[0001] The present invention relates to field effect transistors and more particularly to
modulation-doped high electron mobility field effect transistors.
[0002] Field effect transistors (FET'S) using gallium arsenide (GaAS) are known to be capable
of high speed operation. Devices using gallium arsenide/aluminum gallium arsenide
(GaAs/AlGaAs) heterostructures form a two-dimensional electron gas (2DEG). In such
devices the donor impurities are confined away from the active channel in which electrons
may flow. This separation of donor impurities from the electron layers causes the
device to exhibit extremely high electron mobilities and thereby to be capable of
extremely fast operation, for example, in high speed switching.
[0003] Nevertheless, since the electrons in the device must still transit the channel length,
an ultimate switching speed depending on the channel length will exist, so long as
the conductivity modulation of the device is achieved by varying the carrier density
in the channel.
[0004] In accordance with a first aspect of the invention, a modulation-doped field effect
transistor comprises a first semiconductor layer being substantially undoped, an arrangement
on one side of the first semiconductor layer for supplying charge carriers into a
first region of the first semiconductor layer adjacent the one side of the semiconductor
layer, and an arrangement for locally modulating hole and electron density in a second
region of the first semiconductor layer adjacent another side thereof in a repeating
pattern of alternations so as to substantially inhibit conduction therethrough in
the direction of the alternations.
[0005] In accordance with a second aspect of the invention, the modulation doped field effect
transistor includes a gate arrangement for shifting the charge carriers between the
first and second regions.
[0006] In accordance with a third aspect of the invention, the arrangement for locally modulating
doping comprises alternating p and n doped semiconductor regions adjacent the second
region.
[0007] In accordance with a fourth aspect of the invention, the arrangement for supplying
charge carriers comprises a second semiconductor layer adjacent the first region.
[0008] In accordance with a fifth aspect of the invention, the first semiconductor layer
is gallium arsenide and the second semiconductor layer is aluminum gallium arsenide.
[0009] In accordance with a sixth aspect of the invention, the first and second semiconductor
layers are separated by a third semiconductor layer of aluminum gallium arsenide,
the third semiconductor layer being thin compared with the first and second semiconductor
layers.
[0010] In accordance with a seventh aspect of the invention, the alternating p and n doped
semiconductor regions are formed in respective layers of aluminum gallium arsenide.
[0011] In accordance with an eighth aspect of the invention, a modulation-doped field effect
transistor includes a first semiconductor layer being substantially undoped, a second
semiconductor layer of a first conducting type, formed over the first semiconductor
layer, and a gate electrode arrangement formed over the second semiconductor layer
for controlling conduction through a conduction channel formed in the first semiconductor
layer. The transistor includes a third semiconductor layer of the first conductivity
type and a fourth semiconductor layer of a second conductivity type formed next the
third semiconductor layer, the third and fourth semiconductor layers having respective
ends on one side thereof abutting the first semiconductor layer.
[0012] In accordance with an ninth aspect of the invention, the third and fourth semiconductor
layers form part of a plurality of semiconductor layers of Al Ga As in a sandwich
structure of alternating p and n conductivity type semiconductor layers, the plurality
of layers having respective ends on the one side abutting the first semiconductor
layer and having respective ends on the other side abutting the fifth semiconductor
layer.
[0013] In accordance with a tenth aspect of the invention, a sixth semiconductor layer of
substantially undoped AlGaAs is interposed between the first and second semiconductor
layers, the sixth semiconductor layer being relatively thin compared with the first
and second semiconductor layers.
[0014] In accordance with an eleventh aspect of the invention, a semiconductor layer of
AlGaAs of substantially intrinsic conductivity is interposed between each one of the
plurality of semiconductor layers and the next, such that a repeating pattern of conductivities
is formed in the sandwich structure of n-conductivity, intrinsic-conductivity, p-conductivity,
and intrinsic-conductivity semiconductor layers.
[0015] In accordance with a further aspect of the invention, a modulation-doped field effect
transistor comprises a first-semiconductor layer being substantially undoped, and
a second semiconductor layer of a first conductivity type, formed over the first layer.
