Now, we are at the final lecture. Lecture 13, we're going to learn III-V Gallium arsenide and metal semiconductor field effect transistor called the MESFET instead of the MOSFET, and then we also going to learn bipolar junction transistor, BJT, and finally, we're going to learn short channel effect. Short channel effect occur when your transistor gate lengths becomes a sub-micron regime. In early 1990, the MOSFET transistor go under the channel lengths of the micron regime, and then the MOSFET transistor I-V curve of phenomenon is quite different than long channel device. So they call this new effect is the short channel effect. Then 1980, people think that if we go to the sub-micron, this MOSFET cannot survive and then it will not develop further. But however, clever semiconductor engineer has came with a solution to serve all of the structural effect, and we comes to the nanometer gate length regime right now. So several, among the many, short-channel effect, we are going to learn most important three phenomenon called threshold voltage lowering, drain-induced barrier lowering, DIBL, and subthreshold swing. First, Gallium arsenide MESFET. MESFET is called MESFET because the Metal Semiconductor Field Effect Transistor, that's why it's called MESFET. This is the one type of the junction field effect transistor. So most junction field effect transistor, and you know the field effect means the electric field in MOSFET or either MESFET. There's two types of the field, electric field between the source to drain and voltages supply the source to drain, then there is the horizontal electric field. Another field is the gate voltage. Gate voltage induced the baltic electric field, and then those two combination of electric field transistor is operating. So in MESFET device they use the, let's say that n-type Gallium arsenide. There's a lot of electron in this Gallium arsenide and there's a semi-insulating substrate. There is no carriers here. Then there's a source drain of the Ohmic contact, and gate as the Schottky contact or rectifying contact. These III-V MESFET does not have a gate dielectric as in a MOSFET transistor, and then dielectric contacting to the metal, to the semiconductor. So two form different contact of Schottky contact or Ohmic contact using the matter. So let's say first, they deposit the matter to the source drain, different matter to the gate, then they annealed it in high temperature to form Ohmic contact. After you form the Ohmic contact, you depositing or patterning on another matter to the gate area that has a [inaudible] function can form Schottky contact. Then you have a Schottky contact and two Ohmic contact. Why are we using the MESFET device? Because of MESFET of III_V material has a very high mobility for Gallium arsenide. Electron mobility is 8,500, maybe six times higher than silicon. So we can feel very high-speed, high-mobility transistor for the communication, etc. So the operating mechanism of the MESFET has maybe, we can have two types of MESFET, normally on and normally off. First, normally on. So as you know, the Schottky contact of the metal semiconductor work function, then there is depletion region. So Schottky contact can be think as a p-n junction, people n junction. So therefore, the depletion region of the Schottky contact, but Schottky contact depletion region is very narrow, so that the source drain voltage applied and current can be flowing. Electron move from the source to drain. In gate voltage is zero. So gate voltage is zero, current can be flowing and transistor on. Gate voltage is zero, current flowing transistor on. To shut it up, this MESFET device, you need to extend or widening the depletion region. To do that, you need to increase depletion region by reverse bias of the Schottky contact. Reverse-bias of Schottky contact is a p-n and then you need to apply negative voltage to the pregion. So negative voltage, if you are applying the negative voltage, then depletion region becomes wider. For depletion is our entire [inaudible] n-type Gallium arsenide, current is no longer flowing because depletion region, there is no carriers. So negative voltage, reverse bias transistor off. So these are the normally on MESFET device. Normally off MESFET device is originally depletion region is very wide and entirely occupying the entire Gallium arsenide. So gate voltage is zero. Then transistor is off. Two current flowing between the source and drain. Then you need to reduce depletion region. To reduce the depletion region less than the thickness of the n-type Gallium arsenide, then you need to apply the forward bias. Forward bias of the p-n junction, you need to apply positive voltage. So positive voltage, you're reducing the depletion region and current to on. So positive voltage, current is on. So gate voltage zero, transistor off; positive voltage transistor on. So let's go a little more detail of the normally-off MESFET. In gate voltage zero, depletion region occupying the entire region of the n-type Gallium arsenide region. Then therefore, current is not flowing at the threshold voltage, which is the off-state at the depletion design directory same as the n-type Gallium arsenide. More further of the high gate voltage above the threshold voltage, then the depletion region is still less than the thickness of the n-type Gallium arsenide, current can be flowing between the source and drain. These depletion region is fixed to the space charge region of the enhancement mode of MESFET. So normally off MESFET is also called enhancement mode of a MESFET transistor.
