HOT ELECTRON TRANSISTORS
This invention relates to hot electron transistors.
hen an electric field applied to a semiconductor is large enough to produce a radical change in the velocity distribution of electrons, or electrons are directly injected into states with high kinetic energy, then the resulting conduction is said to be controlled by "hot carriers". In the case of hot electrons, the electrons have a greater velocity than normal electrons which are close to thermal equilibrium Hot electrons exist in either the higher energy states of a continuous conduction band or in allowed levels that are defined by mini-bands derived from a continuous conduction band. Mini-bands are created when Bragg reflection of certain wavelengths which are related to a superlattice periodicity leads to allowed and forbidden energy levels within a normally continuous conduction band.
Normal heterojunction (i.e. junctions made of different materials) bipolar transistors (HJBT) can have a typical d.c. current gain exceeding about 80-100. They typically operate at frequencies up to 20 GHz, although 40GHz has been reported (Chang et al. Electronics Letters j no.22 pl l74 1986). The latest results, using buried implants to reduce collector capacitance, give extrapolated cut-off frequencies of about 80GHz (Nagata et al. Electronics Letters 23_ no.ll p566 1987). Bipolar transistors can be made out of many semiconducting materials, with III-V alloys and Silicon being the most common. Superlattices have also been used as emitter or base material for heterojunction bipolar transistors (Palmier et al. Applied Physics Letters 9_ no.19 pl260 1986).
There are, however, instances when the speed of the device is of more importance than the gain. Typical applications where device speeds in excess of 80GHz are required are those of fast digital switches for the proposed fifth generation of computers and devices operating at microwave frequencies. In order to meet this criterion, hot electrons may be used as the mode of conduction in the transistors (HET). This is because the base transit time can be made to be very short. Hot electron transistors conventionally use unipolar material that is n-type. This can reduce base resistance and thus reduce associated RC time constants.
There are two main di sadvantage s to ho t e l ec t ron transistors. Firstly, hot carrier mean free paths decrease significantly with increasing temperature and with low collector barriers the reverse bias base-collector current can be high at room temperature. Almost all reported devices that are successful have a working temperature of up to 80K [ Inamura et a l . Electronics Letters _22. no.21 pll48 1986, Luriyi et al. IEEE EDL ]_ no.9 p497 1986 and Heiblum et al. Applied Physics Letters 4.9 no.4 p207 1986] . The only known reported exceptions that work at room temperature are a quantum well HET reported by Levi and Chiu (Applied Physics Letters _5I p984 1987), a monolithic Silicon HET reported by Shannon et al. (Applied Physics Letters 35_ p63 1987) and a GaAs monlithic HET of Woodcock et al. (Proc. of the 4th Int. Conf. on Hot Electrons in Semiconductors , published in Physica 134 B/C pi l l 1985). These devices have base widths of 100A, 200A and 300A respectively.
The first of these uses the less conventional materials of AlSbAs/lnAs/GaSb, and so the electrons can be injected in at higher energies due to the large threshold energy for inter- val ley scattering and higher co l lector barriers may be used. This in turn leads to higher possible working temperatures. The second and third reports were of Si and GaAs devices where electron kinetic energies in the base exceed leV and carrier mean f ree paths are extremely short. The measurement of the GaAs device characteristics was by two terminal measurement, and the comparative characteristics between the GaAs and the Si devices were also anomalous to those that would be expected.
The second disadvantage of the hot electron transistors is that in order to achieve a reasonable gain the base width needs to be less than about 300A due to the very short mean free paths for hot e lectrons in the heavily doped base regions of conventional hot electron transistors. This low base width increases the base resistance.
Superlattice bases with discreet doping at the emitter and collector junctions have been proposed as a suitable material for hot electron devices [Lent Superlattices and Microstructures 2_ no.4 p387 1987] , but no working devices have been reported. This structure relies on using low or undoped superlattice material to achieve very long ballistic path lengths and would be unsuitable for high frequency power amplification due to the relatively high base resis tance. I t has now been f ound that minimising scattering and lowering base resistance are important considerations when structuring the base of a hot electron transistor.
This invention provides a hot electron transistor that will operate at high frequencies, room temperature and with acceptable gain.