The transistor further includes a gate electrode formed over the second semiconductor
layer for controlling conduction through a conduction channel formed in the first
semiconductor layer, the conduction channel forming a main controllable-conduction
path of the transistor, and a plurality of semiconductor layers of the first conductivity
type and a second conductivity type, in a layered structure with the first and second
conductivity types alternating. The plurality of semiconductor layers has respective
ends on one side thereof abutting the first semiconductor layer such that the plurality
of semiconductor layers on the one hand and the first and second layers on the other
hand are in substantially mutually perpendicular planes with the first and second
conductivity types in the plurality of semiconductor layers alternating progressively
along a length direction of the conduction channel, the layers in the plurality being
of sufficient thickness to prevent substantially conduction through the plurality
of semiconductor layers in a direction perpendicular to the planes of the plurality
of semiconductor layers.
[0016] The invention will next be described in greater detail, with reference to the drawing,
in which:
FIG. 1 shows a cross-sectional view, not to scale, of a four-terminal modulation doped
field effect transistor structure in accordance with the invention; and
FIG. 2 shows a cross-sectional view, not to scale, of a vertical field effect transistor
structure in accordance with the invention. Features in FIG. 2 corresponding to features
in FIG. 1 are designated by the same reference numeral augumented by 100.
[0017] In the transistor structure of FIG. 1, 100 is a substrate layer of gallium arsenide
(GaAs). Adjoining substrate layer 100 is a region generally designated in Fig. 1 as
102, comprising a composite of aluminum gallium arsenide (AlGaAs) layers lying in
planes substantially perpendicular to the plane of substrate 100. The layers are arranged
in a sequence of n-doped, intrinsic (i), p-doped (p), intrinsic (i), n-doped, intrinsic
materials, and so on, in the same repeating group sequence of n-i-p-i. While more
than one such n-i-p-i group is utilized in the exemplary embodiment, the total number
of such groups is not, in itself, a primary consideration. The lateral thickness of
the n-i-p-i layers is made sufficiently large to substantially prevent superlattice
behavior.
[0018] Adjoining n-i-p-i region 102 in a plane generally parallel to the plane of substrate
100 is a layer 104 of substantially undoped GaAs. Accordingly, layer 100 and layer
104 include n-i-p-i region 102 therebetween, the planes of the n-i-p-i region being
substantially perpendicular to the planes of layer 100 and layer 104. Adjoining layer
104 is a layer 106 of substantially undoped AlGaAs, referred to herein as spacer 106.
Spacer 106 is a relatively thin layer. Adjoining spacer 106 is a layer 108 of n-doped
AlGaAs which forms a gate dielectric for a gate 110 which is of a conductive material
and is adjacent layer 108.
[0019] To summarize, the layer arrangement comprises semi-insulating substrate 100, n-i-p-i
region 102, undoped GaAs layer 104, undoped AlGaAs spacer 106, n-doped AlGaAs layer
108, and conductive gate 110.
[0020] In operation, layer 108 performs the function of an electron donor, contributing
electrons which, having passed through spacer 106, form a two dimensional electron
gas, commonly referred to in the literature as 2DEG, in at least that portion of undoped
GaAs layer 104 which adjoins spacer 106 and which can be thought of as the active
channel.
[0021] The thickness of spacer 106 provides a compromise since increasing the thickness
leads to higher electron mobility but will also cause lower electron densities in
the active channel. Spacer 106 may also be dispensed with altogether, as will be described
later.
[0022] To the extent of separating mobile carriers from the supplying dopant atoms, the
present arrangement resembles known selectively doped hetero-junction transistors
(SDHT), also referred to as high electron mobility transistors (HEMT) in which such
separation is achieved by using the band-gap discontinuity in the conduction band
of two epitaxially grown layers of different composition and doping (e.g. AlGaAs and
GaAs).
[0023] With no bias applied between layer 104 and n-i-p-i region 102, the density of holes
and electrons in layer 104 will be formed or modulation doped in a pattern corresponding
to the n-i-p-i layers. Accordingly, current cannot flow in a horizontal direction
at this interface, i.e. parallel to the interface. Thus, if n-i-p-i region 102 is
allowed to float electrically, and a potential difference is applied in a horizontal
direction, that is, in a direction along the plane of layer 104 substantially no current
will flow in response to such an electric field applied between the ends of layer
104, for example by way of source and drain electrodes, not shown in FIG. 1.
