GB2159664A

GB2159664A - Hot carrier transistor

Info

Publication number: GB2159664A
Application number: GB08513543A
Authority: GB
Inventors: Joseph Alan Barnard; William Alexander Hughes
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1984-05-30
Filing date: 1985-05-29
Publication date: 1985-12-04
Also published as: GB8413726D0; GB2159664B; GB8513543D0

Abstract

Emitter, base and collector regions (7, 9 and 15) consist respectively of relatively wide energy band gap, relatively narrow energy band gap and relatively wide energy band gap semiconductor materials so as to provide heterojunctions between the emitter and base regions and between the base and collector regions. The the base region is sufficiently thin to constitute a quantum well for the carriers. This construction helps to avoid quantum mechanical reflection of carriers at the emitter-base and base-collector hetero-interfaces, and consequent reduction in gain. <IMAGE>

Description

SPECIFICATION Transistors This invention relates to transistors.

More particularly the invention relates to socalled hot carrier transistors, that is transistors wherein the current carriers, after injection into the transistor, travel through the transistor to a collector region ballistically. In contrast, in a conventional junction transistor the current carriers diffuse from one region to another under the influence of applied bias potentials.

One proposed form of hot carrier transistor utilises emitter, base and collector regions of semi-conductor material of different energy band gap widths with heterojunctions between the emitter and base and between the base and collector. However, such an arrange ment tends to suffer from quantum mechanical reflection of carriers at the emitter-base and base-collector hetero-interfaces, with consequent reduction in gain.

It is an object of the present invention to provide a hot carrier transistor wherein this difficulty is avoided.

Accordingly, the present invention provides a hot carrier transistor of the kind comprising emitter base and collector regions consisting respectively of relatively wide energy band gap, relatively narrow energy band gap and relatively wide energy band gap semiconductor materials so as to provide heterojunctions between the emitter and base regions and between the base and collector regions, wherein the base region is sufficiently thin to consititute a quantum well for the carriers.

It will be understood that in order for the base to be sufficiently thin to constitute a quantum well, the effective thickness of the base region will be small compared with the wavelength of the carriers in the base region.

In a transistor according to the invention the emitter region preferably includes a thin layer adjacent the base-emitter interface which is substantially undoped.

In a transistor according to the invention the collector region and the emitter region preferably consist of material of substantially the same energy band gap.

Preferably the combined thickness of the emitter and base regions is appreciably less than the mean free path of the carriers.

Preferably the collector region is substantially thicker than the emitter region.

A transistor in accordance with the invention is preferably arranged so that injection of carriers into the emitter region occurs by a tunnelling mechanism through a forbidden energy band barrier of thickness dependent on the emitter base potential.

To this end the emitter and base regions are preferably connected respectively to emitter and collector contacts via respective further regions of relatively narrow energy band gap material.

One transistor in accordance with the invention will now be described by way of example with reference to the accompanying drawings in which:~ Figure 1 is a diagrammatic sectional view of the transistor; and Figure 2 is a diagram illustrating the variation of the energy band gap through the transistor.

Referring to Fig. 1, the transistor has an emitter 1 provided with an emitter contact 3.

The emitter 1 comprises a first region 5 of narrow energy band gap material, for example, gallium arsenide, and a second region 7 of a relatively wide energy band gap material, for example, aluminium-gallium arsenide, the first region 5 extending between the contact 1 and the second region 7.

The end of the region 7 of the emitter 1 remote from the region 5 is in contact with a base region 9 of the transistor which consists of a very thin layer of a narrow energy band gap material e.g. gallium arsenide and is provided with a base contact 11.

The collector 13 of the transistor, which is of similar construction to the emitter but appreciably thicker, comprises a first region 15 of relatively wide energy band gap material e.g. aluminium-gallium arsenide, which contacts the base region 9, and a second region 17 of relatively narrow energy band gap material, e.g. gallium arsenide, which extends between the region 15 and a metal contact 19 to the collector.

The emitter 1 and region 17 of the collector of the transistor have n-type doping so that the current carriers in operation of the transistor are electrons, except that the second emitter region 7 has a thin layer 7a of undoped material adjacent the base emitter interface.

The base region 9 and region 5 of the collector are also undoped.

In operation of the transistor the base region 9 is biassed positively with respect to the emitter 1 and the collector 13 is biassed positively with respect to the base region, this being indicated in Fig. 1 by showing the emitter contact 3 grounded and biassing potential sources 21 and 23 of VBE and VCE volts connected respectively between the base and collector contacts 11 and 19 and ground.

In operation, the variation of the conduction band minimum energy level and the valence band maximum energy level through the transistor are of the general form indicated by lines Ec and Ev respectively in Fig. 2. Thus, in the second emitter region 7 adjacent the interface between the first and second emitter region 5 and 7 there is a triangular barrier to conduction electrons through which electrons can pass by tunnelling action, the tunnelling probability T, being given by the expression

where: mis the effective mass of a mobile electron E is the peak energy of the barrier F is the electric field gradient across the barrier q q is the charge on an electron; and y is Planck's constant divided by 2 It will thus be seen that as the emitter-base potential is varied, so the effective tunnelling thickness of the barrier, that is the electric field gradient across the barrier, is varied.

