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Figure 13. Small-signal RF performance extracted from S-parameters for a single-delta structure showing a cut-off frequency fp>1 GHz and the maximum frequency of operation finax~4 GHz.

Figure 13 Small-signal RF performance extracted from S-parameters for a single-delta structure showing a cut-off frequency fp>1 GHz and the maximum frequency of operation finax~4 GHz.

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Figure 1. Single-crystal diamond plates manufactured by CVD and suitable for use in mechanical and optical applications.
Figure 1. Single-crystal diamond plates manufactured by CVD and suitable for use in mechanical and optical applications.
Table 1. Electronic properties of diamond.
Table 1. Electronic properties of diamond.
Figure 2. Comparison of diamond’s combined carrier mobility, bandgap and thermal conductivity with other semiconductors (area of circle is proportional to the material’s thermal conductivity).
Figure 2. Comparison of diamond’s combined carrier mobility, bandgap and thermal conductivity with other semiconductors (area of circle is proportional to the material’s thermal conductivity).
Table 2. Common dopants in CVD diamond. Reports claiming that sulphur (S) or oxygen (O) constitute comparatively shallow donors remain to be substantiated at the time of writing. A boron- hydrogen complex has also recently been reported to constitute a shallow n-type dopant (Chevallier et al. 2002), but the long-term stability of this complex is uncertain. As boron is readily incorporated into CVD diamond as a dopant impurity, there has been much activity focused on unipolar devices using boron. However, boron acceptors are only weakly activated at room temperature due to the ionization energy of 0.37 eV. To mitigate this, the boron solid concentration must be increased to very high levels where the conduction mechanism changes first from band-type conduction to hopping, and then eventually to metallic-like as the acceptor bands begin to overlap the valence-band maximum and the activation energy approaches zero (Nebel & Stutzmann 2001). At the same time, the presence of compensating donors (shallow or deep) must be minimized. This metallic-like conduction results in a much lower bulk resistivity. Tt ean he ennelided that while intringie diamond with nronerties eiitahle for
Table 2. Common dopants in CVD diamond. Reports claiming that sulphur (S) or oxygen (O) constitute comparatively shallow donors remain to be substantiated at the time of writing. A boron- hydrogen complex has also recently been reported to constitute a shallow n-type dopant (Chevallier et al. 2002), but the long-term stability of this complex is uncertain. As boron is readily incorporated into CVD diamond as a dopant impurity, there has been much activity focused on unipolar devices using boron. However, boron acceptors are only weakly activated at room temperature due to the ionization energy of 0.37 eV. To mitigate this, the boron solid concentration must be increased to very high levels where the conduction mechanism changes first from band-type conduction to hopping, and then eventually to metallic-like as the acceptor bands begin to overlap the valence-band maximum and the activation energy approaches zero (Nebel & Stutzmann 2001). At the same time, the presence of compensating donors (shallow or deep) must be minimized. This metallic-like conduction results in a much lower bulk resistivity. Tt ean he ennelided that while intringie diamond with nronerties eiitahle for
Figure 3. Schematic of an externally pumped high-voltage switch.
Figure 3. Schematic of an externally pumped high-voltage switch.
Table 3. Parameters used in simulation. than 225 nm) or by a strongly absorbed e-beam. In both cases, owing to the lov penetration depth, a space-charge region near the front contact will be created When charge carriers are injected (using photon or e-beam excitation) into ar intrinsic semiconductor, or alternatively into the depletion region of a reverse- biased diode, a high enough electric field can accelerate the charges to velocities where the impact ionization becomes important. This can lead to avalanche ionization in the semiconductor as shown in figure 4. An avalanche could be usec to extend a current pulse over time, even if the trigger pulse that generates the initial charge carriers is short. Thereby the necessary power in the trigger puls¢ can be reduced and a shorter trigger pulse is needed. +r, 1 +,171 : 1,7 er ae 11
Table 3. Parameters used in simulation. than 225 nm) or by a strongly absorbed e-beam. In both cases, owing to the lov penetration depth, a space-charge region near the front contact will be created When charge carriers are injected (using photon or e-beam excitation) into ar intrinsic semiconductor, or alternatively into the depletion region of a reverse- biased diode, a high enough electric field can accelerate the charges to velocities where the impact ionization becomes important. This can lead to avalanche ionization in the semiconductor as shown in figure 4. An avalanche could be usec to extend a current pulse over time, even if the trigger pulse that generates the initial charge carriers is short. Thereby the necessary power in the trigger puls¢ can be reduced and a shorter trigger pulse is needed. +r, 1 +,171 : 1,7 er ae 11
Figure 4. A sample of a two-dimensional transient simulation (three successive times are shown): carriers (in this case, holes) injected at the front contact drift in the electric field towards the back contact while generating more carriers by impact ionization.
Figure 4. A sample of a two-dimensional transient simulation (three successive times are shown): carriers (in this case, holes) injected at the front contact drift in the electric field towards the back contact while generating more carriers by impact ionization.
Figure 5. Four transient simulations showing current through a device for four different injection levels. The bias is 20 kV. Clearly, there is a threshold injection level (in this case, between 55 and 70 nC) that must be exceeded in order to achieve a breakdown.
Figure 5. Four transient simulations showing current through a device for four different injection levels. The bias is 20 kV. Clearly, there is a threshold injection level (in this case, between 55 and 70 nC) that must be exceeded in order to achieve a breakdown.