[0024] On the other hand, when a positive bias voltage is applied to gate electrode 110,
electron current responsive to a horizontally applied potential difference can flow
horizontally in the upper portion of layer 104, that is, the portion distal from n-i-p-i
region 102. The transition from non-conduction to conduction in layer 104 and vice
versa occurs as a result of the 2DEG in layer 104 being switched from near the bottom
interface to the top interface and vice versa. Since the thickness dimension of layer
104 is extremely small in comparison with the lateral dimensions, such switching is
very fast in comparison with the classical FET action and does not suffer from the
effects of transit time delay. No gate current or substrate current will flow, other
than capacitive displacement current. The device described therefore functions as
a very fast switch, a small vertical shift of the electron current between the upper
and lower interfaces being sufficient to effect switching.
[0025] Thus, so long as the electrons are confined to the vicinity of the top horizontal
interface between layer 104 and spacer 106, the device in accordance with the present
invention functions similarly to a conventional HEMT. On the other hand, when the
electron wave is towards the lower interface, the heterojunction between layer 104
and n-i-p-i region 102, then the channel current depends on the condition at this
lower interface where horizontal mobility is constrained, as has been described. Moreover,
transfer of the electron wave from the top interface to the lower interface, achieved
by a relatively high negative bias on gate 110 can also occur as a result of a relatively
high positive bias on susbtrate 100, used thus as a back gate, in contrast to top
gate 110. Also, such a transfer will result from a high positive potential being applied
to one end of layer 104, corresponding to a high positive applied drain voltage. Thus,
as the applied drain voltage is first increased from a low value, the horizontal or
drain current through layer 104 will at first increase in the aforementioned normal
FET manner. Thereafter, as the electron wave is shifted toward the lower interface
responsive to a higher drain voltage, the drain current will decrease, corresponding
to a negative incremental or differential resistance. Such a characteristic is useful
in high frequency oscillators and in logic switching. In the device in accordance
with the present invention, the negative resistance is moreover controllable by changing
the gate potential.
[0026] The transistor structure of FIG. 2 has a vertical configuration and represents a
convenient configuration for fabrication. Adjacent a semi-insulating substrate 200
is an n-doped GaAs substrate 201 which is provided with drain contacts 212, 212′ and
adjoins n-i-p-i region 202. Undoped GaAs layers 204, 204′ adjoin n-i-p-i region 202
on respective sides thereof to provide active channel areas. N-doped layers 208, 208′
adjoin layers 204, 204′, respectively and support respective gate electrodes 210,
210′. A source region 214 surmounts n-i-p-i region 202 in a plane parallel to the
plane of the n-i-p-i layers and has an electrode 216 coupled thereto.
[0027] The transistor in accordance with the present invention has at least two principal
fields of application. One field is as a high speed logic switch element, e.g. as
an inverter. Another application is as a microwave oscillator, using differential
negative resistance controllable by gate bias.
[0028] While the invention has been described in terms of preferred embodiments, various
changes which do not depart from the scope of the claims following will suggest themselves
to one skilled in the art. For example, the intrinsic layers in n-i-p-i region 102
or 202 are not essential, since appropriate modulation doping can also be produced
by an n-p-n-p-n-etc. sequence.
1. A modulation-doped field effect transistor including a first semiconductor layer
being substantially undoped, a second semiconductor layer of a first conducting type,
formed over said first semiconductor layer, and gate electrode means formed over said
second semiconductor layer for controlling conduction through a conduction channel
formed in said first semiconductor layer, comprising:
a third semiconductor layer of said first conductivity type; and
a fourth semiconductor layer of a second conductivity type formed next said
third semiconductor layer, said third and fourth semiconductor layers having respective
ends on one side thereof abutting said first semiconductor layer.
2. A modulation-doped field effect transistor according to Claim 1 wherein said first
semiconductor layer is gallium arsenide (GaAs), and said second, third, and fourth
layers are aluminum gallium arsenide (AlGaAs).
3. A modulation-doped field effect transistor according to Claim 2 wherein said first
conductivity type is n and said second conductivity type is p.
4. A modulation-doped field effect transistor according to Claim 3 including a fifth
layer of a semi-insulating semiconductor, said third and fourth semiconductor layers
having respective ends on the other side thereof abutting said fifth semiconductor
layer.
5. A modulation-doped field effect transistor according to Claim 4 wherein said third
and fourth semiconductor layers form part of a plurality of semiconductor layers of
AlGaAs in a sandwich structure of alternating p and n conductivity type semiconductor
layers, said plurality of layers having respective ends on said one side abutting
said first semiconductor layer and having respective ends on said other side abutting
said fifth semiconductor layer.