According to this invention a Structured Base Hot Electron Transistor (SBHET) comprises in serial order an emitter, base and col lector whereby base and emitter are constructed such that:-
a base is formed as a superlattice structure of layers of at least two materials repeating sequentially, arranged to provide a built in field for accelerating electrons from the emitter to the collector with at least two mini-bands in the conduction band structure below the threshold for inter-valley scattering,
an emitter is configured to inject electrons into the second or higher mini-band of the base material,
This invention may be used in the cons truction of both unipolar and bipolar devices. For a unipolar SBHET a col lector is constructed to provide an energy barrier height sufficient to block electrons in the first mini-band occuring in the base.
A hetero- or homo- junction bipolar SBHET is constructed with a col lector which exhibits normal reverse bias junction characteristics.
A suitably designed superlattice structure is used for the base because of its ability to suppresss inter-mini-band scattering from plasmons and polar optic phonons. This is advantageous because it allows longer energy relaxation times for the electrons injected into the higher mini-bands. A built in field is incorporated into the superlattice structure in order to accelerate the electrons (i.e. setting up a drift field) from the emitter to the collector. One possible method of incorporating a built in field in the superlattice is to progressively dope the superlattice from the emitter end to the collector end. The built in field may also be achieved by varying the dimensions and compositions of the selected superlattice components. A typical built in field is 1.5*104 v cm-1 for a base width of about 800A.
Silicon is a possible n-type dopant for a superlattice component such as GaAs. A typical doping level for such a dopant
17 —3 18 -3 would be 10 cm at the emitter junction and 3*10 cm at the collector junction, and thus providing a built in field.
Typical suitab l e super l attice comp onents are III-V materials, their alloys and III-V strained layer superlattices. Silicon based super lattices could also be considered for generating drift fields and suppressing plasmon emission, though no suppression of inter-valley phonon scattering is predicted.
Possible methods for constructing the superlattice to provide a non-doping built in field would be to vary the width of each component and/or to vary any or al l of possible component alloy compositions.
A buil t in field in a superlattice base of a SBHET may be provided by each method independently, or using a combination of the methods available. Where unit cell dimensions are uniform throughout the superlattice and a built in field is provided by a doping gradient between the emitter and collector junctions, this gives a uniform superlattice. A built in field resulting from dimension and/ or component variations gives a non-unif orm superlattice. Use of a combination of methods for producing a built in field also gives a non-uniform superlattice.
A unit cel l within a superlattice is def ined as the repeat unit within the superlattice of two or more components. The unit cell dimension is required to be short comparedto the mean free path lengths, but sufficiently long to ensure that at least two mini-bands are present at energies below the threshold for inter-valley scattering.
1
Materials suitable for this invention of a SBHET need two or more mini-bands in the conduction band structure of the base. The electrons at the forward biased emitter are given enough energy to overcome the emitter energy barrier. This energy barrier is selected to enable the electrons to be injected into the most energetically favourable mini-band for hot electron conduction in this invention. The limits imposed upon this energy range can be specified by the need for an energy lower than the threshold energy for inter-valley scattering (e.g. 0.34eV in a typical GaAs/AlGaAs superlattice) and a high enough energy to promote the electrons into the most energetically favourable mini— band. A typical AlGaAs emitter energy barrier would be 0.3eV to promote the electrons into a higher mini-band spanning this energy.
The electrons are then accelerated through the superlattice base under the built in field imposed by the structuring and/or doping of the superlattice. The doping wil l influence the gradient of the allowed energies in the lowest mini-band as well as all other mini-bands. However , by varying the component dimensions or component alloy composition of the superlattice components it is possible to affect the second mini-band or higher while having little effect on either the lowest mini-band or the lowering of the lowest mini-bands associated with the L or X minima in the superlattice. Although the hot electrons are injected into the second mini-band or higher, there are still electrons in the first mini-band because the doping in the base region leads to a high electron density giving occupancy of the first mini-band level. Phonon-type scattering also takes place between the higher mini-bands and the first mini-band. .
In a unipolar SBHET the collector energy barrier height is selected such that only electrons in the most energetically favourable mini-band or higher for hot electrons in the SBHET have enough energy to overcome this barrier. These electrons then become the collector current. A typical barrier height in a GaAs SBHET with a uniform superlattice base would be 0.2eV.
