NORMALLY-OFF FIELD-EFFECT SEMICONDUCTOR DEVICE, AND A METHOD OF INITIALIZING THE SAME

Abstract

A HEMT-type field-effect semiconductor device has a main semiconductor region formed on a silicon substrate. The main semiconductor region is a lamination of a buffer layer on the substrate, an electron transit layer on the buffer layer, and an electron supply layer on the electron transit layer. A source and a drain overlie the electron supply layer. A carrier storage layer overlies the electron supply layer via an insulator, and a gate overlies the carrier storage layer via another insulator. Upon application of an initializer voltage to the gate, the carrier storage layer has stored therein a sufficient amount of carriers to hold the device off even without voltage application to the gate. An initializer circuit is also disclosed whereby the device is initialized automatically for normally-off operation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a HEMT-type field-effect semiconductor device built on the novel principles of the present invention, shown together with a schematic diagram of associated initialization and power supply circuitry.

FIG. 2 is a block diagram showing in more detail the initializer circuit included in the electric circuitry of FIG. 1.

FIG. 3, consisting of (A) and (B), is a diagram of waveforms useful in explaining how the HEMT-type device of FIG. 1 is initialized by the method of this invention.

FIG. 4 is a view similar to FIG. 1 but showing another preferred form of HEMT-type device embodying the invention.

FIG. 5 is also a view similar to FIG. 1 but showing still another preferred form of HEMT-type device embodying the invention

FIG. 6 is also a view similar to FIG. 1 but showing a MESFET-type field-effect semiconductor device embodying the invention.

Claims

1. A field-effect semiconductor device capable of normally-off operation, comprising: (a) a main semiconductor region having a major surface;(b) a source on the major surface of the main semiconductor region;(c) a drain on the major surface of the main semiconductor region spaced from the source;(d) first insulating means on the major surface of the main semiconductor region and between the source and the drain;(e) a carrier storage on the first insulating means, the carrier storage being capable of accepting and storing carriers;(f) second insulating means on the carrier storage; and(g) a gate on the second insulating means;(h) the carrier storage having carriers stored therein to such an extent that the source and the drain are held electrically disconnected from each other even without application of a bias voltage to the gate.

2. A normally-off field-effect semiconductor device as defined in claim 1, wherein the main semiconductor region comprises two neighboring semiconductor layers having different band gaps for generating a two-dimensional carrier gas layer as a channel between the source and the drain.

3. A normally-off field-effect semiconductor device as defined in claim 2, wherein the semiconductor layers of the main semiconductor region are both made from compound semiconductors.

4. A normally-off field-effect semiconductor device as defined in claim 1, wherein the main semiconductor region comprises a single semiconductor layer of a prescribed conductivity type.

5. A normally-off field-effect semiconductor device as defined in claim 4, wherein the semiconductor layer of the main semiconductor region is made from a compound semiconductor.

6. A normally-off field-effect semiconductor device as defined in claim 1, wherein the first insulating means comprises an insulating film having such a thickness that, upon application of a sufficient voltage to the gate to give rise to hot carriers in the main semiconductor region, the hot carriers are capable of tunneling through the insulating film.

7. A normally-off field-effect semiconductor device as defined in claim 1, further comprising a field plate formed on the major surface of the main semiconductor region via at least either of the first and the second insulating means and positioned between the carrier storage and the drain, the field plate being electrically coupled to the gate.

8. A normally-off field-effect semiconductor device as defined in claim 1, further comprising initializer means for initializing the device by causing the carrier storage to store a sufficient amount of carriers for normally-off operation of the device.

9. A normally-off field-effect semiconductor device as defined in claim 8, wherein the initializer means comprises means for applying between the gate and the source an initializer voltage which is higher than a gate-source voltage to be applied therebetween in order to turn the device on in normal operation, thereby causing carriers to be stored in the carrier storage to such an extent that the source and the drain are held electrically disconnected from each other even without voltage application to the gate.

10. A normally-off field-effect semiconductor device as defined in claim 9, wherein the initializer means further comprises means for ascertaining, after application of the initializer voltage, whether the carrier storage has stored therein a sufficient amount of carriers to hold the source and the drain electrically disconnected from each other even without voltage application to the gate.

