", Microwave and Guided Wave Letters, Vol.5 No.2 February 1995, pp.56-58

[3] A.L.Gutierrez-Aitken, J.Cowles, P.Bhattacharya, G.I.Haddad, "High bandwidth InAlAs/InGaAs PIN-HBT monolithically integrated photoreceiver", Proceedings of the 6th International Conference on Indium Phosphide and Related Materials, pp. 247-250, 1994

This work is supported by US Army Research Office (Contract no. DAAL-03-92-G-0109) and Daimler Benz AG.