Table 4. Data for a 20 kV switch in example below.
Table 4. Data for a 20 kV switch in example below.
Figure 6. The circuit simulations include the finite-element model of the optical (or e-beam) switch ‘OS’, a capacitor in parallel with the switch ‘C,’, a 50Q resistive load ‘Rj ,q’, some stray inductance ‘Lgtray’ and a fixed voltage bias (e.g. a large primary energy storage capacitor).
Figure 6. The circuit simulations include the finite-element model of the optical (or e-beam) switch ‘OS’, a capacitor in parallel with the switch ‘C,’, a 50Q resistive load ‘Rj ,q’, some stray inductance ‘Lgtray’ and a fixed voltage bias (e.g. a large primary energy storage capacitor).
Figure 7. Current through a 50Q load for two values of the capacitor C,. The bias voltage Va =20 kV; the injected charge by the trigger pulse Qi,3=3X10~‘ C.
Figure 7. Current through a 50Q load for two values of the capacitor C,. The bias voltage Va =20 kV; the injected charge by the trigger pulse Qi,3=3X10~‘ C.
Figure 8. Detail of the rising flank in figure 7 showing an approximately 1 ns rise time assuming an infinitely short trigger pulse at time zero. For a realistic trigger pulse of (say) 1 ns duration, the rise time will increase correspondingly.
Figure 8. Detail of the rising flank in figure 7 showing an approximately 1 ns rise time assuming an infinitely short trigger pulse at time zero. For a realistic trigger pulse of (say) 1 ns duration, the rise time will increase correspondingly.
Table 5. Summary of data for the 150 um, 20 kV switch with two different values of C, To achieve the avalanche, our simulations suggest that a charge in excess of approximately 70 nC is required, regardless of how the charge is generated. However, sources of deep UV photons with sufficient output intensity and fast enough turn-on times are not currently commercially available (other than excimer lasers that are prohibitively expensive), so the only practical trigger source mechanism would be irradiation with electrons. Suitable grid-switched electron guns are commercially available.
Table 5. Summary of data for the 150 um, 20 kV switch with two different values of C, To achieve the avalanche, our simulations suggest that a charge in excess of approximately 70 nC is required, regardless of how the charge is generated. However, sources of deep UV photons with sufficient output intensity and fast enough turn-on times are not currently commercially available (other than excimer lasers that are prohibitively expensive), so the only practical trigger source mechanism would be irradiation with electrons. Suitable grid-switched electron guns are commercially available.
Figure 9. Basic unipolar device design: doped channel diamond MISFET. to use the superior properties of intrinsic diamond. In particular, it would be highly desirable to achieve some degree of spatial separation between ionized acceptors and holes. Several workers have made use of the unique ability in diamond to create a conductive (p-type) layer on the H-terminated surface (so-called ‘surface transfe1 doping’). We will not discuss surface devices in this paper, but impressive progress has been made by several groups with headline figures, which include an output power density of 2.1W mm ‘cw at 1 GHz (Vps=—16 V, L,=0.3 pm. W.=2X50 um; Hirama et al. 2006) and fr=45GHz and finax=120 GHz (Vps=—18V, L,=0.1 um, W,=50 um; Ueda et al. 2006). Although these results are promising, the long-term stability of these surface FETs remains an issue, particularly at elevated temperatures and in harsh environments.
Figure 9. Basic unipolar device design: doped channel diamond MISFET. to use the superior properties of intrinsic diamond. In particular, it would be highly desirable to achieve some degree of spatial separation between ionized acceptors and holes. Several workers have made use of the unique ability in diamond to create a conductive (p-type) layer on the H-terminated surface (so-called ‘surface transfe1 doping’). We will not discuss surface devices in this paper, but impressive progress has been made by several groups with headline figures, which include an output power density of 2.1W mm ‘cw at 1 GHz (Vps=—16 V, L,=0.3 pm. W.=2X50 um; Hirama et al. 2006) and fr=45GHz and finax=120 GHz (Vps=—18V, L,=0.1 um, W,=50 um; Ueda et al. 2006). Although these results are promising, the long-term stability of these surface FETs remains an issue, particularly at elevated temperatures and in harsh environments.
Figure 10. MESFET design concepts: (a) P-i-P MISFET, (6) delta MESFET and (c) double-delta MESFET. double-delta MESFET as shown in figure 10c. Here an additional boron-doped layer is deposited on top of the structure, which is used to make contact to the delta layer and thus improve the source and drain ohmic contact properties.
Figure 10. MESFET design concepts: (a) P-i-P MISFET, (6) delta MESFET and (c) double-delta MESFET. double-delta MESFET as shown in figure 10c. Here an additional boron-doped layer is deposited on top of the structure, which is used to make contact to the delta layer and thus improve the source and drain ohmic contact properties.
Figure 11. SIMS data for boron-doped delta layers: (a) single-delta structure and (6) double-delta structure.
Figure 11. SIMS data for boron-doped delta layers: (a) single-delta structure and (6) double-delta structure.
Figure 12. DC output characteristic for a single delta device with Lg=0.7 pm and Wg=30 pm measured at room temperature.
Figure 12. DC output characteristic for a single delta device with Lg=0.7 pm and Wg=30 pm measured at room temperature.
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