In a bipolar SBHET the collector exhibits normal reverse bias junction characteristics. A typical n-p-n bipolar SBHET would be constructed to have a GaAs collector doped with Silicon at a concentration of 1018cm~3 or greater. Beryllium would normally be used as the p-type dopant for the base.
Known techniques may be used to construct all parts of a SBHET apart from the structured superlattice base.
Construction of a Structured Base Hot Electron Transistor (SBHET) as outlined above would provide a transistor capable of operating at frequecies of 100GHz or greater.
w
This invention will now be described by example only with reference to the accompanying figures of which :-
Figure JL_ is a schematic cross-section of a unipolar SBHET.
Figure 2_ comp r is e s o f Figure 2( a) , a s chema t ic representation of the energy barriers created within the emitter, base and col lector regions as described in Figure 1. Band bending variations in uniform (as described in Figure 1) and non- uniform superlattices are shown . -in f igures 2(b) and 2( c) respectively.
Figure 3^ is a schematic cross-section of a SBHET as described in Figure 1, utilising a horseshoe construction for the collector terminal and a semi-insulating substrate.
Figure 4 is a schematic cross-section of a SBHET as shown in Figure 3 where the superlattice base is connected to the base terminals by ion implanted material.
Figure 5_ is a schematic cross-section of an alternative construction to that described in Figure 4 using a collector contact with lower capacitance.
Figure 6_ is a schematic representation of the energy barriers created within the emitter, base and collector regions in a bipolar SBHET.
As shown by Figure 1, a unipolar SBHET is grown on a n+Qa^s substrate 1 about 0.5mm thick, with a doping concentration of
1 Α —.'5 1 Q ___. «±.
3*10 cm of Silicon. A 10 cm Silicon n GaAs buffer layer 2 of about 0.5 m thickness is grown by epitaxial means on the substrate 1. Epitaxial GaAs buffer layers provide superior quality material for subsequent layer growth than standard GaAs bulk material. A collector 3, superlattice base 4, emitter 5 and n GaAs emitter contact layer 6 (about 0.5 thick and doped with
18 —3 a concentration of about 10 cm of Silicon) are also grown epitaxially, e.g. by vapour phase epitaxy or molecular beam epitaxy. Emitter and collector contacts are provided by standard goId/germanium/nickel metallised layers 7 and 9 respectively.
Each metallised layer is about 0.5 thick. Base contacts 8 are
AuGe/Ni/Ti/Au which provides shallow ohmic contacts that are about 0.5 m thick.
The emitter 5 is constructed of graded Al Ga, As where x 1-x
0 1 that provides an energy barrier of 0.3eV when forward biased over about 500A.
72.
The superlattice base 4 of about 800A thickness has alternating layers of GaAs and Al Ga]_xAs» where O l, in a unif orm superlattice cons truction. The unit cell of the superlattice base 4 is 40A GaAs and 30A Al Ga, As. The Al X Ga, _L X As provides an energy barrier height of 0.2eV relative to bulk GaAs. To provide a built in field the GaAs is doped with Silicon at a concentration of 10 17 cm -3 at the emitter junction, and this is increased exponentially to 3*10 18 cm —3 at the collector junction. This provides. a potential drop across the superlattice base 4 from the emitter 5 to the collector 3. The superlattice base 4 and emitter 5 have an angle allowing metallisation down an exterior side of superlattice base 4. This angle is determined by an etching solution and is required to facilitate deposition of metallised contacts 8.
The col lector 3 is about 2000A thick and made of undoped graded Al Ga1 As. The grading construction is such that the first 300A of collector 3 next to the superlattice base 4 has an energy barrier height of 0.2eV under bias conditions. The latter 1700A is graded to provide a compatible band structure to that of the n GaAs buffer layer 2.
The energy barriers constructed in the SBHET of Figure 1 are shown in Figure 2(a). Figure 2(b) represents the contribution towards band bending by Silicon doping of GaAs giving a uniform super lattice , as described in Figure 1. In a unif orm superlattice the doping wil l inf luence the gradient of the al lowed energies in the lowest mini-band as well as al l other mini-bands, c. and c, represent band edges of superlattice first and second mini-bands respectively.
