GB2202373A

GB2202373A - Field effect transisitor structure

Info

Publication number: GB2202373A
Application number: GB08706564A
Authority: GB
Inventors: Wong Sang Lee
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1987-03-19
Filing date: 1987-03-19
Publication date: 1988-09-21
Anticipated expiration: 2007-03-19
Also published as: GB2202373B; GB8706564D0

Abstract

The absolute magnitude of the frequency dependent conductance component of a primary Schottky field effect transistor (11) is reduced by the provision of an auxiliary transistor 12 coupled with its source-drain path in series with the primary transistor 11. The threshold voltage of the primary transistor is more positive than that of the auxiliary transistor. The structure has improved high frequency properties and can be used as an amplifier, as the current source of a differential amplifier, and in switched capacitor filters. <IMAGE>

Description

FIELD EFFECT TRANSISTOR STRUCTURE This invention relates to field effect transistors, and in particular to Schottky field effect transistors (MESFET's).

A major limitation inherent in the basic MESFET structure fabricated e.g. in gallium arsenide, is the existence of a frequency dependent component of drain conductance. This frequency dependent component has a significant effect on the performance of the device at high frequencies where the gain of a single amplifying stage is limited by the extra drain conductance. This affects the performance accuracy of circuits such as switched capacitor filters. Moreover, under pulsed conditions, the frequency dependent component gives rise to excessive drain-source transient currents which are detrimental to high resolution circuits such as analogue to digital converters.

The frequency dependent conductance component is thought to arise from an electrostatic feedback effect between the drain and the source of the MESFET via the bulk semi-insulating substrate. The feedback is modulated by the capture and thermal emission of free electrons by defect centres disposed at the channel-substrate interface region. Attempts have been made to reduce the number of defects but have not proved successful. In an alternative approach a buried channel structure formed by two implanted p-layers has been employed. However, extra process steps are then required to expose the gate and drain-source contact areas.

The object of the present invention is to minimise or to overcome these disadvantages.

According to the invention there is provided a Schottky field effect transistor structure, including a field effect transistor, and means for providing negative feedback to said transistor whereby the absolute magnitude of the frequency dependent drain conductance component of the transistor is reduced.

According to the invention there is further provided a Schottky field effect transistor structure, including a primary transistor, and an auxiliary transistor coupled with its source-drain path in series with the primary transistor, wherein the threshold voltage of the primary transistor is more positive than that of the auxiliary transistor, the arrangement being such that the absolute magnitude of the frequency dependent conductance component of the primary transistor is reduced.

The technique reduces the absolute magnitude of the frequency dependent conductance component by providing localised negative feedback to the primary transistor. As the threshold voltage of the primary transistor is more positive than that of the auxiliary transistor, the primary transistor operates in the saturation current region. This also allows the gate of the auxiliary transistor to be biased near the high conductance (gm) condition thus providing a high extrinsic transconductance.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a circuit diagram of the field effect transistor structure, Fig. 2 illustrates the frequency dependent drain conductance of the structure of Fig. 1; and Figs. 3 and 4 shown, in schematic form, amplifier circuits using the transistor structure of Fig. 1.

Referring to Fig. 1, the transistor structure comprises a primary transistor 11 and an auxiliary transistor 12 found in a common semiconductor body.

Typically the transistors 11 and 12 are found in an epitaxial gallium arsenide layer disposed on a semi-insulating gallium arsenide substrate. The transistors are arranged with their drain-source paths coupled in series, there being a common concentration applied at the transistor gates. The transistors 11 and 12 are so constructed that the threshold voltage of the primary transistor 11 is more positive than that of the auxiliary transistor 12. In use, this allows the gate of the auxiliary transistor 12 to be biased near the high conductance (gm) region thus providing a high extrinsic transconductance. Small signal analysis of the circuit of Fig. 1 shows that the effective drain conductance go of the structure is given approximately by the expression: go = gol . go2 gm2 where gol and go2 are the drain conductors of the transistors 11 and 12 respectively.

The effect of the feedback introduced by the auxiliary transistor 12 is illustrated in Fig. 2 which shows the frequency dependent drain conductance characteristic of the structure of Fig. 1. For comparative purposes Fig. 2 also shows corresponding characteristics for a single transistor without feedback. As can be seen from Fig. 2 the addition of the auxiliary transistor 12 provides a reduction in the high frequency drain conductance by a factor of 14. This implies a corresponding increase in voltage gain of 23 dB in a single inverter amplifier using identical driver and load structures.

The structure can be employed in a variety of circuit applications e.g. as a current source or an active load. A particular application is the current source of a differential amplifier as shown in Fig. 3 of the accompanying drawings. In this circuit the transistor structure 31 improves the common mode rejection ration in comparison with a conventional single transistor current source. By connecting one input IP1 of the circuit to an analogue signal source and the other input 1P2 to a reference voltage, the circuit of Fig. 3 may be employed as an analogue to digital converter stage.

Another application of the transistor structure is shown in Fig. 4 where one structure 41 is employed as a load and a second structure 42 is used as an amplifier stage of a high gain amplifier. The arrangement provides a significantly higher gain than that obtained with conventional devices.

A further application of the transistor structure is in the construction of switched capacitor filters where the high frequency characteristics of the structure allow the use of high clock rate and provide a high degree of resolution.

Claims

CLAIMS:

1. A Schottky field effect transistor structure, including a primary transistor, and an auxiliary transistor coupled with its source-drain path in series with the primary transistor, wherein the threshold voltage of the primary transistor is more positive than that of the auxiliary transistor, the arrangement being such that the absolute magnitude of the frequency dependent conductance component of the primary transistor is reduced.

2. A Schottky field effect transistor structure, including a primary transistor, and an auxiliary transistor coupled with its source-drain path in series with the primary transistor, wherein the threshold voltage of the primary transistor is more positive than that of the auxiliary transistor, the arrangement being such that the absolute magnitude of the frequency dependent conductance component of the primary transistor is reduced.

3. A transistor structure as claimed in claim 2, wherein a common connection is provided to the gates of the primary and auxiliary transistors.

4. A transistor structure as claimed in claim 2 or 3, formed in an epitaxial gallium arsenide layer disposed on a semi-insulating gallium arsenide substrate.

5. A transistor structure substantially as described herein with reference to and as shown in Fig. 1 of the accompanying drawings.

6. An amplifier or a switched capacitor filter incorporating one or more transistor structures as claimed in any one of claims 1 to 5.

7. An amplifier circuit substantially as described herein with reference to and as shown in Fig. 3 or Fig. 4 of the accompanying drawings.