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Threshold Compensation Circuit and Process Process GaAs Digital Logic

IP.com Disclosure Number: IPCOM000045109D
Original Publication Date: 1983-Feb-01
Included in the Prior Art Database: 2005-Feb-06
Document File: 3 page(s) / 45K

Publishing Venue

IBM

Related People

Andrade, TL: AUTHOR

Abstract

The threshold voltage for gallium arsenide (GaAs) MESFET devices is controlled by the circuit shown below. A difference amplifier senses the difference between a reference voltage and the output voltage, and the output of the difference amplifier is applied to a level shifter and a substrate voltage driver. This achieves a feedback control of the threshold voltage by adjusting the substrate voltage.

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Threshold Compensation Circuit and Process Process GaAs Digital Logic

The threshold voltage for gallium arsenide (GaAs) MESFET devices is controlled by the circuit shown below. A difference amplifier senses the difference between a reference voltage and the output voltage, and the output of the difference amplifier is applied to a level shifter and a substrate voltage driver. This achieves a feedback control of the threshold voltage by adjusting the substrate voltage.

Fig. 1 illustrates a circuit schematic diagram of the invention, and Fig. 2 is a cross-sectional view of the semiconductor structure for carrying out the invention. In Fig. 1, a reference voltage is obtained from the Schottky diode D(1) and the resultant reference voltage V(REF) is determined by the ratio of the resistances R(2) to R(3) and hence depends only on the geometry of the semiconductor layout. The reference voltage V(REF) is set at the mid-point between the logical 1 voltage level and the logical 0 voltage level, in other words V(REF) equals (VH+VL)/2.

An inverter is formed by the MESFET device Q(1) and its load resistor R(4), which normally will have a transfer gain of approximately three and an output voltage approximately equal to V(REF). The output voltage depends on the threshold voltage of the device Q(1) which depends, in turn, upon the substrate voltage itself. Hence, the substrate voltage V(SUB) may be used to control the output voltage V(OUT), as shown in Fig. 1.

MESFET devices Q(2) and Q(3) and their respective load resistors R(6) and R(7) serve as a difference amplifier which senses the difference between V(REF) and V(OUT). The P channel devices Q(2) and Q(3) are used for this amplifier because they also may then serve as drivers and level shifters. In addition, Q(2) and Q(3) are depletion-mode devices and have greater threshold tolerance and are more independent of substrate bias. The difference amplifier has a normal mode rejection which depends upon the value of the resistor R(5) in Fig. 1. Since the amplifier is a differential amplifier, the absolute value...