MOSFET depletion mode

We are calling the new model My. Note that the symbols for enhancement- and depletion-mode MOSFETs are the same. PSpice knows that a MOSFET is either an enhancement- or a depletion-mode MOSFET by the value of the threshold voltage parameter, VtO. 

The first SiC MOSFETs were fabricated by Suzuki et al [1] in 3C-SiC. At the present time, SiC MOSFET research is carried out by several groups, including Cree Research [2-4] and Westinghouse [5]. Enhancement mode and depletion mode (3-SiC MOSFETs have been fabricated by Palmour et al [3]. Enhancement mode devices with 5 pm gate lengths had a maximum transconductance (gm) of 0.46 mS mm 1 at room temperature (see FIGURES 1 and 2). The devices were operational up to 823 K. 

The depletion mode 6H-SiC MOSFETs fabricated by the General Electric group [8] exhibited a fairly low leakage current of 5 x 10 " A at 23 °C at -10 V gate bias and drain bias of 8 V with the on-current as high as 1.6 mA. However, the subthreshold was quite low. These devices operated up to 350 °C. 

FIGURE 5 Cross-section of a depletion mode n-channel (3-SiC MOSFET [3],... 

FIGURE 7 Current-voltage characteristics of a depletion mode n-channel a-SiC MOSFET [7]. 

Analogous to these field-effect devices is the buried-channel, depletion-mode, or normally on MOSFET, which contains a surface layer of the same doping type as the source and drain (opposite type to the semiconductor body of the device). As a result, it has a built-in or normally on channel from source to drain with a conductance that is reduced when the gate depletes the majority carriers. 

The MOSFET described here is a depletion-mode p-type. A depletion-mode n-type is also possible, wherein the n- and p-regions of Figure 18.26 are reversed. 

Another important difference between MOSFETs and junction transistors is that although majority carriers dominate in the functioning of MOSFETs (i.e., holes for the depletion-mode p-type MOSFET of Figure 18.26), minority carriers do play a role with junction transistors (i.e., injected holes in the n-type base region. Figure 18.25). 

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