The present invention is concerned with a device capable of detecting a magnetic field. More specifically, the present invention is concerned with a high-electron-mobility transistor (HEMT) configured to detect a magnetic field. The need to sense magnetic fields is common in many technical fields. Magnetic sensors are used to measure current and magnetic field strength in aerospace and automotive fields. For example, current protection devices may utilize magnetic sensors to protect sensitive equipment should current rise to unacceptable levels. Another example is in the detection of the frequency (and therefore speed) of a rotating component such as a shaft. Shaft position can also be detected with magnetic sensing. Battery condition sensors use magnetic sensing. So called "magnetic cameras" utilizing a matrix of sensors are also used in the art for e.g. non-destructive testing.

Such devices have the ability to sense magnetic fields in the order of lOOnT to lOOmT, which is the range with which the present invention is concerned.

Magnetic sensing techniques exploit an extensive range of ideas and phenomena from the physics and material science fields. The widely used magnetic sensors based on integrated circuit (IC) compatible sensing devices, such as the one based on complementary metal-oxide-semiconductor (CMOS) and bipolar technologies have modest sensitivity in the range of few mT comparing with the non-IC compatible magnetic sensors such as giant magnetoresistors and/or superconducting quantum interference device (SQUID) having a sensitivity bellow nT. There are also other fields, such as spintronics and nanomagnetism, where accurate magnetic sensing is highly important. Consequently, the increase of sensitivity of IC compatible magnetic sensors is always highly desirable for various applications.

The CMOS compatible Hall-effect sensors mostly rely on using either the p-n junction isolated diffused Hall plates or the split-drain magnetic sensitive (MS) metal-oxide-semiconductor field-effect transistors (MagFETs) as magnetic sensitive elements. Both sensitive devices exploit a physical phenomenon that an electron moving through a magnetic field experiences a force, known as the Lorentz force, perpendicular to its direction of motion and to the direction of the field. It is the response to this force that creates the Hall voltage in silicon plates or a variation in electron current distribution detected as the current or voltage difference between two drain outputs of MagFETs. Recently proposed silicon based Magnetic Sensitive LDMOSFET with integrated Hall plate (Jankovic, Igic et al.) achieved ten times higher sensitivity comparing to conventional MagFET dual drain devices. The device has also been compatible with the high-voltage power integrated circuits (<600V). A problem with traditional narrow-bandgap semiconductor devices is that they have limited sensitivity, and do not operate well at elevated temperatures. Many of the applications mentioned above e.g. automotive or aerospace feature environments with elevated temperatures.

In order to further increase sensitivity and allow for even higher operating voltages at elevated temperatures different semiconductor material(s) may be used to design magnetic sensing devices. GaN properties such as spontaneous and piezoelectric polarizations resulting in two-dimensional electron gas (2DEG) with densities above 10 ¹³ cm ^"2 , relatively high mobility (up to 2000 cmW ¹ ), a large energy band gap (3.4 eV), a good thermal conductivity (160 WK ^" ½ ^_1 ) ensuring good heat dissipation, and a very high breakdown field (3500 kV/cm) make it an ideal candidate for all devices requiring fast carrier transport with high breakdown and high temperature operation.

According to a first aspect of the invention there is provided a device capable of detecting a magnetic field according to claim 1.

According to a second aspect of the invention, there is provided a method of manufacture according to claim 12. By "wide bandgap semiconductor" we mean a semiconductor with a bandgap of greater than 2eV, more commonly 2eV to 4 eV.

The device may be gated or un-gated.

Advantageougly, MagHEMTs such as the GaN MagHEMT can be employed for different high-voltage and high-temperature applications. It has been reported in the literature that GaN device can be operational at around 500degC and for some material combination at even 900degC (InAIN/GaN HEMTs for example).

An example device according to the invention will now be described with reference to the accompanying Figures in which:

Figures la and lb are perspective views of a GaN gated MagHEMT with two drains; Figure 2a is a perspective view of the structure of a simulated GaN gated MagHEMT with two drains;

Figures 2b and 2c are graphs of the transfer characteristics of the simulated device of Figure 2a in presence of the magnetic field and without the magnetic field; Figures 3a and 3b are contour plots of density distribution without magnetic field (a) and when magnetic field of BY= 1 T is applied (b);

Figure 4 is a simulated transfer function ΔΙ ₀ vs. By of a MagHEMT in accordance with the invention; Figures 5a to 5f represent steps in a simplified manufacturing process for a MagHEMT in accordance with the present invention;

Figures 6a to 6e are views of respective second, third and fourth MagHEMTs in accordance with the present invention; and,

Figure 7 is a graph showing the sensitivity of a MagHEMT constructed in accordance with the present invention.

In what follows we described a dual drain magnetic sensitive high electron mobility transistor suitable for high-voltage and high-temperature operation. Device concept has been confirmed by employing industrial standard SILVACO TCAD toolbox. Initial results predict device sensitivity above that of LD MagFET with integrated Hall plate. Finally, the device is fully compatible with the current GaN technology and does not require any micromachining or additional material layers.

The structure of a device 100 in accordance with the invention is shown in Figures la and lb, together with the principle of operation. The device 100 comprises a substrate 102, a lower layer 104, a first intermediate layer 106 and an upper layer 108. The layers are constructed as follows:

• Lower layer 104 - GaN;

· First intermediate layer 106 - AIGaN; and,

• Upper layer - GaN.

