Electronic devices

A Gunn effect oscillator comprises a body of semiconductor material in which electrons are injected from one region to another region via a very thin intervening system. The thin region has a thickness which is less than the mean free electron path length and is typically of the order of 100 .ANG., which results in hot electrons being transferred from the injection region into the other region in which electron bunches form.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to electronic devices. In particular the invention 
relates to electronic devices employing the Gunn effect to produce 
coherent electronic oscillations. 
2. Description of Related Art 
Such devices have been known for a number of years, and typically comprise 
a quantity of a Gunn effect material, for example a material such as GaAs 
having two conduction bands of different curvatures, together with means 
for injecting electrons into the quantity. By application of an electric 
field of sufficient magnitude across the quantity, the electrons injected 
into the quantity may be excited from the lower to the upper conduction 
band. As the electrons within the upper conduction band will have a higher 
effective mass than those in the lower band, they will have a lower 
mobility than the electrons in the lower band. Thus, as these lower 
mobility electrons drift across the quantity, along the electric field 
gradient, they will form bunches amongst the higher mobility electrons, 
these bunches being known as domains. Since only one domain will be formed 
within the quantity at any one time, the output of the device will 
comprise a series of current pulses, whose frequency is dependent on the 
length of the quantity through which the domains drift, i.e. the transit 
region of the device. 
Such a device suffers the disadvantage however that it is difficult to 
define the exact length of the transit region, due to the fact that it is 
difficult to control the region where the excitation of the electrons from 
the lower to the upper conduction band takes place, this imprecision 
causing noise in the output signal of the device. Furthermore the "dead 
space" within the quantity of Gunn effect material in which the energy of 
the electrons is insufficient to enable them to be excited into the upper 
conduction band causes parasitic resistance in the device. Attempts have 
been made to overcome these problems by using a material for the charge 
injection region which has a sufficiently large band gap offset to that of 
the Gunn material that the electrons injected into the quantity of Gunn 
material have nearly all the energy required to excite them into the upper 
conduction band at the injector material/Gunn material interface. Such a 
device suffers the problem however that there will be a relatively wide 
depletion region within the quantity of Gunn material adjacent to the 
injection region. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an electronic 
oscillator utilising the Gunn effect wherein the above difficulties are, 
at least, alleviated. 
According to the present invention an electronic device comprises: a charge 
injection region and first and second regions of a Gunn effect material, 
the charge injection region being effective in use of the device to inject 
charge of a predetermined energy which is less than the energy difference 
between the minima of the pair of energy band edges rise to the Gunn 
effect in said Gunn effect material into said first region, the first 
region having a thickness which is not more than the mean free electron 
path length in said first region, and being doped to an extent that in use 
of the device the sum of the depletion energy in the first region and said 
predetermined energy is in the order of said energy difference, the second 
region being doped to a lesser extent than said first region such that the 
electric field across the second region is sufficient for the formation 
and propagation of domains. 
The mean free electron path length in said first region is primarily that 
due to scattering from the lower to the upper conduction band of the 
material, and is typically of the order of 200 .ANG.. 
The charge injection region suitably comprises a graded gap injector the 
composition of the charge injection region being the same as said first 
and second regions at its edge remote from said first and second region, 
and varying in the direction towards said first region such that the 
direct band gap increases linearly with distance from edge remote from 
said first and second regions, and so that it forms a heterojunction with 
said first region. 
Other structures could be used for the charge injection region. The common 
feature of such structures is that under external bias they will 
accelerate electrons over a short distance (less than the mean free path 
for scattering within the lower conduction band, typically 1000 .ANG.) to 
a specified energy h which is determined by considerations referred to 
subsequently. 
As an alternative to the graded gap injector a planar doped barrier could 
be used for the charge injection region. Such a structure consists of a 
selectively doped region of semiconductor material which would be the same 
material as that used for the Gunn effect region. The type and amount of 
doping in such a structure are chosen so that under bias an appropriate 
electric field is generated to accelerate electrons according to the 
requirements discussed above. 
Said Gunn effect material is suitably GaAs. 