Hence the number of electrons tunnelling through the barrier, i.e. emitter current, varies with the emitter-base-potential.

Having tunnelled through the barrier the electrons travel ballistically towards the collector. Some of these electrons will lose energy due to collisions and become trapped in the energy well in the base region, these electrons being swept out of the base region via the base contact 11 and constituting the base current. However, the majority of electrons will pass into the collector 13 and to the collector contact 19.

The proportion of electrons trapped in the base region energy well is reduced by making the base region energy well sufficiently thin to constitute a quantum well, i.e. a well having small thickness compared with the wavelength of a mobile electron while passing through the base region. Compliance with this requirement ensures that mobile electrons will suffer negligibly small quantum mechanical reflection from the base-emitter or base-collector heterointerface, especially when the emitter and collector material systems are identical.

The required quantum well base is achieved by making the base region 9 appropriately thin. In addition, to ensure that the base region 9 is highly conductive so that a uniform potential is established in the base region 9 via the base contact 11, a thin layer of the second region 7 of the emitter 1 adjacant the base 9 is undoped, as mentioned above.

Consequently, mobile electrons from the remainder of the second emitter region 7 will leave their parent ions and congregate in the base region, spaced from their parent ions by the undoped layer to form a two-dimensional electron gas in the base region 9. The electrostatic coulomb tails associated with the ions in the emitter are thus attenuated with consequent reduction in electron scattering in the base region as they move parallel to the baseemitter interface.

To further reduce the possibility of collisions in the emitter 1 and base region 9 the combined thickness of the emitter and base region is made appreciably less than the mean free path of the mobile electrons.

Fewer collisions in the emitter and base region result, of course, in lower base current for a given emitter current with consequent larger current gain.

The collector 13, more particularly region 15 thereof, is made relatively thick compared with the emitter 1, more particularly region 7 thereof, to reduce the electric field gradient in the collector region 15 and so reduce the possibility of electrons tunnelling from the base region 9 to the collector region 15.

The relatively narrow energy band gap material regions 5 and 1 7 of the emitter and collector are provided to reduce the contact resistance between the metal emitter and collector contacts 3 and 19 and the emitter and collector 1 and 1 3.

It will be understood that satisfactory operation of the transistor at very high frequencies may be expected due to the very short transit times of the carriers, electron tunnelling being an intrinsically fast quantum mechanical transport process. Furthermore, the base resistance is low due to the use of a two-dimensional electron gas quantum well base, and the base-collector capacitance is kept low by use of a relatively thick, lightly doped, collector region 15. The parasitic RC element associated with the base resistance and collectorbase capacitance is thus kept small allowing for a high unity power gain frequency.

Whilst in the transistor described above, by way of example, the current carriers are electrons, in other transistors in accordance with the invention the current carriers may be holes, the emitter base and collector regions then consisting of appropriately chosen materials with p-type doping instead of n-type doping.

It is also pointed out that whilst in the transistor described above by way of example each region of each of the emitter and collector has a substantially constant energy band gap throughout its thickness, in other transistors in accordance with the invention the energy band gap in all or part of one or both of the emitter and collector may vary to improve the operating or other characteristics of the transistor.

It will be understood that in other transistors in accordance with the invention material systems other than that used in the transistor described above by way of example may be used, for example, a GalnAS/AIlnAs system or a GaSb/GaAlSb system.

Claims

1. A hot carrier transistor of the kind comprising emitter, base and collector regions consisting respectively of relatively wide en ergy band gap, relatively narrow energy band gap and relatively wide energy band gap semiconductor materials so as to provide heterjunctions between the emitter and base regions and between the base and collector regions, wherein the base region is sufficiently thin to constitute a quantum well for the carriers.

2. A transistor according to Claim 1 wherein the emitter region includes a thin layer adjacent the base-emitter interface which is substantially undoped.

3. A transistor according to Claim 2 wherein the collector region and emitter region consist of material of substantially the same energy band gap.

4. A transistor according to any one of the preceding claims wherein the combined thickness of the emitter and base regions is appreciably less than the mean free path of the carriers.

5. A transistor according to any one of the preceding claims wherein the collector region is substantially thicker than the emitter region.

6. A transistor according to any one of the preceding claims wherein said base and collector regions are lowly doped compared with at least the major part of said emitter region.

7. A transistor according to any one of the preceding claims arranged so that injection of carriers into the emitter region occurs by a tunnelling mechanism through a forbidden energy band barrier of thickness dependent on the emitter-base potential.

8. A transistor according to Claim 7 wherein the emitter and collector regions are connected respectively to emitter and collector contacts via respective further regions of relatively narrow energy band gap material.

9. A transistor according to Claim 8 wherein said further regions are relatively highly doped compared with the base and collector regions.

10. A transistor according to any one of the preceding claims wherein said wide and narrow energy band gap materials are respectively aluminium-gallium-arsenide and gallium arsenide materials.

11. A transistor according to any one of Claims 1 to 9 wherein said wide and narrow energy band gap materials are respectively aluminium-indium-arsenide and gallium-indium-arsenide materials.

12. A transistor according to any one of Claims 1 to 9 wherein said wide and narrow energy band gap materials are respectively gallium-antimonide and gallium-aluminium-antimonide materials.

13. A hot carrier transistor substantially as hereinbefore described with reference to the accompanying drawings.