Slide 38 of 64
So what happens as the emitter shrinks to a width comparable to base thickness? Several effects come into play. One of them is the Kirk effect which tends to limit the current in the emitter. But in addition there is the Van der Ziel lateral base pushout effect. Although this diagram is for a traditional nested diffusion vertical NPN bipolar transistor, notice that some of the current travels over longer paths out laterally from the emitter shadow or intrinsic region. Similar effects can occur in HBTs if the base is in contact with the collector outside of the intrinsic region. Often this is done to provide the base contacts.