Figure 6.34 Rectifying diode behavior obtained from a lateral vacuum diode with nanodiamond 6-finger emitter array as the cathode and nickel as the anode; inset: corresponding F-N plot.
These diode characteristics demonstrate that the nanodiamond lateral device with the non-diamond anode configuration can perform as an excellent diode with rectification ratio >104, suitable for practical microelectronic circuit applications.
0 1 2 3 4 5 6
-12 -9 -6 -3 0 3 6 9 12
Electric Field (V/µm)
6.2.3 Field emission characteristics of monolithic nanodiamond lateral vacuum triodes
Field emission diodes and triodes are critical building blocks in the development of ultra- fast, temperature- and radiation-immune vacuum microelectronics. Having examined the nanodiamond lateral device as a vacuum diode, the research was continued to study the device in a triode configuration. Lateral triodes with a gate-cathode spacing as small as 2 µm, and anode- cathode spacings ranging from 10 µm to 1 mm were developed. The different three-terminal lateral configurations were tested to investigate the triode characteristics of the device. It was identified that the position of the electrodes in the device design plays a significant role in determining its characteristics.
The functionality of the lateral emitter device as a triode or a transistor could be manipulated by lithographically altering the design of the three-terminal structure. The lateral anode, when placed closer to the cathode (~ 10 µm) can induce the electron emission and result in gate-modulated triode characteristics. By moving the anode farther (> 100 µm) from the cathode and gate electrodes, the anode effectively acts as a collector, with the gate, owing to a very small cathode-gate gap (≤ 2 µm), principally controlling the electric field at the emitter finger-tip and significantly shielding the collector field from the emitter, leading to transistor- type characteristics. The operation of the nanodiamond lateral field emission device in triode and transistor modes is described in this section.
(a) Vacuum triode characteristics
A 1-finger nanodiamond lateral microtriode with 3 µm gate-cathode and 12 µm anode- cathode spacings (device structure shown in Figure 6.35 (a)) was characterized for electron field emission in a vacuum condition of 10-6 Torr. The gate electrodes are separated laterally by an
equal distance of 3 µm from the cathode to have effective emission modulation at low gate voltages, while minimizing intercepted gate current. The device was operated similar to the classical thermionic triode, where the electron emission from the cathode is induced by the anode and modulated by the gate electrode. The only difference in this case is the gate voltage modulates the electric field at the emitter tip and hence the tunneling probability, whereas in the thermionic triode, the gate controls the space charge near the emitter. Figure 6.35 shows the I-V characteristics of the triode device as a function of the gate and anode biasing voltages. The gate- controlled current modulation behavior is clearly observed, whereby higher applied gate voltages give rise to higher emission currents induced by a given anode voltage. A large anode current of
~ 4 µA was thus obtained from a single nanodiamond lateral emitter at an anode voltage of ~ 65 V, when the gate bias was 40 V. The low applied electric fields required to initiate and modulate the electron emission between the electrodes are attributed to the geometry and material composition of the nanodiamond lateral emitter. For a given anode voltage, the anode current was found to change by more than an order of magnitude with a ±10 V alteration in the gate bias.
The transconductance, gm, relates the current driving capability, voltage gain, and high frequency response when the diamond triode is operated as an amplifier. It is defined as the change in the anode current due to a change in the gate voltage at a given anode voltage:
(6.7)
This transconductance parameter was determined to be ~ 0.3 µS at Va=65 V, one of the highest reported for lateral-type devices involving a single emitter, especially with the cathode-gate separation in microns. This value can be further improved by applying a larger nanodiamond
Va=const
g a
m V
g I
∂
= ∂
finger array as emitter. The gm was also found to increase exponentially with Vg, as the device electron current is strongly affected by the gate voltage.
Figure 6.35 (a) Structure of the integrated nanodiamond lateral field emitter vacuum microtriode with 3 µm gate-cathode and 12 µm anode-cathode spacings; (b) Triode characteristics of the 1- finger lateral device; inset: F-N plot of one of the I-V curves shown in the main figure.
0 1 2 3 4 5
0 20 40 60 80 100 120
V
a(V) Ia ( µ A )
Vg = 20 V Vg = 30 V Vg = 40 V
-37 -35 -33
0 0.02 0.04
1/Va
ln(Ia/Va2 )
Cathode
Anode Gate
Gate
The device characteristics were found to follow Fowler-Nordheim relationship (inset, Figure 6.35), which confirms that the extracted anode current is due to the field emission mechanism. For optimum device performance, the gate-intercepted current must be small relative to the anode current. Operation in triode mode can allow for a low gate current, especially for negative and moderate positive gate voltages. The gate current for this lateral triode was below the measurable limit of 0.1 nA. The achieved triode device behavior is congruent with the simulated electrical characteristics [90] reported from lateral field emitter triodes, where an increase in gate voltage leads to a subsequent increase in the anode current, owing to the enhancement in the electric field at the emitter tip.
(b) Vacuum transistor characteristics
The demonstration of a lateral field emission transistor at microelectronic scale is essential to the development of the vacuum IC. Transistor characteristics have been reported from vertical emitter devices utilizing different cold cathode materials [164-166,218-220]. This work involves the development of a vacuum transistor, designed and fabricated in a completely integrated, planar, lateral configuration, achieving an advanced, alternative electron device.
A 1-finger nanodiamond lateral device with 2 µm gate-cathode and 500 µm anode- cathode spacings (device structure shown in Figure 5.18 (c)) was tested under vacuum for dc field emission characteristics in a common emitter amplifier configuration. The device was operated in a gate-induced emission mode, with the anode (collector) potential contributing an insignificant component to the electric field at the tip of the emitter-finger. A positive gate voltage (Vg) was applied on the gate electrode to extract electrons from the emitter and a fixed voltage (Va) was applied to the anode to collect the emitted electrons. The anode emission
current (Ia) was then recorded as a function of the anode (collector) voltages while holding the gate voltage constant. The measurements were repeated for different Vg to obtain a family of Ia- Va-Vg curves. Figure 6.36 shows the dc characteristics curves of the lateral vacuum device.
Gate-controlled current modulation behavior was clearly observed, whereby higher applied gate voltages, by means of field enhancement at the emitter-finger tip, gave rise to higher emission current collected by the anode. The gate turn-on voltage was 40 V, defined as the voltage required to obtain 1 nA of emission current from the 1-finger cathode. Saturation of the collector currents was observed at anode voltages above ~ 180 V, where the current stays constant, independent of the applied collector voltage. Overall, the field emission data demonstrated basic transistor characteristics of the nanodiamond lateral device with distinct cutoff, linear, and saturation regions in a triode configuration.
Figure 6.36 DC transistor characteristics of the nanodiamond lateral field emission device.
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
10 60 110 160 210 260 310 360 410 460