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REDUCED VARIATION GaAs MESFETs THROUGH THE USE OF AN ON-CHIP RESISTOR

IP.com Disclosure Number: IPCOM000006736D
Original Publication Date: 1993-Mar-01
Included in the Prior Art Database: 2002-Jan-28
Document File: 3 page(s) / 191K

Publishing Venue

Motorola

Related People

Neal Mellen: AUTHOR

Abstract

State-of-the-art RF Power Amplifier Modules used for portable communication transmitter applications use discrete, recessed gate, gallium arsenide (GaAs) MESFET devices for the driver and foal output stages as opposed to the silicon bipolar transistors used just a few years ago. Advantages of discrete recessed gate MFSFFT stmc- tures are an improvement in noise tigure, gain, and break- down voltage resulting in improved efficiency of the mod- ule and, consequently, the radio. One of the disadvantages of the recessed gate MESFET is the excessive variabil- ity of pinchoff voltage and saturated current due to the variation in channel thickness that is introduced during the recess etch process step. A method is presented here to compensate for some of that variability.

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MOTOROLA INC. Technical Developments Volume 18 March 1993

REDUCED VARIATION GaAs MESFETs THROUGH THE USE OF AN ON-CHIP RESISTOR

by Neal Mellen

  State-of-the-art RF Power Amplifier Modules used for portable communication transmitter applications use discrete, recessed gate, gallium arsenide (GaAs) MESFET devices for the driver and foal output stages as opposed to the silicon bipolar transistors used just a few years ago. Advantages of discrete recessed gate MFSFFT stmc- tures are an improvement in noise tigure, gain, and break- down voltage resulting in improved efficiency of the mod- ule and, consequently, the radio. One of the disadvantages of the recessed gate MESFET is the excessive variabil- ity of pinchoff voltage and saturated current due to the variation in channel thickness that is introduced during the recess etch process step. A method is presented here to compensate for some of that variability.

A diagram of a single fmger recessed gate gallium arsenide (&As) MESFET structure is shown in Figure
1. In atypical power application many fingers are paral- leled to form a cell. The major advantage of the recessed gate in this structure is the reduced channel resistance that results in a reduced noise figure and improved gain and breakdown voltage as compared to a planar, or nonrecessed, structure. The major disadvantage to this structure is the inherent nonuniformity associated with the etching chemistry required to form the recess area.

The drain current (IJ of the FET is related to V, through the following equation:

I, = q vSat N, (t-w) Zg (1)

Where,W= dv, (2)

is the depletion region depth under the gate, F is the dielectric constant, q is the'electron charge, v, The elec- tron saturation velocity, Cr, is the barrier height, N, is the carrier concentration of the n region, t is the thick- ness of the channel under the gate, and Z, is the channel width. It is implicitly assumed in this equation that the device is operated in the saturation region.

  Figure 2 shows the calculated relationship for V, t and I, assuming typical device geometries and an n chan- nel that is uniformly doped to 2 x 1017/cm3. Pinchoff of the device (I, = 0) occurs when the depletion depth equals the channel thickness and the gate voltage at which this occurs is called the punchoff voltage (VJ.

I Chamd ll%+me% @ml

Figure 2. Relationship of I,J on Vg and t

State-of-the-art portable cellular radios incorporate high efficiency RF Power Amplifier Modules that uti-

Figure 1. Recessed Gate MESFET

22 0 Motorola, 1°C. ,993

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MOTOROLA INC. Technical Developments Volume 18 March 1993

achieve this specification. It can be seen that in order to meet the V,, specification the total allowed variation is approximately .005 pm or 50 A. To achieve a 6 sigma process the recess etch must have a standard deviation of approximately 4 A! The initial channel thickness of a typical device for this applicat...