IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 49, NO. 6, JUNE 2001 1101 A -Band Indium–Antimonide Junction Circulator Chin K. Yong, Member, IEEE, Robin Sloan, Member, IEEE, and Lionel E. Davis, Fellow, IEEE Abstract—Following a brief overview of the underlying theory, experimental results are presented for the first time showing circu- lator action in a semiconductor junction structure. An axially mag- netized indium–antimonide disc fixed in a three-port finline struc- ture and cooled to the temperature of boiling nitrogen, 77 K gives circulation across -band. For a dc magnetic bias of 0.73 T, a 15-dB isolation is recorded from 28 to 40 GHz, or a fractional band- width of at least 35%. Typical insertion loss is less than 1.5 dB from the WG22 reference plane at the test fixture ports. Continued op- eration above 40 GHz is predicted, but has not yet been measured. Measurement suggests that circulation is evident even where the effective propagation constant is imaginary, although better theo- retical agreement is achieved when this is a real quantity. This new device makes millimeter-wave broad-band circulation a possibility and confirms the current model based upon the Drude–Zener ap- proximation. A theoretical example is then given for a design op- erating to 140 GHz, yielding a fractional bandwidth of 110%. Index Terms—Finline, InSb, millimeter wave, semiconductor junction circulator. I. INTRODUCTION J UNCTION circulators employing ferrites have been widely studied and are well understood [1]–[4]. Ferrite designs provide good isolation, low insertion loss, and broad-band solutions at microwave frequencies. However, since the max- imum saturation magnetization available is about 5500 G [5], increasing the frequency into the millimeter-wave region begets progressively narrow-band performance for ferrite junc- tion circulators unless very high static bias fields are applied [6]. To date, the most broad-band ferrite junction circulator reported has a 10-dB isolation bandwidth of 117% between 5–19 GHz [3], [4]. This performance was achieved by extending the useful frequency into the region where the effective permeability is negative, and by using a unique device configuration that as- sures a nearly uniform internal magnetic field. There are also millimeter-wave junction circulators that exploit the high in- ternal anisotropy magnetic field of hexagonal ferrites [6], [7]. Although such devices have the advantage of not needing ex- ternal magnets, their bandwidth is narrow. For example, the cir- culator reported in [6] has a 20-dB isolation bandwidth of 5%, centered at about 31 GHz. The semiconductor junction circulator was first proposed by Davis and Sloan [8], [9]. The structure of their device is the electromagnetic dual of the ferrite circulator studied by Bosma Manuscript received October 19, 1999; revised August 11, 2000. C. K. Yong was with the University of Manchester Institute of Science and Technology, Manchester M60 1QD, U.K. He is now with the Wireless Semicon- ductor Division (Research and Development), Agilent Technologies Malaysia, 11900 Penang, Malaysia (e-mail: chin-kong_yong@agilent.com). R. Sloan and L. E. Davis are with the Department of Electrical Engi- neering and Electronics, University of Manchester Institute of Science and Technology, Manchester, M60 1QD, U.K. (e-mail: sloan@fs5.ee.umist.ac.uk; l.davis@umist.ac.uk). Publisher Item Identifier S 0018-9480(01)03978-3. [1]. Employing the Drude–Zener model of semiconductor and adopting the Green’s function approach, a few narrow-band designs at millimeter-wave frequencies were produced. The magnetized semiconductor does not exhibit the demagnetiza- tion phenomenon experienced with the ferrite. In [9], a term representing the electron collision frequency was introduced into the analysis to model the losses due to electron collisions in the semiconductor. The simulated results suggested that the electron collision frequency must be small in order for the device to have low-insertion loss. Examination of these losses facilitates the choice of semiconductor for circulating action. In later papers [10], [11], Sloan et al. illustrated that the semi- conductor junction circulator exhibits the frequency-tracking behavior analogous to that of the ferrite circulator demon- strated by Wu and Rosenbaum [2]. By utilizing this property, millimeter-wave semiconductor junction circulators with theoretical bandwidths greater than an octave were shown to be theoretically possible. In [11], the perfect circulation conditions for the region where the effective permittivity is negative were presented, and it was illustrated that frequency tracking of a semiconductor junction circulator is also feasible in such a region. In this paper, the relevant material parameters are discussed, the theory of the semiconductor junction structure is reviewed, and intriguing experimental results demonstrating such a device and confirming the validity of the Drude–Zener model are pre- sented for the first time. A theoretical design is then presented showing circulation to 140 GHz. II. PERMITTIVITY TENSOR OF THE MAGNETIZED SEMICONDUCTOR The gyromagnetic behavior of a magnetized ferrite arises from the precessional motion of the spinning electrons. However, for a magnetized semiconductor, it is the cyclotron motion of the mobile electrons that gives rise to its gyroelectric behavior. Using the Drude–Zener model [12], [13], the tensor permittivity can be written in the form (1) where (2) (3) (4) The symbols , , and denote the permittivity of free space, tensor permittivity, and static relative permittivity of the semi- 0018–9480/01$10.00 ©2001 IEEE