6. A modulation-doped field effect transistor according to Claim 5 wherein a sixth
semiconductor layer of substantially undoped AlGaAs is interposed between said first
and second semiconductor layers, said sixth semiconductor layer being relatively thin
compared with said first and second semiconductor layers.
7. A modulation-doped field effect transistor according to Claim 5, wherein a semiconductor
layer of AlGaAs of substantially intrinsic conductivity is interposed between each
one of said plurality of semiconductor layers and the next, such that a repeating
pattern of conductivities is formed in said sandwich structure of n-conductivity,
intrinisc-conductivity, p-conductivity, and intrinsic-conductivity semiconductor layers.
8. A modulation-doped field effect transistor according to Claim 6 wherein a semiconductor
layer of AlGaAs of substantially intrinsic conductivity is interposed between each
one of said plurality of semiconductor layers and the next, such that a repeating
pattern of conductivities is formed in said sandwich structure of n-conductivity,
intrinisc-conductivity, p-conductivity, and intrinsic-conductivity semiconductor layers.
9. A modulation-doped field effect transistor comprising:
a first-semiconductor layer being substantially undoped;
a second semiconductor layer of a first conductivity type, formed over said
first layer;
gate electrode means formed over said second semiconductor layer for controlling
conduction through a conduction channel formed in said first semiconductor layer,
said conduction channel forming a main controllable conduction path of said transistor;
and
a plurality of semiconductor layers of said first conductivity type and a second
conductivity type, in a layered structure with said first and second conductivity
types alternating, said plurality of semiconductor layers having respective ends on
one side thereof abutting said first semiconductor layer such that said plurality
of semiconductor layers on the one hand and said first and second layers on the other
hand are in substantially mutually perpendicular planes with said first and second
conductivity types in said plurality of semiconductor layers alternating progressively
along a length direction of said conduction channel, said layers in said plurality
being of sufficient thickness to prevent substantially conduction through said plurality
of semiconductor layers in a direction perpendicular to the planes of said plurality
of semiconductor layers.
10. A modulation-doped field effect transistor according to Claim 9 including a third
semiconductor layer, being substantially undoped and being interposed between said
first and second layers.
11. A modulation-doped field effect transistor according to Claim 10 including a fourth
semiconductor layer of semi-insulating conductivity, said plurality of semiconductor
layers having respective ends thereof, on a side thereof opposite said one side thereof,
abutting said fourth semiconductor layer.
12. A modulation-doped field effect transistor according to Claim 10 wherein said
first semiconductor layer is gallium arsenide, said second, third, and fourth semiconductor
layers are aluminum gallium arsenide.
13. A modulation-doped field effect transistor according to Claim 10 wherein said
third semiconductor layer is thin compared with said first and second semiconductor
layers.
14. A modulation-doped field effect transistor comprising:
a first semiconductor layer being substantially undoped;
means on one side of said first semiconductor layer for supplying charge carriers
into a first region of said first semiconductor layer adjacent said one side of said
semiconductor layer; and
means for locally modulating doping of holes and electrons in a second region
of said first semiconductor layer adjacent another side thereof in a repeating pattern
of alternations so as to substantially inhibit conduction therethrough in the direction
of said alternations.
15. A modulation-doped field effect transistor according to Claim 14 including gate
means for shifting said charge carriers between said first and second regions.
16. A modulation-doped field effect transistor according to Claim 15 wherein said
means for locally modulating doping comprises alternating p and n doped semiconductor
regions adjacent said second region.
17. A modulation-doped field effect transistor according to Claim 15 wherein said
means for supplying charge carriers comprises a second semiconductor layer adjacent
said first region.
18. A modulation-doped field effect transistor according to Claim 15 wherein said
first semiconductor layer is gallium arsenide and said second semiconductor layer
is aluminum gallium arsenide.
19. A modulation-doped field effect transistor according to Claim 18 wherein said
first and second semiconductor layers are separated by a third semiconductor layer
of aluminum gallium arsenide, said third semiconductor layer being thin compared with
said first and second semiconductor layers.
20. A modulation-doped field effect transistor according to Claim 16 wherein said
alternating p and n doped semiconductor regions are formed in respective layers of
aluminum gallium arsenide.