GUARDED GATED CHANNEL RESISTOR IN GaAs
Original Publication Date: 1994-Jun-01
Included in the Prior Art Database: 2002-Mar-06
High values of resistance may be obtained by using a FET N+ (contact) implant in an ungated FET structure. The sheet rho for this process is con- trolled primarily by the implant dose and rapid ther- mal anneal temperature and shows excellent con- trol. The disadvantage is that a N+ resistor has low sheet resistivity and the construction ofa high value resistor requires considerable length (RaL/W). A high value resistor can also be made using bulk GaAs resistors with an N- (FET channel) implant (Figure la). However, due to the surface properties of GaAs, the exposed N- implant area is more sensitive to process induced surface depletion change and may easily exhibit *20% resistance variation. This sen- sitivity to process variation can be mitigated by a layout with several tloating gate Schottky (gate metal) strips across the resistor area (Figure lb). The gate metal blocks the N+ implant and protects the sen- sitive N- implant area surface from later process dam- age so that a high value resistor which is less sensi- tive to process variation can be obtained. Many gate strips are used to avoid a single large gate which would have a very low knee voltage and would tend to capacitively bypass the resistive channel at high frequency.