Radio-Frequency Power Transistors Based on 6H- and 4H-SiC
- PDF / 1,199,596 Bytes
- 7 Pages / 576 x 777.6 pts Page_size
- 103 Downloads / 214 Views
Radio-Frequency Power Transistors Based on 6H- and 4H-SiC
* out
50
i
J in
(1)
(2)
Pdc
The maximum efficiency for an rf transistor is 50% under class A (transistor is biased at 50% of its open channel current) or 78.5% under class B (transistor is biased at pinchoff or quiescent drain current /DQ = 0 A) operation; however the maximum possible output power for the device remains unchanged with bias so long as the device is operated at 50% or less of its open channel current. Qualitatively, any rf drive in the region below the knee voltage will result only in resistive loss, which leads to lower output power, decreased gain, and lower efficiency. The increased resistive loss also leads to device heating that further degrades device performance. Thus the ideal rf transistor would have a knee voltage of 0 V.
Basic Operation of rf Transistors Virtually all rf systems require active circuit elements for use as oscillators, amplifiers, and so on. These elements permit conversion of energy from dc bias sources to rf bands where the energy can be used to provide useful gain at specified frequencies. The ideal rf transistor has high current, high breakdown voltage, and a low "knee" voltage (the voltage at which the transistor current saturates), as illustrated in Figure 1. The device is given dc bias at one-half its maximum operating voltage and one-half or less of its maximum operating current, and any rf signal superimposed over the device is IDS
RL
where VDS is the drain bias, VKnee is the knee voltage, and Rh is the load resistance, which is determined by the current level and bias point in the device. The power added efficiency (PAE) -q of the device is another important figure of merit, and quantifies the amount of dc bias that is converted to rf power:
Karen Moore and Robert J. Trew Introduction In recent years, SiC has received a great deal of attention as a nearly ideal material for the fabrication of highspeed, high-power transistors. The high electric breakdown field of 3.8 X 106V/cm, high saturated electron drift velocity of 2 X 107 cm/s, and high thermal conductivity of 4.9 W/cm K indicate SiC's potential for high-power, high-frequency operation. A wide bandgap should also allow SiC field-effect transistors (FETs) to have high radio-frequency (rf) output power at high temperatures.1 These material qualities have been verified through outstanding device performance. Recent results for SiC metalsemiconductor field-effect transistors (MESFETs) have included superior frequency and power performance, with power gain at frequencies as high as 40 GHz2-3 and power densities as high as 3.3 W/mm.4 This represents significantly higher operating frequencies and power densities than current Si rf power FET technology, and nearly three times the power density of GaAs MESFETs, which are currently used in many commercial rf power applications. Similarly, SiC static induction transistors (SITs) have much higher power densities than their Si counterparts and have recently been demonstrated in modules with as much as 47
Data Loading...