BRIEF DESCRIPTION OF THE DRAWINGS 
One electronic device in accordance with the invention will now be 
described by way of example only, with reference to the accompanying 
figures in which: 
FIG. 1 is a schematic side view of the device, the electron energy levels 
of the various regions of the device also being indicated in the figure; 
and 
FIG. 2 is the DC current voltage characteristic of the injection region of 
the device of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring firstly to FIG. 1, the device comprises a layer of Al.sub.x 
Ga.sub.1-x As 1 where x is 0 at one edge of the layer, and varies linearly 
over the width of the layer to a maximum value of 0.3 at the other side of 
the layer, this layer being of a few 100 .ANG. thickness. The direct band 
gap of Al.sub.1-x Ga.sub.x As varies linearly with x. Adjacent to the 
layer 1, at its edge where x is equal to 0.3 there is provided a thin 
layer of GaAs 3 of approximately 100 .ANG. thickness, this layer being 
doped with Si to a level of 10.sup.18 cm.sup.-3. At the side of the layer 
3, remote from the layer 1 there is provided a further 1 .mu.m thick layer 
of GaAs, this being lightly doped with Si to a level of 2.times.10.sup.16 
cm.sup.-3. At the free edges of the layers 1, 5 there are provided 
respective capping layers of GaAs 7, 9 these both being heavily doped with 
Si to a level of 5.times.10.sup.18 cm.sup.-3. Respective metal contact 
layers 11, 13 are provided on the capping layers 7, 9. 
In use of the device with an appropriate electrical bias applied across the 
device by means of the contact layers 11, 13, the layer 1 constitutes an 
electron injector, injecting electrons into the GaAs layer 3 at an energy 
h above the energy of the lower conduction band of the layer 3. The energy 
gap h is chosen to be less than the intervalley separation .DELTA.E 
between the two conductive bands in each of the GaAs layers 3 and 5, the 
concentration doping of the layer 3 being set such that the sum of the 
depletion voltage within the layer 3 adjacent to the heterojunction formed 
at the interface between the layers 1 and 3, together with the energy h is 
of the order of .DELTA.E. Thus the region of intervalley transfer of 
electrons from the lower to the upper conduction band with the regions of 
GaAs in the device is set within one mean free path, .about.200 .ANG., of 
the interface between region 3 and 5, the layer 5 taking the form of a 
transit region across which the lower mobility domains drift, the length 
of the layer 5 thus determining the frequency of the current pulses 
forming the output of the device. 
FIG. 2 shows the DC current voltage characteristic of the injection region 
1 of the above device a region of negative differential resistance being 
evident for bias voltages in excess of 5 volts, such negative differential 
resistance together with the further regions of Gunn effect material 
leading to the required oscillatory output of the device as described 
above. 
It will be appreciated that whilst particular layer widths and doping 
levels have been specified in the device described above by way of 
example, these may readily be varied. Generally however the thickness of 
the injector region 1 will be greater than 50 .ANG. to prevent tunnelling 
of the potential barrier constituted by the graded composition of the 
layer 1. Electro-migration and other lifetime effects will determine the 
necessary thickness of the layer. The thickness of the layer 3 must be 
less than that of the mean free path for intervalley scattering between 
the two conduction bands in GaAs, but must be thicker than the depletion 
region within the layer 3 caused by the adjacent injector region 1, when 
an operating bias is applied across the device. The doping level of the 
layer 3 is of course set by the requirement that the sum of the energy 
acquired by the injected electrons in the depletion region, and the 
injector height h is of the order of .DELTA.E. The doping level and length 
of the transit region constituted by the layer 5 are chosen such that 
their product is greater than 10.sup.12 cm.sup.-2, the condition for 
domain formation, it being necessary that the electric field in the 
transit region is just sufficient to maintain the electron population of 
the higher conduction band. 
It will also be appreciated that whilst the Al.sub.x Ga.sub.1-x As/GaAs 
system described above is a particularly convenient system as Al.sub.x 
Ga.sub.1-x As is capable of being expitaxially grown on GaAs layers, the 
invention is applicable to devices employing other systems of materials, 
for example In Al As/In P, the materials being doped to the required 
levels with Si. 
It will also be appreciated that whilst in the device described 
hereinbefore the charger transfer is by means of electrons, the invention 
is also applicable to devices in which the charge transfer is by means of 
holes. The appropriate regions within the device, will then be suitably 
p-doped, the necessary energy gap .DELTA.E within the Gunn material being 
defined by appropriate valence bands.