## Q. 3.6

A nonequilibrium electron transistor (NET) of the type shown schematically in Fig. 3.16 is to be designed for use in a high-speed switching application. In this situation the NET must have a high current drive capability. To achieve this one needs to ensure that spacecharging effects in the emitter and collector barriers are avoided. How would you modify the design of the NET to support current densities in excess of $10^{5} A cm^{-2}$ ?

## Verified Solution

To avoid space-charging effects in the emitter and collector barriers of a NET, it is necessary to dope the emitter and collector barriers to an $n$-type impurity concentration density $n\gt j/e\upsilon _{av}$ , where $\upsilon _{av}$ is an appropriate average electron velocity in the barrier and $j$ is the current density. For typical values $\upsilon _{av} =10^{7} cm s^{-1}$ and $n\gt 10^{17} cm^{-3}$ this gives a maximum current density near $j=1.6\times 10^{5} A cm^{-2}$ .