Figure 2(c) presents the effect on the mini-bands of varying component dimensions and the ratio of Aluminium to Gal l ium in Al X Ga,~X As, giving a non-uniform superlattice. In contrast to a uniform superlattice, a non-uniform superlattice can be designed so as not to influence the gradient of the allowed energies in the lowest mini-band.
The SBHET of Figure 1 may also be constructed with a horseshoe collector contact formation as shown in Figure 3. All aspects of the device are as stated for Figure 1 except that collector contacts 11 are formed in a horseshoe formation on a
18 —3 •+■
10 cm Silicon doped n epitaxial buffer layer 12. The active layer of buffer layer 12 is about 0.5mm wider in diameter than the collector 3. The device is grown on 0.5mm semi-insulating
GaAs substrate 13. Energy barriers created in the emitter, base and collector regions are as shown in Figure 2(a).
«.
The SBHET as shown in Figure 4 is grown with emitter contact 6, and terminal 7 as shown in Figure 1 , and buffer layer 12, substrate 13 and collector terminals 11 as shown in Figure 3. The superlattice base 16 is constructed of alternating layers of GaAs and Al Ga l,— x As for a uniform superlattice. Unit cell dimensions are 40A for GaAs and 30A for AlχGa1_χAs. Ohmic contacts 17 are fabricated by ion implanting a 0.5mm anular outer region of superlattice base 16. The base terminals 18 are provided by standard gold/germanium/nickel shal low ohmic contact metallisation of about 0.5mm thickness. Emitter 19 and collector 20 are Al Ga, As. The energy barriers provided by the emitter, base and collector regions are as shown in Figure 2(a).
An alternative construction to that shown in Figure 4 is shown in Figure 5. This device lowers the active collector area and so minimises associated capacitance f or use at high frequencies. It is fabricated with emitter contact 6 and terminal 7 as shown in Figure 1, substrate 13 and col lector terminals 11 as shown in Figure 3 and superlattice base 16, ion implanted region 17, base terminals 18, emitter 19 and collector
20 as shown in Figure 4. During fabrication a buffer layer 25 of 10 18 cm —3 n + Silicon doped GaAs is grown 0.5mm thicker than buffer layer 12 as shown in Figure 3. Selective proton implantation takes place to with a thickness of 0.5mm and is etched away to provide insulation layers 26 and n collector region 27 of radius
0.25mm.
Specific superlattice component dimension construction will now be described by example only:-
The non-uniform superlattice components are Al Ga As and x 1— x
GaAs. The electrons are injected into the second mini-band (c„) from an emitter contact. The unit cell parameters are graded in order to achieve a high built in field in the second mini-band without inducing a built in field in the lower mini-band. The data in table 1 gives the grading dimensions of the GaAs and AlGaAs in columns 1 and 2 respectively. Column 3 detail s the barrier energy height, whilst columns 4 and 5 lists the mini-band edges of c and c_ respectivel y in units of meV. No doping gradient is present in the non-uniform superlattice.
TABLE : 1_
GaAs AlGaAs Barrier Mini-band Edges (eV)
(A) (A) Energy(eV)cιC2
40 11 0.30 59 289
40 11 0.27 54 280
40 17 0.25 64 268
34 23 0.17 62 255
34 23 0.15 56 243
34 28 0.15 60 233
28 34 0.12 61 224
34 34 0.14 61 213
28 40 0.11 60 200
28 45 0.11 63 187
28 45 0.10 58 179
28 51 0.10 60 168
Alternatively a uniform superlattice may be fabricated by exponential doping of GaAs layers with Silicon. The concentration of Silicon increase exponentially from 10**-'cm~3at t^e emitter
18 —3 junction to 3*10 cm at the collector junction. The unit cell is made of 40A layers of GaAs and 30A layers of Al Ga, As.J x 1-x
A bipolar SBHET may be produced as a n-p-n emitter-base- collector transistor. The superlattice base may be intrinsically
18 —3 doped with Beryllium at a concentration of 10 cm or greater to produce a p-type region. A schematic diagram of the energy barriers created in the emitter, base and co l lector regions of such a device are shown in Figure 6.