An effective mean-free path of hot electrons in the conduction band of SiO2 in a Si-gate metal–oxide–semiconductor (MOS) electron tunneling cathode was measured and found to be about 0.7 nm. Following these observations, we proposed and fabricated a depletion gate MOS electron tunneling cathode. The highest transfer ratio of 13.3% was achieved in the cathode at the low emission current level, which was considerably higher than that of tunneling cathodes studied in the past. However, the ratio decreased drastically at high current due to the hole injection into the depletion region from the gate.
Tel. 22-211-8817 F a . 22-21 1-8817At the last conference, we reported the emission characteristics of an Al-gate MOS tunneling cathode (Figure 1) and pointed out the potential as pressure insensitivity of emission current, low voltage opcration, reproducibility of the cathode and so on1). However, deterioration of the oxide in an Al-gate MOS cathode was remarkable at high field and resulted in rapid reduction in emission current.We fabricated an amorphous Si-gate MOS cathode instead of an Al-gate and showed the emission current was very stable in the cathode, The paper describes the emission characteristics of the Si-gate MOS cathode. Figure 2 shows a typical emission characteristic of the cathode as a function of gate voltage.The emission occurred at nearly 4V corresponding to the work function of Si-gate. Figure 3 shows the transfer ratios for several cathodes with different oxide thicknesses (with a Si-gate thickness of 3Onm), as a function of gate voltage. The transfer ratios for tunneling emission are nearly same at high field for thick oxide cathodes suggesting the scattering process of hot electrons in the conduction band of the oxide is remarkable and makes a steady state energy distribution of electrons after traveling through the conduction band of the oxide. Figure 4 shows the energy distributions of emitted electrons for several gate voltages. The result clearly supports the previous speculation.Additionally, we investigated the pressure dependence of the emission current for the Sigate MOS cathodes and confirmed that the emission current was nearly independent of pressure and occurred even at atmospheric pressure.Following these experimental results, we proposed a new structure of tunneling cathode to increase the transfer ratio and to reduce the energy spread of emitted electrons, as shown in Fig. 5. Electrons tunnel directly through the oxide potential barrier without scattering, after which they are accelerated in depletion layer of the semiconductor, pass though the low work function gate metal and finally they are emitted into vacuum. We expect the cathode is able to generate stable and high current density emission.
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