11. A normally-off field-effect semiconductor device as defined in claim 8, wherein the initializer means comprises: (a) an initializer pulse generator for applying between the gate and the source any required number of initializer pulses of greater amplitude than a gate voltage to be applied therebetween in order to turn the device on in normal operation and hence for causing carriers to be stored in the carrier storage; and(b) means for ascertaining, after application of each initializer pulse, whether the carrier storage has stored therein a sufficient amount of carriers to hold the source and the drain electrically disconnected from each other even without voltage application to the gate.

12. A normally-off field-effect semiconductor device as defined in claim 11, wherein the initializer pulse generator is capable of generating initializer pulses of variable amplitude and reversible polarity.

13. A normally-off field-effect semiconductor device as defined in claim 11, wherein the ascertaining means of the initializer means comprises: (a) a first threshold pulse generator for applying between the gate and the source a first threshold pulse representative of one limit of a target range in which the threshold of the device is desired to be;(b) a second threshold pulse generator for applying between the gate and the source a second threshold pulse representative of another limit of the target threshold range; and(c) means for ascertaining whether the device has turned on or off upon application each of the first and the second threshold pulse.

14. A method of making a field-effect semiconductor device capable of normally-off operation, comprising the steps of: (a) providing a field-effect semiconductor device comprising a main semiconductor region, a source and a drain on a major surface of the main semiconductor region, a carrier storage formed on the major surface of the main semiconductor region via first insulating means, and a gate formed on the carrier storage via second insulating means;(b) applying between the gate and the source an initializer voltage which is higher than a gate-source voltage to be applied therebetween in order to turn the device on in normal operation, thereby causing carriers to be stored in the carrier storage to such an extent that the source and the drain are held electrically disconnected from each other even without voltage application to the gate.

15. A method of making a normally-off field-effect semiconductor device as defined in claim 14, further comprising the steps of: (a) ascertaining, after application of the initializer voltage, whether the carrier storage has stored therein a sufficient amount of carriers to hold the source and the drain electrically disconnected from each other even without voltage application to the gate; and(b) re-applying an initializer voltage between the gate and the source until the sufficient amount of carriers becomes stored in the carrier storage.

16. A method of making a normally-off field-effect semiconductor device as defined in claim 15, wherein whether the sufficient amount of carriers has stored in the carrier storage is ascertained by: (a) measuring the threshold of the device after application of the initializer voltage; and(b) ascertaining whether the threshold measurement is within a target range.

17. A method of making a field-effect semiconductor device capable of normally-off operation, comprising the steps of: (a) providing a field-effect semiconductor device comprising a main semiconductor region, a source and a drain on a major surface of the main semiconductor region, a carrier storage formed on the major surface of the main semiconductor region via first insulating means, and a gate formed on the carrier storage via second insulating means;(b) applying between the gate and the source a first initializer pulse of greater amplitude than a gate-source voltage to be applied therebetween in order to turn the device on in normal operation, thereby causing carriers to be stored in the carrier storage;(c) ascertaining, after application of the first initializer pulse, whether the carrier storage has stored therein a sufficient amount of carriers to hold the source and the drain electrically disconnected from each other even without voltage application to the gate;(d) applying a second initializer pulse of the same polarity as the first initializer pulse between the gate and the source if the carrier storage has proved to have stored therein less than the sufficient amount of carriers at step (c);(e) ascertaining, after application of the second initializer pulse, whether the carrier storage has stored therein the sufficient amount of carriers;(f) applying a third initializer pulse of opposite polarity to that of the first and the second initializer pulse between the gate and the source if the carrier storage has proved to have stored therein more than the sufficient amount of carriers at step (e); and(g) ascertaining, after application of the third initializer pulse, whether the carrier storage has stored therein the sufficient amount of carriers.

18. A method of making a normally-off field-effect semiconductor device as defined in claim 17, wherein whether the sufficient amount of carriers has been stored in the carrier storage by the application of each initializer pulse is ascertained by: (a) applying between the gate and the source a first threshold pulse representative of one limit of a target range in which the threshold of the device is desired to be;(b) ascertaining whether the device is on or off in response to the application of the first threshold pulse;(c) applying between the gate and the source a second threshold pulse representative of another limit of the target threshold range;(d) ascertaining whether the device is on or off in response to the application of the second threshold pulse; and(e) determining whether the device has been initialized or not according to whether the device is on or off in response to the application of the first and the second threshold pulse.