Contacts in the form of a source electrode S, a gate electrode G and two drain electrodes Dl and D2 are defined on the upper layer 108. The device 100 is therefore a MagHEMT - a magnetic-field- sensitive high-electron-mobility transistor. With no magnetic field B present the current is equally split between two drain contacts Dl and D2 as shown in Figure la. Once the magnetic field B is present (Figure lb), electrons in the two-dimensional electron gas (2DEG) experience a Lorentz force perpendicular to their direction of motion and to the direction of the field. It is the response to this force that creates the variation in electron current distribution detected as the current or voltage difference between two drain outputs of the MagHEMT 100. A second GaN MagHEMT 200 in accordance with the present invention as simulated is shown in Fig. 2a.

The MagHEMT 200 comprises a 1.5 μιη thick GaN layer 202 on top of which a 25 nm thick AIGaN layer 204 is formed. A 2DEG high mobility channel 206 is formed between the AIGaN layer 204 and the GaN layer 202. In the simulation, the lengths of source and drain electrodes are 0.5 μιη making the device channel length of 5 μιη. A 2μιη long gate electrode was placed on top of the AIGaN layer at a distance of 1.5 μιη from the source. The spacing of 2 μιη between drain electrodes Dl and D2 was considered in the simulation. The total device width of 10 μιη was considered. The work function of 3.93 eV was used for source and drain electrodes to form highly conductive Ohmic contact and the work function of 5 eV was used for the gate electrode to form Schottky contact. To simulate the formation of 2DEG, polarization effects between AIGaN and GaN were included in the simulation. Polarization charge is calculated as a sum of spontaneous and piezoelectric polarizations and it is calculated as a function of composition, strain and lattice constants.

The resulting output and transfer characteristics of the simulated normally on device are shown in Figs. 2b, 2c, 3a and 3b. When no magnetic field is applied (Fig. 3a) the current through Drain 1 and Drain 2 electrodes is identical. Once the magnetic field is applied, the current density through Drain 1 and Drain 2 is no longer symmetrical (Fig. 3b) due to the effect of a Lorentz force on the moving carriers in the channel and the split drain current difference can be detected (Figs. 2b and 2c).

The simulated dependence of difference in drain currents on the applied magnetic field is shown in Fig. 4. In the saturation regime the ΔΙ ₀ of the designed magnetic sensor is around 60 times higher than the previously reported MagFET sensor, making the MagHEMT according to the invention more sensitive even at the lower magnetic fields.

Fig. 5 summarises the simplified manufacturing process that could be employed to manufacture GaN MagHEMT. A typical GaN on Si substrate wafer is shown in Figure 5a. Firstly, the electrical isolation between the devices is achieved by an etching process through AIGaN/GaN layers, followed by selective etching used to open source and drains contacts (only Dl is visible on a 2D schematic) and ohmic contact metal deposition (either single metal or stack metal deposition)- Figure 5b. This is followed by the silicon nitride (c) and photo resist layer depositions (d). Finally, the window for Schottky gate contact is open (e) and gate contact has been formed (in the case of MOS gate structure, the gate dielectric deposition will precede the gate contact formation).

Figures 6a to 6e show alternative form of MagHEMTs according to the invention. Each MagHEMT is viewed from the top, although it will be understood that the general architecture of each is per the present invention. In each case, a strategy has been adopted to attempt to reduce electron "drift" between the drain contacts Dl / D2 and thus further still increase the sensitivity and accuracy of the device.

Figure 6a shows a MagHEMT 300 having a substrate 302, a lower layer (not visible), and upper layer 304 a source contact S, a gate contact G and two drain contacts Dl, D2. An area of increased resistivity A is provided betwewen the drain contacts Dl, D2. In this embodiment, the area A is created by etching away the layers down to the non-conductive substrate 302. In Figure 6a, the area A is a rectancular slot extending past the drain contacts Dl, D2 towards the gate G.

Figure 6b shows a similar MagHEMT 300', but the area A is triangular in shape. In Figure 6c, instead of area A being etched, it has been ion doped with e.g. Mg or Si. This increases the resistivity of the area between the drain contacts Dl and D2.

In Figure 6d, an additional contact E is provide between the drain contacts Dl and D2. Current can be fed through this contact to to reduce drift between the drain contacts, by repelling the charge from each of the drain contacts Dl / D2. In Figure 6e, the gate G has an extended region E provided between the drain contacts Dl and D2. As with Figure 6d, current can be fed through this contact to to reduce drift between the drain contacts, by repelling the charge from each of the drain contacts Dl / D2.

Referring to Figure 7, the sensitivity of a MagHEMT constructed in accordance with the invention is around 15%T ¹ . This experimentally achieved sensitivity of prototype device is much higher the one found in conventional silicon MagFETs having reported relative sensitivity in the range of 1-3%T ^_1 .

Variations fall within the scope of the present invention.

The MagHEMTs shown in the above embodiments are shown with two separate drains. In an alternative embodiment, a MagHEMT has more than two drain contacts (i.e. 3 or more).

In another embodiment, the MagHEMT is designed without the gate electrode (known as an un-gated MagHEMT).