In/sub 0.53/Ga/sub 0.47/As junction field-effect transistors as tunable feedback resistors for integrated receiver preamplifiers

Abstract

Absfmcf-We have fabricated I I I O .~&~O .~~A S active feedback junction field-effect transistors (JFET's) for use in integrated transimpedance photoreceivers. By varying the gate-to-source voltage VGS, the resistance can be continuously tuned between 3 and 40 kO with a drain-to-source capacitance of <lo fF. The temperature coefficient of resistance is hetween -5 and -20 OPC (for V~S less than the pinch-off voltage). The combination of large resistance and low capacitance can result in high receiver sensitivity without sacrificing amplifier dynamic range. The feedback FET was fabricated adjacent to l.&pm gate length JFET's with transconductances of 110 m S h m , gate-to-source capacitances of 1.3 p F h m , and dc amplifier voltage gains of 100. The compstsbility of these transistor structures indicates that an integrated preamplifier with dynamically tunahle bandwidth can be realized. N INTEGRATED optical receiver fabricated using InPA, nd related compounds that has a sensitivity equivalent to that of comparable hybrid circuits is yet to be reported. Several groups have attributed the experimentally observed low sensitivity values of such optoelectronic integrated circuits to excessive noise currents-a dominant source being the Johnson noise produced by a small passive feedback resistor [ 11, [ 2 ] . A problem with passive feedback is that large resistors can have a parasitic shunt capacitance, which decreases the receiver dynamic range and bandwidth [ 3 ] . In contrast, the drain-to-source resistance R D~ of a field-effect transistor (FET) operated in its linear regime can be dynamically tuned over a wide range with negligible variation in drain-to-source capacitance CDS , which, by appropriate device design, can be made extremely small.An additional advantage of active feedback is that it is substantially more compact than a conventional passive resistor. For example, the feedback resistor in integrated preamplifiers is commonly fabricated using the FET channel layer [4], [ 5 ] . The resistance per square of such an element is equal to the ratio of channel resistivity p to thickness t. Accordingly, a 10-kR resistor requires 35 squares assuming p = 0.01 R . cm and t = 0.3 pm. With these same design parameters, a 10-kR active feedback FET requires as few as two squares, since the Manuscript channel thickness can be modulated by varying the gate-tosource bias VGS . Smaller passive resistors have been demonstrated, but require more complex growth and processing [ 2 ] .A final advantage of active resistors is their voltage tunability. The method for tuning a passive resistor is to physically change the length of the device, commonly done by shorting sections, or by trimming the metal contacts. The active feedback transistor, however, can be tuned over a continuous range of resistances by simply adjusting VGS . An increase of reverse bias decreases the channel thickness, resulting in an increase of resistance, and vice versa. This continuous tunability allows one to trim the receiver gain to optimize performance at ...

An analysis of inductive peaking in photoreceiver design

“…1. This design employs active feedback, which has been shown to have performance superior to that of a passive feedback resistor [9]. As is typical of photoreceiver preamplifier designs, the transimpedance amplifier is followed by a source follower stage to provide output impedance matching to 50 R. In order to concentrate on the electrical characteristics of the photoreceiver, the reduced circuit of Fig.…”

“…Since the function of the feedback and load transistors is resistive in nature [9], these transistors can be represented 0733-8724/92$03.00 0 1992 IEEE -- by resistors along with the associated transistor parasitics in order to model their small-signal behavior. Figs.…”

Monolithic large-signal transimpedance amplifier for use in multi-gigabit, short-range optoelectronic interconnect applications

“…Provided the source and drain voltages remain close, the MOSFET behaves as a resistor with a value determined by the voltage between the gate and the channel. The advantage of this arrangement is a significantly smaller parasitic capacitance than can be achieved by using either integrated or external resistors with the additional facility of automatic gain control [7], [8].…”